Irregular Image Brightness - Confused!

[aaronmichael]

Hello All,

I have a question that I can not figure out for the life of me. When shooting with my home made pinhole camera that takes an 8x10 negative, I always get this weird 45 (roughly) degree area of brightness towards the bottom of the image and it lasts for about 1/4 of the picture length. It doesn't start on the bottom left corner or bottom right, but it's sort of in the middle. Here are some links to demonstrate what I'm talking about. I'm not sure whether I like this effect or not, if its something I want to fix, or if it can be fixed at all. Any ideas as to what could be causing this would be greatly appreciated - thanks.

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Just occurred to me -could it be a "lens" flare?

 

[Jim Jones]

Lens flare or a reflection from an undarkened or shiny surface within the camera are two possibilities.

 

[Vaughn]

Perhaps "lensflare" -- did you make your own pin-hole, or did you buy a one? I am just wondering if it may be caused by a hole that is not perfectly round or has a rough edge.

 

[DWThomas]

It seems pretty consistent -- that's probably a Good Thing(tm) in terms of tracking down a cause.

Occasionally prone to off the wall blue skies speculation, I find what's left of my mind wondering about a few things ...
1) The camera itself -- Totally homemade? Modified regular camera? Pinhole to lens distance vs pinhole diameter? Has it ever worked or is this the 1st try?
2) The pinhole -- Homemade? If so, how? or purchased?
3) Film? paper? Loading camera? Developing and handling? Are these prints or inverted scans?

What attempts to pass for reasoning ...

1) A homemade (and maybe even a modified regular camera), especially if it's trying to cover a wide angle, might have stray "junk" interfering with the optical path. That may not be happening here as I would expect obstructions to be sharper in a pinhole scenario. Reflections, as Jim suggests are quite possible. If the attempted angle of view is very wide, it's possible for miscellaneous parts of the camera -- even mounting brackets or shutter flaps -- lifted corners of pieces of tape -- to cause a vignette effect or bounce some light around.

2) A very rough and ragged pinhole, such a one might get by just pushing a pin through foil might cause some odd distortions. Although I'd say these look a little too clean and regular to make that likely. A pinhole in very thick material would tend to increase light fall-off away from the center, though the sky area in those shots looks pretty evenly exposed.

3) In handling, there could be patterns exposed by various light leaks or the like. If doing prints, that offers an additional opportunity for something to sneak in; e.g. dark areas could be light struck paper in the printing and developing process.

I'd say in general the consistency seems to make the typically random effects of some of my speculations unlikely. You might try a shot of a uniformly illuminated light colored wall and see if there's more pattern detail that is recognizable. You might develop a piece of paper/film that has not been exposed to check some issues. That could be expanded to never loaded in the camera, versus loaded in the camera, camera parked in light, but no exposure made.

It's one of those "there's gotta be an explanation" things, but it strikes me as a bit inobvious to track down!

 

[ChuckP]

Have you turned the camera upside down or 90 degrees to see if it stays or follows?

 

[aaronmichael]

Thanks everyone for the great replies. Some more information on the camera - It's a completely home made camera made out of plywood, the inside is spray painted black, and the seams on the inside were caulked because they didn't lay perfectly together which means there were light leaks. The pinhole was home made as well. I took aluminum from a soda can, sanded it down with an electrical orbital sander to get it more thin than it already was, then used a sewing needle to "drill" a hole through, and used a light grit piece of sandpaper to sand down the burr on the side that got poked through. What confuses me is that I've used the same pinhole on a smaller home made camera that takes a 4x5 negative and have had no problems. Here are a couple shots with that camera.

http://www.flickr.com/photos/aaronmichael/5499026210/

http://www.flickr.com/photos/aaronmichael/5475648370/

 

[aaronmichael]

I have not tried turning it upside down or 90 degrees. That's an interesting idea, I'll have to try that out when I get a chance.