Bellows compensation for S-K 300mm f5.5 Tele-Xenar

[JimboWalker]

I just purchased a nice S-K 300mm f5.5 Tele-Xenar for my Pacemaker Graphic 4x5. How would I calculate infinity and bellows factor? When focused on some trees about 3 blocks away, I am getting a measurement from shutter to film plane of only 170mm! Should I focus on the horizon for infinity, then measure the bellows draw and use that as my standard? I have always used the "shutter to to film plane" method and seems close enough for my needs. Thanks!

[Dan Fromm]

Should I focus on the horizon for infinity, then measure the bellows draw and use that as my standard?

Yes.

[JimboWalker]

Thought so. I'm really amazed that this lens can also focus so closely! I have not run any real test, but I focused on my window curtains at about 5 feet with my Pacemaker! Thanks for the response .

[Emmanuel BIGLER]

Hello from France!

If you want to compute your bellows factor, or exposure compensation factor (ECF in short here) with a (simple) mathematical formula, you need, in principle, to know an additional parameter of your lens named: pupillar magnification factor (PM in short).

This PM factor is fixed by design for a given non-zoom telephoto lens i.e. a telephoto with non-moving optical elements and consequently a fixed focal length.
PM=1 for all quasi-symmetrical lenses. PM<1 for telephotos.

Unfortunately, the PM factor was not published in manufacturers's data-sheet for large format lenses in the past.

But as explained in detail below, in most photographic situations of landscape and portrait work, except in close-up at 1:1 image magnification ratio, you can safely ignore the strange behavior of the Tele Xenar.

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Now more details about the pupillar magnification factor and additional exposure corrections.

I have estimated the PM factor for my 360 mm Schneider Tele-Arton to be PM ~= 0.57.

In order to do this estimation, you shoud look at the iris from the front of the lens and then from the back of the lens. In a telephoto, the image of the iris appears smaller when seen from the back. For the 360 tele-Arton the image of the iris is 0.57 times smaller seen from the back (the exit pupil) than seen from he front (the entrance pupil).

The PM factor can also be estimated knowing the focal length and estimating the position of the entrance pupil and the exit pupil with respect to the lens barrel and the position or the focal planes.

Although if I do not know what happens in reality for the 300 mm Tele Xenar, it is likely that the PM factor for the Tele Xenar 300 mm 5.5 is somewhere between 0.5 and 0.7, just to get an idea of a very rough estimate.

The general formulae for the exposure compensation factor (ECF) read as

ECF = ((M+PM)/PM)² = ( 1 + M/PM)²

where M is the (image)/(object) magnification factor. M=0 at infinity, M=1 at 1:1 ratio.

For quasi-symmetrical lens formulae like many large format lenses (wide-angle or standard lenses) PM is close to unity. Hence when PM=1 we get the usual formula for the ECF:

ECF = ( 1 + M)²

expressed in terms of f-stop full-clicks we get

f-stops = log₂(ECF) = 3.32 log₁₀(ECF)

e.g. if ECF = 2, open the lens by one full f-stop.

For any lens formula, it happens that the image magnification factor M is given by the same expression, independant of the PM factor.
Let EXT be the additional bellows extension beyond the focal point needed to get a sharp image of the subjet, we have

EXT = M.f
M = EXT/f

Hence in order to estimate your image magnification factor M, you can easily measure the additional bellows extension beyond the infinity setting needed to properly focus.
With a lens of focal length 300 mm, be it quasi-symmetrical or asymmetrical like a telephoto, M = EXT(in mm)/ 300 mm.
For example if EXT = 100 mm or 4", your image magnification factor with the 300 mm Tele Xenar is 100/300 = 1/3 = 0.33

For a 300 mm quasi-symmetrical lens, the shutter-to image distance @M=0.33 is about 300 + 100 = 400 mm.
For the 300 mm Tele Xenar with a flange-focal distance of 170 mm, @M=0.33 the distance between the shutter and the sharp image is 170+100 = 270 mm.

