A quick way to get the corrected f stop for bellows extension for all formats is to convert the focal length in mm to inches by dividing by 25.4.
If you then make up a list of F stop numbers as follows:
2.8* 3.0 3.5 4* 4.5 5 5.6* 6.3 7.1 8* 9 10 11* 13 14 16* 18 20 22* 25 28 32*
The numbers with asteriks are whole stops. These conversions of lens mm to inches can be rounded to nearest corresponding stop.
If we were to use a 75 mm (2.95 inch) lens on the Bronica and add the 28 mm extension tube to come to an actual 4.05 inch lens to film plane distance the exposure increase would be 2/3 stop increase in exposure.
With a 150 (6 inch) mm lens on the Bronica adding the 28 mm extension tube would give the lens an effective 178mm (7 inch) lens to film plane distance. The exposure compensation would be an additional 1/3 stop.
In the case of a 210 (8.25 inches) Symmar lens with a lens to film plane distance measurement of 300 (12 inches) would require a 1 and 1/3 stop increase in exposure.
In another case of a 90 mm Super Angulon (3.54 inches) with a actual lens to film plane distance of 120 (4.72 inches) would require slightly more then 2/3 stop of increase in exposure.
Don, I see you too are suggesting that adding a tube enlarges focal length. What is your definition of focal length, please. Mine must be different.
Add some exposure and shoot a polaroid. Do a polroid test keep track. then do a film test. Make sure your chemistry is balanced how you like it. as your test subject use a subject your comfortable with then slide a grey scale in the set. Eventually you'll know by instinct,
The focal length of the lens is not modified in and of itself through the use of extension tubes with fixed focal length lenses. In the case of view cameras the same effect is realized by increased bellows extension beyond infinity focus.
Through these alterations (extension tubes and bellows extension beyond infinity focus) lens to film plane distance is increased and the lens to subject distance is decreased which affects (increases) magnification. As magnification is increased exposure must be increased to compensate.
To answer your question about focal length. The focal length of an asymetrical lens would be the distance from the lens nodal point to the film plane when the focus is at infinity.
Really? Huh. When I was taught/told about focal length by the builder of my cameras, he said the focal length represents the diameter of flat of field for the lens. For example a 780mm lens divided by 25.4 gives just shy of 30". So I can safely use this lens to receive on film or PMT an image up to 21"sq with zero edge distortion, or print from an image 21"sq with zero distortion. I wonder if that explanation is because my cameras can't see to infinity. The frame work limits the lenses from getting close enough to the film plane. For those of you who just went, HUH!?. I am not a photographer, my cameras are reproduction cameras in a darkroom.
By the way, I am in no way arguing with your definition. It actually makes more sense than the one I mention. And I now understand also that the point to the bellows/tube addition is a way to a end; not an actual indication of increased focal length. thank you for clearing that up.
Well, that is an interesting point. Since I am not well versed on process lenses I can not speak to the definition that you propose. I do have a 600 mm Apo Nikkor process lens. This came from a graphics application and is barrel mounted without shutter. This lens on my 8X10 camera requires a bellows extension of approximately 24 inches when focused at infinity. I will say though that this lens will project a very large image circle. It will certainly cover a 16X20 negative and possibly larger when stopped down. However it would not cover that large a projection for a graphics application since the requirements for resolution would be much higher in my understanding.
Your builder was just giving a simplistic explanation of the coverage of the lens, which can vary with the construction of the lens. If you were using the same lens in a view camera shooting landscapes you would most likely have about half the coverage, shooting 1:1 increases your coverage ability with any given lens. If you want a really good technical book on the subject of view cameras try Leslie Stroebels book View Camera Technique. That will get into the subject of coverage of various lens designs as well. The length of the lens barrel of any focal length lens has a large effect on the covering power as well as stopping down the aperture. J.L. Woodens Large Format Optical Reference Manual gives very specific details on lens specifications.
Thanks, glbeas, I will go in search of one or both asap. The better I understand, the better I can make best choices.