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07-23-2009, 09:28 AM   #1
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Difference between Pentax fish eye @ 10mm and Sigma @ 10mm?

Hi all,

I'm starting to have a re-think about my Pentax 10mm-17mm fish eye lens, and my current landscaping lens the 17mm-70mm.

I'd like to go a bit wider than 17mm, but the distortion on the Pentax fisheye is as you would expect - it's a fish eye! This isn't ideal for landscape photographs as the barrel distortion is unbearable. The other obvious choice is the Sigma 10mm-20mm which seems to be highly spoken of for landscaping due to the focal length of the lens.

My question is: what makes a fish eye at 10mm different from a non fish eye at 10mm? Is the fish eye deliberately distorted? Is it something to do with the lens element setup?

Here is a sample of the Sigma at 10mm - notice the lack of any barrel distortion.

http://www.dpreview.com/lensreviews/sigma_10-20_4-5p6_n15/Samples/issues/DSC_0369_acr.JPG

07-23-2009, 09:52 AM   #2
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QuoteOriginally posted by Big G Quote
My question is: what makes a fish eye at 10mm different from a non fish eye at 10mm? Is the fish eye deliberately distorted? Is it something to do with the lens element setup?]
A fisheye lens needs that kind of distortion to achieve its 180 degree fielf of view. The point is, if you compare the images of the fisheye versus the Sigma 10mm ones (that's a rectilinear lens), you will find some more differences, not just the barrell distortion:

A fisheye uses a special projection (most use the equisolid angle projection) to fit the 180 degrees view into the image frame. So the "barrell distortion" is not a fault of the lens, but a design goal.

Rectilinear lenses, like the Sigma 10-20mm zoom aim to prevcent the curved lines and produce no or less visible distortion. BUT at the expense of a changing projection ratio, which gets very visible (and annoying) near the edges. Photograph a person with a rectilinear lens, which is placed near the frame edge and you'll distort the person's head to some ugly egg-form. BUT this is usually not or hardly noticeable in landscapes.

A short wrap-up cayn be found here: Fisheye Projection - PanoTools.org Wiki

Some people use software tools to "correct" the distortion of the fisheye lens, which is counterproductive. You not only interpolate many pixels (which reduces resolution), but also end up with a much diminished angle of view, which a rectilinear lens provides anyway.

Simply think of a fisheye lens versus a rectlinear lens as two very different lenses, which serve different purposes.

Ben
07-23-2009, 09:53 AM   #3
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The field of view of the fish eye at 10mm is much greater than that of a rectilinear 10mm, such as the Sigma 10-20. If you use a tool such as PT_Lens, you can "defish" the fish eye picture and still have a much wider FOV than on the Sigma at 10mm.
I was bored at work one day and actually compared these two lenses directly, here are the results:

DA 10-17 @ 10mm, no correction


Same shot at 10mm, "defished" with PT_lens. (this isn't totally "defished", only defished enough to suit my taste, PT_lens does allow you to correct it more if you wish)


Sigma 10-20 at 10mm


And to quench curiosity, a shot from the DA 10-17 at 17mm, no correction.


I did manage to sell some stuff between the shots, so they may not be all perfectly aligned.
07-23-2009, 09:58 AM   #4
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Ahh thanks guys, so I need a rectilinear 10mm lens then, i.e. the Sigma.

Better get the wallet out (looks like i'm getting into LBA).

07-23-2009, 10:02 AM   #5
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The Pentax 10-17 has a wider field of view, a full 180 degrees at 10mm. If you want a very wide lens for primarily landscape shots and the distortion bothers you then the 10-17 probably isn't the lens to buy. Yes it is built that way on purpose, thats why it's called the fisheye. I am not an engineer but I don't think you can get 180 deg view without some of the fisheye distortion. With a little care in framing , you can get good landscape shots and interior shots with the FE that won't appear too distorted and can be straightened in PP. I kind of like the effect I get with the lens and I carry it with me almost everywhere.That is my preference. This shot is a landscape shot of the St. Lawrence River. It was from my first weekend with the lens and I shot it mainly as a lens test/learning shot. It was shot at 10mm. The distortion decreases a lot as you move toward the longer end of the zoom.

Last edited by reeftool; 12-24-2009 at 07:59 AM.
07-24-2009, 01:10 PM   #6
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QuoteOriginally posted by Ben_Edict Quote
A fisheye lens needs that kind of distortion to achieve its 180 degree fielf of view. The point is, if you compare the images of the fisheye versus the Sigma 10mm ones (that's a rectilinear lens), you will find some more differences, not just the barrell distortion:

A fisheye uses a special projection (most use the equisolid angle projection) to fit the 180 degrees view into the image frame. So the "barrell distortion" is not a fault of the lens, but a design goal.

Rectilinear lenses, like the Sigma 10-20mm zoom aim to prevcent the curved lines and produce no or less visible distortion. BUT at the expense of a changing projection ratio, which gets very visible (and annoying) near the edges. Photograph a person with a rectilinear lens, which is placed near the frame edge and you'll distort the person's head to some ugly egg-form. BUT this is usually not or hardly noticeable in landscapes.

A short wrap-up cayn be found here: Fisheye Projection - PanoTools.org Wiki

Some people use software tools to "correct" the distortion of the fisheye lens, which is counterproductive. You not only interpolate many pixels (which reduces resolution), but also end up with a much diminished angle of view, which a rectilinear lens provides anyway.

