# Coupled lens exposure

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• 09-24-2010, 03:03 PM
markjames
Coupled lens exposure
Hi,

I would like to trying some macro photography with my Mamiya RZ67 using a 110mm lens on the body and a 180mm lens reversed in front of this. I've got a double threaded coupling ring but I can't work out how to calculated the exposure using a handheld meter when you have the two lens mounted on the camera.

Can anyone offer an explanation of the corrections needed to allow for the second lens mounted on the front?

Thanks
• 09-24-2010, 04:11 PM
ic-racer
• 09-24-2010, 04:45 PM
Q.G.
So what would the focal length and f-stop of the combined lens be, IC-racer?
Or: how would that disc help?
• 09-25-2010, 10:13 AM
ic-racer
Quote:

Originally Posted by Q.G.
So what would the focal length and f-stop of the combined lens be, IC-racer?
Or: how would that disc help?

Yes that system only calculates your bellows factor only.

The OP also needs to calibrate the aperture scale of the new lens irrespective of the bellows factor. There are many threads on making f-stop scales, but it involves measuring the the empiric derived focal length (obtained with 'Cheetos' or by other means) http://www.apug.org/forums/forum44/5...h-my-lens.html and dividing by the observed entrance pupil in millimeter to get various f-numbers.
• 09-25-2010, 05:03 PM
Dan Fromm
A. A. Blaker gives a lucid explanation of using coupled lenses for closeup photography in his book Field Photography. Used copies are easily found and quite inexpensive. Buy the book and read it.
• 09-26-2010, 03:13 AM
markjames
Yes there are some second hand copies available but as I understand it the formulae are in a booklet that comes with the book and this is the sort of that is bound to be lost from a second hand copy.

All I want to know is how I correct the measured exposure when I have a second lens mount on the front of a camera with no TTL metering.
• 09-26-2010, 04:36 AM
Q.G.
Measure the magnification (size of image of subject on the screen / size of the subject in real life); add 1 to the result; square the result of that; and multiply the shutterspeed (the actual time) by that.
If you want to have a compensation not as a shutterspeed factor, but in stops, take the log of that factor and divide it by the log of 2.
• 09-26-2010, 05:37 AM
markjames
Thanks,

so I take the shutter speed on the meter and multiply it by (magnification + 1)squared to give me the shutter speed I use.

That's great. :)
• 09-26-2010, 05:44 AM
Q.G.
Try it first, i.e. test.
It may be great, but it also may turn out not to be so great.
• 09-26-2010, 10:42 AM
Dan Fromm
Mark, buy a copy of the book and read pages 298-303. Blaker explains what you want to know. I'm not going to type it for you.

Why do you think I'd give advice that couldn't be acted on? Do you have any idea how insulting you've been? I have the book. How do think I could give page numbers? We're not all as ignorant as you.

Be skeptical of old wives' tales and don't propagate them.

What motivated you to try the stacked lens approach? I ask because as I understand it it gives fixed magnification. Fixed magnification is rarely appropriate, not all subjects are the right size.

Quote:

Originally Posted by markjames
Yes there are some second hand copies available but as I understand it the formulae are in a booklet that comes with the book and this is the sort of that is bound to be lost from a second hand copy.

All I want to know is how I correct the measured exposure when I have a second lens mount on the front of a camera with no TTL metering.

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