Is perspective determined by distance or focal length?

[rick325]

I know this has been discussed before, but I just cannot seem to find a definitive answer as it relates to a crop vs. ff sensor (most of the discussion is purely focal length / distance related):

Mock scenario:

1) 30mm lens on a 1.6x crop sensor
2) 50mm lens on a ff sensor

In both cases: Distance to subject is 1m

Both will give a similar FOV. Which of the following is true:

a) the pictures will look the same because the distance to the subject is the same (ignore the 2mm effective focal length discrepency)
b) the pictures will look very different, since the 30mm lens is "wider" you will get a more "3d" look while the 50mm will give a "flatter" image
c) none of the above. please explain

In my non-scientific testing, I believe "b" to be true. However, I keep reading on the forums that "a" is in fact correct--perspective is a function of distance to the subject and nothing else.

Thoughts?

[rick325]

Can someone please move this thread to the "sharing knowhow -> talk about photography" section?

Thanks in advance.

[Perry Ge]

Perspective is determined entirely by distance from your subject (camera positioning) and not by focal length or your sensor size. Focal length affects your field of view. A cropped sensor crops that field of view. None of these have any bearing on perspective.

Move your hand close to your face, then move it far away. Notice its relative size in your field of view to objects around it. That's what moving closer to your subject does. When you're far from the subject, you tend to use a tele lens, hence the 'compression' effect, where the subject looks smaller than its surroundings. Vice versa with being close up and using wide angles. It's not focal length, it's camera position. All the cropped sensor does is crop off 40% of the edges of the image. It does nothing else.

This question has been asked a lot though, you should be able to find a lot of information by running a search.

[n1as]

The relative size of objects in the picture is completely controlled by 3 things:
- Distance from the camera to the objects.
- How far it is from the camera to the objects.
- Amount of linear space from the objects to the camera.



And here are the things that do NOT affect the relative size relationships between items in an image.

- Sensor size (crop vs. FF)
- Lens focal length
- Shutter speed
- ISO
- Aperture
- etc.

Do a search for "portrait distance" and you'll find a thread recently that got into this quite heavily, including example pictures.

[Madweasel]

Think about it this way. Perspective is the relative sizes of things at different distances from your viewpoint. This is a propotional thing, so that if you have two identical objects and one is twice as far from you as the other then it will look half as big.

So, imagine these two identical objects are 4 feet apart and you are viewing from a point 2 feet from the first and 6 feet from the second. The nearer one will look three times bigger than the farther, giving a 'strong' perspective effect. Now move our imaginary viewpoint back to 20 feet from the first, 24 feet from the second. Now the first one is only 1.2 times bigger than the second, giving a 'compressed' perspective effect.

These observations will be true regardless of the lens and sensor size you use. It's just that if you use a wideangle lens from the distant viewpoint, both objects will be small and near the centre of the field of view and the things that you notice will be much nearer, making you think you're getting an exagerrated perspective. But if you use a telephoto or crop the centre portion out of a wider shot, you'll see the relative sizes (and therefore perspective) of those two objects will be exactly the same.

I've seen this demonstrated countless times in books and magazines and apologise for the clumsiness of doing it in words.

[Wilt]

http://photography-on-the.net/forum/...69#post5023469

photos proving that FL does not change perspective...
http://photography-on-the.net/forum/...5&postcount=17

[rick325]

You all are great, thanks! What I had gotten myself hung up over was a degenerate case:

Imagine a picture taken with a 8mm fish-eye, cropped to give you an image that replicated a 50mm FOV taken from the same distance. I would expect that the 8mm cropped would be distorted perspective relative to a 50mm lens at the same distance.

In any case, I'll stop the idle chatter, and thank you all for the perspective

[Jon]

Originally Posted by rick325

You all are great, thanks! What I had gotten myself hung up over was a degenerate case:

Imagine a picture taken with a 8mm fish-eye, cropped to give you an image that replicated a 50mm FOV taken from the same distance. I would expect that the 8mm cropped would be distorted perspective relative to a 50mm lens at the same distance.

In any case, I'll stop the idle chatter, and thank you all for the perspective

Well, visualize a 15 mm fisheye compared to a 15 mm UWA. Fisheyes aren't measured by the same rules as other lenses.

[Wilt]

Originally Posted by rick325

You all are great, thanks! What I had gotten myself hung up over was a degenerate case:

Imagine a picture taken with a 8mm fish-eye, cropped to give you an image that replicated a 50mm FOV taken from the same distance. I would expect that the 8mm cropped would be distorted perspective relative to a 50mm lens at the same distance.

In any case, I'll stop the idle chatter, and thank you all for the perspective

'Perspective' distortion is one beast, plain old distortion like fisheyes create is something entirely different. Fisheyes depict straight lines as curves, and show them as straight lines only if they pass thru the optical center of the lens!