A solution to resolution – Part 2 (Image Size explained)

NOTE: This post was prepared and edited from a book I designed, wrote and attempted to publish in 2017. -P

A typical consumer DSLR camera might come with an image resolution of 24.3 MP (24 megapixels). So exactly what does that mean anyway? It means that at the highest quality setting – Camera Raw or the .jpeg “L” large setting – an image file can be produced that contains 24,160,256 pixels. This will, at an input resolution of 300 ppi (Nikon D750 default input resolution) produce an acceptable 11 x 17 inch print at 360 ppi.

How do I know this? A look at the Image Size Dialog (Figure 1) after opening the image in Photoshop, tells us exactly what we need to know about the image file. I’ll illustrate four examples typical of how I use the Image Size dialog to prepare images: 1. Default camera data; 2. Inkjet reference print (after image editing); 3. 4 x 6 inch print; 4. Same 4 x 6 image for use online (downsampled).

The Image Size dialog is divided into two sections: Pixel Dimensions and Document Size. The Pixel Dimensions section contains two boxes, Width and Height. The dimensions and file size shown in this screen grab are of the full-size version of my image as it came out my camera BEFORE I gave it purpose, such as resizing it to something more suitable like the inkjet print for my reference image, or for an image on my website, Blog or Facebook page, for example. Photoshop is telling me that my photo has a width of 6016 pixels and a height of 4016 pixels. In other words, it contains 6016 pixels from left to right, and 4016 pixels from top to bottom. To find out exactly how many pixels I have in my photo, I simply multiply width x height, which gives me a total of 24,160,256 pixels, a very large file size at 138.2 Mb. Rounded up that equals 24.3 megapixels – the same megapixel image resolution of my camera. Pretty easy so far, right?

The second section is the Document Size and contains three boxes: Width, Height and Resolution. It says that at a resolution of 300 ppi (pixels per inch) I can produce a print roughly 13 x 20 inches. So now, lets give our image a purpose, a photo-quality inkjet print. At a resolution of 360 pixels/inch – optimal resolution for a photo-quality inkjet printer) – I can make a print at 11 x 17 inches. Why is that?

Its because Resolution and Document Size are inversely proportional to each other. Take a look at the revised Image Size dialog (Figure 2). So long as the Resample/Constrain Proportions box is UNCHECKED, any number can be entered into the resolution box and the Document Width and Height will change proportionately. And, the Pixel Dimensions remain unchanged as does the File Size, which is 138.2 Mb. This is very important since the resolution of an image is a number without any meaning – an invented number – totally dependent on the image being given a purpose. In the above case, the purpose is proper resolution for photo-quality output to my inkjet printer.

Do the math
Looking at the new image size at 360 ppi, the Image Size dialog shows the pixel dimensions at a width of 6016 pixels. That means for every inch in my output there will be 360 pixels, and for every inch in height there will be 4016 pixels. In other words, Photoshop is telling me that my photo contains 6016 pixels from left to right, and 4016 pixels from top to bottom.

So then:
6016 ÷ 360 = 16.71; 4016 ÷ 360 = 11.16
Haven’t we seen those numbers before? That’s right, those are the same numbers in the Width and Height boxes. If we had changed the resolution number to 72 ppi, the resolution normally used for viewing images online only, the Document Size width and height would jump to 83.56 inches x 55.78 inches respectively. Unfortunately, 72 pixels per inch is just not enough to produce the sharp, high quality images we need as reference images when printed and to create a painting from. It’s not even close.

So, with resolution now increased from 300 to 360 pixels per inch, this means that out of the 6016 pixels that make up our image from left to right, 360 of them will now print inside every inch of paper, and out of the 4016 pixels contained in our image from top to bottom, 360 of them will now print inside every inch of paper. With so many more pixels squeezing into each inch of paper, the resultant photo must print much smaller, and sure enough, the Document Size section is now showing that the image will now print at a size of approx. 17 inches wide by 11 inches high. Once again in all cases, the higher the number, the better the image will appear. Even better, Photoshop automatically takes care of resizing the pixels for us.

