Focusing Scales for LF Lenses

[Steve Smith]

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.

[Wade D]

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.

[Ian Grant]

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

[Steve Smith]

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.

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.

[Steve Smith]

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

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.

[salihonba]

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.

[Ian Grant]

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

[largeformat pat]

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

[Steve Smith]

however wouldn't the hyper focal equation cover this?

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.

[RalphLambrecht]

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

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.

http://www.darkroomagic.com/DarkroomMagic/Camera.html