How to Calculate Bellows Extension correction for a true Telephoto lens?

[Drew Bedo]

I have a 380mm Telephoto lens that I would like to use on my Zone VI 4x5. Infinity focus is at about 8".

I am not sure how to figure the exposure correction for bellows extension with this lens.

My most frequently used lens for outdoor field work is a 150mm (6 inch) lens. With this lens, an extension beyond infinity of three inches ( nine inches total) requires one full stop of additional exposure . . .or 1/3 stop per inch. This works quite well outside with changing light and a cold wind blowing.


So how can I use this approach with the 380mm/15 inch telephoto? Seven and 1/2 inches beyond infinity would be one full stop Do I measure from infinity focus point at 8 inches of bellows draw?

[Doremus Scudder]

I have a 380mm Telephoto lens that I would like to use on my Zone VI 4x5. Infinity focus is at about 8".

I am not sure how to figure the exposure correction for bellows extension with this lens.

My most frequently used lens for outdoor field work is a 150mm (6 inch) lens. With this lens, an extension beyond infinity of three inches ( nine inches total) requires one full stop of additional exposure . . .or 1/3 stop per inch. This works quite well outside with changing light and a cold wind blowing.


So how can I use this approach with the 380mm/15 inch telephoto? Seven and 1/2 inches beyond infinity would be one full stop Do I measure from infinity focus point at 8 inches of bellows draw?

Drew,

The "formula" you are using for your 150mm lens works only coincidentally and approximately, but well enough for your purposes.

With your telephoto, and using your approximate approach, you just need different numbers and, most importantly a different starting point.

Assume that whatever extension required for your telephoto lens at infinity focus is 380mm (or 15 inches) regardless of what it really is. You say your lens focuses at infinity at "about 8 inches," so take that as your starting point. An additional extension of 15 inches from your 8-inch starting point would then be 23 inches measured, but for the purposes of exposure calculation you would consider it 30 inches and a two-stop adjustment. To be clearer: just add 7 inches to your total bellows extension measurement and calculate from there.

If you want the "halfway" point, you would add 7.5 inches to your starting point (8+7.5=15.5) for infinity focus and then mentally add another 7 inches (15.5 plus the phantom 7 inches = 22.5 inches and a one-stop adjustment.) Or you can just measure from your infinity focus point, whatever is easiest for you. You can estimate smaller increments from there using 2.5-inch increments for each 1/3 stop.

I have to point out, however, that your method is just a practical working method based on an approximation. A doubling of extension from the infinity position does indeed need a two-stop adjustment, but the "halfway point" that you are working with is not really accurate (but close enough for most purposes). The true intermediate point (i.e. the one-stop adjustment point) is really the focal length multiplied by the square root of 2.

For example, for a 6-inch lens, you'd multiply 6 by 1.414.... and get approximately 8.5 inches, not the 9 inches you are figuring from. But, no matter, it's close enough.

For the 15-inch lens, the numbers are 15 * 1.414... = approx 21.2, not the 22.5 I used above. Still, it's close enough for most purposes.
Finding the exact 1/3-stop positions requires more math.

Bottom line, you can apply the method you already use by doing what I described above. If you really need full exposure, add a third or two-thirds of a stop to what you figure and you should be fine.

Best,

Doremus

[Dan Fromm]

Drew, extension is measured from the infinity focus position, i.e., where the front standard sits when the lens is focused at infinity.

The magic formula for exposure compensation given extension is independent of focal length. It is in two steps:

(1) magnification = (extension/focal length) - 1 Extension and focal length have to be in the same units, such as mm, cm, m, inches, feet, rods, furlongs, miles, light years, ...

(2a) for a symmetrical lens, exposure compensation in stops = magnification + 1

(2b) for an asymmetrical lens facing normally, exposure compensation in stops = ((magnification/pupillary magnification) + 1) Pupillary magnification is (diameter of exit pupil, seen from the rear/diameter of entrance pupil, seen from the front). This is your tele lens.

(2c) for an asymmetrical lens reversed, exposure compensation in stops = (1/pupillary magnification)*(1 + magnification * pupillary magnification)

There's no mindless rule of thumb for asymmetric lenses. You have to measure pupillary magnification and do the calculations. But you only have to do it once.

If you want a really easy way out, use a 4x5 Horseman Exposure Computer. This is an averaging exposure meter that slips in like a sheet film holder. For most subjects, with a long lens a 2x3 Horseman Exposure Computer is a 4x5 adapter will give good enough results with a lens normal or longer, can give underexposure with lenses shorter than normal. This because it doesn't see the outer part of the 4x5 frame, where falloff with a lens shorter than normal can be problematic.

