# Thread: Math: Calculating distance scale based on focal length, FFD, helical travel

1. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

How interesting this is. I'm putting a 47mm S-A XL on my 4x5. The lens is replacing a lesser 47mm S-A f5.6 (pic).
My helical has distance markings but I'm almost certain they are arbitrary.

I plan on first getting the lens board just right so that when the helical is fully retracted it is on infinity. Initial infinity spacing will use brass shims and gaskets. Then I will focus using on the ground glass at my favorite distances, 3', 12', 24', mark them and be happy.

2. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Originally Posted by Corran
Running the calculations for my 47mm and 58mm XL lenses my results are way off from reality.
As I said before, the thin lens equation is only accurate for nodal points effectively coincident within the lens.
That is only true for lenses close to the "normal" focal length for a format.

You're trying to use it with lenses that are grossly shorter than "normal" for 4x5 (150mm normal).
Of course there are errors.

- Leigh

3. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Jac - if you haven't seen, I posted an image of my Mercury w/ the 47XL on the "Show off your LF Camera" thread.

It's a nice setup, especially since I'm not inclined to build my own like you . I can also swap lenses somewhat easily. Took it out last weekend and it worked great for 4x5 and 6x12.

I'm most likely to use just a few distances (or GG focus) but this is a "universal" calibration to assist other users of the camera...so trying to do the best I can.

Here's a rough image of the resulting focus points as seen on the GG as best I could, not checked against calculations:

Leigh: yes, I know, hence why I'm going to check with a normal soon. I have only done the actual testing with extreme wides. Do you think using the values as seen at inf and at 3ft, solving for the rest in between, would work to check my accuracy?

4. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Originally Posted by Corran
Jac - if you haven't seen, I posted an image of my Mercury w/ the 47XL on the "Show off your LF Camera" thread.

Leigh: yes, I know, hence why I'm going to check with a normal soon. I have only done the actual testing with extreme wides. Do you think using the values as seen at inf and at 3ft, solving for the rest in between, would work to check my accuracy?
I'll look for your Mercury camera. Leigh may be able to help you. I don't have the math skills to extrapolate.
I can offer a tip regarding measuring distance for testing using the ground glass. I use a laser range finder
in evening light to set the actual distance, then focus and mark the helical.

5. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

I've been looking all over my house for an old laser measuring tool this past week. Wish I could find it, and don't want to buy a new one, but yes that would help for sure!

Since I'll be "focusing" via a digital image for calibration on normal lenses, it's easier. The ultrawides are especially hard because the focus targets are tiny specks on the GG. My 38mm XL calibration could be totally wrong because the huge DOF at any focus point farther than a few feet and because it's not really that sharp to begin with at f/5.6.

6. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Originally Posted by Corran
Running the calculations for my 47mm and 58mm XL lenses my results are way off from reality. Perhaps the error accumulates as the focal length moves farther away from "normal" when using a simplified formula. Will test again with a normal lens soon.
Bryan, interpolating is incorrect.

There's no magic to it. If you can establish the infinity position and focal length, you're set. From there, magnification given extension from the infinity position is ((focal length + extension)/focal length) - 1. Film plane to subject distance will be rear node to film plane distance (= focal length * (magnification +1) + front node to subject distance (= focal length * (magnification + 1)/magnification) + internodal distance (get it from the lens' data sheet). Just do a little algebra to get extension given film plane to subject distance.

7. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Thanks Dan! I will give the numbers another run when I have a chance. For now I've gotta get back to filling some erosion and replacing my downspouts in prep for this hurricane.

8. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

I used Dan's formula to make an excel spreadsheet and redo the helical calibration for my home-made 6x9 camera with 58mm SA XL. The focus mount is a Fotoman, marked for 65mm. I had previously used the thin lens formula which worked pretty well down to about 7 ft. The formula taking internodal distance into account is much more accurate at close distances though. The data for Schneider's lenses are still on their website, for example, here for the 58mm. There's an explanation of the optical terms here.

Does anyone know if similar data is available anywhere for Rodenstock or Nikon lenses? The Fujinon brochures give all the data necessary for their lenses.

9. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Nikon, yes, Rodenstock apparently no. You can find data on LF Nikkors by following the link in my list, see the Lens section's stickies.

10. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Thanks, Dan!

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•