Image size, resolution and resizing images for the web

This is a simple yet ridiculously complex topic (yes I realise that’s a contradiction, but it’s nonetheless true), and one where there is an awful lot of misinformation floating around. Even from professionals (actually, a lot from professionals). But it is an area where a lot of non-imaging professionals have need of a different perspective and some foundational skills when it comes to resizing digital images for various purposes. So, this is my attempt to counteract some of the misinformation out there. Huge caveat here: My main field of knowledge is in photography. I have been fiddling with cameras my whole life, and have taught photography and digital imaging at local colleges for the last five years, and I feel like I have a  reasonably authoritative voice on the topic. I also make WordPress websites. While I can read and write HMTL and CSS, I am more a designer than a coder, and I don’t profess to be know everything about the technical intricacies of web design. And I certainly don’t profess to knowing everything there is to know about pixels either, particularly about pre-press and the printing industry. So if, in spite of my experience and research, I am misinformed or mistaken about some point or other, I welcome any corrections.

Only pixels matter!

Pixels

This is a great starting point. Digital imaging capture devices (aka cameras, but also scanners) have a fixed number of pixels they can capture in one image. A Canon EOS 5D MkIII for example can capture an image at 5,760 x 3,840 pixels. If we multiply those together we get 22,118,400 pixels, which reflects that it is a 22 Megapixel camera (22 million pixels). A Nikon D800 captures a whopping 7,360 × 4,912pixels (a 36.3MP camera). The top of the line Phase One Digital Camera back, the IQ180, captures a staggering 10328 x 7760 pixels (over 80 million pixels!). So pixels are finite and are linked to the capture device. Pixels are our starting point. They are the raw material of our images, whether we are are printing or just displaying the image on a screen.

The other main player in the story is resolution. Resolution has many different meanings, and is quite complex when you look into it. Broadly speaking though, resolution is basically referring to the detail an image has. The higher the resolution, the more detail there is. But in this introduction, the type of resolution concept I want to introduce is pixels per inch, otherwise known as ppi. The best way to think about how resolution works together with the number of pixels captured in an image, is that “the number of pixels is determined by the capture device, but pixels have no fixed size”. A pixel is a unit of a digital image, but overall resolution depends upon the size of the pixels (and the viewing distance). The smaller the size of a pixel, the higher the resolution of the image will be and the clearer the image will be.

PPI vs DPI

Before we go any further, I think it’s important to clarify the oft misused terms dpi and ppi. DPI is a printing term, and it stands for DOTS PER INCH. Printers lay their ink down in dots, and this figure is not the same as PPI. Dots have space in between them, and pixels don’t. DPI doesn’t have any real relevance to our discussion today. PPI on the other hand, which stands for PIXELS PER INCH, is a term that relates to digital images. Pixels, or picture elements, as we have established, are what digital images are made up of, and are square. The terms DPI and PPI are NOT interchangeable, and PPI is the correct term to use when talking about digital images. I’m not going further into the whole DPI / PPI thing here, but there are some good articles going into much detail on this subject here, here, here and here. Enjoy! 😉 I really like Wikipedia’s simple definition: In printing, DPI (dots per inch) refers to the output resolution of a printer or imagesetter, and PPI (pixels per inch) refers to the input resolution of a photograph or image. DPI refers to the physical dot density of an image when it is reproduced as a real physical entity, for example printed onto paper, or displayed on a monitor. A digitally stored image has no inherent physical dimensions, measured in inches or centimeters.

Resolution for print

So before we look at images for the screen, it will be instructive to look quickly at images intended for printing. So let’s look at how the resolution (ppi) of an image combines with the pixel dimensions when printing an image. (Again, I am a photographer, and here I will refer to c41 process photographic printing, not inkjet or other dot based system) As you can see here in the Photoshop Image Size dialog, Figure 1.1, digital image information is separated into two sections. The top section shows the aforementioned pixel dimensions of the image, and below that is the Document Size area. This shows the physical size of the image if it were to be printed, by taking the number of pixels, and dividing it by the number of pixels per inch. In this example, we have 5220 pixels on the longest length, and if we divide that by 300 we get an image of 17.402 inches. What this shows us is that print size is simply a result of how tightly we squash together the pixels we have. If we increase the ppi, say to 400, without resampling the image (adding to or removing from the total number of pixels), the document size goes down to just over 13 inches. If we spread the pixels further and reduce the ppi to say 240, it becomes a 21.75 inch document. That’s what I mean when I say pixels have no fixed size.

