Graflex.org

[Doug Kerr]

I'm sure this is well known by you experienced Graffies, but I just had to figure it out so I thought I'd "publish" it.

Sometimes we are not sure what focal length a certain focusing scale is based on. It is easily determined this way.

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

(The extension 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.

Then the focal length for which the scale is intended can be calculated (with sufficient accuracy for this purpose) this way:

f = sqrt(305De)

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

[Again, this is an approximate equation, fairly accurate for the numbers usually involved here. The precise equation is quite cumbersome, and in fact cannot really be applied, as it involves a distance we don't usually know, the hiatus (internodal) distance of the lens.]

Working the other way, if we are trying to figure out which of a number of focusing scales matches a certain lens focal length, we can reckon the extension to be looked for this way:

e = f^2/305D

where the quantities are as defined above.

_________________
Best regards,

Doug

[Doug Kerr]

Hi, Gang,

Well, I simplified that equation more than I meant to (lost a term).

Here is the entire thing with the proper (still simplified) equation. (The equation for going the other way is bummed too, and I have left that out for now).

*************

I'm sure this is well known by you experienced Graffies, but I just had to figure it out so I thought I'd "publish" it.

Sometimes we are not sure what focal length a certain focusing scale is based on. It is easily determined this way.

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

(The extension 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.

Then the focal length for which the scale is intended can be calculated (with sufficient accuracy for this purpose) this way:

f = sqrt(305De) - e/2

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

[Again, this is an approximate equation, fairly accurate for the numbers usually involved here. The precise equation is quite cumbersome, and in fact cannot really be applied, as it involves a distance we don't usually know, the hiatus (internodal) distance of the lens.]

*********

Again, please keep in mind that this is still an approximation, so don't expect to get agreement closer than perhaps 5 mm or so.

Sorry about the error.

Best regards,

Doug

[alecj]

Or, for Graflex scales, you can post the (a) focal length of the lens you want a focusing scale for, or (b) the number of the scale you have to find out which lens it is for. While you wait for my answer, have a cold, wet one!

Or, of course, you can fire up the old calculator [or slide rule]. Your choice.

[ This Message was edited by: alecj on 2006-08-17 19:45 ]

[Doug Kerr]

Greetings,

[Doug Kerr]

Well, it seems as if I am really having some trouble with my algebra here, and the bad news is that the equation I gave above is still in error.

The good news is that it turns out that in its corrected form is in fact the precise equation, except for the fact that it ignores the hiatus distance of the 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. These is very little error contributed by ignoring h altogether.

So, one more time, from the top:

*************
Sometimes we are not sure what focal length a certain focusing scale is based on. It is easily determined this way.

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

(The extension 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.

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).

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.

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.

**********

Well, thanks everyone for your patience as I got my algebra right!

Best regards,

Doug


[ This Message was edited by: Doug Kerr on 2006-08-18 06:43 ]

[alecj]

It's hot here in Alabama too, but not as hot as you've been having.

I have a complete Pacemaker list, but have never seen nor heard of an Anny list. You've stumped the chump with that one. At least you got an extra drink between measurements.

Let us know what your computations reveal for that number. We'll start a database for Anny #s too.

Stay cool.

Alec

[Doug Kerr]

Report of a focusing scale:

Camera: Anniversary Speed Graphic (4x5)
Focusing scale p/n: 28944
Calculation for focus distance (D): 6'
Measured extension at 6' focus (e): 35.3 mm
Assumed hiatus (h): 15 mm
Calculated focal length (f): 218 mm

I do not know what lens was originally on this camera; when I got it, it had been equipped with an Optar 135 mm f/4.7.

Best regards,

Doug