What is mFT FoV?

Started Jan 27, 2013 | Questions thread
Detail Man
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Re: What is mFT FoV ?
In reply to Detail Man, Jan 28, 2013

Detail Man 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?

I agree that the diagonal FOV (thus the 35mm equivalent focal length when the crop factor is considered) of the default (rectilinear distortion corrrected) 4608x3456 pixel size is designed to be relatively close to the intended specification.

There do exist additional photo-sites on M43 image-sensors. They are in the horizontal sections. Some of those photo-sites are unusable - because they are masked-off, and used in order to report black-level offset information only. There are only a few extra photo-sites in the vertical dimension (but they are apparently set aside for use in implementing rectilinear distortion corrections).

The image-file meta-data for your E-M5 (4:3 AR) only reports "Image Dimensions" the calculated diagonal FOV of which is only very marginally different from the default value of 4608x3456. Those pixel-dimensions (for the 3.73 Micron pixel-pitch of the E-M5's image-sensor) calculate out to be only very marginally "longer" (in 35mm equivalent focal length), meaning "narrower" (in FOV) than the specification value.

See: http://forums.dpreview.com/forums/post/50738679

However, it is clear (from your posted example as well as another poster's example) that the E-M5 image-sensor does have some extra (horizontal) photo-sites that are not reported in the E-M5's image-file meta-data.

GH2 RW2 (4:3 AR) image-file meta-data reports "Sensor Dimensions" of 4760x3472.

See: http://forums.dpreview.com/forums/post/50738731

DxO Optics Pro 7.23 (only when automatic distortion corrections are enabled, and "Keep Aspect Ratio" is unchecked) widens the horizontal field (only), and produces output images that are 4668x3457 (as compared to it's default output image pixel-size of 4609x3457). The wider horizontal pixel-dimension yields a diagonal dimension that is 0.821% larger than default diagonal pixel-dimension.

Have been playing around (using RAW Therapee 4.0.9.184) with the same GH2 RW2 image-file used with the DxO Optics Pro experiment (above). It allows a maximum of 4616x3464 pixel-dimensions (with or without rectilinear distortion corrections being changed in value), and with or without "Auto fill" of the frame (so that no sign of blackened areas outside of image-data being used shows).

It appears (I think) that both DxO and RT are (in the course previewing in-process images) are re-sampling the original de-mosaiced image-data in order to cause them to comport with maximum processed image pixel-dimensions that the application developers have chosen for the particular camera model (with or without rectilinear distortion corrections being applied).

While it may well be the case that camera manufacturers are attempting to ensure that their distortion-corrected in-camera JPGs are (typically) providing a diagonally referenced 35mm equivalent focal length (corresponding to a diagonally referenced Angle of View) - at least where it comes to their own manufactured lenses, anyway - that is something close to the rated AOVs of (their) lenses ...

... it appears that theses RAW processors (at least those not provided by the manufacturer for use with their own camera models, anyway) are (by re-sampling the original de-mosaiced image-data) taking liberties based on their own independent judgments of how many extra image-sensor photo-sites can (safely) be utilized in the case of image-sensors on any particular camera model without running into "masked" photo-site locations on the horizontal egdes, and without running into photo-site locations needed for rectilinear-distortion corrections on the vertical edges.

DxO Optics Pro does not use the rectilinear-distortion correction data that exists in RAW image-file meta-data. They use their own data derived from characterizing camera/lens combinations.

RAW Therapee appears to utilize the rectilinear-distortion correction data that exists in RAW image-file meta-data (in the case of their experimental "Auto Distortion Correction" button that works in the case of some camera models only, the DMC-GH2 included), but that is an optional control existing outside of their own manual rectilinear-distortion correction tool.

.

Due to optical differences that exist between various lenses, as well as the re-sampling of the original de-mosaiced image-sensor (even in cases where rectilinear distortion corrrection is not being applied at all), it seems that one is not able to accurately predict the Angle of View (hence, the 35mm equivalent Focal Length) by performing calculations based upon known pixel-pitch specifications of the camera's image-sensor and the expected to exist active photo-site dimensions.

Further, the lens-systems are multi-element (and in the case of telephoto lenses, asymmetrical with a Pupillary Magnification value less than unity), and exhibit "focus breathing" and effects which make the actual AOV hard to predict for any given Focal Length and Focus Distance (as I vaguely understand such things).

The commonly used formula:

AOV = ( 2 ) x Arctangent ( ( Frame Diagonal ) / ( ( 2 ) x ( Effective Focal Length ) ) )

... descibes a single, symmetrical lens only.

An excellent reference: http://www.panohelp.com/thinlensformula.html

.

Thus, as Anders has referenced, using the following emprical method seem the only approach:

http://www.panohelp.com/lensfov.html

while also using a specific RAW processor configured to yield the widest (usable) observable AOV.

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