Focusing Scales for LF Lenses

This is more of a theoretical question than a practical one but I will ask it by giving an example.

If I fit a camera with a lens (for example a 135mm Zeiss Tessar which I have) and focus it on infinity I can make a mark somewhere so I can return it to that position. I can then focus it at 10' and make another mark.

For the sake of discussion lets say that the difference between the two marks is 1" (it doesn't really matter what the actual figure is for now).

If I were now to take off the Zeiss lens and fit another type of lens of the same focal length and do the same test would the difference in film to lens position be the same for infinity and 10' focusing?

Whilst I would expect to have to re-align the infinity stop (or position) for a different lens I would also expect that the focusing scale would be the same for any lens of the same focal length regardless of the lens design as the mathematics of it would treat any such lens as a 7.41 Dioptre lens.

Am I correct in this assumption or are there differences?


Steve.

Different lenses of the same focal length will most likely have slightly different focus points. Most of the difference is due to the shutter depth and the lens board it is mounted on. If the boards are identical then the focus should be quite close for the lenses. Measuring from the lens nodal point would be the best way to ensure that the lenses are in fact identical in focus.

In theory yes, but in practice there may be slight variations. Many Focal lengths are nominal and a 150mm lens may actually be 149mm or even 151mm

Ian

Wade D said:

Different lenses of the same focal length will most likely have slightly different focus points. Most of the difference is due to the shutter depth and the lens board it is mounted on.

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I appreciate that the film to lens position may be different for various lenses. I am asking about the difference between infinity and close focusing.

i.e. to change from infinity to 10' with one lens may require an extra 1" of bellows length. Will the extra extension needed to go from infinity to 10' be the same with any type of lens of the same focal length?


Steve.

Ian Grant said:

In theory yes, but in practice there may be slight variations. Many Focal lengths are nominal and a 150mm lens may actually be 149mm or even 151mm

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I suppose a roundabout way of getting the same answer would be to consider the cams for Speed Graphic rangefinders. Are they specified only by focal length or are they specific to lens type?

My reason for asking is if I were to make a simple sliding box camera with a mechanism for focusing and I calibrated this mechanism and later fitted a different lens of the same focal length, once I had set the new lens in the right position for infinity focusing, would the rest of the scale remain correct?


Steve.

Yes, other distance scale remain correct, in somewhat in-noticeable tolerance, I measured many different lens in same focal length, and found out that one curve and round out those lenses, actually they are 149.3mm or 151mm.... but in their DOF range, you can say they are identical. my personal experience.

Steve, some of the rangefinders used by Graflex are specific to a focal length, others are adjustable or use different cams.

The Hugo Meyer rangefinder on my Crown graphic is not adjustable and can only be used with a 150mm lens, it's surprisingly accurate with my 1950's Tessar.

In practice scale focussing works well provided you are stopping the lens down and I use my 6x17 camera that way with no problems with focus etc. So yes in practice if the lenses are set correctly for Infinity then slight variations in Focal length are most likely to be inconsequential.

Ian

Different design lens have different elements, however wouldn't the hyper focal equation cover this?

largeformat pat said:

however wouldn't the hyper focal equation cover this?

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Yes. That's what I was thinking. Mathematically, any formula used to work out focusing distance couldn't care less who made the lens or how many elements are in its construction. As far as the maths is concerned it's just a 135mm lens.

Thanks to all. My assumed logic (this time) appears to have been correct!


Steve.

Steve Smith said:

...Thanks to all. My assumed logic (this time) appears to have been correct!...

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Yes, your assumption is correct.

The large-format depth-of-focus ruler on my site is based on a similar logic, but it goes further. It even works for all focal lengths.

Dead Link Removed

Actually the Horseman FA 4x5 camera works that way. It has multiple infinity stops and focusing scales for six different focal lengths.

Linhof and Leica focusing cams are ground individually for the lens (and older Linhof cams are also customized for one camera body), because the focal length of any particular lens of even the same brand and model depends on the refractive indices of the glasses used, so there are slight sample to sample variations. As long as everything is calibrated, you should be able to focus more accurately at wide apertures with this kind of system than with a camera that uses the same cam for all lenses of a certain nominal focal length.

Scale focusing is generally less precise than rangefinder focusing unless you can actually measure the distance with a tape measure, so scales tend to be generic for the focal length. Linhof doesn't customize focusing scales for each lens. On movie sets they do measure with a tape measure and cine lenses are often marked in finer increments than still camera lenses, and follow focus units are designed to be marked with a china marker for the shot, so scale focusing can be made more precise than rangefinder focusing.

A useful trick with a rangefinder press camera that can be calibrated to only one lens, like a camera with a Kalart rangefinder, is to mark infinity stops and scales on the camera bed for each lens that is to be used with that camera. Then if the rangefinder is calibrated, say, for a 135mm lens, you can use the rangefinder normally with that lens, but if you want to rangefinder focus with a 210mm lens, you would focus with the rangefinder, read the distance from the 135mm scale, refocus so the index mark points to the same distance on the 210mm scale, and you should be in focus.

A bit more information then:

At the weekend I went to see an exhibition of marine photography by Beken of Cowes they also had on display the first two home made cameras which they used. The first seen here: http://www.cowes.co.uk/zonexml/story?story_id=8776;cp=0-731 used 6x8 glass plates and was a twin lens design. Not a reflex though, just a ground glass to view with. The lens is mounted in a sliding box arrangement and apparently had just three focusing positions marked dinghies, yachts and liners.

The next camera was 5x4 and had a large viewfinder - just a look through type with a centralised cross for lining up.

Both of these cameras were made of mahogany painted black and were designed to put up with the harsh conditions encountered at sea. They are very simple and sturdily built and between them are responsible for some of the finest marine photography ever made (do a Google images search for Beken and you will see what I mean). Towards the end of the 1970s they started to use Hasselblads and later - that other technology!

Anyway, I was thinking about building something similar, possibly not so weatherproof but along the same lines.


Steve.

Steve Smith said:

The lens is mounted in a sliding box arrangement and apparently had just three focusing positions marked dinghies, yachts and liners.

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Now that's a distance scale that more cameras ought to have.

For those kinds of shots, outdoors, generally at least 15 feet from the subject, zone focusing is good enough. If you want to shoot with the lens wide open and/or at close distances, then you need more precision.

It's the marine version of the close up portrait, groups of people and mountain icons seen on some simple cameras.


Steve.

Here are a few distance scales for focusing with various focal lengths.
Also, the Horseman FA, in case people don't know that that is.