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12-07-2010, 04:56 PM   #1
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Fisheye question

Does the focal length on a fisheye lens affect how round the image appears. I have seen some fisheye images that look like a ball and some that are a traditional rectuangular shape with just a little distortion of the straight lines in the image.

12-07-2010, 07:00 PM   #2
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Some (like the 8mm) are circular fisheyes. Others like the Zenitar 16 andf the Pentax 10-17 are rectilinear fisheyes.
12-07-2010, 07:15 PM   #3
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I have the 10-17 and yes 10mm will have more of a fishey look than 17. It also depends on where you center the horizon as to how much curvature you get. I have been considering getting the 12-24 and selling the 10-17 but after my last vacation, I really find the 10-17 a very useful lens in most circumstances calling for extreme wide angle.
12-07-2010, 09:37 PM   #4
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Look at the Wikipedia FISHEYE entry for details. But generally, there are full-circle fisheyes, and full-frame fisheyes. Lenses like my Pentax DA10-17 and Zenitar 16/2.8 are full-frame fishy's -- the have a wide angle of view (AOV) with circular distortion at the edges. Ultra-wide rectilinear lenses try to straighten the edges, with introduces another sort of distortion. A full-circle fisheye doesn't try to fill the image frame.

A true full-circle fishy lens will be sharp but expensive compared to a fisheye adapter like the 180 Degree that may be labeled as Kenko or Spiratone or whatever, which often goes for under US$50. You can also simulate a full-circle fishy effect in many photo editors, but that just ain't the same, eh?

Your question is, does the focal length affect the amount of fishiness? The answer is, yes and no. It also depends on how the lens design projects the image. A 12mm lens might produce a full-circle image on a full-frame camera, while a wider 10mm lens may have a frame-filling projection. And I've calculated that it takes a 6mm lens to project a full circle on an APS-C dSLR sensor like in a Kx or K7. I haven't tried a new 8mm fisheye so I don't know if it is really full-circle, or if the image top and bottom are cut off a little.

Using a full-circle fisheye is a little tricky because of exposure metering. In a rectangular frame, much of the frame remains black, which throws off the exposure. Also, very slight changes in distance and angle can have major effects on the captured image. It's all so much fun!

12-07-2010, 09:52 PM   #5
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QuoteOriginally posted by GaryH Quote
I have been considering getting the 12-24 and selling the 10-17 but after my last vacation, I really find the 10-17 a very useful lens in most circumstances calling for extreme wide angle.
Those are actually pretty different lenses. Here's a comparison. Sorry I did not do 10mm on the fisheye...
12-07-2010, 10:06 PM   #6
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SpecialK, are you sure about that "rectilinear fisheye"? I thought a lens was one or the other.
RioRico, using a .15X adapter on a 50mm lens on an APS-c sensor yields a circle with the top and bottom cut off. That should be about 7.5mm, which would square with your calculations. On the DA 40 (6mm equivalent) it is a full circle.
12-07-2010, 10:21 PM   #7
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QuoteOriginally posted by Nick Siebers Quote
SpecialK, are you sure about that "rectilinear fisheye"? I thought a lens was one or the other.
I think SpecialK meant to type "diagonal fisheye".

QuoteOriginally posted by RioRico Quote
I haven't tried a new 8mm fisheye so I don't know if it is really full-circle, or if the image top and bottom are cut off a little.
Depends on the lens and sensor/film size. An 8mm circular fisheye (such as the Sigma 8mm) shows a full circle on flim.


On APS-C digital, the top and bottom are cut off. It's pretty common to crop out the remaining black areas on the left and right (resulting in a diagonal fisheye image with a flexible aspect ratio).

RE metering with a circular fisheye:
It is usually sufficient to use center-weighted metering and substitute meter from a light gray object.
12-07-2010, 10:48 PM   #8
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QuoteQuote:
I think SpecialK meant to type "diagonal fisheye".
Rectilinear is the term I grew up with, meaning rectangular rather than circular. In other context it refers to straight lines :-) It may have been replaced by diagonal fisheye, which I heard for the first time tonight.

12-07-2010, 11:16 PM   #9
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. . . actually the term "full frame fisheye" is older and more common than "diagonal fisheye", although potentially confusing now that "full frame" commonly refers to sensor size.
12-08-2010, 02:34 AM   #10
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Circular fisheye lenses are projecting 180 degrees FOV (half sphere) onto a circle with about the same diameter as sensor height.

However, these fisheyes are usually designed for full frame sensor so their circles look like black corners on the 1.5x crop sensors.

Then there are diagonal fisheyes, which are supposed to have diagonal FOV of 180 degrees. If these are designed for full frame, then on crop sensors they will produce significantly less FOV.

There are however both circular and diagonal fisheye lenses designed for 1.5 crop sensor, but they are very expensive.

Another kind of fisheye is zoom fisheye (Pentax 10-17 and I think there is Tokina analog) - those are 180 degree diagonal (cropped) and when zoomed in, it's like a cropped FF fisheye,

As of the focal length, well they say little for fisheyes since there are different projection types.

Traditional fisheye projection has an near-edge effect opposite to rectilinear lenses projection (which is stretching objects along radius). On the traditional fisheye image, objects near edge look compressed along the radius so they look very disproportional. If you try to correct such distortion, objects will become blurred along original radius.

