Sizing digital images for output may be one of the most difficult to grasp concepts in digital photography. Even after more than a decade of moderately hardcore Photoshop use I still find new ways to confound myself in this area. This topic is steeped in misunderstanding, urban legend, faulty logic and general confusion. Screen resolution vs. print resolution? What is the correct resolution for the web? To resample or not to resample? How big can I enlarge an image for printing? Why shouldn’t I upload a bunch of full resolution 21 megapixel images to Facebook or email them to grandma? In the next few paragraphs I hope to make it all clear…although I have my doubts.

Before you read any further, if you are a recreational photographer who isn’t concerned with optimal image output, you should maintain your blissful state of mind and avoid reading this article at all cost. If you are quite happy using the basic software that came with your camera to automatically size images for email or upload them to Facebook, then stick with that. If you are having no problems uploading your images to an online printing service (Picasa or Mpix for example) and allowing them to figure out how to generate different size prints from whatever you give them, don’t break what isn’t broken.

However, if you have already gone down the road of using Photoshop to size images for different uses you are already in too deep. You have peered around the other side of the curtain. While you can never really go back, the following information may help you keep your head above water…and hopefully keep you from mixing your metaphors.

Let us begin by setting a couple of things straight. First, if you aren’t already, you should maintain sized output image files separate from your master image files. I never change the size of my master image files. They remain, forever, the size at which they emerged from my camera (unless I do creative cropping). When I’m ready to output the image I create a duplicate of the master and I size the duplicate. After the duplicate has served its purpose, it can either be saved with a different file name or deleted, but the master remains at it’s original size.

Second, it is important to wrap your mind around two different types of sizing and not allow the rules for one to bleed over into the rules for the other. One type of sizing is for screen output. The other type of sizing is for print output. Let us begin with sizing for screen output.

A common misconception is that screen images must have a resolution of 72 ppi. As the freakishly intelligent digital photography guru, Tim Grey, recently pointed out in his daily Q&A, when it comes to screen size the only thing that matters is pixel dimensions. To reduce the size of an image on the screen, all that needs to be done is to reduce the number of pixels in the image. We often get caught up in creating 72 ppi resolution images for the screen, but in reality ppi (pixels per inch) only matters when it comes to print size. For example, a 300 x 200 pixel image at 72 ppi will appear the same size on screen as a 300 x 200 pixel image at 300 ppi. Furthermore, the file size for each will be the same. They both are constructed of 300 x 200 pixels.

If you don’t believe me, take a look at the images below. One was saved at 300 x 200 pixels at 72 ppi and the other was saved at 300 x 200 pixels at 300 ppi. They both measure 300 x 200 pixels in size on the screen, both are 46.9 kb in file size and they both look identical. The only place where ppi makes a difference is in print output size. In this case the 300 ppi image would print at 1” x .667”, while the 72 ppi image would print larger at 4.167” x 2.778”. We will take a closer look at how ppi and print size are related in a minute.

So why are we all sizing our screen images using a resolution of 72 ppi if resolution doesn’t matter? It probably stems from the way the controls in the Image Size window in Photoshop are set up. Inches make more sense to us than pixel dimensions. In the Image Size window it would be intuitive to go directly to the Document Size area and reduce the image by entering smaller dimensions in inches, such as 3” x 2”. At 3” x 2” and 300 ppi the file is smaller, but still 900 x 600 pixels, or nearly 3 million pixels in total size. The next logical step might be to try decreasing the Resolution to further shrink the image. Setting the Resolution to 72 ppi does reduce the image to 182 thousand pixels; a much more manageable size. This is due to the fact that the pixel dimensions have been further reduced to 216 x 144 when the resolution was changed.

Instead of messing with inches and ppi at all, if the pixel dimensions were decreased to 216 x 144 to begin with, and the resolution was left at 300 ppi, the image size on the screen will still be the same…216 x 144 pixels. What would be different at 300 ppi is the size the image would print, but since we aren’t going to print this image it doesn’t matter.