Simply think of a fisheye lens versus a rectlinear lens as two very different lenses, which serve different purposes.

Ben
Thanks Ben for a great explanation! Fisheye lenses are much misunderstood. I use a Zenitar 16/2.8 on my K10D as a landscape lens and seldom have a need or desire to "correct" anything. The circular projection of the cropped image is actually quite natural appearing. Curved horizons are another matter, but that is not related to the fisheye per se.

Steve
10-07-2010, 08:29 AM   #7
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Are there any full-sized examples of the fisheye de-fished vs the sigma at 10mm? I'd like to see how much the resolution suffers: overall and in the corners where de-fishing is extreme.
10-07-2010, 08:37 AM   #8
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Hah, my old thread - I could try and arrange it this weekend for you as I have both lenses now .

10-07-2010, 08:43 AM   #9
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QuoteOriginally posted by Mock Quote
The field of view of the fish eye at 10mm is much greater than that of a rectilinear 10mm, such as the Sigma 10-20. If you use a tool such as PT_Lens, you can "defish" the fish eye picture and still have a much wider FOV than on the Sigma at 10mm.
I am not sure about that last part. If you totally defish then you will get the rectilinear image of a 10 mm I think, but I have no problem with an example that proves that I am wrong...
10-07-2010, 08:48 AM   #10
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Doh! I wish this old thread hadn't been revived. I was considering selling my DA 10-17mm, now that I had the Sigma 10-20mm. I had not done any comparative testing, so I didn't appreciate just how much the FOV differed between the 2 at 10mm. Oh well....guess I'll keep both.
10-07-2010, 09:13 AM   #11
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QuoteOriginally posted by Big G Quote
Hah, my old thread - I could try and arrange it this weekend for you as I have both lenses now .
That would be awesome. Could you live with just the FE, using it to produce rectilinear images?
10-07-2010, 09:31 AM   #12
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My experience having done this exercise is that yes you initially get a wider FoV on the fisheye, but doing the correction to make it rectilinear rather negates this. Distortion on either lens is pretty extreme at the peripheries and overall whilst I currently still have both and really just keep the fisheye for novelty value, I find it hard to previsualise or shoot a pleasing image using a fisheye so have rarely used it in anger this year after getting the Sigma. I strongly suspect it'll be sold off sooner or later. Mind you, YMMV, I bet there are folk out there that'd be great with the curvy perspective that a fisheye gives you - I just don't happen to be one of them
10-07-2010, 10:26 AM   #13
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QuoteOriginally posted by Eruditass Quote
That would be awesome. Could you live with just the FE, using it to produce rectilinear images?
Posted a few examples on the other forum a while ago comparing 12-24 and 10-17 in various defished alternatives:

Examples: Pentax SLR Talk Forum: Digital Photography Review
10-07-2010, 10:58 AM   #14
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Background: It was the desire for an affordable fisheye zoom that drove me to Pentax in the first place -- my initial K20D kit were the DA10-17, DA18-250, and FA50/1.4. I now use a Zenitar 16/2.8 mild fisheye more than the 10-17, mostly because of speed (I've used it a lot indoors). But my Vemar 12, and the Pentax @ 10, and a Kenko 180 adapter on an 18-55 or 28-80 @ 40, still deliver results impossible for rectilinears. NOTE: a rect 10-20 or 12-24 is definitely in my future.

As mentioned, ultrawide rectilinears and fisheyes are different tools for different purposes. Barrel distortion is built into a fisheye. Geometrical distortion is built into an ultrawide rectilinear. That's how optics work. That's how 3D-to-2D (globular to flat) projections work. Think of a photo of Terra from Luna (fishy) vs a Mercator projection of the globe (recti). For ultrawide without geometrical distortion, shoot panoramas.

Ah, panoramas. There are still projection limitations. You can put a camera on a tripod with a pano head, shoot a 180 or 27 or 360 degree scene, stitch them together -- but you're still going globular-to-flat. It works as long as you don't have any large rectilinear objects in the image. But say I'm on the street and I'm shooting a long building opposite -- the ends will be pinched together. For an undistorted pano, I'd take a shot at one end, move parallel 50 feet or whatever, take a shot, move parallel another distance, shoot again, etc. Now, *those* images stitched together correctly will give a totally rectilinear view.

Every image, every projection, has its own problems. Life ain't perfect. Bother...
10-07-2010, 11:15 AM   #15
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QuoteOriginally posted by Eruditass Quote
That would be awesome. Could you live with just the FE, using it to produce rectilinear images?
Simply put, absolutely not .

The Sigma and the Pentax are different beasts. First and foremost, you can't attach filters to the Pentax 10-17 as it has a built in metal petal hood - I use filters for most, if not all of my landscape shots so the Pentax 10-17 is no use here. Additionally, the Pentax 10-17 suffers heavily from CA and PF which is much better controlled on the Sigma.

I suppose you could try to de-fish the Pentax, but I reckon you're going to lose a fair amount of image quality. Both are great lenses, they just have different uses.

Edit: you may want to look at this review:

http://www.photozone.de/pentax/132-pentax-smc-da-10-17mm-f35-45-edif-fisheye...report?start=1

Resolution figures at 10mm aren't that great plus this review touches on the nasty CA and PF that this lens exhibits.
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