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02-05-2009, 01:24 PM   #1
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Fisheye Question: 35mm x Cropped

A discussion regarding the Pentax 17mm Fisheye (35mm) popped up in another forum and few questions came up:


- it is my understanding that a fisheye lens has about 180degree FOV, so if I mount a FF lens on cropped sensor, I'd lose some of that... so can I still say that I have an "official" fisheye picture?

- is it correct to say that for APS-C one must buy APS-C fisheye lenses to get the real fisheye effect?


I guess it boils down to what one considers fisheye effect. I guess there is no standard regulating this (as we see "macro" on some lenses that are merely "close focus") but when we talk about fisheye, we expect to see around 180-degree FOV, no?


Any inputs welcome!

02-05-2009, 01:39 PM   #2
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QuoteOriginally posted by BBear Quote
A discussion regarding the Pentax 17mm Fisheye (35mm) popped up in another forum and few questions came up:


- it is my understanding that a fisheye lens has about 180degree FOV, so if I mount a FF lens on cropped sensor, I'd lose some of that... so can I still say that I have an "official" fisheye picture?
I think the popular Zenitar 16mm fisheye is actually 180 degrees on the diagonal on 35mm film, a bit less horizontally. I don't know how other fisheyes do 180 degrees, but the Sigma 16mm I have is similar to the Zenitar. On APS-C, both these lenses have fisheye distortion, just not as much.

QuoteOriginally posted by BBear Quote
- is it correct to say that for APS-C one must buy APS-C fisheye lenses to get the real fisheye effect?

I guess it boils down to what one considers fisheye effect. I guess there is no standard regulating this (as we see "macro" on some lenses that are merely "close focus") but when we talk about fisheye, we expect to see around 180-degree FOV, no?
I would define the effect with the amount of distortion, not the field of view. But on APS-C, both are less for the 16mm primes I've used. I think the 17mm Pentax or Takumar would be similar to these. The DA 10-17 zoom has a much greater fisheye effect at 10mm.

Look through the fisheye thread for examples.
02-05-2009, 02:00 PM   #3
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QuoteOriginally posted by Just1MoreDave Quote
On APS-C, both these lenses have fisheye distortion, just not as much.
I understand the 1.5x crop wont eliminate the distortion completely, but because we WANT the distortion and technically I think fisheyes have 180degree, hence the question.

02-05-2009, 02:04 PM   #4
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yes. they had to make the 10-17mm in order to provide the equivalent field of view.

however keep in mind that it is fundementally easier to make fisheye lenses, since the reason they distort is because they are lacking corrective elements

which is why wide angle primes that offer no distortion (like the zeiss 18mm f3.5) are so expensive.

02-05-2009, 02:34 PM   #5
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QuoteOriginally posted by BBear Quote
I understand the 1.5x crop wont eliminate the distortion completely, but because we WANT the distortion and technically I think fisheyes have 180degree, hence the question.

There are two different kinds of fisheye lenses:
  • 180 degree diagonal
  • Circular

The 180 degree diagonal provides 180 degrees along the diagonal axis of the picture frame. The circular provides 180 degrees in all directions, but projects a circular image. Both can be used with a cropped sensor with some loss of FOV. Both have significant distortion, even on a cropped sensor. Most fisheye lenses are 180 degree diagonal in order to fill the frame.

So, the question comes...Why is it that some pictures taken with fisheye lenses don't look the least fishy? The answer is two-fold:
  • If the lens axis is level to the earth, the horizon will look flat
  • In that absence of straight lines, the eye tends to ignore distortion
I have the Zenitar 16/2.8 Fisheye. On 35mm film, there is no denying its fishiness. On APC-C, it all depends how I hold the camera and/or compose the shot. Consider these (all with the Zenitar):

Fishy...


Fairly fishy...


Not so fishy...


Also not so fishy...


Steve

(bought the Zenitar for general landscape use...would really prefer a nice 16mm or 18mm rectilinear for the same amount of money...)
02-05-2009, 02:54 PM   #6
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Given the definition of Fisheye lens - Wikipedia, the free encyclopedia I would argue that this lens is not a 'fisheye' on an APS sensor camera. It will have barrel distortion, but not the angle of view - perhaps the worst of both worlds?
02-05-2009, 03:22 PM   #7
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QuoteOriginally posted by jfsavage Quote
Given the definition of Fisheye lens - Wikipedia, the free encyclopedia I would argue that this lens is not a 'fisheye' on an APS sensor camera. It will have barrel distortion, but not the angle of view - perhaps the worst of both worlds?
That was exactly my point, of course, if there were a standard guide stating that "fisheyes have to cover 180 degrees".



@ Steve: I was purely talking about full-coverage/180 degree fisheyes. But thanks for the class on the subject!
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