Resampling
That’s all well and fine if you are going to produce an image that’s the same size as the raw digital image file, but that is rarely, if ever, the case. Lets say I need an image to output to fit a small photo album that accepts 4 x 6 inch prints. Obviously the Pixel Dimensions AND Document Size are way too large for a 4 x 6 inch output size. Our image file now has another purpose!

I know the new output size is 4 x 6 inches and I know I will printing to an Epson P700 photo-quality inkjet printer at 360 ppi, the proper resolution for high-quality photographic output. The maximum output resolution of this printer is 5760 DPI (that’s right, dots per inch) although 1440 dpi is more than enough output resolution to make a superior quality reference print.

New numbers will have to be entered into the proper boxes in the Document Size part of the dialog to reduce the Pixel Dimensions, hence file size. This is called resampling — in this case downsampling — and is easily accomplished by calling up the Image Size dialog once again. You can also upsample an image – within a reasonable limit – but I don’t recommend it until you really understand a lot of other advanced Photoshop skills, which I’ll discuss later in future Posts.

One thing you’ll soon learn is that there are many ways to accomplish the same thing in Photoshop. Some are the correct way, some the wrong way, and eventually some way will become your way. This method assumes that I do not want to apply some kind of special cropping to my image in which case I would use another method to downsample my image, which I will also discuss in a future Post.

Downsampling is a simple two-step process within the same Image Size dialog. After opening the Image Size dialog, I make sure to UNCHECK Resample Image. Immediately the Width box is highlighted and ready to accept my new width, 6 inches. I notice that when I do this, the height automatically changes to 3.985 inches and the resolution changes to 714.667, way too much resolution for output to my printer. It is an inversely proportional relationship – if one number goes up the other goes down, and vice-versa. The file still has not been given its purpose yet – output to a photo printer – so you’ll notice that the Pixel Dimensions numbers at the top of dialog did NOT change, and remain at 69.9Mb.

Since I want a 4 x 6 inch print the height did not quite make it to 4 inches (dialog not shown) so instead I entered the height first – 4 inches – which automatically reset the width to 6.022 inches and changed the resolution to 1002 pixels/inch. (Figure 3).

It was a tiny bit over, and I knew that cropping to 6 inches from 6.022 would not aesthetically affect the image in any way, so I called up the Canvas Size dialog (No. 4) and trimmed it down to exactly 6 inches. It does not matter which dimension you enter first, the other dimension will automatically change as you adjust the numbers until you get what you are after. Note that the image size Pixel Dimensions is still unchanged at 69.9 Mb. But, our image file still has no purpose.

Now we’ll give our image another purpose – or repurpose – to that of a 4 x 6 photo album inkjet print. Now, you need to CHECK Resample Image and then enter 360 into the resolution box (No. 5). You’ll see right away that the Pixel Dimensions Width is now 2160 pixels and Height 1440 pixels. The Pixel Dimensions file size is now 17.8 Mb. Since we reduced the physical size of the image by roughly 1/4, the file size was reduced from 69.9 Mb down to 17.8 Mb.

Lastly, if the image was also destined (repurposed ­again) for viewing online, I would have entered 72 (Figure 6) into the Resolution box and the result would have been Pixel Dimensions Width of 432 pixels and Height of 288 pixels, with Pixel Dimensions file size changing to a mere 729 Kb, much less “weight” than the original hefty 138.2 Mb original file, making for quick uploads and downloads.

Now you can call up the Save As dialog, name and save the new file in Photoshop’s native .psd format. Please be sure to NEVER save over an original working master file. The reason I suggest saving to native format (.psd) is because you’re not quite done with the new files. Since, for example, by drastically downsampling one file, a lot of data (pixels) was thrown away, so the integrity of the image has been compromised – it got much softer, and now requires Sharpening.

You still have some or a lot more work to do. Assuming you already did all of your other mandatory editing like Color and Tonal Balance and White Point adjustment using tools like Curves, Shadow & Highlight Adjustment, Selective Color and others, along with all of your creative and compositional editing, there is still Sharpening, the last edit before saving the final image with a new file name. AND you still have to choose a Colorspace by selecting Assign or Convert Profile (you actually have to do all of these things to all of your image files all of the time). Please read an upcoming Post to find out why and how.