Especially among hobbyist photographers, I feel that focal length gets overlooked as being unimportant. I feel this stems because of a misunderstanding that the focal length just represents how “zoomed in” the lens is. Commonly people think wide (zoomed out) for close subjects and telephoto (zoomed in) for subjects that are far away. The fact is, however, that the focal length of your lens as a huge impact on the image, much more so than appearing closer or further away.
What is focal length.
At the lowest level, the focal length represents distance the point of convergence in your lens, to the image sensor in your camera.
Classifications of focal length.
There is a wide range of focal lengths, and we generalize them with the labels wide angle to telephoto.
Wide Angle represents focal lengths below 35mm, Normal focal lengths are ones that fall between 35mm and 70mm, and Telephoto makes up everything above 70mm.
Focal lengths impact on a photo.
Telephoto lenses have the effect of flattening a photo, and this can make or break a photo. Having this in your head when composing a shot will help you decide if you should take a few steps back from your subject with a longer focal length (for a flatter photo) or put on a wider lens and get right up to your subject.
Because I don’t have a good telephoto lens right now, I borrowed a picture from https://expertphotography.com that does a great job of showing the different looks focal length can make. You can see at wider angles, the subjects seem to be further apart than they do when using a lens with a longer focal length. This is why choosing the right focal length is absolutely critical, and your zoom lens isn’t a replacement for actually getting up close to your subject.
Additionally, as you can see in the GIF image above, the compression of a lens can have a significant impact on the image. At very wide angles the subjects face seems narrow and deep where at 200mm the subjects face seems much wider and flatter.
As hobbyist photographers, it’s likely that we’re using a camera that has a crop sensor like Nikon’s DX cameras and Cannon’s APC-C cameras. Full-frame cameras have a sensor that is the same size as a single frame of 35mm film. Cameras with cropped sensors are much cheaper and accessible for non-professional photographers.
It should be known that crop sensors do impact your effective focal length. Meaning that they will reduce the field of view to that of a more telephoto length (and also increase your depth of field), however, they wont change the flattening effect (compression).
Thanks for reading, I hope it helped . Lett me know if you feel like I have missed anything or made a mistake!