Resampling

To consider resampling, let’s use an example. If we wanted to print an 8″ x 12″ print from the image this dialog represents, how do we go about resizing it correctly? Firstly, we have to understand what resampling is. In a nutshell, resampling is resizing an image by reducing or increasing its number of pixels. So if we want to change the size of the print but keep the resolution at 300 ppi, which is a common standard from a lot of professional photo labs, we simply type the new dimension into the dialog, and Photoshop will throw away or create the needed number of pixels to get the dimensions to work. When the Resample Image box is ticked, changing the resolution (ppi) or the document size will affect how many pixels there are in the image, but not each other. Now look at Figure 1.2. If, for example, we type 8 inches into the width field, the resultant dialog would look like this. Note how changing the width to 8 from 11.602 has two effects. One is that the length now becomes 12. This is simply because the aspect ratio of the original image (2:3) was the same aspect ratio as an 8×12″, so changing one to 8 meant the other one automatically became 12″. If the image did not have the same aspect ratio as our desired 8 x 12″, then cropping would be needed instead, but that’s another story for later. The second change we can see is that the pixel dimensions have changed. Photoshop has thrown the un-needed pixels away (by way of a very complex algorithm no doubt), resulting in an image that is now only 3600 pixels on the longest length. This gives us the correct relationship between the document dimensions, the resolution and the number of pixels. 3600 pixels, spread out at 300 pixels per inch gives us 12 inches of image. If we were to change the resolution of the image with resample ticked, the document size would remain the same, and again, the pixel dimensions would be altered to suit. Now look at the third dialog screen grab, Figure 1.3. Let’s say we want to print our original image as an 8×12″ but we also want to print it on an Epson inkjet at home, and that 240 ppi was sufficient. From our original image, we have to change both the resolution and the document size. We can do this in one sweep in this manner: First with Resample Image checked, we type in our desired document size (the pixels get reduced to 3600 on the longest length at this stage). But before we click OK, we now change the 300 ppi to 240 ppi. The pixel count decreases yet again, leaving us with 2880 pixels on the longest length (12 x 240 =2880) and an 8 x 12″ image printed at 240 ppi.

Image Size Dialog 1.1 - resizing images for the web

Figure 1.1

Image Size Dialog 1.2 - resizing images for the web

Image 1.2

Image Size Dialog 1.3 - resizing images for the web

Image 1.3

It should be clear by now that resampling is a dangerous activity. 😉 It’s OK to resample, but you have to understand what you’re doing. If you are resampling down, you are throwing away information from the file, and this can’t be regained (unless of course you are working on a copy). But as long as you know what you’re doing, it’s fine and is totally necessary in many situations. You can also resample upwards, which is known as interpolation, but this is more limited. In this process, Photoshop (or other software) is “creating” pixels from nothing (again, those complex algorithms come into play) and so there are limits to how much you can do this. In the right hands, it’s actually quite amazing how much you can interpolate files,  and combined with judicious sharpening and printing, you can make a rather large and awesome looking print out of a typical DSLR file. A final note here is that we can always change the resolution of an image without it affecting the number of pixels in the image. This is important to grasp. The resolution (ppi) is simply a printing instruction. If we change the resolution without changing the number of pixels in an image (by making sure Resample Image is unchecked), we DON”T change the size of the file either. The only two factors controlling file size are number of pixels and image format and compression (OK, there’s arguably a third factor which is the amount of detail in the photo, which affects compression). When we uncheck the Resample Image checkbox, the pixels are greyed out in the dialog box and the only thing that can change when changing the ppi is the document size. And as I’ve said, this has no effect on file size. But it is useful and important to be able set the desired ppi when printing to different devices, and doing it this way doesn’t affect the pixels.

The 72 ppi myth

The 72ppi Myth - resizing images for the web

 You might have heard the suggestion that it’s important to save images for web at 72 ppi (or perhaps they even said 72 dpi if they didn’t understand the difference between ppi and dpi). The reasons for this are given as many:

  • It reduces the file size – Rubbish. PPI doesn’t affect file size at all. Compression does.
  • That’s what resolution screens are at – You don’t have to match the image ppi with the screen ppi. And besides, most monitors are more than 72 ppi anyway.
  • It’s the industry standard – This is probably the best argument of the lot, as it is a sort of industry standard. It’s just that it’s an industry standard for no good reason.

