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Old 30th March 2017, 04:41 PM
Pete Bedell Pete Bedell is offline
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Default Hotspot on pinhole images

I have recently constructed a 12"x16" pinhole camera for use with multigrade paper. The paper holder is curved to avoid vignetting but results show a marked hotspot in the centre, ie an area of gross over-exposure whereas the rest of the image is exposed correctly.
This is my first foray into pinhole.
Any ideas?

Pete
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Old 30th March 2017, 06:22 PM
Martin Aislabie's Avatar
Martin Aislabie Martin Aislabie is online now
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How thick is the material the pinhole is made from ?

If for instance - you take a 0.5mm diameter hole - but the material the hole is in is 3 mm thick - you get a significant tunnel effect - which will translate in to a hot spot in the middle of the print.

Allegedly, beaten gold leaf is the best - being only a few molecules thick.

Cheap Aluminium cooking foil is an decent economical alternative.

Also - are you getting your pinholes drilled or better still laser cut.

Its really important to get a very clean (sharp) edge to the hole.

Martin
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Old 31st March 2017, 09:30 AM
JOReynolds JOReynolds is offline
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I made pinhole cameras for school use out of tins, which were loaded with paper in the darkroom and sealed with black electrical tape. A textbook I read years ago suggested piercing the base of the tin (not the lid, which held the paper) and sanding off the burr with emery cloth to avoid the tunnel effect. This worked. The exposures were even except around the edge, where the reflective inner surface had distorted the illumination. We vowed to repeat the experiment with a lining of black cloth but, to my knowledge, it was never done.
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