Can someone point me to a good reference to understand this relationship?
thanx
Or, can it explained simply here?
Can someone point me to a good reference to understand this relationship?
thanx
Or, can it explained simply here?
I've long used the Calumet Exposure Calculator. Its a very simple and straight forward way to get to the bellows factor. I think they still sell them. It's a two piece (target & scale) set. Put the 2" sq. target into the scene & focus. Place the scale on the ground glass according to the directions and read the increase in f-stops. Very simple & direct.
Theres a rule of thumb approach that works well.
For every 25% increase in the length of the bellows add one 1/2 stop of exposure, for a 50% increase in the lenth of the bellows add 1 full stop of exposure etc.
e.g It you were using say an 8 inch lens and the bellows were extended 10 inches (25% more than the focal length of the lens) you would add 1/2 stop, if the bellows were extended to 12 inches (50% more than the focal length of the lens) you would add 1 full stop of exposure. If the bellows were 16 inches (1:1 reproduction) you would add 2 stops of exposure.
Ahhh, good point... I almost forgot about that handy tool.
If one goes to www.southbristolviews.com , they once had a PDF of a very similar tool.
There are some good tips in this relatively recent thread.
http://www.largeformatphotography.in...Bellows+factor
The basic principle is that the further the film is from the lens, the dimmer the image, so you have to compensate. Quantitatively, this dimming follows the so-called inverse square law. The best way to understand that is to realize that as you move the film away from the lens, the image area increases. If the same amount of light gets spread over a larger area, any particular point in that area will get less light. The intensity is inversely proportional to the area which in turn is proportional to the square of the bellows extension.
You can check this yourself, if you have a spot meter, as follows. Shine a flashlight in darkened room against a white wall and using your spotmeter read the EV at the bright spot in the middle of the illuminated area on the wall. Then double your distance from the wall and again measure the EV. If you do it correctly, you should notice that the EV drops by two units. Similarly, if you halve the distance, the EV goes up by two units. EV is measure in f-stop units, so one EV change corresponds to a doubling of the time. In this case you would have two such doublings so you have to multiply the time by 4.
The bellows extension factor is the ratio of the actual extension to the focal length, which would be the extension at infinity. To compensate for the dimming, you need to multiply the exposure time which would be appropriate at infinity by the square of the bellows extension factor.
It is often more convenient to change the f-stop. The intensity of the light source is proportional to the area of the lens opening, which in turn is proportional to the square of the diameter of the lens opening. So f-numbers go linearly with the bellows extension factor rather than with its square. That means you just divide the f-number by the bellows extension factor to compensate for the dimming if you keep the exposure time constant.
There is also a formula which relates f-numbers to f-stops which reads
Number of f-stops = 2*Log(Bellows extension)/Log(2)
For exact calculations, there is no way around this table, but
you can get a good approximation by interpolating in the following table
1/3 stop corresponds to a BEF of 1.12
1/2 stop ................................... BEF of 1.19
2/3 stop ................................... BEF of 1.26
1 stop ....................................... BEF of 1.414
1 1/3 stop ................................ BEF of 1.59
1 1/2 stop .................................BEF of 1.68
1 2/3 stop ................................ BEF of 1.78
2 stops ..................................... BEF of 2.00
Let me do an example. Suppose the original exposure is f/16 at 2 sec, and suppose you have a bellows extension factor of 1.2. That means you should multiply the exposure time of 2 sec by 1.2^2 = 1.44 to get 2.88 sec, or just under 3 seconds. On the other hand, you can change the f-number from 16 to 16/1.2 = 13.3, or equivalently, interpolating in the above table, by opening up about 1/2 stop.
Anyone know if this is available as an iPhone App?
Be nice to be able to key in the focal length of the lens, the shutter/aperture settings as per your initial exposure reading and the total draw of the bellow and get the compensation listed. Guess you'd need to check a box that links the list to either shutter or aperture priority...
Lachlan.
You miss 100% of the shots you never take. -- Wayne Gretzky
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