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Dave miller
14th February 2009, 10:35 AM
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?

Argentum
14th February 2009, 11:36 AM
the bellows extension and focal length each have to be squared before the division

Dave miller
14th February 2009, 11:52 AM
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. :slap:

So for my example it will be 450x450/300/300 = 2.25.
So 3 seconds meter reading will become 7 in exposure time.

Argentum
14th February 2009, 12:03 PM
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.

Dave miller
14th February 2009, 12:06 PM
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.

Bob
14th February 2009, 09:02 PM
I use a Quickdisk for such situations - works well over most close-up distances. Download link at bottom of this page (http://www.salzgeber.at/disc/index.html).

Argentum
14th February 2009, 09:59 PM
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. :slap:

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

TonyMiller
30th November 2010, 09:46 PM
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?

MartyNL
30th November 2010, 11:56 PM
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.

paulc
1st December 2010, 12:12 AM
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.

TonyMiller
1st December 2010, 04:13 AM
Thanks - understand now.
I was making it more complicated than it needs to be.

Neil Smith
1st December 2010, 10:52 AM
I used this information from Large Format photography website, it has some quick calculation formula listed and when I did a test it worked. I used the method listed by John Cook.

http://www.largeformatphotography.info/bellows-factor.html


Neil

TonyMiller
1st December 2010, 11:49 AM
Good info thanks. I think I'm ok with the maths. It was the when and where to measure the bellows that hadn't clicked until now.
(Yeah I'm slow on the uptake sometimes :confused:)
And then the question of what is the 'nodal point' of the lens. I've just pulled out my copy of a book by Harvey Sharman called The View Camera and in there he reckons measure to the front of the lens board from the film plane. As John Cook says in your link - photography isn't an exact science, so I'll be thereabouts.

Maris
2nd December 2010, 12:18 AM
I've given up trying to measure the bellows extension for lenses focussed on infinity. The location of the rear principle plane, the rear nodal point, or the flange to film distance compared to rear focal distance is all too complicated.

An accurate rule for a lens focussed on infinity is that the nominal bellows extension is the focal length. This holds good even for retrofocus and telephoto lenses. All that is needed is to measure the extra extension necessitated by close focussing. Any consistent convenient point on the camera or lens that moves forward for focussing can be used. Then add the extra extension to the focal length and the number thus obtained is the new bellows extension. After that the the calculations are routine.

Just to make measurements easy my large format focussing cloth has a dressmaker's tape sewn into the edge. No more guessing numbers!

Mark-NY
2nd December 2010, 12:00 PM
Here is a fun tool for bellows extension that I ran across on the web -- great for anyone needing to check the accuracy of their formula or arithmetic:

http://www.cookseytalbottgallery.com/bellows_compensation.php

DaveP
2nd December 2010, 01:48 PM
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.

This is exactly the rule of thumb I would use. Even if you just do it roughly in your head it works (i.e. I tend to forget my 25.4 times table....), say 300mm is about a foot, 450mm is about a foot and a half, so f/12 - f/18, about a stop.

JimW
2nd December 2010, 05:56 PM
http://www.salzgeber.at/disc/ Provides a nice easy tool.