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Tunnel/Fish Bowl effect in pictures?
Old 12-25-2004, 06:44 PM   #1 (permalink)
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Ok ... another newbie question (hope you don't mind). You guys were VERY helpful with my ISO/moire pattern question, so I thought I would ask this one since I can't find the answer in the manual.

I just got back from my trip home for Christmas (Merry Christmas by the way), and my new Canon EOS Digital Rebel worked great. I did notice for some of the pictures that I took of the kids this morning opening the presents, that the lens created a kind of tunnel/fish bowl effect. I was sitting at one end of the room by the tree and took pictures of them as they walked in the room. I had the lens pulled back (wide angle) and noticed a fish bowl type effect in the picture. I also got this effect when standing above and slighting away from the kids when they opened the gifts.

So ... what causes this and how do I adjust for it? I have the 18-55mm lens and assume it has something to do with the zoom, distance, angle, etc., but I don't know how all these variables play into the final result of the picture. Is there a quick answer for this?

Thanks again!

Earl
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Re: Tunnel/Fish Bowl effect in pictures?
Old 12-25-2004, 09:36 PM   #2 (permalink)
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Earl,
This sounds like it's caused by the wide angle. I'm not sure about the conversion factor with digital lenses. But, I have a 7mm lens for a film camera that causes all corners to vingette and gives an extreme "fishbowl" look. I would guess that your 18mm is fairly wide for digital. If you don't want/like this look, just don't shoot that wide. There's really no other way to correct it.
Hope this helps.
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Re: Tunnel/Fish Bowl effect in pictures?
Old 12-25-2004, 09:59 PM   #3 (permalink)
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Jason,

Thanks for the reply!

Earl
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Re: Tunnel/Fish Bowl effect in pictures?
Old 12-26-2004, 04:47 AM   #4 (permalink)
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When you go wide angle, perspective is exaggerated. Things closer to the lens appear MUCH bigger than things even a little further away.

If you are shooting from above your kids, then their heads are closer to the lens than their bodies, so you get a distorted body image. Likewise, if you are pointing the camera downwards, all the upright lines in the room will distort drastically because of the same phenomenon of exagerrated perspective. You will notice this problem less with wide angle if you try and shoot very close to horizontal when there are obvious uprights to confuse the eye (things like walls, corners, chairs, tables etc).

The other possibility is that the lens has something called barrel distortion. This is where straight lines will look straight at the centre of the image, but bend outwards towards the edges of the image. This is a common problem with most wide angle lenses (some of the very expensive primes and maybe a good copy of the sigma 12-24 don't suffer badly). How straight the lines stay at the edges of the image is a measure of how rectilinear the lens is.
If you want a clear test to see if your lens is barreling: take a close-up photo of a coke can or something. Fill the frame with it. The trick is to get the straight lines of the can on or near the edge of your frame. If it looks like a barrel instead of a can, its got barrel distortion.

Move back, zoom in - it should go away as you use longer lens lengths.

I hope some of this helps a bit.

James
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Re: Tunnel/Fish Bowl effect in pictures?
Old 12-26-2004, 09:19 AM   #5 (permalink)
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James,

Thanks! It helps a lot!

Earl
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Re: Tunnel/Fish Bowl effect in pictures?
Old 12-27-2004, 12:25 PM   #6 (permalink)
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Maybe its something as simple as the flare shield on the lens. Had this crop up several years ago, while shooting wide angle with the flare shield on. Remove the shield, and problem went away...

Good luck

Shadow
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