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Same Lens on Different sensors. Do they match?


Anaconda_
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This is a hypothetical situation, so assume the lenses are absolutely identical in every way, and the footage will be graded to a perfect match.

Gear list:

2 x Canon FD 28mm F1.8

Full frame camera

MFT camera

If I adapt the lenses to have one on each camera, aside from crop factor, will the lens characteristics be the same? Or will the MFT have 'sharper' corners, since it's only using the middle part of the lens? Would this even more noticeable, or worth thinking about?

An example setup would be an interview in a very small room, with both cameras next to each other. The FF would be the wide/mid shot, and MFT a close.

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Assuming sharpness is the main property you're looking at,

The simple/theoretically-correct-but-not-really answer is, look up the MTF chart for your lens, and you'll see how many line pairs per mm your lens resolves at any given distance from its center. Multiply lp/mm by the size of the sensor in mm, and you know how many line pairs your lens is resolving for the entire image. Example: if the lens resolves 100 lp/mm, then the FF lens resolves 3600 line pairs across its width, and the MFT lens resolves 1800 line pairs over its width.

Even with this simplified view, it depends on the MTF curve of a specific lens, which also depends on the aperture you're shooting at.

The complex answer is that the MTF chart doesn't show the whole picture. It's a 1D representation, but lenses project 2D image. And it might be different at different focus distances. And it varies from lens copy to lens copy. LensRentals' Roger Cicala has a number of great articles about this topic.

So really the best way to answer is to take the lens in question and run some tests with the cameras and lens in question.

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@Anaconda_

The differences mentioned above will be real, but you can compensate for them in post, and even if you didn't almost no-one would recognise them anyway.

If you're unsure, get any camera and any fast prime and shoot a test clip wide open and stopped down a few stops.  If the lens is fast then the wide-open shot should be softer.  Now pull them onto a timeline, put a grade on them, and play with sharpening the wide-open shot and potentially softening the stopped-down shot (a small radius blur applied at a low opacity is a good trick and simulates the halation from a soft lens without actually blurring the detail).

You can also experiment with using different amounts of diffusion on the cameras.

If you do this then likely the lenses won't be identical, in which case, choose which lens goes on which camera by making them look more similar.

If your question is actually a "should I use this approach" question, then the answer is yes.

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If you already have a FF camera (one that has a crop mode anyway) you can sort of test it yourself somewhat by using the lens both FF and APSC.

Not exactly the same as M43 and FF but close and would give you an idea (and closer if the FF crop mode is not 1.5 or 1.6x but has a larger crop still).

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Honestly, under the circumstances you're going to use them under, you'll only notice a difference if you're looking for one. There will be slight differences in sharpness, DOF, background compression, etc. but really these aren't things the average person will notice unless it's thrown in their face. The biggest concern is color matching the two cameras, but it sounds like you're set with that. You might wanna play around with sharpness in post, but I think you'll find they cut together nicely as long as their properly color matched. 

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8 hours ago, newfoundmass said:

Honestly, under the circumstances you're going to use them under, you'll only notice a difference if you're looking for one. There will be slight differences in sharpness, DOF, background compression, etc. but really these aren't things the average person will notice unless it's thrown in their face. The biggest concern is color matching the two cameras, but it sounds like you're set with that. You might wanna play around with sharpness in post, but I think you'll find they cut together nicely as long as their properly color matched. 

Actually, if it's two cameras with the same lens then there won't be a difference in background compression at all.  But I get what you're saying - that had the OP used identical cameras and different lenses then there would have been a difference..

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Agree with others. My number one worry also would be color matching footage from both cameras.

For an interview, your background will be thrown out of focus and blurred to some degree even if you shoot at f4 on FF. Never see background in focus during interviews or youtube presentations. Out of focus background is the look almost everybody goes for. Sharpness in this case will be important only for your main subject / person and this will be the central or near central area of the lens.

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Thanks all - Colour matching isn't really an issue. Both cameras will record in the same codec, and with the same colour science. Some tweaking will be necessary, but they will be pretty close to start with.

My main concern was background compression and the softness / vignetting in the corners on the FF angle. But it seems like that won't be causing any big distractions. 

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