closer focusing with telephoto lenses question

[Arne Norris]

I want to use a 240-270mm lens on a 4x5 to do head or head and shoulder portraits.

I was thinking that a telephoto design like a Tele-Arton would save me considerable bellows draw for portrait work, but maybe I'm wrong?

Will a telephoto design (which obviously has a reduced flange focal distance for shots at infinity) save me a significant amount of bellows draw used at this distance?

Is there an easy way to calculate the amount of bellows draw for a telephoto design if I know the FFD, etc. at different scales of reproduction, area covered or lens to subject distance?

Thanks!

[Leonard Evens]

I'm not sure what you mean by "bellows draw". If you use a lens of telephoto design, the distance between the lens and the ground glass will be less, for any subject distance, than for a conventional lens of the same focal length. but the distance from the infinity position that you would have to move the standard would be the same.

The distance from the front of the front standard to the film i.e., the rear of the shutter, when focused on infinity, is usually called the rear flange focal length. For a normal lens this is pretty close to the focal length. For a telephoto lens, it is short of the focal length. The rear flange focal length is usually listed in the specifications for the lens, or it can be estimated by measuring the distance between the standards when focused on infinity.

The relation ship between the subject distance and the image distance is given by the lens equation:
1/u + 1/v = 1/f.
where u is the subject distance, v is the image distance, and f is the focal length.

If you solve for the image distance in terms of the subject distance and focal length, you get

v = f u/(u - f)

So if you know the subject distance, you can calculate the image distance For a normal lens the distance between the standards would be (pretty close to) the image distance. For a lens of telephoto design, you would have to offset the image distance back by the difference between the focal length and the rear flange focal length.

For example, suppose you are using a 240 mm lens and the subject is at 2 meters = 2,000 mm from the lens. The lens equation would yield

v = 240 x 2000/(2000 -240) = 272.72.. or about 273 mm.

If the rear flange focal length were 210 mm, you would have to correct this by 240-210 = 30 mm, so the distance between the standards or actual physical extension of the bellows would be about 243 mm.

I you know the scale of reproduction M, i.e., the ratio of image size to subject size, then you can use the following formula to calculate the image distance

v/f = M + 1.

So, , for example let the scale of reproduction be 1:10, i.e., 1/10 = 0.1. then v/f = 1/10 + 1 = 11/10 = 1.1. If f = 240 mm, then v = 1. 1 x 240 = 264 mm. Again you have to correct this by the difference between the focal length and the rear flange focal length. If that were 30 mm as before, the actual physical bellows extension would be 234 mm.

The scale of reproduction is also equal to the ratio of the image distance to subject distance, so if you know M and u, you can easily find v.

There is one further quibble. The image distance is measured from a point called the rear principal point and the subject distance is measured from the front principal point. These need not be the same, and for a telephoto lens they could be some distance apart, depending on the design of the lens. For exact work in close-ups you might need to know the positions of these points with some precision. But in portraiture it wouldn't make that much difference. You would probably be safe measuring from the front of the lens. If you use the formula deriving the image distance from the scale of reproduction and you have measured the latter carefully, you don't need to know the subject distance, so the issue doesn't arise.

[Leonard Evens]

Let me just add that if you want to do head or head and shoulder shots with a lens with focal length in the range 240-270 mm, you are going to have to get pretty close to the subject. In portraiture, you usually want to be at least six feet or two metersfrom the subject to avoid over emphasis of facial features such as the nose, and being even further back is often advisable. From the top of my head to just below my neck is about 300 mm. Doubling this would yield about 600 mm. The long dimension of a 4 x 5 frame is 120 mm. So the scale of reproduction would be about 120:600 = 1:5, so M = 1/5. The relevant formula is u/f = 1/M +1, so u/f = 5 + 1 = 6.

For a lens of focal length 270 mm, you would have to place the subject at

u = 6 x 270 = 1620 mm = 1.62 meters. That is a trifle close.

Such calculations suggest that it is not easy to do good head and shoulder portraiture with the usual lenses one has available in large format. In addition for a subject at the same distance, and aiming for a final print of the same size, you have less depth of field at any given aperture than you would with a camera with a smaller format. Moreover, relatively longer lens are easier to find for medium format and 35 mm cameras. For this reason, I think it is more natural to do the kind of thing you want to do with a good medium format camera.

Many large format photographers do environmental portraiture in which the subject is a full figure in a natural environment instead of head or head and shoulder portraiture. This takes advantage of the strengths of larger formats, and avoids the weaknesses.

[Emmanuel BIGLER]

In addition the detailed explanations by Leonard, may be we could put here a very short summary
- with telephotos, only the infinity-focus bellows draw is shorter
- the additional bellows draw required to get a given magnification is the same for all kind of lenses, either quasi symmetrical, retrofocus or tele designs
if we denote by ABD the Additional Bellows Draw we have
ABD = M.f
where M is the magnification factor (image size)/(object size) and f the focal length as mentioned by Leonard
from Leonard's formula (v/f)=M+1 we get (v-f)/f = M
ABD = v-f
again valid for all thick coupound lenses even the most eccentric onces.

An example
take a 360 apo ronar. the infinity-focus bellows draw is approx 360mm. To reach the 1:1 ratio you need an additional bellows draw of one focal length i.e. plus 360mm extra, total 720mm, almost 30" !
Take a 360 tele-Arton. the infinity-focus bellows draw is approx 210mm. ABD to reach 1:1 is 360mm, total draw 570mm. this is still quite long.
In other words, for landscape use, telephotos require only a short bellows draw.
For close-up work this advantage dissapears gradually.
Telephotos are not designed to deliver optimum performance in close-up except if they have floating elements inside (too bad no such lens exists for large format !) but this is another story !!

[John O'Connell]

Arne, I use a 600mm T Nikkor on 8x10 for mild closeups and portraits. The above responses are correct, and that the flange-to-film distance advantage for telephotos decreases as you increase magnification.

The good news is that you still retain a significant advantage at portrait distances. My camera can’t focus a normal 600mm at 1:2 at all—but it can focus my 600mm telephoto at 1:2. Yes, you have to add exposure because of the pupillary magnification problem. Yes, telephotos aren’t as good as macro lenses close up. But they still work.

You want to do headshots and head-and-shoulders portraits on 4x5. That means you need to get between 1:6 and 1:3. If you use a 270mm tele with a 190mm flange-to-film distance, you can do all of your intended work on a single-extension field camera without coming close to the ~12" bellows limits. (325mm required for 1:2 with that lens, and a bellows limit of a nominal 300mm.)

As far as the 2-meter subject distance goes—in my experience, you have to choose your battles carefully when doing LF portraiture. A lot of standard solutions to problems aren’t as easy or simply don’t work, and you just manage to work around them. I wouldn’t let that stand in your way.