How does crop factor apply on lenses?

Question

I understand that the crop factor of a sensor multiplied by the focal length of the lens equals the focal length of an equivalent 35mm lens.

Also, there are well known differences between wide, normal and tele lenses, mainly the wide lenses "spread" objects on the axis of shooting, while tele lenses "compress" them.

Yet I wonder, if a shoot with a MFT camera (crop factor x2) and a 25mm lens, what will I get? An image equivalent to a 50mm lens, or a croped image of a 25mm lens? (compared to an image taken from a 35mm camera)

That is, does a 25mm lens used by a crop factor of x2, feel like a normal 50mm lens? (Besides the size of the image) Also, does a 50mm lens used by a crop factor of x2, feel like a tele 100mm lens?

I know that similar questions have already been answered but I am still confused, and would prefer a somehow simpler answer.

Any thoughts would be appreciated.

See this - my 'idiot's guide to crop factor & perspective'. When I first started, all the maths totally confused me. It's easier than you think one you grasp it. The lens neither compresses nor expands, only moving your position does that. https://photo.stackexchange.com/questions/97718/how-does-crop-factor-affect-perspective?rq=1 — Tetsujin, Apr 17 '22 at 17:47
The focal lengths and crop factors are different, but the issue in this question is identical to the marked duplicate, and the answers there are just as applicable to the differences between using a 25mm lens on an MFT camera and a 50mm lens on a FF camera. — Michael C, Apr 18 '22 at 01:11
Also related: relationship between focal length, perspective projection and camera distance and Why is the background bigger and blurrier in one of these images? and When do the differences between APS-C and full frame sensors matter, and why? — Michael C, Apr 18 '22 at 02:27
Here is a list of the 1,305 existing questions and answers we have that mention "crop factor". Here is a list of the 98 existing questions we have with the [crop-factor] tag. — Michael C, Apr 18 '22 at 02:35

score 2 · Accepted Answer · answered Apr 17 '22 at 17:56

A 25mm lens is a 25mm lens. The angle of view depends on the size of the sensor you stick behind it. So, a 25mm lens on MFT gives you the same angle of view as if you had shot a 25mm lens on a full-frame camera and chopped out the central part. And this is also equivalent to the angle of view you'd get if you shot with a 50mm lens on the full-frame camera.

When you talk about "spreading", "compressing" and "feel", you are talking about perspective. Perspective is not related to the focal length, it is related to where you are standing and the distance to the object. Imagine you shoot a photo of a distant church spire against a mountainous backdrop using a 16mm lens on a full-frame camera... and then you chop out a tiny portion of the photo, showing just the spire with the mountains in the background... this will give you exactly the same "compressed" perspective as if you had shot the scene with, let's say, a 200mm lens.

The point is, the perspective doesn't intrinsically come from the focal length, but from your position and the object's position and how much of the scene you grab. So, your 25mm on MFT "grabs" the same image as a full-frame 50mm does, which is the same image as the chopped-out central portion that a full-frame 25mm grabs. And each image has the same perspective because you are standing in the same position, photographing the same object.

Cropping an image does not change it's perspective nor add any "compression." It has nothing to do with "how much of the scene you grab;" it is only affected by relational distances. — Steven Kersting, Apr 17 '22 at 20:16
@StevenKersting I never said any of those things. It seems you just misunderstood my explanation. My point was exactly that - the perspective is the same with a 16mm lens and with a 200mm lens - you just might have to crop out the same segment of the scene in order to see this clearly. — osullic, Apr 17 '22 at 23:13
the first sentence in the last paragraph ends with "and how much of the scene you grab"... so yeah, I did misunderstand that part... and it ends correctly. — Steven Kersting, Apr 18 '22 at 13:13

score 1 · Answer 2 · answered Apr 17 '22 at 20:53

Yet I wonder, if a shoot with a MFT camera (crop factor x2) and a 25mm lens, what will I get? An image equivalent to a 50mm lens, or a croped image of a 25mm lens? (compared to an image taken from a 35mm camera)

You get a cropped 25mm lens image.

That is, does a 25mm lens used by a crop factor of x2, feel like a normal 50mm lens? (Besides the size of the image) Also, does a 50mm lens used by a crop factor of x2, feel like a tele 100mm lens?

How does the "feel" of a scene change when you look out a window of a smaller size?

It doesn't; because your physical relationship to the things w/in the scene does not change (relative distances remain constant).

A "normal lens" is considered normal because it records an ~ 55˚ diagonal FoV (FL ~= to sensor diagonal); which is ~ equivalent to a human's primary FoV, where focus and object recognition is effective (central-near peripheral, ~ 60˚). And the "normal display" is to then view the output image from a distance so that it also occupies the same ~ 55˚ diagonal FoV (viewed from a distance ~= to the image diagonal). Because, when all of this is done, the apparent relative distances (perspective) remains constant and looks "normal."

BTW, the reason the diagonals are of concern is because, for any rectilinear image format, the image diagonal dictates the circular FoV required to cover it; which also correlates to the human's circular primary FoV at distance (circular eyes, circular pupils, monocular vision).

But you cannot change the perspective once it has been recorded into 2D. You can only change how much of it you view... i.e. through a smaller or larger window (crop).

score 0 · Answer 3 · answered Apr 17 '22 at 17:44

Yet I wonder, if a shoot with a MFT camera (crop factor x2) and a 25mm lens, what will I get? An image equivalent to a 50mm lens, or a croped image of a 25mm lens? (compared to an image taken from a 35mm camera)

Both.

If you shoot with a full frame camera using a 25mm lens, but then crop three fourths of the image away (so both image width and image height are halved), you get an image that effectively has 50mm focal length.

If you shoot with a full frame camera using a 50mm lens, or a MFT (or any crop factor 2.0) camera using a 25mm lens, you get an image that effectively has 50mm focal length.

Similarly, if you shoot on a full frame camera using a 25mm lens that supports using extenders (I'm not aware of any such lens though) and attach a 2x extender, you get an image that effectively has 50mm focal length.

There is no difference between cropping and using a longer focal length, unless you go into the realm of depth of field, background blur and ability to collect light in a low-light image.

Also, does a 50mm lens used by a crop factor of x2, feel like a tele 100mm lens?

Yes, 50mm on MFT camera is like 100mm on full frame camera.

How does crop factor apply on lenses?

3 Answers3