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[Davewl]

I have allot of focusing scales with part numbers on the back them. Is there a way to find out what lens these might match up to? Most of the part numbers start with 30882- or 30881- and end in a number 2 through 34.

Thank you kindly for your help, Dave

[alecj]

I can help you. Let's do this - tell me what lenses you have and we'll narrow the list down.

[Davewl]

Hi, Thank you for your help, I think I have got the right part numbers for the 90mm and 162mm. (30882-17 and 30881-20) (I think). Although I won't be using a scale for each lens, the lenses I have are 90,105,127,135,150,162,210. Mostly I'd like to match the scale part numbers to lens focal length so I might get rid of some of these and pass them out to my friends in need. I hate to burden you with too long a list, but here are some numbers: 30882-2,3,4,5,6,7,8,10,11,12,13,14,15,16,17,
18,19,20,21,23,24,25,26,27,34,38 and
30881-2,3,5,10,12,14,15,16,34,37,47 and
30817-2,3,4
Any help you can give is greatly appreciated.
If you would like to email me directly my address is chsdavid@earthlink.net

[alecj]

You should know that there is no single scale for most lenses - the FL designation on the lenses themselves were only approximate. Therefore, you have the situation where a 90mm might have a scale 30992- 17 to 20. The 135 would be 30881 - 7 to ll. No scale at all for 210. To match them up accurately requires precise FL measurement.

[Davewl]

Thank you for the information, so if I find the actual focal length of a lens, is there a formula or measurement to determine the scale or range of scales to use? For example, the measurement from inf. to 8' or 6' on the scale would approximate the focal length to use it for? Also, was there a scale for the 203mm lens? Thanks again....

[Doug Kerr]

Hi, Dave,

We can determine the focal length lens a certain focusing scale is based on this way:

1. Determine the hyperextension (in mm) the scale provides for focus at its shortest marked focus distance (in feet).

(The hyperextension is the amount the lens is moved forward, when the camera is focused at some distance of interest, from its position for focus at infinity.)

This can be measured this way:

a. For a non-vernier focusing scale: Measure the distance (in mm) from the infinity mark to the mark for the shortest focus distance indicated.

b. For a semi-vernier focusing scale: (1) Align the infinity index mark with the infinity mark on the scale. (2) Measure the distance (in mm) from the "short distance" (unmarked) index mark to the scale mark for the shortest focus distance indicated.

2. Then the focal length for which the scale is intended can be calculated this way:

f = sqrt(305De)-e

where f is the focal length (in mm) of the lens for which the scale is suited, D is the focusing scale distance involved (in feet), and e is the extension of the scale for that distance (in mm).

3. This is the precise equation except that it does not take into account the hiatus distance of the intended lens (the distance between its principal/nodal points). Since we don't know what lens was intended, we can't know h, and even if we did know the lens, we probably wouldn't know its h distance. There is very little error contributed by ignoring h.

4. If we can make a reasonable assumption as to the value of h, then the fully precise equation is:

f = sqrt((305D-h)e)-e

I suspect that for most of the lenses of interest, the value of h is probably around 15 mm.

**********

You will perhaps to need to work this in the opposite direction. If you can't make the transformation, let me know and I'll derive the equation for that direction.


_________________
Best regards,

Doug