In fact, it is not important at all to set images to be 72 ppi for the web. It doesn’t hurt, but it’s not important. The ppi of an image makes no difference to the file size and neither does it make any difference to the visual appearance of an image on a monitor. It would of course have an effect on how big an image was printed as it is a printing instruction, but when viewed in a browser, it is the monitor itself that determines how much space the pixels take up on the screen. Let’s look at the three images below. The first has been saved at 72 ppi, the second at 300 ppi and the third at 1000 ppi. See any difference? No, of course you don’t. They all share the same physical pixel dimensions (300 x 450 px), and that’s the only thing the browser cares about, so the logical instruction of ppi makes no difference unless we print them. So hopefully, that’s all you need to put that old chestnut to bed. PPI has no effect on file size, and neither does it affect on-screen appearance.

Asher @ 72ppi - resizing images for the web

Asher @ 72ppi, file size: 54kb

Asher @ 300ppi - resizing images for the web

Asher @ 300ppi, file size: 54kb

Asher @ 1000ppi - resizing images for the web

Asher @ 1000 ppi, file size: 54kb

Where the 72 ppi myth originally comes from is that screen resolution, in the first Macintosh computers back in the mid 80s, used to be 72 ppi (follow this link to an interesting more in-depth article on the subject). But it’s not a fixed number and nowadays it’s a lot more on most monitors. Some screens are around 96 ppi, some are up around 120 ppi, while the latest screens on tablets are achieving mind blowing resolutions of over 500 ppi (Retina eat your heart out). To figure out exactly what resolution your monitor has, simply measure the width of the screen area in inches (not the whole monitor) and then find out what the native resolution is. My monitor’s native resolution is 1920 x 1080 (full HD) and my screen width is 18.622 inches. This gives me a screen resolution of  just over 103 ppi. In other words, 103 pixels in an inch of screen real estate. Just to be thorough, I tested this by resizing an image to 1030px and opened it. And yes, it measured exactly 10 inches across on my screen. This is an area of much debate, and it is generally print designers who argue about these numbers. Design is not just screen based, and ppi and dpi have major implications in the print world. But my essential point remains. For web based viewing of images, it’s the screen resolution, in conjunction with the number of pixels, that determines how big the image is displayed on a  monitor, not the inbuilt ppi instruction in the image. Another very interesting article discussing this topic is The Myth of DPI. It has its own inconsistencies (a pretty big one even in the title, as he confuses dpi with ppi from the start) but it is essentially a good read. Even perhaps more interesting and informative than the article itself though, are the comments. There are loads of them, many from quite educated graphic designers with a wealth of knowledge, and the discussion gets quite heated at times. If you have a spare half an hour and the interest, you will learn a lot by reading the various perspectives of web and print designers, and those whose workflow does require a bit more consistency and awareness about the ppi of images.

Resizing images for the Web

When you understand that there is no real need for setting the ppi in images destined for the web, resizing becomes a lot simpler. Basically, you resize the image in pixels at the size you want it to display. Did I say simple? Well unfortunately, here we enter a completely different world of pain, that of pixel based web design. But at least you don’t have to worry about ppi ;). If, for instance you had a WordPress or HTML template that you know is 940 pixels in width and you wanted an image to fill the content area, then around 900 pixels in width would be a good size (you might want to leave a bit for padding). With resizing images for the web, the number of pixels is the only relevant detail. Forget the ppi of the image, and remember it’s the resolution of the screen and the number of pixels in the image that will determine the size of the presented image on screen (disregarding CSS and Retina and some JavaScript at this point). You could argue that it makes sense to set the ppi to 72 anyway. Fair enough, I suppose. It would give consistency. I only rail against the mistaken belief that it is important to do so. If you use Save for Web in Photoshop, you might find it doing it for you anyway. A quirk of the software is that if you save the image with metadata, it won’t change the ppi (as that’s in the metadata), but if you strip the metadata out when saving (by choosing None), and then open the image in Bridge or Photoshop again, you will see that it’s now set to 72 ppi. I believe this is because the field can’t be empty, and so Save for Web just puts in the default value (which Adobe has set at 72 ppi). I use Photoshop to resize my images, but you can use virtually any decent image editor. You can even do it online with services like http://www.shrinkpictures.com/ and http://www.picresize.com and many others.

