Is my lens calibration off?

Ulophot · Sep 2, 2023

koraks said:
That's pretty much it, really. I usually place a piece of ground glass (can be improvised in several ways; transparent plastic with some scratches will do, too) on the film plane with the back open and then check focus on near and far/infinity settings. The viewfinder focus should agree with the ground glass/film plane focus. It doesn't require shooting any film. Just the back of the camera open and the shutter on bulb. It helps to set up the camera on a tripod indoors near a window where a distance object (e.g. the horizon) is in view as well as something inside a few feet away (the window frame for instance). No ruler necessary this way.

Thanks, Koraks; much appreciated. It's the kind of thing I would have come up with myself, were I not so, um, challenged when it comes to matters involving measurement or construction. Well, frankly, anything with numbers. Or geometry. Anyway, I'll do this tomorrow wtih some frosted mylar, once I figure out how to keep it flat. Maybe everything will be fine and then I'll sleep better tomorrow night.

koraks · Sep 3, 2023

I can relate my approach fortunately does not involve numbers, calculus, measurement and just straight observation in a What You See Is What You Get kind of way!

I usually repurpose any piece of flat and rigid transparent polymer I happen to get my hands on and then improvise a way to get it to stay flat and snug on the film plane. Last time I think I used a cutout from the transparent lid of a disposable food container, treated with a leftover scrap of sandpaper.

Ian C · Sep 3, 2023

Assuming that the viewing system is properly calibrated, what you see is what you get when you make the exposure.

Marked focus and DOF scales values on cameras and lenses don’t necessarily give you this accuracy. For this reason, it’s prudent to depend on visual focus to be sure. Unfortunately, this isn't applicable to rangefinder cameras.

If you calculate a hyperfocal distance or an ideal subject distance in zone focusing, it’s most reliable to set the focus visually.

Either focus on a target of opportunity at the required subject distance, or temporarily place a focusing target into the scene at that point. I sometimes use my camera bag as a focusing target.

Ulophot · Sep 3, 2023

For those who think they might try the test suggested by koraks in #23, here is a suggestion:

While searching for a piece of plastic to use, I finally hit upon a CD case cover. Perfect! The plastic can be cut with a mattte knife, though I used an inexpensive acrylic knife I happen to have, which is a little quicker. I first snapped off the side rim with pliers to make measuring the needed piece easier. Mine is 2 1/2 x 2 1/4 inches.

Once measured, cut about 3/4 through the plastic. You can then snap off the unwanted piece. With the camera tilted slightly forward, it stays in place against the rails without tape.

I started sanding with some 180 sandpaper, but found the scratches too coarse. Emory paper is better, and you can polish the edge and corners easily with it to remove any tiny pieces from the rough cut that don't belong anywhere near your shutter.

My preliminary, window and infinity test, as recommended by koraks, shows all in good order, but I am going to confirm with a ruler test as well.

Thanks again to all for your help. I will sleep better tonight.

koraks · Sep 3, 2023

Sounds good! The CD ('jewel') case is indeed a great donor for a piece of suitable plastic.

Sirius Glass · Sep 3, 2023

Ulophot said:
For those who think they might try the test suggested by koraks in #23, here is a suggestion:

While searching for a piece of plastic to use, I finally hit upon a CD case cover. Perfect! The plastic can be cut with a mattte knife, though I used an inexpensive acrylic knife I happen to have, which is a little quicker. I first snapped off the side rim with pliers to make measuring the needed piece easier. Mine is 2 1/2 x 2 1/4 inches.

Once measured, cut about 3/4 through the plastic. You can then snap off the unwanted piece. With the camera tilted slightly forward, it stays in place against the rails without tape.

I started sanding with some 180 sandpaper, but found the scratches too coarse. Emory paper is better, and you can polish the edge and corners easily with it to remove any tiny pieces from the rough cut that don't belong anywhere near your shutter.

My preliminary, window and infinity test, as recommended by koraks, shows all in good order, but I am going to confirm with a ruler test as well.

Thanks again to all for your help. I will sleep better tonight.

Yet another success story for a subscriber. Time for the rest of you to shell out a few dollars and become a subscriber too.

Is my lens calibration off?

Ulophot

koraks

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Ian C

Ulophot

koraks

Moderator

Sirius Glass

Is my lens calibration off?

Ulophot

koraks

Moderator

Ian C

Ulophot

koraks

Moderator

Sirius Glass

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