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just_another_actuary
23-Nov-2013, 05:53
Hi everyone,
I have a Fuji 90mm f/8 lens that's supposed to have an image diameter of 216mm at f/22. Going by this, I should be restricted to a maximum rise/fall of 36.56mm SQRT((216/2)^2-(127/2)^2)-101.6/2,
Where 127mm = 5inches and 101.6mm = 4 inches.

When I focus at full aperture in the ground glass, I'm able to comfortably clear about 50mm fall before it vignettes. This leads to an image diameter of about 235mm. Also, the lens says Fujinon 90mm f/8 but it has aperture opening up to f/5.6....why is this so?

Bob Salomon
23-Nov-2013, 06:08
Because you bought a used lens that had a bad shutter and someone re-mounted the lens into a used shutter and whoever that was either did not know how to get new scales or how to make new scales.

The stated circle of illumination is the wear of acceptable resoltion to the manufacturer. Not the area where it starts to vignette. That is well past the aera of acceptable resolution.

just_another_actuary
23-Nov-2013, 06:15
Because you bought a used lens that had a bad shutter and someone re-mounted the lens into a used shutter and whoever that was either did not know how to get new scales or how to make new scales.

The stated circle of illumination is the wear of acceptable resoltion to the manufacturer. Not the area where it starts to vignette. That is well past the aera of acceptable resolution.

Ah! Thank you so much. So f/5.6 is actually f/8 in this?

Jim Jones
23-Nov-2013, 06:49
Ah! Thank you so much. So f/5.6 is actually f/8 in this?

Maybe. It depends on what lens the shutter was originally intended for. If the original lens was about 90mm, the scale may read correctly from f/8 up. To check this, set the scale to an indicated f/8 and measure the apparent diameter of the aperture while looking through the front of the lens from a distance. It should be about 11.25mm. If not, you can either use the existing scale and correct for the error, or make a new scale. Changing the scale did not change the maximum aperture of the lens. If the shutter is not original, there is a possibility that the spacing between front and rear lens cells or between the cells and the aperture is incorrect. This will affect lens performance.

just_another_actuary
23-Nov-2013, 07:21
Maybe. It depends on what lens the shutter was originally intended for. If the original lens was about 90mm, the scale may read correctly from f/8 up. To check this, set the scale to an indicated f/8 and measure the apparent diameter of the aperture while looking through the front of the lens from a distance. It should be about 11.25mm. If not, you can either use the existing scale and correct for the error, or make a new scale. Changing the scale did not change the maximum aperture of the lens. If the shutter is not original, there is a possibility that the spacing between front and rear lens cells or between the cells and the aperture is incorrect. This will affect lens performance.

Perfect. It's slightly more than 11mm at f/5.6, measured with a ruler. Thanks Jim!

On another note, how did you arrive at 11.25mm calculation? Thanks.

Edit: ok, that's 90/8. NVM!

Jac@stafford.net
23-Nov-2013, 07:45
Ah! Thank you so much. So f/5.6 is actually f/8 in this?

If it is engraved f/8 on the front lens retaining ring, then it is probably f/8 because so few people would muck with that part. Replacement of shutter is much more likely. I would be concerned as well whether the rear lens module is correct.

There are a few different models of that lens. Which is it, exactly? SW? SWD? Neither? And what brand of shutter?

just_another_actuary
23-Nov-2013, 08:03
If it is engraved f/8 on the front lens retaining ring, then it is probably f/8 because so few people would muck with that part. Replacement of shutter is much more likely. I would be concerned as well whether the rear lens module is correct.

There are a few different models of that lens. Which is it, exactly? SW? SWD? Neither? And what brand of shutter?

It's a SW-S model, the seller mentioned Seiko shutter. How do I check if it affects lens performance?

Edit: It is a Seiko shutter. I see a marking "LS57" on it too, if that means something.

mdarnton
23-Nov-2013, 09:02
Here's another way to look at it. If you don't see the aperture closing until you get to f/8, then the scale is working, even though it's wrong. Be sure that when you see 11mm at f/5.6, as you say you're seeing now, that the aperture isn't more open than open, and that it doesn't actually start to close down until f/8. In other words, between f/5.6 and f/8 you may be seeing the effect of opening the curtains wider than the window--the curtains are getting more open, but the window isn't