Using step-down rings while lens is stopped down?

Discussion in 'Medium Format' started by 10984403, Apr 6, 2021.

  1. Basically how badly using a step-down ring will cause vignetting if the lens is stopped down as well?

    Specifically, I have got a Mamiya-Sekor C 80mm f/1.9 lens, which has 67mm filter ring, and I would like to try IR photography in medium format. The problem is that my IR filter is 52mm, and as Heliopan filters aren't exactly cheap, I'd prefer to use a step down ring instead if I can get away with it.

    Now obviously it would vignet a lot if you used it at full aperture, but how about stopping it down to say f/5.6? If I have understood correctly, that would basically mean that only the centre third of the lens would be used, so would this reduce the vignetting to manageable levels?
     
  2. It depends on the lens, step down rings, aperture and focussing distance. So something you will have to test.
    Stopping down will make vignetting more prominent, making the transition from non-vignetted to vignetted part more abrupt.
     
  3. It sounds surprising that stopping down will make vignetting more prominent, usually vignetting is reduced when a lens is stopped down.

    Regardless, I bought a cheap step-down ring from eBay, I guess I can just test it with it.
     
    Last edited: Apr 6, 2021
  4. Stopping down will reduce the inherent vignetting arising in the lens itself, caused by compromises in the lens design. It will tend to accentuate vignetting caused by external obstructions.
     
    michael_frangos and rodeo_joe|1 like this.
  5. I think it's worth a try because when the lens is stopped down only the central parts of the elements are passing relevant light. If you try it please let us know.
     
  6. There are IR filters made in China for very reasonable prices.
    (And also step-down rings.)

    There are now Opteka 67mm IR720 filters, $12.79 including shipping.

    Those are shipped from New York, but some are actually mailed from China.
     
  7. Huh?
    Stopping down does not change the angle-of-view of a lens. Therefore any obstruction of the peripheral field will still be an obstruction - only more sharply in focus.

    Consider: Say you have a wideangle lens that takes in a 70 degree horizontal angle-of-view. You fit the wrong lenshood that restricts that view to 60 degrees. Will stopping down help at all?
     
  8. Cowboy, stopping down will reduce mechanical vignetting caused by the rear of the lens' barrel. Do the experiment. It does the same for the front of the lens' barrel. I just tried it. Do the same.
     
  9. Looking through the lens with your eye - maybe.
    As for stopping down removing vignetting caused by too small or too deep a filter - no way! All that happens is that the image of the intrusion into the image field gets sharper.

    Try it yourself!

    And nobody was talking about lens barrels, iris shape, iris vignetting or anything else intrinsic to the lens.
     
  10. The step down ring arrived by post. Against my expectations, I can't see any vignetting in the viewfinder. Neither at full aperture or when stopped down to f/22.

    It isn't quite what I expected with as extreme step-down as 67mm -> 52mm is. Well, I guess I should take a test picture with and without the step down ring.
     
  11. Have you tried it with a filter fitted though? The depth and inner diameter of a filter might be a step too far... pun intended.
     
  12. I just tried that, and I still can't see vignetting. I guess I just got lucky...
     
  13. It shouln't be too surprising. The f/2.8 version of Mamiya-Sekor 645 80mm lens only has a 58mm filter thread, but has exactly the same angle-of-view.

    In theory, a front element of only a little more than 42mm diameter could have been used, which would easily fit in a 58mm diameter barrel. But I suspect that the larger 67mm filter diameter did more to impress and improve sales than it being an engineering necessity.
     

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