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Old 14th February 2009, 10:35 AM
Dave miller Dave miller is offline
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Default Bellows extension compensation

I assume that if I use a 300mm lens with bellows extension of 450mm, then I have a bellows exposure factor of 450/300 = 1.5?

That means that I have to multiply the exposure by that amount in addition to any other factors?
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Old 14th February 2009, 11:36 AM
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the bellows extension and focal length each have to be squared before the division
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Old 14th February 2009, 11:52 AM
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Quote:
Originally Posted by percepts View Post
the bellows extension and focal length each have to be squared before the division
Thanks Rob,
I should have realised that because that's the same formula that I use for print enlargement size changes based on the inverse square law.

So for my example it will be 450x450/300/300 = 2.25.
So 3 seconds meter reading will become 7 in exposure time.
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Old 14th February 2009, 12:03 PM
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The bellows extensoin should be measured from the rear nodal point of the lens which on a view camera is usually close enough to the lensboard that measuring from the lens board to the film plane is good enough. On your mamiya with your 140 lens, things may be different so you need to know where the rear nodal point is located to do the measurement unless it is marked and tells you how much extension there is.
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Old 14th February 2009, 12:06 PM
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Quote:
Originally Posted by percepts View Post
The bellows extensoin should be measured from the rear nodal point of the lens which on a view camera is usually close enough to the lensboard that measuring from the lens board to the film plane is good enough. On your mamiya with your 140 lens, things may be different so you need to know where the rear nodal point is located to do the measurement unless it is marked and tells you how much extension there is.
This was for an exercise on a 10x8, the Mamiya dosn't extend quite that far, and in any case the exposure factor required is shown on the front calculation ring on the RB67 140mm macro lens.
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Old 14th February 2009, 09:02 PM
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I use a Quickdisk for such situations - works well over most close-up distances. Download link at bottom of this page.
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Old 14th February 2009, 09:59 PM
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Quote:
Originally Posted by Dave View Post
Thanks Rob,
I should have realised that because that's the same formula that I use for print enlargement size changes based on the inverse square law.

So for my example it will be 450x450/300/300 = 2.25.
So 3 seconds meter reading will become 7 in exposure time.
You can calculate it as (450/300)^2 which may be easier to do without a calculator i.e. 450/300 = 1.5^2
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Old 30th November 2010, 09:46 PM
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Need some clarification please. Just reading up about bellows extension compensation. How do I know what is the 'normal' bellows length of my camera before I have to apply any compensation? I have this 310mm lens on my 13x18 camera and their are no markings. Does this mean I measure 310mm from the lens to the film plane and anything beyond that is compensated for?
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Old 30th November 2010, 11:56 PM
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I knew there was a quick and dirty method of calculating the bellows extension factor and I've just looked it up to make sure I wasn't giving you false information, and here it is;

For every 25% more bellows extension than lens focal length
Increase Exposure time by 1/2 stop
OR
Multiply Exposure time by a factor of 1.5

And measurement is normally between the film plane and the centre of the lens. And yes there are some wierd and wonderful mathmatics out there if you want to be absolute.
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Old 1st December 2010, 12:12 AM
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The quick'n'dirty method I use (found on one of the large format sites) - Using the focal length and extension expressed as inches, convert to f-stops. the difference between the two is the number of stops to add to the exposure.

For the example in question:

300/25.4 = 11.8 (11 1/4 stops)
450/25.4 = 17.71 (16 1/3 stops)
...difference (as near as damit) one stop.
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