What is mFT FoV?

Started Jan 27, 2013 | Questions thread
Anders W Forum Pro • Posts: 21,466
Re: What is mFT FoV?

gollywop wrote:

Anders W wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

The diagonal FoV in the official specs is the one you get after correction. This follows from the fact that the FoV specs for MFT lenses are the same as for other lenses with the same EFL (e.g., 84 degrees for the 12/2) as well as from the formula for calculating diagonal FoV based on the FL for a rectilinear lens described here.

The FL and the horizontal and vertical FoVs are the same before and after correction. Geometric distortion implies a departure from strict rectilinearity that affects the diagonal FoV only.

I'm not so sure, Anders; but thanks, by the way, for a very thoughtful answer. Here is an image taken with my 12-35 at 12mm. The first image is the dng as rendered in ACR, which does the lens corrections automatically. The second is as rendered in Photo Ninja, which takes the full raw data and makes no lens corrections. (The PN image is identical in size and distortion to the rendering obtained with RPP, which also makes no lens corrections.)

You can see that the FoV as seen in the PN rendering is both wider and taller. It's not just the diagonal that's affected. Once the lens-correction crop has been made by ACR, the FoV is less in every way.

Yes. They crop away more than required in this case, perhaps to bring the FoV in line with the official specs.

My question is whether it is the cropped FoV from ACR that corresponds to what should be the angle of view appropriate to a focal length of 12mm on a mFT camera -- or is it the FoV as seen in the PN image that does.

In case of doubt, you can use the method described here to measure the FoV yourself. I tried it with the 20/1.7 (after correction) and found that the FoV was exactly what it was specified to be.

Obviously, any given lens can depart from the FoV specs in the one or other direction. That's nothing new. But such deviations is one thing and the systematic issue you are talking about (before versus after correction) another.

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