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Old 30th June 2009, 06:09 PM
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Smile Calculating exposure using extention tubes

Recently purchased old stock but brand new Bronica 42mm extention tubes from (Jessops of all places) - silly price on their website £34 but on reading the manual, it states you need to increase exposure because they is less light entering the lens and you are probably closer to you subject.

I have a Bronica ETRSi (no metering) but do have WLF and a plain prism. I was wondering if I could still use my external meter as I would normally and still get correct exposure?

I assume I won't need to make any exposure compensation if I stick to either incident or spot using my sekonic. Am I correct in this assumption and if so, which external metering pattern would be better. If I went spot I could place a grey card in front of my subject - any tips!

The manual seems to reference exp. compensation when using an AE prism.

Can't afford a macro lens so, I thought I'll have some fun and experiment with using tubes instead.

Thanks for any tips and advice

apxuser
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Old 30th June 2009, 06:38 PM
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There will be some tables somewhere on the net (or in the Broni's manual) that you can use with those tubes as you do need to apply a + compensation factor with your exposure readings. The AE metering prism would do this automatically for you of course if you had one. Using your Sekonic without correct compensation for the loss of light falling onto the film during exposure would lead to under exposed negs. To sum up: you need to establish the light loss with a tube, or a combination of tubes, fitted and apply that to you meter settings.

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Old 30th June 2009, 07:34 PM
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I have used extension tubes in the past,not on a bronny but an old mamiya that hs long gone,and I used to increase exposure between half to one and a half stops depending on which extension tube,or combination I was using,this was a rule of thumb, but it worked and should give you a starting point,Richard
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Old 30th June 2009, 08:16 PM
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I haven't used tubes for a while but roughly speaking if you double the focal length of the lens you increase exposure by two stops.

There are formulae and calculators on the web for large format users - you might search for bellows extension since the principle is the same.
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Old 5th July 2009, 12:10 PM
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Thanks for all your suggestions and guidance.

I plan on going out later today with my bronica kit and tripod to take some close-ups shots and I'll update you all when I've processed the film. It's great fun learning about a new aspect of photographic technique.

apxuser
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Old 5th July 2009, 12:23 PM
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Best of luck and write everything down as you will never remember what you did later! I look forward to seeing how you got on.

Neil.
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