When we choose a lens to take an image with, most of the time we are looking at how much we want to fit into the frame, and far away from our subject we are. For example, if you are shooting indoors in a small room, but you want to leave in some of the surroundings to give the image context, then you are going to want to choose a shorter focal length. If you are photographing something farther away, like your child at sports day, you are going to want to have a longer length lens on your camera, so you can be farther back but still have the whole of your child in the frame.
In the post I did yesterday, you can see just how much more you can fit into the frame with a 35mm than a 200mm. (Check out the post on focal length comparisons if you missed it!)
As I mentioned in that post, there is another reason for using certain lenses over and above how near or far we are from our subjects, and that is the lens compression.
If we frame our subjects the same way in the frame (by keeping them the same size in the viewfinder but just moving ourselves farther back or closer to subject) we can still get two very different images purely depending on the lens you choose because of lens compression.
Lens compression changes how close or far away elements look within the frame. if you shoot with a wide angle lens, objects that are closer to the frame will appear larger than objects that are farther away, and items in the frame that are farther away will look smaller and farther back. They have the affect of spreading everything out, pushing the elements further apart.
Now if I stick on a telephoto lens, the opposite happens. Subjects closer to the camera look smaller, and items in the background look larger and closer to the subject. Longer lengths tend to flatten an image more - bringing the foreground, subject and background elements closer together - the opposite of what a wide angle lens does. With a longer length lens, depth of field also appears much shallower, blurring out your background and helping isolate your subject.
It's probably going to be easier to explain using some examples,, featuring my lovely model Dave again :)
First example, and in these I am using a 35mm as my wide angle lens, and an 85mm for the longer length lens (because that is as far as I could back up!) I've got Dave on the dining room table. I've tried as best as I can to frame them in the same way, and keep them the same size in the frame.
Can you see the difference in the picture frames that are on the wall in the background? Using the 35mm, those frames look farther away, and smaller, and I am able to fit more of them in. With the 85mm, the frames look much closer to him, almost pulled right up against him, and they look larger and take up more space within the image. You can also see a difference in how Dave looks - it's not glaringly obvious, but he looks a little slimmer in the second photo :)
This next image shows the same thing again - in the first image taken with the 35mm the door at the end of the hallway looks quite far away (you can just see the patch of light from it over his camera right shoulder) and we can see that the door closest to him is made up of individual panes of glass (which are a nightmare to clean I can tell you). However, in the image taken with the 135mm lens, the doorway at the end of the hallway looks right up against him, and the field of view has narrowed - I can't see the door closest to him at all now. Although the subject is taking up roughly the same amount of room as the frame in both images, the results are very different.
I had to seriously curb my OCD tendancies not to straighten those lines but it would have knocked off my framing!
One more example (I do like to labor a point home) and in this example I added the nearest thing that came to hand into the background of the image - in this case a skittle (which due to background blur just looks like a red blob) You can see the affect in action again - with the 35mm the red blob looks farther back and quite small, with the 200m, that red blob ooks larger and right up against him.
So, longer lengths brings background elements closer, wide angles will make the background appear smaller and farther away. With items closer to the lens, shorter focal lengths make them look larger, with longer focal lengths, they look smaller. (This is why portrait lenses are often longer lengths - for that slight slimming affect)
It is not that one is better than the other, more that each has a different "look" and it will largely depend on what you are photographing as to which lens you want to choose. For example, in that last image of Dave, I prefer the one taken by the 35mm - it looks more comical and quirky, which, of course, is perfect for a minion....but if I were taking a photograph of a bride, I'm not so sure she'd want that look, and I'd go with a 85mm or larger.
You can also use the affect of lens compression compositionally, since it affects how the various elements in a scene are positioned within the frame - so you can use it to bring elements closer or make them farther away, and therefore bring them into certain areas or just imply a closer connection.
I think that's everything there is to know about lens compression, but if you have any questions just leave a comment below (or on Facebook) and I'll try to answer!