Image Resizing and Screen Resolution

Let’s come back to screen resolution, as it majorly affects how big or small we resize our images. One of the biggest difficulties for web designers over the last decade or so was always what size images should be. Web design is a mixture of rendered, flexible elements like text and fixed, pixel based elements like images. Screen resolutions come in all sorts of sizes, but to design a web site with pixel based elements, you have to determine how wide your content area is going to be. There’s no point putting a 2000 px wide image in a webpage if 90% of the population are using screens with a  maximum 1024px resolution. The image is just going to be too big to be seen on the screen. So determining your audience and designing to them was always a big part of the process. It has been changing a lot though in the last decade. You can define image size with CSS or HTML (i..e upload an image that’s 800px wide and tell the browser to display it at 400px wide), and there are also java scripts that can automatically resize an image to fit different screen resolutions and new vector based formats like svg are coming through. But designing for the most common resolution of the day is still a major consideration. The most common resolution today is 1366 x 768 (http://www.w3schools.com/browsers/browsers_display.asp) but with the explosion of different form factors, even that particular resolution has only 25% share. Obviously with responsive templates and the like, a lot of changes are going on in the web design space when it comes to images and how to deal with them. But the most basic part of getting started in this area is understanding that unless you have a technology at play that will repurpose images on the fly, you are best off resizing your images to fit your template and your audience. If you look at the current WordPress templates for example, most have a content container width of 940 or 1000px. This is to ensure that the content container will never exceed the resolution of the screen. As less than .5% of all users have resolutions less than 1024px, this is a safe bet. In the next year or two, my guess is we’ll see a jump to 1300px or so, as 1024px resolutions comprise only 9% of users and is falling fast, while Retina style resolutions are increasing quickly. For years I had a photoblog, and I uploaded photos several times a week. It was my practice to upload images at 800px width if they were landscape oriented, and 650px if they were portrait. At the time, that meant that the images would largely fill the screen of up to 95% of my audience. These days, an 800 px image on 27″ iMac looks rather small. But it would still look pretty good on most laptops and tablets. Retina displays have made things even more complex.

CSS and JavaScript

Another complexity I am going to very quickly touch on, is CSS and Javascript. With CSS you can tell an image to fit a certain percentage of the container, and with some javascript (Lightbox and the like) has the ability to dynamically increase or decrease the size of the image depending on the resolution of the screen. I don’t know a whole lot about this side of things, not being too much of a coder, but I can see a time when we just upload a large image and the code helps us display it in all all sorts of sizes, We’re pretty much there already, but not quite. You can also use HTML as I’ve said earlier to display an image at a size other than it’s original size. Trouble with this, is you often get artifacts (jagged lines) and lose quality in the image. You are also downloading a bigger file, only to present it at a smaller size. Responsive templates obviously do this sort of thing today, as images get squished down to fit on a  tablet or  mobile phone. All this sort of dynamic resizing is way beyond the scope of this article, and any knowledge I have, so I invite someone who knows about that side of images for the web to step up and educate us all with a similar article.

Save for Web

Save for Web dialog - resizing images for the web

Save for Web dialog The Save for Web dialog in Photoshop is a tool to compress images for use on the web. It outputs to one of four file formats, gif, png and jpg, plus the basically unknown Wireless Bitmap format (wbmp). Let’s start with the most commonly used format, jpg. This is the best format for continuous tone images for the web. They can be heavily compressed, while still retaining a lot of their detail. As noted before, there are two main things that determine file size – the number of pixels in the image and the compression applied when saving as a jpg. Save for Web is designed to be used on images that are already resized, as it is primarily a compression tool, but even if you load a full size image into it, it will load anyway after warning you it wasn’t designed to do so. You can then resize the image as well, but I prefer to resize the image first using the image size dialog and then compress for web using Save for Web. The basic compression choice comes in the form of either presets (high, medium low etc) or a slider that you set anywhere from 0 to 100. In the example above, it took a 1.91MB image and reduced it anywhere from 180kb at 70 level quality right down to around 27kb at level 1 quality. How much you compress is up to you, but I find the best balance for size and quality to be around the 50 mark. The compression will also depend on the amount of detail in the image (the more detail, the less redundancy is possible). Gifs is the traditional format for graphics, like logos and such, as they support transparency and a smaller colour palette, but these days most designers are using 8 bit png files instead. They also support animation, but are not recommended for continuous tone images. They can support 256 colours but are much larger than an equivilant jpg. Png files support transparency, which is very useful in web design, but pngs are rather large even out of the Save for Web dialog. That said, there are ways to further reduce the size of pngs, including a great little online tool https://tinypng.com/ which compresses the png file while retaining transparancy. As a basic rule of thumb, use jpgs for photographs on the web and png for graphics. For a much more in-depth look at these formats, check out this excellent  article on sixrevisions.com

