# Focal Distance Calculation

• 04-11-2013, 02:08 PM
bascom49
Focal Distance Calculation
I would like to be able to calculate how much bellows extension that I would need for a given focal distance.

As in knowing the lens size, how much bellows would it take to focus at two feet ?

Is it possible to calculate the image size as well ? 1:1, 1:4, etc ?

I have three lenses in my arsenal - A 210mm Caltar S II, A 300mm W Nikkor and A 165mm Schneider Super Angulon.

I have a Cambo 8x10 legend and a Cambo 4x5 legend. Using the pieces parts from each I have just about four feet of bellows.
The rails from each screw together and I can use the front standard from my 4x5 and couple the two bellows together.
I would like to do some close up work with my 8x10 and would rather plan ahead than use trial and error given my limited amount of experience
in the studio. Up to this point I've been a landscape guy.

Any help, advice, or opinions would be most appreciated.
• 04-11-2013, 02:30 PM
Kevin Kehler
Generally speaking, when you are at twice the focal length of the lens, you will have 1:1 magnification. Thus, for a 210mm - when the bellows are at 420mm, you have 1:1. It is impossible to say how much it will take to focus at two feet since it will depend on the subject and focal length of the lens. After this, every time you increase the bellows by the focal length amount, the magnification increase by one power; so for the 210 - if 210mm of bellows is at infinity, 420mm is at 1:1 and 630mm is 1:2 (two times life size). Alternatively, 315mm of bellows draw is 1.5:1 sized. You are looking at the ground glass when focusing? Or are you wanting to do a P&S type of thing, where you focus without looking through the glass?

I have found it much easier to use a calibrated focus target to determine exposure increases necessary due to increased bellows draw; some people like measuring but it never seems to work for me in the field.
• 04-11-2013, 02:56 PM
bascom49
Kevin, that helps. What I'm shooting for is to be able to fill the entire film sheet with a given subject. So if life size is three inches long, how much bellows and at what distance to get to nine inches or 1:3. So for a 210 mm lens I'll be at 620 mm of bellows, for 300 mm I'll be at 900 mm of bellows. Just knowing this helps me greatly.

I am focusing on the glass, but I'm thinking that it would be helpful to know how close to set the lens to the subject initially. Working inside moving the camera around is not that difficult. However, if I decide to go out into the field with my 8x10 cambo legend thats a lot to contend with. But, if I get a bit of experience inside I should be able to get my initial setup pretty close when I'm working outside. It would still be nice to calculate it though.

So as for a calibrated focus target do you mean something like this :http://www.salzgeber.at/disc/
• 04-12-2013, 12:03 AM
Kevin Kehler
Yes, you can use the salzgeber disc or as similar one (that I use) which is found in Ralph's book Way Beyond Monochrome (I thought he had it available online but I can't find it at the moment). Jay Brunner also has video on this topic on his website.

I have found it much easier for macro photography, to extend the bellows to a reasonable amount and then to move the subject closer/farther rather than trying to move the camera. It seems I have more control and am better able to capture what I want - of course, this is in studio. Also, if you can use the rear standard to focus instead of the front, this is easier as moving the front standard changes the size of the subject on the glass. I generally start with the subject 9-12 inches away and then move it closer. You also need to be aware that at these magnifications, even at very small apertures, there is very little depth of field and movements are critical.

It sounds a lot more complicated than it is, but if you have been doing photography for a while and understand the concepts, it makes sense once you start to practice. I put a small toy up on a ledge and spent an afternoon practicing to become better, all without taking a picture. A little boring but a big boost to the confidence in actual usage.
• 04-12-2013, 12:37 AM
Dr Croubie
Quote:

Originally Posted by Kevin Kehler
Generally speaking, when you are at twice the focal length of the lens, you will have 1:1 magnification. Thus, for a 210mm - when the bellows are at 420mm, you have 1:1

So here's a sort of adjunct to the Original Question that I've wondered.

Does it make a difference to this 'generally speaking' depending on the formula of the lens in question?
ie, as far as I know, most LF lenses are pretty well balanced, the flange distance at infinity is the same as the focal length of the lens. Eg, a Super Angulon is a Biogon-style, so a 90mm lens with the pupil 90mm from the film is focussed at infinity. Then (theoretically) 180mm from the film you're at 1:1.

But what if the lens design is a retrofocus, where flange distance is longer than the focal length (like a wide-angle on a 35mm SLR, focal length 12-35mm with flange distance of 40-50mm, or wides on MF 30-65mm with flange-distances of 70-80mm). Afaik some of the widest and latest Alpa 4x5 lenses (like anyone can actually afford them) are also retrofocus.
Or if it's a Telephoto, where pupil distance is longer than the flange distance. (are there any Telephoto LF Lenses? I know focal lengths can get huge sometimes, do those Fine Art XXL 1100mm lenses have to have a 1.1m-long bellows just to get to infinity?)
• 04-12-2013, 10:59 AM
Kevin Kehler
I do not have extensive experience with different lens designs, so I can give limited advice. In it's proper configuration, my original statement is true that a 150mm lens will require 300mm of bellows draw. Now, there are a number of sources which indicate that lens which are non-symmetrical (like your retro-focus examples) may work better at macro work if you reverse the elements (rear elements in front, front elements in back) in that they give you more magnification. A symmetrical lens would not change, since it's front and rear elements are identical. So, a reversed non-symmetrical 300mm might only need 400-450mm of bellow draw to get to 1:1 or it might get 1:1.5 on 600mm of bellow draw.

Some people report poorer image quality when reversed (i.e., more distortion or diffraction) and some people report no difference in quality of images. Using such a long lens (such as the 1100mm) would require enormous bellows, which is evident when you see these cameras - there is a photographer in Chicago making 4x6 foot negatives who needs 30 feet of bellows in order to a photo of a model (probably more than 1:1).