Can anyone tell me the proper way to determine the bellows factor for telephoto type lenses?
In other words is the BF determined by the flange focal length (which is far less than for the same length non-tele lens) or by the focal length? An example is the Tele-Arton 5.5/270 which has a FFL of less than 160mm.
Thanks!
same formula for determining bellows factor for any lens
measure your bellows draw from the nodal point of the telephoto lens to the film plane
(bellows draw / lens focal length)squared = bellows factor
(540 mm draw divided by 270)squared = factor of 4
of course, with a telephoto lens, the trick is locating the nodal point...
If you determine bellows factor by magnification, you don't need to know where the nodal point is or what type of lens you have. Calumet makes a device for this purpose, or look on the main lfphoto.info page for the "Quik Disk," and also there's a similar tool designed by Jason Brunner, and you can find it on his website (and there's a thread on it on apug.org). They all use a target that you put in the scene and a "ruler" marked in bellows factor for measuring the target on the groundglass.
What I do is I have a table with magnification factor in one column and bellows factor in the next column taped to the backs of all my cameras, on my light meter, and in my notebook, so it's always handy. I usually estimate the magnification factor by comparing the width of the scene at the subject position to the width of the frame, and then I just look up the bellows factor in the table. If I want to be more precise, I'll put a ruler in the scene and measure its image on the groundglass. This has the advantage of working with any camera or format, even if you can't access the groundglass directly.
same formula for determining bellows factor for any lens..
... almost any lens provided that the pupillar magnification factor is not very different from unity.
This is the case for most wide-angle and normal LF lenses, but strictly speaking, not true for telephotos which exhibit a pupillar magnification factor that can be as low as .5 for some old lens designs like the Voigtländer Telomar. However in real life when the object is located no too close, there is almost no difference.
Use the Quickdisc and be happy !
The quickdisk instructions mentions : do not forget to remove the quickdisc from the scene before taking the picture ; exactly like the famous legal warning : do not drive with the sun shade in place at your windscreen
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For those of our readers who want to know ALL technical details (I know that here we have some experts on splitting hairs), here they are :
The general formulae are not that complex to use if you know the pupillar magnification factor of your lens :
bellows factor = (P_M+e/f)^2 / (P_M)^2
expressed in terms of aperture stops = log_2(bellows factor) = 3.32*log_10(bellows factor)
where P_M = pupillar magnification ratio, a given fixed constant for a given lens.
e = additional extension / bellows draw with respect to the infinity-focus position
f = focal length as usual.
Note that e/f is equal to the image/object magnification ratio M = e/f ; this formula is valid for any thick compound lens. Probably the only one to remember
M = (image size) / (object size) = e/f
M is easy to measure on the ground glass with a ruler ! Alternatively, with a l LF camera it is easy (even easier with a monorail camera) to determine the additional bellows drag "e" with respect to the infinity-focus position.
when P_M is close to unity the formula reduces to (1 + e/f)^2 = (1+M) ^2 as usual
Actually, there is no need to know where the rear nodal point is located, it is located at one focal length ahead of the focus point, i.e. image position for far distant object, but you do not have to know this precisely if you know the magnification factor M or if you can measure the additional bellows drag "e".
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After all this boring stuff comes the Happy End : forget about the General Formulae
In practice, corrections with respect to a quasi-symmetrical lens (i.e. with a pupillar magnification factor close to one) are small except if you insist on using a Telomar at 1:1 magnification.
I have approximately determined the cardinal elements of my 360 Schneider Tele Arton
and I found a P_M factor close to .57 which is, admittedly, very different from unity.
If we work at a magnification of 1:5 (e/f = 0.2) with this tele-arton, which is already quite close for a telephoto work, the object will be located at 6 times (5 + 1) the focal length i.e. about 6 feet in front of the camera (not really, in fact : 6 feet in front of the front nodal point, this one is floationg somewhere in air ), we find that the approximate formula differs from the exact formula by only 1/3 f-stop
namely : the exact formula with e/f = 0.2 and P_M = 0.57 predicts a bellows factor of 1.82
the approx formula with P_M = 1 predicts 1.44, the ratio of both factors corresponds to 1/3 f-stop
However if we get the strange idea to use the same lens at 1:1, then we have to use the general formulae if we insist on being very accurate on exposure.
With e/f = 1 and P_M = .57, the bellows factor is 7.5, almost 3 f-stops, whereas the usual formula predicts 2 f-stops only.
The only situation where you have to worry about general formulae is when you want to use special telecentric lenses fo which P_M is either very large or very small. Those lenses are common in optical measuring instruments and vision/robotics systems, but, to the best of my knowledge, not used by photographers.
A general rule is to add 1/2 stop of exposure for every additional extension of 1/4 the FL. The 270mm Tele Arton has effective focal length of 265mm and flange focal distance of 152mm, so the front nodal point is 113mm in front of the lens board. A tape measure or a scale on the bed/rail completes the simple method.
Thanks for all of the greaat information. It still blows me away how much knowledge people here have to share.
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