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There was recently a video posted about how different focal lengths affect portrait photography, specifically how distortion at smaller focal lengths have more distortion and are less flattering.

So the question is that, because we always talk about how Canon's cropped sensors have a 1.6x crop factor so the lenses are essentially longer, does the distortion caused by the lens change when used on a cropped sensor?

My initial thought is "no" given that my idea of how a cropped sensor works is like printing off a 8x10 picture and then cutting out the 4x6 that I like: the picture doesn't change, only my view of it.

However, if distortion is least prominent at the center of the photo and most prominent at the edges, then the perception of the net effect of the distortion would be changed by using a cropped sensor and I get confused again.

Any thoughts?

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Check out What does it really mean that telephoto lenses “flatten” scenes? for some great visualizations of what's going on as focal length changes. –  mattdm Aug 23 '12 at 2:21
    
Add to that this answer about angle of view, which explains visually how focal length relates to crop factor, and you'll have the perfect background for this question. –  mattdm Aug 23 '12 at 2:23

4 Answers 4

up vote 2 down vote accepted

What changes is perspective. It's not lens distortion (barrel distortion) that makes wide-angle closeups unflattering, it's how close you need to get to the subject to fill the frame. For a given focal length lens, you need to be closer to fill the frame using a full-frame camera than you do with a crop-sensor camera. That means that, proportionally, the nose, say, is going to be much nearer the camera than the eyes with a full frame than with a crop sensor, and the ears will be off in the (comparatively) far distance.

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http://photographylife.com/equivalent-focal-length-and-field-of-view

I have to contradict with the reply's to this post

The only thing that changes is the field of view when you use a cropped sensor. But the angle of view is what causes the distortion. And it remains the same for a lens irrespective of the cropped factor.

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Are you sure about that? It's subject to camera distance that caused perspective distortion. –  Andy Blankertz Aug 21 at 19:00

Does sensor size affect lens distortion?

Short answer: Yes.

The reason is only because most if not all distortion happens on the edges of the lens glass. Using a cropped sensor is indeed

like printing off a 8x10 picture and then cutting out the 4x6.

and therefore by trimming the edges you will most likely get rid of some of the distortion.

Just to rectify/precise Stan's statement:

What changes is perspective.

The size of the sensor does not change the perspective. The perspective is only affected by the distance between the lens and the subject no matter its focal length.

As a matter of a fact should one take an exposure with a 200mm lens and then from the same location take the same exposure with a 20mm lens the perspective will be the same. What changes is the amount of information you will see in the 2 exposures. Crop the 20mm lens exposure to match the same field of view of the 200mm lens and you will see the perspectives are identical. The same logic goes for cropped sensor. a 50mm lens on a cropped sensor might have a FOV of a 75mm lens on a full frame body but the perspective remains unchanged nonetheless.

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I realized something key in your question:

... a cropped sensor works [...] like printing off a 8x10 picture and then cutting out the 4x6 that I like: the picture doesn't change, only my view of it.

That's actually exactly right, and the only key part you're missing is that that changing your view of the picture by cropping off the edges is exactly what you're doing by zooming in.

This type of distortion isn't really a lens artifact. It's the actual, real perspective distortion there in the world. You just don't notice it when looking with your eyes, because your brain is actually doing the seeing, and it spends rather a lot of processing power glossing over things like this and using memory, pattern recognition, and of course stereo vision to build on an impression of a three-d world. Even when you try to sit very still and be mindful of the exact way the scene looks, your brain is working hard to foil you. (This is one of several areas where a drawing class could be very beneficial to a photographer!)

When you project an image through a lens onto a flat sensor, though, the light rays are all frozen, and perspective distortion can be very clear in a way that's jarring to, well, people used to looking at the real world. When you crop or use a longer, tighter focal length, you flatten that out.

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