An aspect ratio is a relationship between the width and the height of a photograph. Most DSLR and MIRRORLESS cameras have a default setting of 3X2. Sometimes 5X4.
Example Of Aspect Ratios
If your camera takes photos at 6000 X 4000 pixels, this is a 3:2 format. The ratio is always simplified to its lowest numbers...
6000 x 4000 = 6 x 4 = 3 x 2
If your camera takes photos at 5000 X 4000, this is simplified to 5 X 4.
5000 x 4000 = 5 x 4
Cropping is selecting a portion of the image you'd like to keep and "cropping" the rest out. Deleting it! In post-production, you can remove parts of a photograph to improve framing or fit the desired ratio.
Editing, Cropping & Aspect Ratios
Sometimes a content creator or editor may have a 6000 X 4000-pixel photo, but the project they're working on requires a different aspect ratio. They CROP the photo to get the ratio they desire...
Examples of Common Crop Sizes
A 3 x 2 Crop – A Common Default Aspect Ratio for Most Cameras
= 6 X 4, 12 X 8, 15 X 10 and more
A 5 x 4 Crop – A Common Aspect Ratio in Headshots & Portraits
= 8 X 10, 16 X 20 and more
A 16 x 9 Crop – The most Common Aspect Ratio in Video
=1920 X 1080 (video), 3840 X 2160
Cropping For Composition
One of the most common reasons for cropping is to strengthen the composition.
If a portion of the photo near the edge is distracting (maybe a port-a-john!) you can crop it out. Just not include it!
If your main subject is centered in the frame, you may decide to crop in order to place the subject on the rule of thirds line.
Its always better to frame things up the way you want, and keep all of those pixels! You probably spent a lot of money on those pixels. Do what you can to keep ALL OF THEM!