Cropping

You may have shot a relatively small object, such as M51, the Whirlpool Galaxy, with a wide-field instrument, and ended up the galaxy with a lot of sky around it. If you are only interested in the galaxy, you can discard the unwanted sky area by cropping the photo.

There are basically two ways to crop an image in Photoshop. We can use the crop tool, and click and draw a crop selection around the area we want to keep. Or, we can use the rectangular marquee tool to make a selection and then go to Image > Crop.

Resizing and Resampling

The original image from a Canon 20Da DSLR camera is 3504 x 2336 pixels (1,185,334 total pixels). This is a 23.4mb file in 8-bits per RGB channel of color depth.

If we are going to print the image, this file will make a very nice 11 x 17 inch print at about 200 pixels per inch of resolution.

If we were going to print it with a printer that printed at 300 pixels per inch, the exact same file would make a nice 8 x 11.5 inch print.

In this case, the file size and total number of pixels remain exactly the same, only the dimensions of the image have been resized, and the number of pixels per inch have changed. The pixels are just displayed larger or smaller.

When resizing an image in Photoshop's Image > Image Size dialog, make sure the "Resample Image" check box is not checked at the bottom of the dialog box. This will preserve the file's original data.

But if we are going to display the image on the web, or send a smaller version to a friend via email, we have to resample the image when we resize it.

Depending on the resolution they are running at, most monitors display images at about 72 - 96 pixels per inch. If we want to display an image on a computer monitor and make it 4 inches high by six inches wide, we only need (4 x 80ppi) x (6 x 80ppi) pixels. This is only 320 x 480 pixels. But the original image is 3504 x 2336 pixels. So we need to resample the image.

Resampling means changing the number of pixels in an image by mathematical algorithms that examine neighboring existing pixels and create new ones based on this analysis.

Photoshop's Image Size Dialog

In Photoshop select Image > Image Size. Be sure that the box next to "Resample Image" is checked on, and Bicubic is selected in the pull down box next to it.

Then, under "Document Size" put in the number of inches you want for the width or height, and the number of pixels per inch of resolution you need. Figure on about 80 pixels per inch for display on a monitor or for email, and about 200 to 300 pixels per inch for an extremely high quality print.

You'll notice that the pixel dimensions and size of the image file listed at the top of the box change as you make changes in the Document Size details below. This is correct, so OK the dialog box.

If you decide to save the image at this point, remember to give it a new file name and save it as a separate file and do not overwrite your high-resolution original image that you have worked so hard on!

When resampling an image in Photoshop's Image > Image Size dialog, make sure the "Resample Image" check box is checked on at the bottom of the dialog box and that "bicubic" is selected as the method.

Resizing, Resampling and Cropping at the Same Time

The Crop Tool Options

It is also possible to resize, resample and crop, all at the same time. Just enter the image dimensions and number of pixels per inch that you want the final image to be in the crop tool's options. Note that the units will default to those specified in the preferences. If you want to use a different unit, just type it in after the number, such as 576px if you want to crop the image to 576 pixels.