Cropping and Aspect ratios

This is a topic close to my heart. (OK, OK, I’m a nerd. Get over it). As I mentioned before, aspect ratio is inherent in the captured image, as it comes from the capture device. If you want a square image, you either have to shoot it with a 1:1 aspect ratio (Hasselblad anyone?) or you need to crop, which is totally different from resizing. Cropping is both resizing and changing the aspect ratio. Cropping is something to keep in mind when shooting. You might know you want a 1:1 shot, and so you compose a certain way in camera as if you were shooting with a 1:1 ratio. Or you can creatively crop after you have taken the image, finding new images in the image by selective cropping (as long as you have plenty of pixels to start with you won’t lose too much quality unless you crop away most of the image, and particularly not if you’re just going to web). Over the course of centuries, we have become used to certain aspect ratios in our two dimensional visual art. Many have their foundation in mathematics, or in that strange place where aesthetics and mathematics meet. The Golden Ratio is a good example of this (this is the “perfect” ratio). The beauty and simplicity of the square is of course another. Digital SLRs create images in a 3:2 ratio (close to the Golden Ratio but not exactly). TVs and computer monitors were commonly in the 4:3 ratio, as are most compact cameras an 16:9 is now a common wide screen ratio. The point of all this is that aspect ratios matter. When you crop, I think it’s much better to stick to an established aspect ratio than to just crop willy nilly. This is where some web designers, particularly WordPress template designers, could do a lot more work. Images in WordPress is another complex topic, as there are so many other things at play when dealing with templates and columns and forced resizing and WordPress image resizing etc, but personally I think it’s best to stick to a bunch of established aspect ratios so that if you have to crop an image to re-purpose it, you always do it in a consistent manner.

Colour profiles

This is another huge area, and one that my interests have never taken me to. I’m very interested in digital asset management, but colour management: yawn…. I’ll do my best to not make a fool of myself. In fact, to be safe, I will simply say that when saving images for the web, it is best to ensure they are saved with an sRGB colour profile, as this matches most closely the gamut of a monitor. Adobe RGB has a larger gamut, but images in this colour profile can look very flat and washed out on the web. Most cameras shooting jpgs are set to shoot in sRGB anyway, but if you are shooting RAW, you have to set the profile yourself when the data becomes an image. If you use Save for Web, you can get it to convert to sRGB when saving, which is probably the easiest way to go about it. It can also embed the colour profile as well, for those few browsers that actually check to see if an image has one.

Retina Displays

This is an area where I have no expertise whatsoever. and so I will refrain from making a fool of myself. As I learn more, i might make some cautious statements here.   OK, that’s it. I hope you have learned a few things along the way. Thank you for reading. If you dispute anything, please bring it up in the comments. As long as you are reasonably polite, I will reply, and we might all learn something.

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22 Comments

  1. Robert June 1, 2017 at 2:58 am

    Nice article – I am just getting started and much of my previous work was all done at 16:9 aspect ratio (72 dpi), some of it is at 5312 x 2988, other work is at 3264 x 1836. I am trying to find out what the best optimum way is to crop photos for standard size printing, ie. 8×10, 11×14, 16×20, etc. Also, based on the aforementioned figures, what is the largest size prints I can have made (while still keeping the image quality decent).

    Is this something I should crop on an image by image basis, or as long as the majority of the subjects are centered, is there a free cropping / conversion tool that would batch handle photos? Later – if it’s what I need to do, I can shoot raw and import photos, but years of pictures I have taken has typically been at 16×9 aspect ratio. Now that I am getting into printing – is there a commerical clearinghouse that handles printing / drop shipping, possible with online tools available to already handle this, or am I just going to have to do it all by hand?