To further flog the point, as long as you size an image for the screen based on pixel dimensions, the resolution in ppi can be anything you want. I still tend set the resolution of my screen images to 72 ppi out of pure stubborn force of habit. Nothing wrong with that as long as I understand 72 ppi is no different than if I used a resolution of 300 ppi or 3,000 ppi.

So, what are the correct steps for down sizing an image for the screen using the Image Size window? Usually the goals are to make the image fit its intended screen space and also decrease file size so the image will email efficiently or load faster over the Internet if needed. Here is my suggestion.

Go to Image>Image Size.

Make sure the Constrain Proportions and Resample Image boxes are checked.

Change the Pixel Dimensions for either width or height to suit your need.

Many social media sites limit images to 720 pixels on the long side and this is also a good size for email. For an image that will be a bit larger on the screen 1000 to 1500 pixels works well, but the best size depends on the resolution of the viewer’s monitor. For slide shows you can select pixel dimensions that match the resolution of the projector.

Click OK and Photoshop will size the image to your specs.

Of course all of this confusion is avoided if you simply go to File>Save For Web & Devices instead. In this window you can take care of a variety of screen output settings, such as color space and file type as well as screen size in pixel dimensions, all in one handy location. Notice that nowhere in the Save For Web & Devices window is there an option to set the ppi…because ppi doesn’t matter for screen output.

Now that we have established best practice when sizing an image for screen output, let’s take a look at sizing an image for print output. Remember that these rules are different than, and must be kept separate from, the rules for sizing for screen output.

While screen output almost always involves reducing the image size and the file size along with it, sizing for print can involve either increasing or decreasing the image output size depending on the size of the print you want and the resolution of the camera that made the image. The size of the print and the resolution of the camera determines whether you will need to resample the image or not. What is this resampling I speak of? Resampling is changing the size of an image by either adding or removing pixels. This is what we were doing previously when we were downsizing an image for the screen, we were removing pixels.

While there is just one way to size an image on the screen, there are two ways to size an image for print. 1) As with screen size, you can change the print size by changing the number of pixels in the image. More pixels produce a larger print and fewer pixels produce a smaller print. 2) You can also change the print size by changing the size of the pixels (it’s actually how closely spaced the pixels are, but it helps me to think of it as pixel size). If the number of pixels in an image is kept the same, bigger pixels will produce a larger print and smaller pixels will produce a smaller print. How do you get larger pixels? When you reduce how many pixels fit in an inch (ppi) each pixel is larger and vice versa. So, the fewer pixels per inch, the larger the pixels and the larger the print size.

OK, I lied. There are actually three ways of sizing an image for print. 1) You can change the number of pixels (resampling). 2) You can change the size of the pixels (resolution). 3) You can change both the number and the size of the pixels at the same time.

My preference is to size an image for print without resampling (adding or removing pixels) if possible. While all recent versions of Photoshop do an excellent job of resampling, at some point the process of adding or removing pixels to change the size of an image does start to degrade the quality. Upsampling usually causes more problems than downsampling when it comes to printing.

Based on testing done by Mac Holbert, as well as my own experience, I have developed some basic guidelines for deciding whether I’m going to change the resolution, change the number of pixels (resample) or both when sizing for print output. As previously stated, my first choice is to size the image by changing pixel size (resolution) and avoid resampling if possible. We have all heard that 300 ppi or perhaps 360 ppi is the optimal print resolution. The reality is that the human eye can’t tell the difference between 200 pixels per inch on a page and 400 pixels per inch. In fact, I have used resolutions as low as 150 ppi on canvas with no noticeable difference in quality from 300 ppi. A colleague says he often prints at 100 ppi and is happy with the results. However, at some point the enlarged pixels at lower resolutions will become apparent and will degrade the image quality. Where this point is precisely, I don’t know. Depending on image detail, some images may be able to hold up at lower resolutions better than others. To be safe I use 180 ppi as my minimum print resolution on paper and 150 ppi on canvas.

