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09-10-2011, 08:40 AM   #1
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FFF question - fisheye, focal length and FOV

I've posed this question before but never had a straight answer...so I'll try again!

The scenario is as follows: there are 4 fisheye lenses with 4 different focal lengths, but a common FOV. What's going on here?

Examples:
Peleng 8mm Fisheye - 180 degree field of view (circular)
Sigma 15mm fisheye - 180 degree field of view
Zenitar 16mm fisheye - 180 degree field of view
Pentax 17mm Fisheye Takumar - 180 degree field of view

What's the deal? Is distortion the deciding factor here, with shorter focal length giving more barrel distortion?

09-10-2011, 08:48 AM   #2
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The sigma, zenitar, and pentax give diagonal field of views of 180 degrees. I'm guessing the difference in focal length but same field of view means that there is varying degrees of barrel distortion. I'd assume shorter focal length actually gives less distortion. The Peleng is a circular field of view, so it gives an image 180 degrees in all directions. Very different than the others.
09-10-2011, 09:06 AM   #3
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Those 180 degree FOV's are *only* on FF cameras. On APS-C they're closer to 100 degrees -- except the Peleng, whose image circle gets chopped-off a bit. These lenses also all likely use different projections. That's the old mapmaker's problem: projecting a 3D reality onto a 2D frame. Be aware that ALL wide lenses distort, same as all non-trivial maps.

There is no single fisheye projection. The full-circle fisheye is one thing; various frame-filling projections exist otherwise. I can play with such projections (and their distortions) by putting an FE adapter onto various zooms within the 28-100mm range. At ~40mm, it's full-circle; at ~60mm it's frame-filling; different zooms project differently in the 50-100mm range.

Then there's a possibility that with the 15-16-17mm frame-filling FE's, the lensmakers fudged their numbers a little, claiming 180 degrees when the truth might be closer to 175. That's the same sort of fudging as calling an f/2 lens f/1.9 because it *seems* faster; like pricing at 99 cents to feel cheaper than one buck. My FA50/1.4 actually tests to 52/1.5 or thereabouts. Big deal.
09-10-2011, 09:58 AM   #4
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Interesting, I'm a GIS / mapping professional and never really thought about optics like this it in terms of mapping projections. I guess one could have a different degree of distortion between the centre and edges, or basically "map" the image in dozens of different ways.

Thanks for the insight. I guess photographing a grid with the various lenses, comparing FOV and distortion could be a really interesting exercise!

09-10-2011, 09:59 AM   #5
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QuoteOriginally posted by RioRico Quote
Those 180 degree FOV's are *only* on FF cameras. On APS-C they're closer to 100 degrees -- except the Peleng, whose image circle gets chopped-off a bit. These lenses also all likely use different projections. That's the old mapmaker's problem: projecting a 3D reality onto a 2D frame. Be aware that ALL wide lenses distort, same as all non-trivial maps.
The Samyang 8mm (which the OP does not have listed in the list but I am posting about its FOV ) is 180 on APS-C sensors, really cool to shoot with but can be a pain if you are not careful - I have gotten plenty of images with my foot and even part of my shirt (loose fitting shirt) in the picture without knowing until processing time...




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09-10-2011, 10:00 AM   #6
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Oh and don't you hate it when people say that rectilinear wide angles don't "distort" images? It kills me! As per mapping projections, rectilinear lenses distort the representation of area instead of lines...it's just "different" distortion.
09-10-2011, 10:16 AM   #7
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QuoteOriginally posted by d.bradley Quote
Oh and don't you hate it when people say that rectilinear wide angles don't "distort" images? It kills me! As per mapping projections, rectilinear lenses distort the representation of area instead of lines...it's just "different" distortion.
Quite right. I see area distortion in standard 24mm lenses on APS-C.
09-11-2011, 08:13 PM   #8
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Are u using film camera on those lenses?

The pentax 10-17 is a very good lens and you can use it on film camera on 15-17mm

09-11-2011, 08:48 PM   #9
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QuoteOriginally posted by MegaPower Quote
The pentax 10-17 is a very good lens and you can use it on film camera on 15-17mm
The DA10-17 can be used on 135/FF cams at its shortest focal length too -- if you tear off the mini-hood! I haven't had the nerve to do that yet.
09-11-2011, 10:53 PM   #10
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Well here are a few links with some explanations and illustrations to support them. Hopefully together they provide a cohesive answer.
09-11-2011, 11:21 PM   #11
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QuoteOriginally posted by RioRico Quote
The DA10-17 can be used on 135/FF cams at its shortest focal length too -- if you tear off the mini-hood! I haven't had the nerve to do that yet.
I doubt it.
The corner to corner APSC length is shorter than FF.
If depends how you define usable, you will see a image circle at 10mm on FF.
It will go beyond the hood at 10mm.
It might work at about 14mm if you remove the hood.
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