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Jan_6568
23-Sep-2006, 14:36
I just received my first telephoto lens which I hope to use more often then the 400 mm Bis-Telar I already had. I have two questions:
1. I am not sure how to calculate bellows extension factor. I have read about the method based on magnification factor but I think it is very inconvenient. Could I just use bellows extension and use actual extension at infininty instead of focal length in my calculations?

2. It is an old lens and I am not sure of it's focal length. So the question is how to establish real FL for a tehephoto design?

Jan

Ole Tjugen
23-Sep-2006, 15:14
1. No, you can't. But you can use (as a first approximation) the actual distance from the film plane to the front nodal point of the lens when focused at infinity combined with the difference in extension (from film plane to lens board) at infinity and your close focus. But for this you need to know the location of the node, or at least the focal length...
To be very precise, you need to take the pupil magnification factor into account too.

2. The size of a projected image with the lens focussed at infinity will be the same for two lenses of the same focal length, regardless of construction (retrofocus, "normal", telephoto). I've made myself a little chart of focal length versus the projected width of a window when projected on a wall at the end of a corridor. One drawback with this method is that you need a "set of calibration lenses" - several lenses with known focal lengths over as wide as possible range of focal lengths.

2b: What lens is it? Maybe someone knows the focal length?

Dan Fromm
23-Sep-2006, 15:19
Why are you worried about adjusting exposure for extension? Remember that the correction factor is 0.0 at infinity and negligible at 1:10, also that most telephotos work better at distance than as close as 1:10. FWIW, I shoot a 12"/4 Taylor Hobson telephoto and get good exposure with it with no correction.

You need to know focal length to calculate the correction factor. f/#(effective) = f/#(set)*(1 + (magnification/pupillary magnification)). With tele lenses, pupillary magnification is often far from 1.0. magnification = (extension from the infinity position /focal length) - 1. If you're not shooting quite close, all of this is irrelevant, of academic interest only, safely ignored.

David A. Goldfarb
23-Sep-2006, 15:35
The method of using magnification factor is quite convenient, I find, because it works the same way with any lens design and any camera or format.

I usually estimate the magnification factor by comparing the width of the scene at the focal distance to the width of the format or sometimes I'll actually put a ruler in the scene and measure it on the groundglass or at least look at it on the groundglass, if the ruler fills the whole frame. Then I've got a table taped to the back of the camera or in my notebook that gives me the exposure factor based on the magnification factor.

As Dan says, you only need to worry about this when you're fairly close to the subject--still lifes and portraits usually, not landscapes or architecture.

Ole Tjugen
23-Sep-2006, 15:45
I've used a telephoto lens for close-ups in the 1:5 to 1:2 range on a couple of occasions, when I needed the long lens-to-subject distance for perspective and the shorter extension of a telephoto lens so I didn't run out of bellows.

There are some cases where only a telephoto lens will do the job!

Dan Fromm
23-Sep-2006, 17:48
Ole, even though we all indulge in it when nothing better is available, bad practice is bad practice ...

Jan_6568
23-Sep-2006, 18:22
Thank you all for the advices, I figured out the focal length. I am thinking of using this lens not only for landscapes but also for still lives, similar to what Ole was doing.
My first thought was: how do I know the real objects size to compare to the image size - that was stupid! of course when I am photographing landscape I do not care about bellows extension and when I am using the lens for close-ups then I can measure the real objects too.
Thank you again, gentelmen

Jan

Ole Tjugen
23-Sep-2006, 19:27
Ole, even though we all indulge in it when nothing better is available, bad practice is bad practice ...

Yes Dan. But when you want 1:2, and you want the lens to be about 1 meter from the subject for perspective reasons (and isolating the subject from a grubby background while shooting through an even worse foreground), and your camera has 42cm of bellows...

That's when a 360mm Tele-Xenar is perfect for closeup work.

Leonard Evens
24-Sep-2006, 07:40
If you know the focal length, just measure the additional extension beyond the position when focused at infinity, and divide that by the focal length to get the magnification. Next add one to that. If you multiply the marked f-number by the number you got, you get the effective f-number. Use that to determine exposure. Alternately multiply the exposure time by the square of that number.

Note that since you are only measuring the extension beyond infinity, you can measure that from any desired reference position and you need not know specific reference points for the lens.

For example, suppose the focal length is 450 mm, and you find the extension beyond infinity to be 100 mm. then the magnification is 100/450 = 2/9. Add one to get 11/9. If the lens is set to f/16, treat it as if it is really set at 16 x 11/9 ~ 19.6. Alternately, if at f/16 you would use 1/2 second exposure, use 1/2 x (11/9)^2 ~ 3/4 seconds instead.

As the above example indicates, the f-numbers or times you get may not be things you can easily set. What you really need to know is the number of stops of fractions of a stop to shift. So, it is useful to have a table which, for each in a list of selected magnifications gives the shift in number of f-stops or fractions of an f-stop. Here is such a table. The first number in each pair gives the magnification and the second gives the shift in stops.

0.1, 0.28; 0.2, 0.53; 0.3, 0.76; 0.4, 0.97, 0.5, 1.17; 0.6, 1.36; 0.7, 1.53; 0.8, 1.70; 0.9, 1.85, 1.0, 2.0

Needless to say, in practice you would round up or down to the nearest half stop or perhaps third of a stop.

Bobf
25-Sep-2006, 16:32
The QuickDisc can be extremely useful if you can fit the disk in the shot: http://www.salzgeber.at/disc/index.html - don't forget to take it out of shot before tripping the shutter....

Cheers, Bob.

Daniel Geiger
25-Sep-2006, 17:15
The QuickDisc can be extremely useful if you can fit the disk in the shot: http://www.salzgeber.at/disc/index.html - don't forget to take it out of shot before tripping the shutter....

Cheers, Bob.

The Quickdisc and their relatives are useful, as long as you can get the target into the scene. When you can not -- there are many instances, e.g., flowing water, anything delicate (snow, sand), out of reach -- that approach does not work. I have used the quickdisc-relatives on my back porch to measure bellows extension at given magnifications with a tape measure, and then I made a a chart that is glued to the back of my clipboard where I take notes on the parameters of the shot. So I measure bellows extension, check the chart, and make exposure compensation accordingly. Works well for me, where I bracket +/- 0.5 f-stops with chromes.