    Any recommendations would be greatly appreciated.

    Thanks!
    Robert

  2. Tom May 3, 2017 at 3:04 am

    ok, here comes the stupid question….my publisher for a new book wants 300DPI images. I have shot them in raw format that downloaded to the computer for minor touch ups as 6000×8000 images, but at 96DPI. From all I have read the images appear to be great for printing a 8×10 book but the DPI number is killing me!

    thank you and yes, you all can laugh all you want, I only need to know they will be fine.

  3. djaef April 6, 2017 at 3:38 pm

    Have a look on my homepage for a link to my new ebook on the subject.

  4. Karen March 30, 2017 at 3:07 pm

    Thank you for your research and advice!
    Now to try to put this into action.

  5. […] color profiles of the images that you post have the tendency to be of a different setting than the one they are going to be […]

  6. […] Image size, resolution and resizing images for the web … – In this article, we will look at digital imaging concepts to do with resolution, image size and resizing images for the web and bust the 72ppi myth forever. […]

  7. djaef January 2, 2017 at 11:26 am

    Torri, they just need to be the same pixel dimensions. So if one is 300 x 300px, all the others need to be cropped to that aspect ratio and size.

  8. Torri December 29, 2016 at 3:37 am

    I am about to show how little I know but that is why I am here. I want to do one thing and one thing only. I can’t figure out how no matter how much I read. I want to have profile pictures of board members on a website. They need to be “physically” the same size. In other words they need to line up neatly in a row. I can’t make it happen.

  9. djaef December 27, 2016 at 2:01 pm

    Bobbo, I would resize to around 2000px wide and compress to 50-40% to achieve a good file size.

  10. bobbo rhino December 24, 2016 at 6:37 pm

    Hey nice article this is kind of what I was looking into. So Im designing my website and ran into the image dilemma. How to get the smallest file size but the best quality, and how to be able to have my images accessible from all devices and fill the width of any screens. My lumix zs-40 camera takes JPGs around 5-7 MB in size, 4896 x 3672. What I did was use a command in mac that runs through photos in a folder and reduces the pixel dimensions while retaining aspect ratio ( sips -Z 1632 *.JPG ) shrinking the pixel dimensions by 3x (1632px wide) which gives me an ok size, 300-1000KB. But its still too big I think for a website. Also for people with widescreen 16:9 1080p monitors I want my images to fill their screen as well and look good, so 1536 or 1632 wide is a little short of this goal. What do you think about this: Shrink my pixel dimensions to around 1920 -2000 px wide, and then compress the jpg to a reasonable size (you mention 50% preset) . This would allow my photos to fill widescreens and hopefully have a smaller size but decent quality. I will do some experimenting.

  11. djaef December 21, 2016 at 12:38 pm

    Hi Cheryl.
    It’s not the resolution you need to increase it’s the pixel dimensions of the images, and the best way to do that is go back to the source file. They would have been bigger than 840 / 900px when originally taken. If that’s not possible, you can interpolate the images but only so far. Don’t know about free programs, sorry. I use Photoshop. Try google for “free interpolation software” or something like that. Good luck.

  12. Cheryl E Nevin December 17, 2016 at 4:06 pm

    Hi, I am working on creating a photo blanket, 50X60″ and I have chosen 6 photos. Some of my photos have low resolution something like 840X480 or in the 900’sX600’s. When I chose the photos that had those resolutions the online site stated that the photos had a low resolution. How can I increase the resolution on photos when they are in png and jpeg file format? Is it possible to do so and if so, is there a free download that offers that? I have Windows 10. Any help/suggestions would be greatly appreciated. Thanks.
    Cheryl Nevin, Waterloo, Ontario

  13. djaef August 17, 2016 at 1:44 pm

    Nathan, my comment was about changing ppi without resampling at the same time. Changing ppi on it’s own does not affect file size at all.

  14. Nathan Crause July 20, 2016 at 7:29 am

    With regards to your claim of “It reduces the file size – Rubbish”, I’ll admit I’m a tad perplexed. And my confusion comes from using ImageMagick under PHP. Using a sample JPEG (a rather large photo of a galaxy), using purely JPEG compression to 60, I get the file to reduce from 35.8MB down to 7.7MB – however, if I resample the image to force it to 72×72 then the resulting file is 3.4MB which seems fairly significant to me. Do you think the actual process of resampling is creating a situation where the compression purely works better (in which case it’s still better to resample since it triggers this additional compression)?