With a quasi-symmetrical lens formula @M=0.33, the ECF would be
ECF = (1+.33)² = 1.78 this corresponds to 0.8 f-stops, open between 1/2 f-stop and 1 f-stop.
Say one f-stop because in black and white negative photography it does not hurt to over-expose by 1/2 f-stop

Now assume that you have a telephoto with PM=0.5
PM=0.5 yields a bigger exposure correction than for PM=1.
We have @M=0.33 : ECF = (1+ .33 / .5)² = (1+0.66)² = 2.76.
2.76 in terms of f-clicks is about 1.5 f-stop. i.e. about 0.5 f-stop more than expected for a quasi-symmetrical lens formula.

SUMMARY

Even if the PM factor for the 300 mm tele-xenar was about 0.5, the additional correction factor is not huge, about 0.5 f-stop for an image magnification ratio of about 0.33

And for landscape and portrait use, when M is smaller than 0.33, this additional correction (with respect to the regular correction valid for quasi-symmetrical lenses) being smaller than 1/2 f-stop can safely be neglected.

Hence you can take benefit of Mr. Salzgeber's "quick disc" device ...

http://www.salzgeber.at/disc/

... to estimate your image magnification factor M, and directly read the ECF without any maths, from the associated scale.
The "quick disc" scale assumes a quasi-symetrical lens formula, i.e. ECF = (1+M)².
You can, except if you work at 1:1 ratio, forget about the additional correction needed to take into account the asymmetry of the Tele Xenar.

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I have attached a diagram showing what happens if you insist on using a telephoto for macro work.
Yes the additional exposure corrections with respect to a symmetrical lens can be huge, but who would insist on using a telephoto for macro work ?

[rdeloe]

Some nice advice from Tim Layton here: https://www.timlaytonfineart.com/blo...ication-ratios

And various tips and tricks here: https://www.largeformatphotography.i...ws-factor.html

I use an app on my Android phone.

[Dan Fromm]

Rob, Emmanuel nailed it.

Tim and the article you linked to presented the most common case, for lenses with pupillary magnification = 1. They're both wrong for telephoto lenses (not that uncommon in LF) and inverted telephoto or retrofocus lenses (rare in LF).

This is a common mistake that is nearly everywhere and, alas, in most macro/closeup books. Lefkowitz (see the list) is the big exception there. Bellow factor is very important for macro work when TTL metering is impossible. Another argument for the Horseman Optical Exposure Computer. See the list for an article about it.

[rdeloe]

That's an important clarification Dan. I use flange focal length where Tim refers to focal length in his formula. I shouldn't have assumed that that was understood when I sent the OP to that post. My bad.

Rob, Emmanuel nailed it.

Tim and the article you linked to presented the most common case, for lenses with pupillary magnification = 1. They're both wrong for telephoto lenses (not that uncommon in LF) and inverted telephoto or retrofocus lenses (rare in LF).

This is a common mistake that is nearly everywhere and, alas, in most macro/closeup books. Lefkowitz (see the list) is the big exception there. Bellow factor is very important for macro work when TTL metering is impossible. Another argument for the Horseman Optical Exposure Computer. See the list for an article about it.

[Dan Fromm]

That's an important clarification Dan. I use flange focal length where Tim refers to focal length in his formula. I shouldn't have assumed that that was understood when I sent the OP to that post. My bad.

Rob, that's a problem but we can't see it in this discussion. And for lenses of normal construction (not telephoto, not retrofocus) in shutter flange-focal distance is usually close to focal length. Not always the case with lenses in barrel.

What's wrong in the links you posted is that they ignore pupillary magnification.

[rdeloe]

One of the links I posted is to the LFF "front page" material on bellows extension. I just had another look and it doesn't discuss pupillary magnification. I don't know how material is added to those, but Emmanuel has gone to some lengths to provide a nice explanation of pupillary magnification (a topic that, to be honest, was news to me!) Who adds material to those pages?

[Jac@stafford.net]

Perhaps one of our experts in optics can answer: a telephoto lens with a positive bellows extension change - does the nodal point change, and if it does, then in what direction does it move?