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  1. #31

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    Quote Originally Posted by Ed Sukach
    One useful flexibility is the depth of focus (remember this is a projection lens). A wide (? "deep"? "extensive"? - a whole lot) is a definite disadvantage in focusing the enlarger. The use of the maximum aperture results in the most "snapping" into place when focusing, and stopping down from there adds a cushion, minimizing errors.
    Unfortunately, some lenses (particularly cheaper ones) suffer from focus shift when the aperture is changed. Focus correctly at the maximum aperture, adjust the aperture, and the focus shifts. I've done some tests with my lenses to verify that this effect is real, and it is real with the 4-element lenses I tested. IIRC, the effect was undetectable with my 6-element Nikon el-Nikkor f/2.8, though.

    Personally, I prefer focusing at the aperture I use for exposing the print simply because when I use the maximum aperture for focusing, I often forget to reset the aperture before exposing the print. That makes for a lot of wasted paper (and chemistry, since I usually don't discover my mistake until I've at least begun to process the print).

  2. #32
    Ed Sukach's Avatar
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    Quote Originally Posted by srs5694
    Unfortunately, some lenses (particularly cheaper ones) suffer from focus shift when the aperture is changed. Focus correctly at the maximum aperture, adjust the aperture, and the focus shifts. I've done some tests with my lenses to verify that this effect is real, and it is real with the 4-element lenses I tested. IIRC, the effect was undetectable with my 6-element Nikon el-Nikkor f/2.8, though.
    I've read this before... but IMHO, there is nothing optically that can cause this to happen, other than having a severely curved field of focus. As one decreases the aperture, it is possible to exclude errant rays - possible, I guess, but I've never seen it happen, in Optical Bench testing of some hundreds of lenses (so - I'm dating myself ... all that is done with lasers and automated sensors now).

    *NONE* of the enlarging lenses I've used over the years have exhibited this "shift", and if I were to find one, I'd sell it on eB... nah! I'm a victim of integrity. I'd take it to my local pond and see how many "skips" I could achieve with it.

    One thing to be wary of... incorrect re/ assembly. Many enlarging lenses have been disassembled for one reason or another (usually over-zealous cleaning) and are re-assembled improperly. Most I've seen contain shims to obtain/ correct element spacing errors, and it is all too easy to forget them, or install the wrong ones. Very bad effect on optical quality - but in no way dependant on the chosen aperture.
    Carpe erratum!!

    Ed Sukach, FFP.

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