Dark spot in center of pin hole pictures

[Grandpa Ron]

Pinhole cameras haves a natural vignetting feature to them. The exposer looks fine but starts to fade towards the edge of the picture.

Using my 4x5 view camera, I can adjust the bellows to give me an aperture between f180 and f500 with a .016" pinhole. However, I seem that when I get above f250 or f360 I start getting a dark spot forming in the center of the negative.

I am not sure if this is diffraction, or a slight light leak or light reflecting off the bellows interior but it only seems to be an issue with long exposures over 3 minutes or so.

Has any one else noticed this phenomenon with longer exposures.

 

[Sirius Glass]

Please post and example.

 

[Grandpa Ron]

Here are some examples of what I am talking about.

4x5 trees, the bright spot is not the sun behind the trees

The 4x5 privy is shot with me and the white privy in the deep shadows . The exposure captured the background very well. Once again you can see the bright spot.

The 4x5 swing image was vignette by my pin hole and shutter mounting method. The bright spot appears on the swing arm on the right.

Finally I have several 35mm shots when I used a black cardboard tube to place the pinhole farther from the film plane. This is a underexposed shot but you can see the bright spot.

I believe I have some sort of reflection bouncing around the 4x5 bellows and the 35mm black cardboard tube. This bright spot appears to some degree in almost all my pinhole shots. My pin holes range from .014" to .025" and the 4x5 shots were with and aperture of f357. I usually shoot on sunny days to reduce the exposure time.

At this point any suggestion would be helpful.

 

[RalphLambrecht]

Here are some examples of what I am talking about.

4x5 trees, the bright spot is not the sun behind the trees

View attachment 232220

The 4x5 privy is shot with me and the white privy in the deep shadows . The exposure captured the background very well. Once again you can see the bright spot.
View attachment 232221
The 4x5 swing image was vignette by my pin hole and shutter mounting method. The bright spot appears on the swing arm on the right.
View attachment 232222

Finally I have several 35mm shots when I used a black cardboard tube to place the pinhole farther from the film plane. This is a underexposed shot but you can see the bright spot.
View attachment 232223

I believe I have some sort of reflection bouncing around the 4x5 bellows and the 35mm black cardboard tube. This bright spot appears to some degree in almost all my pinhole shots. My pin holes range from .014" to .025" and the 4x5 shots were with and aperture of f357. I usually shoot on sunny days to reduce the exposure time.

At this point any suggestion would be helpful.

reflection from the sun would be my guess too.

 

[AgX]

In the photograph of the little hut the sun is on the right hand side above. I hardly think the sun could produce an image at that location.

 

[wyofilm]

What is covering your pinhole when not taking the image? Is it possible that the tape (or whatever you are using) isn't fully light tight?