what is the downside of f/9 lens elements in an f/6.3 shutter?

Somehow I have acquired a Claron-G 150mm f/9 lens in a Seiko f/6.3 shutter. I have only used it a few times but it seems to work fine.
What if any is the problem with this?

The aperture scale will be incorrect.

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Originally Posted by Mark Stahlke

The aperture scale will be incorrect.

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If the original lens was also a 150mm, the scale is correct. Definitely something to check. You can make your own aperture scale.

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Originally Posted by Michael E

If the original lens was also a 150mm, the scale is correct. Definitely something to check. You can make your own aperture scale.

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Not necessarily. Remember that the f/ number is focal length/entrance pupil and that the entrance pupil is the image of the aperture as seen from the front of the lens. Some lenses magnify the aperture more than others.

If it works fine for you just leave it and have fun with it.

I've always been told that its important for the shutter to be shimmed to its lens elements. My guess is that this isn't as important for film as it is an 80 megapixel digital back, but I've got to think it would throw something off at least a little. Am I wrong here?

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Originally Posted by Zndrson

I've always been told that its important for the shutter to be shimmed to its lens elements. My guess is that this isn't as important for film as it is an 80 megapixel digital back, but I've got to think it would throw something off at least a little. Am I wrong here?

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If you can focus, and have a sharp image, it does not matter.

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Originally Posted by Zndrson

I've always been told that its important for the shutter to be shimmed to its lens elements. (...) Am I wrong here?

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It should not be important on reasonably modern lenses - shutters for photographic taking lenses have been in a small set of standard sizes for many decades, and all reasonable makers have negligible production tolerances for the body strength. If there are shims, they generally go with the LENS, not the shutter, as there is more variability in lenses, where small tolerances in the elements might add up or even multiply. Beware, there are a few shutters (mostly for fixed installation) that came in identical width and thread pitch as some common type, but have a different strength.

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Originally Posted by koh303

If you can focus, and have a sharp image, it does not matter.

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I only know from reading that some lenses were optimized for a certain focus range/magnification ratio with shims.

I have experienced myself that incorrect spacing affects the size of the image circle/circle of definition.

The lenses, yes. But hardly ever the shutter. If you transplant whatever shims you found with the lens, you'll be fine. If there are none, it either doesn't need them, or they would be missing on any shutter.