Will the "QuickDisk" work for bellows compensation factor with a telephoto lens?

[Matt Miller]

I would like to do some close-up work with an old telephoto lens that I have. I've done a bit of reading about telephoto lenses and how they are different from non-telephoto lenses. I've read a bit about bellows factors and exit pupil factors and such. There is a lot of math and formulas in these posts that just confuses me more. For calculating the bellows comp factor for my non-telephoto lens, I use the QuickDisc. It's my understanding that this might not work for the telephoto lens. Is this true?

[Sidney Cammeresi]

I believe it would work correctly. The telephoto aspect of the lens moves the nodal point of the lens in front of the lensboard, but the distance between the nodal point and the film to achieve focus at any distance will remain the same. Focussing closer than infinity will require the same increase in extension, which will cause the same factor of enlargement of the image circle, which will cause the same drop of intensity of the light striking the film.

[Emmanuel BIGLER]

Matt

The expo disc will be only very weakly wrong with a telephoto and very usable in practice with your lens. for example if we consider a 400 apo tele wenar, its pupillar magnification ratio is 0.75. With such a value, the maximum error with respect to the simplified bellows factor will be +1/3 f-stop at the magnification 1/2, +1/2 f-stop at 1:1.

The simplified bellows factor is : SBF = (|M|+1)^2 where |M| is the magnification ratio expressed as a positive value (|M| = 0.5 at 1:2 ratio for example) the true bellows factor (TBF) in the general case of an asymmetric lens (retro-focus, PM>1 or tele, PM<1) with a pupillar magnification ratio PM is given by :

TBF = (|M|+PM)^2 / PM^2

Those are multiplicative ("x") factors for exposure time. To convert into f-stops, you should remind that 2X is +one f-stop, 4x are +2 f-stops, 1.4x is +1/2 f-stop etc...

So be confident you can use the expo disc without any major correction with your lens except if you reach the 1:1 ratio.

[Emmanuel BIGLER]

To Sidney : you are right only for lenses where the exit pupil is located in the exit principal plane. This is the case for most quasi-symmetric view camera lenses, but not for telephotos where the exit pupil is located closer to film. Hence the corrective factor ; the pupillar magnification ratio determines the position of the pupils because the longitudinal position of the pupils and their diameter are related by the object-image formula and image magnification formulae.

The exact formulae are extremely simple but I am not interested to carry a pocket calculator in the field ;-) so I prefer to know when I can simply ignore the errors ;-)

[Tim Curry]

Is it not possible to just use the normal "rule of thumb" for bellows extension? For example, if a Fuji 300mm telephoto has an actual focal length of 195mm and there is an additional extension of 50mm to focus closer, then isn't there slightly more than 1/2 stop of extra exposure needed?

[Ole Tjugen]

Very simplified, everything works just as well with telefoto lenses. You just have to remember that the measuring point is not the lens board, it is somwhere in mid-air in front of the lens.

The results you get from the QuickDisk will be accurate to within the variance of the speed of the emulsion, so can be used with the utmost confidence.

[Emmanuel BIGLER]

I agree 100% with Ole, the quickdisc should work with view camera tele lenses because those lenses are not so weird as far as pupils are concerned. I would not say so for small and medium format wide-angle retrofocus lenses but retrofocus designs they are not widely used in large format (the extreme-wide orthoscopic 35mm Apo Grandagon has probably some kind of retrofocus design).

To Tim Curry : you are right about rules of thumb, but for a telephoto the distance to be thumb-taken into thumb-account is not the "back focal distance" of 195mm (distance between the last lens vertex and focal plane), nor the actual focal length of 295 or 305 mm, it is the distance between the exit pupil and film, and this might be substantially different from 300 mm, by many thumb-lengths ;-);-) (1 thumb-lenght = one inch ??). Usually for telephotos the exit pupil is located closer to film than the exit principal plane (at 300mm ahead from film by definition). There is a very detailed explanation on Bob Monaghan's web site. http://medfmt.8k.com/mf/exposecomp.html Zeiss, Schneider and Rodenstock give data for pupillar positions and dimensions so that everybody can compute things by hand if required. As of Japanese view camera lenses I do not know.