Where did My Numbers Go ???

[Steaphany]

In my usual preparation for some astronomical star field shots, part of experiments in film preexposure, I wanted to set the focus between a calculated hyperfocal distance and ∞ so that I know I'd have an optimal focus and not have to rely on setting the focus by eye.

First off, all my lenses focus beyond ∞, which is why I decided to pursue the hyperfocal distance route. That's no big deal, a quick and dirty bunch of calculations in a spread sheet has that done.

Now the problem comes from my lenses, No Numbers !! :eek: At least not where I need them to be ! The "on lens" focal scale for my normal to wide angle lenses runs from 1m or 2m straight to ∞ ! :eek: The best lens is my old Sigma 70-300 Macro Zoom ( it was a kit lens with my SA9 ) that at least goes from 8m to ∞.

First Question: How do I set the focus of my 28mm lens set at f/1.8 between the hyperfocal distance of 87m and ∞ ? Especially when the lens can focus beyond ∞ and I know the scale itself may not be entirely accurate. If the lens focal adjust ring stopped right at ∞, this would be a non-issue.

Note: The hyperfocal distance comes from calculating for a Circle of Confusion of 5um, for my needs the standard 30um is too large. For a star field astronomical photo, a smaller Circle of Confusion means the film can image fainter stars. Equivalent to going from ISO 200 to over ISO 1000.

Second Question: When did lens design and lens manufacturers so extremely compress the ∞ end of the focal scale ? I know the scale is logarithmic, but 1m to ∞ is a bit much.

 

[ic-racer]

This is not the first time I have read of an odd behavior to focus on something other than the subject at hand. If you are taking pictures of stars, they are at infinity. They are not closer than that. In fact the moon is also at infinity and planes in the sky are also at infinity (with a 28mm lens). Very distant stars near the edge of the galaxy are also at infinity.

What to do: focus at a distant object in the daylight. If you have a microprism it will help. Then hold the lens there with some tape or rubber bands until night.

If you are concerned about your lenses focusing beyond infinity, there are a few issues that need to be addrssed. The 'stop' in the lens barrel may no longer be adjusted right. Also, the mirror stop in the camera's mirror box may be not adjusted right. Some cameras have, in addition to a mirror angle adjustment, a mirror vertical height adjustment. Also, if your camera was ever tampered, or 'cleaned,' the focus screen may no longer be in the correct position (shims etc.).

 

[Rol_Lei Nut]

I'l second the suggestion to focus on something at infinity in daylight and either marking or blocking the lens at the spot.

I don't understand why a hyperfocal solution is wanted. Perhaps you're worried about night-time temperatures affecting the infinity setting, but that should be practicaly insignificant on a 28mm...

II: The compressed distance markings are usually tied to a short focusing throw. With a short throw (say, 90° instead of 270°), the lens can be focused more quickly, though perhaps not as accurately.