Questions about how to use prefocus on an FD telephoto lens

The Canon New FD 500mm f/4.5 L has two rings with focus markings - one is a prefocus feature. It has a thumbscrew that can be loosened and tightened.

If I want to take a picture of the moon some night when the conditions are right, how should I use this prefocus to lock in focus now so I don't have to spend time trying to achieve critical focus on the moon and possibly miss the shot? Does it actually work long-term for critical infinity focus? Depth of field is extremely narrow on this lens and infinity focus is very tricky.

Shooting the moon, simply set the lens on Infinity...no need for focus lock!

Even using the more-precise 20/20 vision acuity standard for DOF calculation (vs. 'manufacturer standard'), 500mm FL focused at 10,000m has DOF at f/5.6 of 3,000m - Infinity!
(Focused closer, at 1000m the same combination has DOF from about 800m-1200m, so much more restrictive.)

There are no infinity focus stops on these super telephoto lenses. Trusting the mark on the lens is not accurate - the focus near infinity is extremely sensitive.

Many telephoto lenses have focusing rings which go "past" the infinity mark. This is necessary because the components can expand and contract with temperature changes. I find it easier to focus long lenses using a grid type screen.

dynachrome said:

Many telephoto lenses have focusing rings which go "past" the infinity mark. This is necessary because the components can expand and contract with temperature changes. I find it easier to focus long lenses using a grid type screen.

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Just to add to this reply , it sounds like the OP wants to preset focus during the day at an object a good distance away .
Then use that setting during the night on the moon .
If so , because if the temperature difference, focus will likely be off as the lens is colder , after it's allowed to acclimatise.

If the lens is still warm from being in the house , then thermals might be an issue .

neilt3 said:

Just to add to this reply , it sounds like the OP wants to preset focus during the day at an object a good distance away .
Then use that setting during the night on the moon .
If so , because if the temperature difference, focus will likely be off as the lens is colder , after it's allowed to acclimatise.

If the lens is still warm from being in the house , then thermals might be an issue .

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Thanks, yes, if that's a big enough effect to notice, that would be a critical problem with any prefocusing plan.

I'm also hoping for tips on gaining extremely accurate focus with it in general... should I get some type of magnifier? I'm finding with the 300 and 500mm lenses used with teleconverters, I may have to take several shots to get one where the focus is super-crisp. Both the lenses claim to have technology that increases the focus throw near infinity, but less than a millimeter of focus turn still makes a huge difference. I wonder if there's a simple trick to increase the friction of the turn a little.

loccdor said:

Thanks, yes, if that's a big enough effect to notice, that would be a critical problem with any prefocusing plan.

I'm also hoping for tips on gaining extremely accurate focus with it in general... should I get some type of magnifier? I'm finding with the 300 and 500mm lenses used with teleconverters, I may have to take several shots to get one where the focus is super-crisp. Both the lenses claim to have technology that increases the focus throw near infinity, but less than a millimeter of focus turn still makes a huge difference. I wonder if there's a simple trick to increase the friction of the turn a little.

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If your camera allows you to change focus screens , that's where I'd be investigating.
Also , depending on your tripod and back , a decent angle finder might be beneficial.
Mine has a lever to alter between 1x magnification and 2x .

loccdor said:

Thanks, yes, if that's a big enough effect to notice, that would be a critical problem with any prefocusing plan.

I'm also hoping for tips on gaining extremely accurate focus with it in general... should I get some type of magnifier? I'm finding with the 300 and 500mm lenses used with teleconverters, I may have to take several shots to get one where the focus is super-crisp. Both the lenses claim to have technology that increases the focus throw near infinity, but less than a millimeter of focus turn still makes a huge difference. I wonder if there's a simple trick to increase the friction of the turn a little.

Click to expand...

A classic astronomical technique to get correct focus with a long focus lens (or telescope + camera) on a faint subject like a star is to use a groundglass with a clear spot and center crosshair, and focus on the aerial image by parallax. You move your eye side to side, looking for relative motion between the star and the crosshair. When there is no relative motion, the star is focused on the plane of the crosshair.

A more modern technique is to use a Bahtinov mask (pupil mask) in front. That's intended for a point source and I'm not sure if it would be useful on the moon? You could focus on a star near the moon though.

The moon is in daylight so it shouldn't be as hard to focus on as a star, but in general I think you will want to use the matte area of the screen rather than any focus aids (microprism, split image).

@reddesert Thank you for those star focusing techniques, I hadn't heard of them before. I do want to give star pictures a chance when I next find myself in a low light pollution area.