This was loosely referenced in another thread, but not expanded upon at all. Is it true that a lens with a smaller maximum aperture [f9] will contribute to focus error by providing a larger on glass depth of focus, as compared to a lens with a wider max. aperture [f5.6]? I know Im probably not using the correct terminology, so another way to phrase the question would be; do lenses with a wider max. aperture provide the ability for finer critical focus?

For further clarification, I would be interested in the non-low-light aspect, that obviously favors the bigger glass, but how about side by side in good light?

Thanks.

-- Robert Anderson (randerson1@uswest.net), June 16, 1999

Answers

On your first paragraph: yes, it is easier to focus lenses (and adjust tilt, swing, etc) with larger maximum apertures, because the DoF is smaller, and the image is brighter.

I'm not sure what your second paragraph is asking.

-- Alan Gibson (Alan.Gibson@technologist.com), June 17, 1999.

Hi Robert; your question seems rather simple, but in effect it involves quite a lot of complication. Generally speaking, focusing exactly with a wide aperture is easier, because the depth of focus is smaller and you can make a good distinction among objects with small distance differences each other; of course the job is easier also for the better brightness of the image. But in some case, when the aperture value is changed for taking the picture, the focus of the lens changes a bit: i.e. an object who was perfectly focused at f/4 could be sligthly out of focus at f/16. In effect this fact was frequent with old lenses (expecially using only one element of convertible lenses) and now it happens only rarely, but you never know! Another point of complication in you question arises from the fact of comparing two different lenses: the intrinsic optical performances of the two lenses could be very different, so perhaps you will get a better result with the f/9 maximun aperture lens than with the faster one. If I can give you a suggestion, indipendently from the lens you are using, try to focus at the working aperture; with the help of a good loupe or enlarging glasses, in good light conditions, this operation is not too difficult, and you will have no surprises. Excuse me for my english, and my best regards. Franco

-- Franco Rallo (f.rallo@ele.uniroma3.it), June 17, 1999.

I would say that for "finer critical focus," the answer would be "no." Wider apetures will give you easy rough focusing, but nothing beats focusing at the apeture at which you intend to expose your film. I know that this is not always feasible (poor lighting and f/128 being two reasons), but I have a lens you might categorize as "bigger glass," a 300mm f/4.5, and focusing at f/32 is WAY DIFFERENT (much more subtle) than focusing at f/4.5.

-- Chad Jarvis (chad_jarvis@yahoo.com), June 17, 1999.

Thanks for all the great answers, and I praise your ability to unraveling my question. I have to admit I've always focused wide open, and then shut down the aperture assuming [ass-u-me] the working aperture would compensate for any focus discrepancy. I guess I've been lucky with this sloppy technique!

My thought on this was that faster LF lenses [f5.6] may aid in refining focus by presenting a more knifes edge delineation of focus, as compared to say a f9 lens, when checking focus wide open. Now I know.

Thanks again, and best regards to all!

-- Robert Anderson (randerson1@uswest.net), June 17, 1999.

I would say yes and no. I have a 14 inch f/9 lens that seems to snap in and out of focus with a 4x loupe on the ground glass. It's easier to focus -- and view at f/9 than my 125mm (5 inch) is at f/5.6 .

The long lens has a much less pronounced hot spot on the ground glass. It's easier to view the whole frame without moving my head around. But that's a characteristic of almost 3x the focal length, not of the max aperture.

I do believe that even in good light, a wider aperture is going to make focus easier.

-- mike rosenlof (mike_rosenlof@yahoo.com), June 17, 1999.