Going the other way I can’t say I have ever noticed a quality loss from increasing the resolution too much, although I haven’t tested extremely high resolutions. From my 21mp camera I have sized images down to 6 x 9 inches by increasing the resolution to over 600 ppi and they print fine on my Epson 4880. I generally downsample to 200-300 ppi if I’m uploading the image for printing at a lab. Some online labs, such as those using the Chromira printer, indicate that is what they want, so I oblige. It makes sense to downsample in this instance anyway because a 200 ppi file will upload much faster than a 600 ppi file for the same print size but it will look just as good to the eye.

If I can’t achieve the print output size I want after lowering the resolution to the 150-180 ppi range, then I know it is time to resample. Photoshop does an amazing job of upsampling to quite large sizes. Without any resampling, an image from my 21mp camera will produce a 20 x 30 inch image at 187 ppi. Photoshop can upsample that image to 24 x 36 and even 30 x 45 at the same 187 ppi resolution with very minimal loss of quality. I have even printed several 40 x 60 prints with great results, especially when viewed at a normal viewing distance for that size image. The degree to which you can successfully enlarge an image for print depends on the quality and resolution of your camera sensor, your camera technique, your developing technique, the amount of detail in the image, the media the image will be printed on, the viewing distance and your own expectations.

So, now that I have thoroughly confused you with all that, what does my workflow for sizing for print output look like?

Go to Image>Image Size.

Uncheck the Resample Image box (we want to size without resampling if we can).

Enter the desired print output width or height in inches.

See what the Resolution will be at that print size.

If the Resolution has not dropped below the 150-180 ppi threshold, click OK and you are ready to print without any resampling necessary. Because you haven’t resampled the number of pixels in the image you won’t notice any change in the size of the image on your screen.

If the Resolution has dropped too low you know you will need to do some resampling.

Check the resample box and then change the Resolution to something acceptable (180 ppi or higher).

Remember that the higher the Resolution you choose the more upsampling will be needed to reach the print size you want. Too much upsampling will lead to loss of image quality so you are trying to find the best balance between too little resolution and too much upsampling. Really big (billboard) images can get away very low resolution (50 ppi) because they will only be viewed from very far away.

Now, with the Resample Image box still checked, enter the desired print dimensions in inches if they aren’t already what you want.

If you are making a substantial enlargement via upsampling you can use the Bicubic Smoother rendering option to smooth out edges that become pixelated, but for moderate upsampling the standard Bicubic option works just fine.

Click OK and Photoshop will add (or remove) pixels to generate an image that will print at the size and resolution you have asked for.

At one time it was believed that upsampling in 10% increments gave better results. However, the interpolation software in Photoshop has been improved to the point that, for the last decade or so, there is no noticeable benefit when applying this technique.

I should also point out that Photoshop isn’t the only application available for sizing images. Pretty much any image editing software gives you the ability to size images, although the sizing options in the most basic programs are severely limited. Adobe LightRoom has an export feature that makes the job of sizing images much simpler than Photoshop, although all the general principles and rules discussed above still apply.

So there it is…sizing images for output demystified (or at least not further mystified I hope). Remember to keep the rules for sizing for screen and print separate. Once you have sized an image for any particular output you may want to make further preparations such as sharpening, converting to a different color space, changing bit depth and selecting file type for saving. As we covered earlier, it is important that any sized image be saved separately from the master image file.

Please feel free to leave a comment if you have any questions or anything you would like to add to the discussion for the greater good. This information and much, much more is available in my image developing video tutorials which can be purchased and downloaded at OutdoorExposurePhoto.com.

Posted in Image Processing

Tags: digital image processing, digital photography, digital workflow, Image Processing Tips, image sizing, photo processing techniques