Perspective distortion and APS-C

Started 4 months ago | Discussions thread
prospects Regular Member • Posts: 257
Re: Perspective distortion and APS-C

Sittatunga wrote:

prospects wrote:

Sittatunga wrote:

prospects wrote:

PhotosByHall wrote:

Hi all,

This is a question I thought I knew the answer to - but I've seen LOADS of contradictory answers on this recently.

So I shot a few close up portraits on a M50 on 35mm, which made it a 56mm on my M50. People were moaning about the perspective distortion making facial features look bigger. (turns out I have a bad eye for that)

No problem, now I use the M50 and the nifty fifty. So now I have an 80mm portrait lens (and will also be using a 135mm at some point).

My question is this -

When mounted on a crop at 80mm equivalent, perspective distortion should disappear right - as it is a function of how close you are to the focal plane, not the focal length?

(Obviously, you have to get closer and focus closer to fill the frame with a 50mm rather than an 80mm and its that which causes the perspective distortion - not the lens itself.)

Like I said, I'm pretty sure that is correct, but just need to be sure, so I can go back to shooting portraits on my nifty fifty

First off, I think portraits on your nifty fifty would still look great.

Next, a longer focal length does result in more compression by itself. Hence, 50mm on APSC =\= 85mm on FF fundamentally even if you are standing at a similar distance. So technically I believe overall distortion is not the same.

While using a crop forces you to move back to get a similar FOV, a 50mm will always be a 50mm. It is a function of both distance and the focal length of the lens itself.

The focal length of a lens by itself has negligible effect on perspective. What controls perspective is the ratio of the focal length to the size of the sensor.

I think you meant field of view. Ratio of FL and size of the sensor controls the field of view, not perspective.

Standing at the same place, with two different lenses with their respective sensors will result in 2 different pictures.

The most obvious point would be the background. The FF with the ~80mm will result in the objects in the background of the subject to be seemingly larger due to the lens compression of the ~80mm.

You've obviously not tried the experiment. Standing at the same place the ratio of the foreground to the background will be exactly the same, no matter what lens or sensor you use. The perspective is controlled only by the distances from the subject and the background. The angle of view, controlled by the ratio of focal length to sensor size, controls the framing. Naturally, if you change the lens or the sensor and move to keep the main subject to the same size in the final image, the perspective will change, but that's because you have moved, not because of any magical properties of the lens' focal length.

It's just simple geometry and similar triangles. An 80mm (more precisely an 81mm) lens will produce exactly the same angle of view, framing, relative sizes of foreground, subject and background (perspective) on a 36x24mm sensor as a 50mm lens on a Canon APS-C sensor, provided it's at exactly the same position relative to the same subject.

Perspective compression or exaggeration is caused by the angle subtended by the image at the viewer's eye being different from the angle of view of the lens. In other words, if you are looking at a 250mm wide print from a distance of 250mm or a 20" wide monitor from a distance of 20", you will only get undistorted perspective if you use a lens with the same focal length as the long side of your crop of the sensor. Again, this is just simple geometry and similar triangles.

At the same time though, undistorted perspective would bring the camera well within many peoples' comfort zone or personal space for anything short of a full-length portrait. That is why perspective compression is more flattering for close up portraits, so we prefer to use longer focal lengths for these.

But would people notice in a real world comparison? Only side by side.

Noted. I stand corrected. I’ve tried it and you’re right. The only difference would be the DOF after taking all into consideration.

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