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Wide lens distortion...

 
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Thu Sep 02, 2004 5:20 am    Post subject: Reply with quote

I just finished doing some test pinhole shots on Polaroid film and it reminded me of an oddity that I meant to ask about...

The "lens" of the pinhole is 75mm. I took a couple of shots and things were fine. Then I wanted to reframe and tilted the camera up to get rid of some grass and include some tree. Shot turned out fine, except the wheels of a lawnmower...

If it were not for something round, I would never have noticed. But when the camera was pointed up, the wheels are no longer round. The have elongated into the center of the frame by enough to really notice.

My first experience with this was shooting the same tree with my 8x10 using a 120mm lens. That's quite a bit wider than a 75mm on 4x5, and the distortion was that much more pronounced.

Now, I somehwat understand what's causing the distorion. But what I don't understand is why I never noticed this when shooting 35mm cameras where I routinely use a 17mm lens which is wider than the 75 relatively. As a wider lens causes more distortion, why haven't I ever noticed this with the 35? And it is very obvious, especially on the 8x10 shots (which is very close to the 17mm on 35).

Have I just been oblivious to this for years, or do they have some way of correcting this distortion on 35mm camera lenses??

On the other hand, by controlling the distortion, some unique photos can be made!

On the third hand, using old Polaroid Type 55 rated at ISO/ASA 25 with a pinhole camera that has an f-stop of 231 makes for some very long exposures even in bright daylight....

Just another curiosity I thought someone might like to discuss...


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Les



Joined: 09 May 2001
Posts: 2682
Location: Detroit, MI

PostPosted: Thu Sep 02, 2004 10:04 am    Post subject: Reply with quote

By lens manufacturers standards what you are experiencing is not distortion but wide angle effect. My professor used to say the only distortions were pincushion and barrel. Okay now I won't have his ghost haunting me.

I'm guessing the reason you don't see it AS MUCH on 35 is that wide angle lenses on 35mm have to be retrofocus, ie the image distance is longer on a retro lens than a normal lens of the same focal lenght. (Just the opposite of a tele-photo lens) This allows the light rays to come in at a less acute angle than say a 120mm on 8x10.

They are finding out digital backs don't like normal wide angle lenses. Since the chips have a thickness (with cut off filters, etc. Light ray coming in at a rakish angle creates all sorts of havoc, well color fringing, ghosting, etc.
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Thu Sep 02, 2004 7:57 pm    Post subject: Reply with quote

Ahh, I never thought of the relative lens-to-film plane distance. So 35mm comes up with some small advantage over LF And digital a big disadvantage

I think there was a discussion not too long ago about why we don't have any retrofocus lenses in LF. Boy would that help things! And I always liked being 'retro'

Thanks Les...
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Nick



Joined: 16 Oct 2002
Posts: 494

PostPosted: Thu Sep 02, 2004 8:19 pm    Post subject: Reply with quote

Aren't the real short 4x5 lenses retrofocus?
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glennfromwy



Joined: 29 Nov 2001
Posts: 903
Location: S.W. Wyoming

PostPosted: Thu Sep 02, 2004 8:38 pm    Post subject: Reply with quote

Another thing to think about, while pinhole "lenses" are absolutely rectilinear, it only applies if the film plane is parallel to the subject. Thus, by tilting the camera up, the wheels become oval as with a regular glass lens.

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Les



Joined: 09 May 2001
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Location: Detroit, MI

PostPosted: Thu Sep 02, 2004 8:58 pm    Post subject: Reply with quote

Well my 58mm Grandagon, 65mm SA and 120 Nikor SW certainly aren't. But I don't know about the 47mm lens out that covers 4x5.

Now some of the new digital lenses in the 35mm focal length may become retro focus, but those won't cover 4x5 and weren't intended to. Most of the photographers shooting with a digital back (leaf,Capture One, etc) are using 35mm Nikors on adapted boards. They seem to cover the 4cm x4cm sensor well enough, but without movements.

[ This Message was edited by: Les on 2004-09-02 14:01 ]
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Nick



Joined: 16 Oct 2002
Posts: 494

PostPosted: Thu Sep 02, 2004 10:10 pm    Post subject: Reply with quote

http://www.schneideroptics.com/photography/large_format_lenses/super-angulon/chart.htm

It looks like all the XL series of Super Angulon needs longer to focus then the focal length.
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Sat Sep 04, 2004 4:27 am    Post subject: Reply with quote

I could easily be wrong, especially when I rely on memory, but I do remember when I first mounted my 65 f/8 SA to my Speed. I was worried about it working at all and how far into the box it would go. I was very suprised to see that it sat at about the same place as my 90mm Optar for infinity. 'course I didn't care that much as long as I could use the thing, but that's what I remember. I'll have to check...

It's nice to know that the XL's seem to be retro and look useable at least down to the 58. Not sure the 47 leaves enough room on a Speed? But by the time I could afford one of them, they would be "retro" and I'd be too old to remember how to work the camera
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Sat Sep 04, 2004 4:34 am    Post subject: Reply with quote

Quote:

On 2004-09-02 13:38, glennfromwy wrote:
Another thing to think about, while pinhole "lenses" are absolutely rectilinear, it only applies if the film plane is parallel to the subject. Thus, by tilting the camera up, the wheels become oval as with a regular glass lens.


Interesting. Never thought of it?

I was originally going to mount a pinhole onto the Speed for expense and double usage. But somehow it just didn't make sense to me? If I was going to use a lensed camera, it just didn't seem right to use it without a lens... Seems odd, but a lot of my rationalizations are (more and more as I get older...)

So, I just added a pinhole camera to a film order. Somehow it seems right to shoot pinhole with a pinhole camera...

I never put any thought at all into movements with a pinhole? And in the back of my dark and cobwebbed mind, I think I somhow thought that movement wouldn't apply to a pinhole. maybe because there's no "lens". Although thinking about it now, I don't see how front tilt or swing could accomplish much? Rear tilt or swing should be very similar to a lens camera. Or so I think at this late hour...

Now I might have to mount a pinhole on a View just to see what it does...


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glennfromwy



Joined: 29 Nov 2001
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Location: S.W. Wyoming

PostPosted: Sun Sep 05, 2004 3:05 am    Post subject: Reply with quote

You may have seen more "distortion" with your 35mm rig than you realize. It can be a bit more subtle, but it's there, even with a normal lens. Look closely at the vertical lines.
Wide angle lenses compound things like "keystone" effect, etc.

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