Compression and crop frame v. full frame

[unpopular]

wildlife shooters?

Did you even read the post I was replying to?

 

[fjrabon]

Compression between crop vs full frame. If the distance between the recording medium focal plane and the subject is unchanged, there is no difference regardless the recording medium size.

For example, there are 2 identical trees and they are 5 feet apart. You are standing about 100 feet from them and are almost inline with them (maybe at a angle about 15 degrees). So the tree which is closer to you is about 100 feet away while the other one is between 100 - 105feet. No big different in them as far as size concern since they both appear to be a tree 100 feet away (close to same distance). When you take a photo with a 200mm lens mounted on a crop camera or full frame camera, the trees should appear about the same size in the crop photo, or in the full frame photo.


Now, you walk towards the tree to a point you are 5 feet away from the first tree and the 2nd tree is between close to10 feet. Now, the 1st tree appears to be bigger because it is much closer than the 2nd one. The 2nd one now almost 2x the distance. And because of the size difference, the compression you see when you are 100 feet away is pretty gone. So if you take a photo (crop or full frame) with a focal length in which the first tree is about the same size, the 2nd tree will appear smaller when compare to the photos you take 100 feet away.


So the compression is pretty much the result of the distance between you and the subject, not the recording medium.

Yes! That's right. Perspective is not a function of lens focal length or the size of the recording medium. Perspective in a photo -- the apparent relationship of objects in 3D space -- is a direct function of only one factor: camera placement. Different lens focal lengths matched with different size film/sensors simply facilitate the desired crop from the chosen perspective position. Obviously you need that 300mm lens to get the photo from a distant location and likewise you need that 20mm lens to get in really close. Conversely, the 300mm isn't going to be the lens of choice when working in close to the subject. The problem here is that a transference of causality has taken place in the common jargon (i.e. telephoto/wideangle perspective) and the effects we see that are in fact due to camera placement get described as if they were caused by some property of the lens. Then people get confused.

Joe

Yeah, I totally agree. After I stripped away all the jargon, the concept is mind numbingly simple.

 

[unpopular]

Imagine FOV as rays extending from the edge of the sensor or film plane through the center of the lens and out to infinity. If you shrink the sensor, the angle becomes more oblique. So as a result, distant objects will consume more of the field, while expanding the sensor larger, the angle becomes more obtuse, and objects at the same distance as before will consume less of the field; however, the same is not true of near objects, and as you approach the lens the proportion which near objects consume the frame will become more and more similar.

Since spacial compression is created by the the relative size of foreground objects to background objects, the degree of the effect is determined by the format relative to the magnification, rather than the focal length of the lens.

I was going to draw a nice little diagram, but after brushing up on my trigonometry, it turns out that I am completely and embarrassingly wrong. I plugged some formulas into a graphic calculator, and to no surprise, the difference in magnification between one field of view and another is constant relative to distance.

Looking at it now, perspective is obviously relative to distance only, not affected by magnification, fov or focal length.

So yeah. I'm feeling pretty derpy now myself.

 

[fjrabon]

Imagine FOV as rays extending from the edge of the sensor or film plane through the center of the lens and out to infinity. If you shrink the sensor, the angle becomes more oblique. So as a result, distant objects will consume more of the field, while expanding the sensor larger, the angle becomes more obtuse, and objects at the same distance as before will consume less of the field; however, the same is not true of near objects, and as you approach the lens the proportion which near objects consume the frame will become more and more similar.

Since spacial compression is created by the the relative size of foreground objects to background objects, the degree of the effect is determined by the format relative to the magnification, rather than the focal length of the lens.

I was going to draw a nice little diagram, but after brushing up on my trigonometry, it turns out that I am completely and embarrassingly wrong. I plugged some formulas into a graphic calculator, and to no surprise, the difference in magnification between one field of view and another is constant relative to distance.

So yeah. perspective is relative to distance only, not affected by magnification, fov or focal length.

haha, kudos to you for admitting it. I was unsure enough about what you meant in the first place to challenge you on it, so it probably would have just slid into oblivion had you not brought it up.

 

[unpopular]

LOL. If I left such a glaring and obvious mistake up, I'd look even more stupid when someone else caught it.

I'm not going to be able to talk my way out of SOHCAHTOA!