Before the rush

Before the rush
by evan-pak

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In a recent question it was stated that

You cannot change the focal length of the zone, it is fixed and a characteristic of the physical dimensions of the camera

As far as I know, focal length of a pinhole or zone plate is its distance from the sensor/film; so I'd assume that while going wide is a challenge, attaining focal lengths longer than flange distance should be rather simple, by using extension tubes (or a bellow). I understand that the "lens" would become slower by using same aperture at a larger distance, but the aperture is rather slow from start anyway.

Are there any good reasons to avoid extending pinholes / zone plates further off from image plane?

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up vote 4 down vote accepted

Absolutely, I've done this. The image does get dimmer though so you'll need to adjust your exposure accordingly. I've only toyed with it since much of my interest is in wide, not tele photography.

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I'm curious if you've done this with your zone plate photography or just pinhole. I'd be interested to hear if focus is an issue in this case, and how much leeway you feel like you have. – mattdm Dec 30 '13 at 20:37
I almost never use the pinhole so my test was almost certainly with one of the zones. The subject was far away also and my memory said it was in focus, but that's so subjective with zone! – Paul Cezanne Dec 30 '13 at 20:47
I'd be curious to see. I have the lensbaby zone plate but no extension tubes. – mattdm Dec 30 '13 at 20:57
I just bought some cheapo extension tubes from ebay, no electronics, but I didn't need any for the zone work! – Paul Cezanne Dec 30 '13 at 21:01

This will work for a pinhole camera, within limitations, but zone plates actually have a certain focal length, so it won't there. (Of course, zone plates tend to have a very large depth of field, and aren't particularly sharp anyway, so in the real world you may have some latitude.)

For a pinhole camera, there is an ideal pinhole size (for every wavelength of light) for every distance from the sensor plane. This ideal size will give the optimum focus without loss of sharpness from diffraction. Fortunately, there's a significant range which is perfectly good enough, so the zoom idea works.

For wide angle... we already have a question on that, actually. How can I shoot wide angle zone plate photography?

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