However, there is a better fisheye projection, used in a Samyang 8mm lens. Objects in this projection are little stretched at the edges, like in rectilinear lenses perspective effect, but not so much. And at the same time, this lens is full diagonal 180 degree fisheye. So if you want to correct fisheye distortion at the edges, you will get better result from Samyang 8mm fisheye than from some traditional one. Therefore, there are more details preserved near borders, although objects in the center of frame appear smaller than on the traditional fisheye image.

So, overall, if you want a fisheye for semi-macro and special effects, pick а traditional fisheye (pentax 10-17 is the best since it's a zoom and has close to zero focusing diatance); if all you care about is the wide angle and best detail all over the frame then Samyang 8mm has no competition currently.
12-08-2010, 03:04 AM   #11
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QuoteOriginally posted by Nick Siebers Quote
RioRico, using a .15X adapter on a 50mm lens on an APS-c sensor yields a circle with the top and bottom cut off. That should be about 7.5mm, which would square with your calculations. On the DA 40 (6mm equivalent) it is a full circle.
Adapters are interesting and interact with different lenses in different ways. I have the 'Kenko' Fish-Eye 180 Degree adapter, a great hulking 250g beast. Using a Tak-A 28-80 zoom on my K20D, I got full-circle at around 43mm. With the DA18-55, I get full-circle at 35mm. I haven't tried it on my F35-70 yet.

That TA28-80 was a mediocre lens (I sold it off) as is my A35-80 (which I will sell soon enough) but those are useful test-beds for the 180 adapter, going from small full-circle at the wide end, to frame-filling at around 60mm and beyond. Hmm, I have a good RMC Tokina 35-135 stashed away that I can try the adapter on. Zooming that amount should display a nice range of fishy projections. Hmmm...

Yo Nick: Which .15x adapter, and what did it cost, and how do you like it?

QuoteOriginally posted by troyz Quote
. . . actually the term "full frame fisheye" is older and more common than "diagonal fisheye", although potentially confusing now that "full frame" commonly refers to sensor size.
That's why I prefer the term "frame-filling". The full acronym would be FF-FE or FFF (frame-filling fisheye) which is hopefully distinguishable from FF (full-frame). So we have FF, and FF-FE or FFF, and FC-FE or FCF (full-circle fisheye). [A related acronym is an obscene reference to my former corporate employer, but I'll skip that now.]
12-08-2010, 08:10 AM   #12
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Hello -

I have the same adapter, I think, branded by Samigon. It is huge, I once put it on the end of my K 80-200 and it looked like I was carrying around a light saber... or something obscene. :-) I like its effect when producing a full circle fisheye or thereabouts, as usually in those shots exact fidelity isn't critical. I also have an A 16/2.8 fish eye, and needless to say it is worlds better than the Samigon adapter on a 100mm lens for a frame-filling effect.

I traded a Super-Tak 35/2 for my adapter... I don't think I got the better of the deal, as I had been asking about $100 for the lens. But so it goes! Do you (or anyone) know if there is a way to change the focus point? Mine seems sharpest at a very close distance, and the only way to get more distant objects in focus that I can see is to stop down. Is that right?

Thanks, Nick

eta the best results I have had were with an M 50/1.7, I figure you need a lens that is nice and sharp wide open. The DA 40 would be good too, but then you need to turn SR off.
12-08-2010, 12:26 PM   #13
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The 10-17 zoom gives you a major fish-eye effect at 10mm:


At 17 it gives much less, just a bit more than the DA 16-45 at 16mm.
12-08-2010, 02:43 PM   #14
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QuoteOriginally posted by Nick Siebers Quote
Hello -

I have the same adapter, I think, branded by Samigon. It is huge, I once put it on the end of my K 80-200 and it looked like I was carrying around a light saber... or something obscene. :-) I like its effect when producing a full circle fisheye or thereabouts, as usually in those shots exact fidelity isn't critical. I also have an A 16/2.8 fish eye, and needless to say it is worlds better than the Samigon adapter on a 100mm lens for a frame-filling effect.

I traded a Super-Tak 35/2 for my adapter... I don't think I got the better of the deal, as I had been asking about $100 for the lens. But so it goes! Do you (or anyone) know if there is a way to change the focus point? Mine seems sharpest at a very close distance, and the only way to get more distant objects in focus that I can see is to stop down. Is that right?

Thanks, Nick

eta the best results I have had were with an M 50/1.7, I figure you need a lens that is nice and sharp wide open. The DA 40 would be good too, but then you need to turn SR off.
That adapter is the same across many brands...

I am using mine on a Takumar-F 28-80. I can focus past infinity and as close as an object touching the lens. Macro mode on the 28-80 is usesless with the adapter since I can focus so close already.

The 28-80 also gives me the ability to go from full circle to full frame as well.

I originally was using it on my Takumar-A 70-210 and it didn't give near as good of pictures.

By chance are you using a step-up or step-down ring in adition to the adapter that came with the fisheye adapter? If so, that is probably what is killing your ability to focus.
12-08-2010, 03:46 PM   #15
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I have been using step rings most of the time, but not all the time. How do you adjust focus? With the lens, or with the adapter? Thanks! Nick
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