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Pawlowski6132
22-Sep-2010, 07:16
So...when I convert my 210/370 lens to 370 by taking off the the front cell, do I need to use a different method for calculating bellows extension factor?

Hmmm.

Joe P.

Michael Roberts
22-Sep-2010, 07:54
Joe, not sure I understand the question...you shouldn't need to worry about a bellows extension factor unless you are shooting macros. Are you using your 370 for macros?

If you are doing "normal shooting" distance, i.e., 10-20 feet to infinity, then you shouldn't need a BCF....

Pawlowski6132
22-Sep-2010, 07:58
Hmm. I have to compensate for bellows extension on every shot I take. I take portraits. If I use my lens at 210mm, I'm at 12" to 16" every time to get H&S shot. I know if I convert to 370mm I'm going to have to rack it out even more.

Now, I'm newer at this so, I assume I may be doing something wrong.

Shooting 8x10 BTW.

Peter K
22-Sep-2010, 08:24
So...when I convert my 210/370 lens to 370 by taking off the the front cell, do I need to use a different method for calculating bellows extension factor?
Without the front-cell the Symmar is a nonsymmetrical lens. So the pupillary magnification P changes the exposure factor from

Exposure factor = (M + 1)^2

with the complete Symmar to

Exposure factor = ([M / P] + 1)^2

(M = image size / subject size)

To find the pupil diameters, open the diaphragm fully and measure its diameter as it appears through the front lenses - in this case through the shutter - , and then through the rear lens. Divide

P = front pupil diameter / rear pupil diameter

to get the value P.

BTW the bellows draw - the distance between back of the shutter and ground-glass - with the Symmar 210mm without front cell is 450mm at infinity, 510mm at 3 meter and 570mm at 2 m. And P is appr. 0.82.

Marizu
22-Sep-2010, 09:31
If you're relatively new to this or simply prefer thinking about things like composition than calculations then it might be worth downloading and printing something like the QuickDisc (http://www.salzgeber.at/disc/index.html).
As far as it goes, you can use to determine exposure compensation.
It works on 10x8.

22-Sep-2010, 09:35
Thanks to you Peter, I have learned something today. Can you suggest further reference?

OP please disregard my earlier post (If it is not soon deleted.)

Pawlowski6132
22-Sep-2010, 09:47
Thanks to you Peter, I have learned something today. Can you suggest further reference?

OP please disregard my earlier post (If it is not soon deleted.)

Really? Can I still use your Bellows Length - 100mm advice do you think?

lenser
22-Sep-2010, 10:07
First of all, when you use the 370mm length, use the marked f stops for that focal length. They will be the set that is closer to the lens barrel as marked on the front of the shutter.

With the increase in magnification nearly doubled by the new focal length, you can keep almost the same image size at nearly double the distance away which means your bellows extension for the new focal length will be very close to normal usage and you will likely need no adjustment or very little. Of course, if you are going to want even tighter, close-up images and move the camera well forward, causing the bellows to be racked far out, you will need to recalculate.

Maybe I'm just lazy, but I've never seen the benefit of running the detailed math. I think math is fun, but not when it breaks my concentration from the subject or scene that I'm trying to photograph.

Many years ago I bought a little exposure calculator device from Calumet (they've still got them for \$10.99) that takes seconds to use and gives absolutely accurate results. You simply place the little chip in the image at the point of focus, measure it on the ground glass with the calculator ruler, read the adjustment at that point and reset either the shutter speed or the f stop. It literally takes seconds for the whole process, just be sure to remove the chip from the scene or portrait before exposure.

There are other devices that do exactly the same thing that you can download free and they have appeared in several previous threads about this topic. The only real difference is that they are going to be paper and the one I use is plastic and tough as nails.

Peter K
22-Sep-2010, 10:17
Can you suggest further reference?
What further reference?
About pupil magnification see e. g. "The focal encyclopedia of photography".

Kevin Crisp
22-Sep-2010, 11:00
Quickdisc is great.

jnantz
22-Sep-2010, 17:48
So...when I convert my 210/370 lens to 370 by taking off the the front cell, do I need to use a different method for calculating bellows extension factor?

Hmmm.