[Drew Bedo]

Doremus: Thanks that is a great explination. I had not learned the square root of two times the focal length to get the one stop distance. I will work with that to pre-figure the correction.

To make this clear in my mind now: to get to the one stop distance (or 1/3 stop positions) I must measure from the telephoto's shortened ( or Telephoto) infinity focus distance. Do I have that right?

And yes, I do understand that the method I have described is an on-the-fly sort of thing. I use it for non-macro imaging.

[Drew Bedo]

Dan: Thank you for the detailed explanation of how to do this when it needs to be done meticulously.

[Doremus Scudder]

Doremus: ... To make this clear in my mind now: to get to the one stop distance (or 1/3 stop positions) I must measure from the telephoto's shortened ( or Telephoto) infinity focus distance. Do I have that right? ...

Yep, that's basically it. I was trying to simply apply what you already do to your telephoto lens so you didn't have to change methods. FWIW, I simply made a bellows-extension table for all my lenses and carry it in my exposure-record notebook. When I need, I measure extension and consult the table. My way is pretty accurate, but likely takes more time than your method.

Best,

Doremus

[jnantz]

Doremus
thanks for this explanation !
makes total sense :)
john

[Iga]

Hi !
First of all - sorry for reanimate this old thread, but reading this and this Photonet discussion
https://www.photo.net/discuss/threads/telephoto-lens-bellows-factors.149623/ I'm a bit confused. Maybe too long time in quarantine and my brain went fat...

I got a nice Tele Xenar 360mm f5,5 yesterday. I never used real teles before, so my idea about bellows factor correction is :
I focused at infinity and measured bellows extension - 22cm. So I will call this lens My New 22 and use same old rule - increase 1/2 stop for every 25% of bellows.
For closeup at 2 meters I need 32 cm of bellows what is about 50% of extension. So I just add one stop of exposure and thats all...
Am I wrong ?
Thanks,
Igor.

[Dan Fromm]

There is no simple rule of thumb for asymmetrical lenses. See post #3 above.

[Iga]

There is no simple rule of thumb for asymmetrical lenses. See post #3 above.

Thats where I'm lost ... For me I'm just moving the light source ( back element of my lens ) from the screen ( my ground glass ).
The only problem I see here is that my 36cm lens is not 36 cm, but 22. The rest is the same old square root of 2..... ?
Best,
Igor.

[TimHGuitar]

Hello There - I am new to the forum. To answer your question on how to address the bellow extension as it affects exposure... this is what I came up with. The F stop number is a calculation of the diameter of the len's design. F means fraction in this case. So F8 means 1/8 of one. One being the actual size of the lens opening which would equal the focal length of the lens. The focal length being the distance between the lens and the film plane when it is focused on infinity. That means F8 is only F8 when the lens is focused on infinity. So a 380mm lens will produce a 380mm length of the bellows when focused at infinity. Thus whatever you focus on just measure the length of the lens to film plane to calculate what your "real" f stop is. That is, even if you set the lens on F8, the actual f stop is less if you focus at any distance less than infinity. Hope this makes sense. When focusing at 10 or 15 feet away the difference is negligible. But when focusing up close it is not. I posted a video on this subject some time ago. Here's a link.... https://www.youtube.com/watch?v=mpJWccGXe5E&t=1s

[Iga]

Thank you very much TimHGuitar. Unfortunately we are talking about special design lens here.
Thanks.

[Dan Fromm]

Iga, a lens' focal length is the distance from the lens' rear node to the film plane when the lens is focused at infinity. Telephoto lenses' rear nodes are in front of the diaphragm, not behind it, and are sometimes in front of the front of the lens.

When your 360/5.5 Tele-Xenar is focused at infinity its rear node is 360 mm from the film plane.

Extension is measured from the infinity position. The relevant magic formulas for magnification given extension and focal length and for exposure correction given magnification and pupillary magnification are in post #3 above.

Focus on the subject, measure extension, calculate magnification. Then calculate exposure correction.

[Iga]

Focus on the subject, measure extension, calculate magnification. Then calculate exposure correction.

OK, will do. Thanks Dan !
Best wishes to all,
Igor

[Iga]

Two good things to learn this morning.
First - 360 Xenar illuminates 5x7 with a lot of movements ( rise and fall at least ),
At 3 meters it illuminates Whole Plate with almost 0 movements, but thats good news.
And as for bellows factor, may be I was close enough with my initial idea ?

https://www.shutterbug.com/content/telephoto-lenses-view-camerasbrthe-long-and-short-it

Best,
Igor.