Focusing at predetermined distance...

[chris_venhaus]

Is there a way to focus various can EF lenses automatically to a pre-

set distance? I regularly do landscapes in low light, and would like

to use the corresponding hyperfocal distances with apertures of f1.4

or f2.0. This is nearly impossible to do when it is dark outside, as

the automatic focus is useless, and manual focus is also impossible.

The DOF scale is not accurate enough on wider angle primes to use

reliabilty. It would be the perfect solution to have my 24mm f1.4

calibrated to 70 feet at F1.4 by simply clicking a button or 50 feet

at f2.0, etc... I've tried using nail polish and other types of

marks on lenses in the past, but the new Canon primes have focussing

rings that are not locked into place in relation to the focusing

scale... Any suggestions?

[johnlund]

Good question. You might start on the infinity mark and count how many notches you need to dial back on the grip of the focus ring to get to a desired distance and use that as the starting point for a lot of focus bracketing. Sorry, I know that's not terribly helpful.

 

I naively played with the idea of using extension to focus to a measured near point corresponding to a desired distant focus point when extension was removed, but this proved to be ridiculously impractical. I mention it only in case it helps you brainstorm other ideas. Good luck.

[nelson cotrim]

Chris, based on what John said about counting notches, and what you said, I have an idea: why don't you mark the focusing ring at position oo, focus to whatever it is you want (ie 70 ft) and place a mark on the lens, besides the focusing ring. When you need to focus, turn the focusing ring until the mark matches the oo (as they turn freely) then turn (this time focusing) until you align the two marks... It might be similar to what you used to do... Or maybe two marks on the focuisng ring, one for oo and one for the distance you want, to match the mark of the distance scale... If this is too complicated (mark one aligns to mark two, mark two connects to the leg bone, leg bone connects to... :), write me and I'll try to be clearer.

[johnlund]

Nelson, I think I explained what you suggested in a private email exhange with Chris. I'll share it here:

 

IMHO the biggest issue is the focus throw of the lens in question. Here's a more explicit description of my attempt at a solution. Note how steps 3 & 4 deal with USM.

 

1. Make a fine line the crosses through your focus ring onto an adjacent, non-moving part of the lens.

 

2. Set your camera a measured distance from a focus target.

 

3. Turn the focus ring clockwise until the focus scale stops moving, at the near end of the focus scale.

 

4. Continuing turning the focus ring clockwise until your line segments meet.

 

5. Now turn the focus ring counter-clockwise until your target is in focus.

 

6. Make a second mark on the non-moving part of the lens to note where the focus ring mark lies.

 

Repeat steps 2-6 for other key distances, as desired. In the field, use steps 3-5 (using the lines in conjunction with the viewfinder in step 5), and bracket your focus.

 

You might be able to anchor from the far end of the scale in step 3 instead, but I worry a little about the built-in temperature fudge factor at that end of the scale. Maybe the problem is similar at both ends; I don't know.

[nelson cotrim]

That's it John, you got it right. It did make sense after all :)

 

And I guess you're right about the temperature fudge factor too. Glad we made it.