  15. Michael Scott June 3, 2016 at 9:58 pm

    Fantastic work! Thanks for the info!

  16. Kathy January 28, 2016 at 3:34 am

    Much appreciated. Very helpful. Many thanks.

  17. Dana Powell December 11, 2015 at 5:47 am

    Alan2102 – have you found a solution to the problem you describe above? I find the same thing and do not know how to fix it. Thanks.

  18. Riley Adam Voth November 25, 2015 at 4:21 am

    Wow you put a lot of research, thought, and time into this and I appreciate it. I stumbled onto it doing a search for “why do I have a thin line around my image on high resolution screens” and though I didn’t get that answered, not exactly, I have a better idea and still thoroughly enjoyed geeking out on this article. Haha. Thanks for writing it!

  19. djaef November 10, 2014 at 9:50 pm

    Alan, this shows how complex resolution is. Resolution is also affected by size and viewing distance. Take the lcd on your digital slr. The images appear sharp on there, and when you get them home, sometimes they are not so sharp, right? That’s an example of the same number of pixels not being viewed at 100% but perhaps only at 10% or 20%.So therefore it looks sharper, just as if you enlarge an image too much you will start to see degradation.
    In Photoshop, if I reduce the size of an image (say from 5000px in width to 300px in width) and still view that image at 100%, I do not see ANY loss in visible resolution. Of course if I view it larger than 100% it is jagged and pixelated. You can’t throw away 80% of the pixels and still expect quality at the same size. But if you don’t view it larger than 100% there is no degradation in the visible resolution.
    If you can show me some examples of what you mean, I might be able to better answer you.
    Thanks for commenting.

  20. djaef November 10, 2014 at 9:40 pm

    Sam, I am no expert on InDesign, but I do use it to design printed matter. I just exported on of my layout at 300ppi and it didn’t change the document size at all. But I can see that when you create a new document, you choose an intent – print or web. My guess is this sets the resolution (ppi) at the time of making the document, and if you then change it, it will change the document size as it’s not interpolating. You’d have to do some more research, but perhaps there’s no way to decouple the ppi and the pixels within InDesign like there is in Photoshop. Hope that helps.

  21. Sam November 6, 2014 at 8:38 pm

    Great post, really interesting array of information.

    However I’m trying to understand the ‘resolution does not affect image size’ part. This may be the case in PS and possibly AI (not tried it). But occasionally I have to create an image/icon/button for web using Indesign. I set the document up as a document for web and put in my document size (110x46px) for example… When I export that as a PNG to be used on the web it gives me a small menu where I can input resolution (this by default is set as 72ppi). Upon reading this post I decided to try outputting my images as 96ppi as it should not affect file size and there is no harm in having an image with higher resolution. My result was a PNG file that no longer measured as 110x46px, it had in fact increased the image size due to me changing the resolution? I tried again at 72ppi and low and behold it came out the correct dimensions.

    Whats going on here?!

  22. alan2102 October 25, 2014 at 2:26 am

    Nice writeup. But it does not address what I’ve found to be a very vexing problem, apparently without any solution (that I can find, after quite some hours of searching). That is, loss of resolution when reducing image size. This happens INVARIABLY, with numerous software items (paint, paint.net, irfanview, inkscape, etc.) and online services (picresize.com and the like). I’ve tried all kinds of tricks, converting to pdf and trying to re-scale, and more. NOTHING works. This is so funny, because in the olden days it used to be that reducing the size of an image invariably increased the resolution; even fuzzy images became razor-sharp if you reduced them enough. Back in the 1970s, I worked briefly on a Kenro vertical camera, doing resizings, and saw this up close. Somehow, I cannot wrap my head around this new reality — i.e. of the opposite. Reduce image size and… LOSE resolution?! It seems incomprehensible to me. But that is the way it is, and there seems no way of avoiding it. But then, there MUST be some way of avoiding it! I mean, are ALL photo professionals satisfied with this state of affairs? Of not being able to create, say, a bunch of thumbnail images (from larger originals that of high resolution) without serious deterioration of image quality? It seems bizarre to me that I cannot find a shred of information about this, much less a reliable solution. It is as though no one else really notices it. Crazy.

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