Sizing Up Your Images & Understanding Resolution - Part 1

Using the "Image>Image Size" command to re-size images.

For the purpose of this discussion, let’s assume that we want to put this image up on our web site. But we know that an 8 x 10 inch print at 300 pixels per inch is way too large for anybody’s web site, so, we will have to size this image down to fit within the constraints or parameters and limitations, if you will, of web browsers and screen displays. The first consideration would be to bring the resolution down from 300 to 72, or is it 96? Or better yet, does it really matter. Well, let’s carry on and we will soon see.

In order to understand what I mean we need to go into the Image Size dialog box. So go to the Image menu and from there choose Image Size....

This can be one of the most confusing issues to those new to digital imaging. Back in the film days, you do remember film, right?, not only did we not need to know anything about resolution unless you were working in pre-press, we didn’t care. We only knew that certain films reproduced better at larger sizes than others and the larger the piece of film, the better the enlargement.

Common sizes of film cameras would be 35mm (Nikon , Canon and such), 2 1/4 (Hasselblad, Mamiya and such) and 4 x 5 inch view cameras.

Now, this image has been re-sized, cropped or whatever you want to call it to fit the ever traditional 8 x 10 photographic print that we have become used to seeing everywhere. I also did this because it will make the math really easy to understand. 

This dialog box is divided into well, three sections. The first section deals with pixel dimensions - the Width and the Height of the image and of course it displays the amount of space this image occupies on a hard drive - 20.6 MB.

The second section deals with how large or small this image will print based on the width (in this case, 3000 pixels) and the height ( 2400 pixels) at a resolution of 300 ppi. If we divide the width and height by the resolution you will see that we end up with an 8 x 10 inch print.

The third section deals with what is known as Resampling. There are many options for resampling our images from the drop down menu - the default is Automatic and if you are not up-sampling or down-sampling too much then the default choice of Automatic is just fine in most cases.

When the Resample check box is checked, any change to the width, height or resolution will force Photoshop to either add or subtract pixels from the image and then make the image look as good as it can.

Notice that the width and height are linked together here so if you change the width, the height will change in proportion maintaining the aspect ratio of the image. If you unlink these two dimensions and change one (the other will not change), you will distort the images and it will not result in a good look. Also, up at the top in the Dimension section where you see 3000 px x 2400 px there is a drop down menu that allows you to change the measuring system. I suggest hat you leave this to pixels as it is easier to understand screen sizes.

If you uncheck the Resample check box, (notice that all three variables are now linked together) you are telling Photoshop that you want to change something but to not add or subtract any pixels in this image - just change how they are distributed. In the image below I changed the resolution from 300 to 72. Notice that the width and height in pixels have not changed but the width and height in inches has. 

Here is the key, this section of the dialog box only deals with how the image will print - how big or small will each pixel print. In this case pretty big - 72 pixels for every inch as opposed to previously having 300 pixels for every inch. The fewer number of pixels per inch, the poorer the image will reproduce. On the other side of things, this does not mean that you can add resolution from say 300 ppi to 500 ppi and expect to get a better looking 8 x 10 print - there is a limit and in most cases that limit is around 300 ppi for most printing devices and that is why the industry has stayed with 300 ppi for printing photos.

So with the Resample check box checked our image will print at a whopping 41.667 inches wide and 33.334 inches high but it will not look good.

The reason that I am showing these particular measurements is because it is very common for JPEG shooters, especially from point and shoot cameras, to see this information when they first come to the Image Size dialog box and you need to get this to print properly.

In both previous cases, you should have noticed that the size of the image - not the pixel dimensions but rather the amount of space the image occupies on your hard drive, is exactly the same, 20.6 MB.

Well, how could that be? One is an 8 x 10 inch print and the other is poster size. Isn’t the poster size one larger? Isn’t the 8 x 10 inch print physically smaller. How could they both be 20.6 MB in file size?

The answer in is in the fact that we did not change the number of pixels in the but rather we change how large those pixels would print. That 8 x 10 inch print has 300 pixels per inch and looks really fine but if that poster version was printed at only 72 pixels per inch and looks really poor in quality - no detail - soft edges - let’s face it, it looks like crap even at 6 feet away.

So, if when you go to the Image > Image Size menu and you see that your image has a low resolution of say 72 ppi and the width and height are really high like 3 feet by 5 feet, you now know how to get it to print with great quality. All you have to do is make sure that the Resample check box is not checked and then adjust the resolution value to 300 ppi and verify that the width and height is at your desired print size.

All of the above discussion was dealing with an 8x10 inch image. When it comes to your digital camera image, odds are if you have not cropped the image, the print will be in the 4:6 aspect ratio. This is the same aspect ratio that your sensor is which guarantees that you should get either a 2x3, 4x6, 8x12 or a 12 x 18 inch print.

Next, Part 2 of understanding image resolution ...