Hi there Thanx to this service once again, I really wonder what is the exact use of the enlarger's lens's apeture, besides lengthening the exposure time, is it responsible for the print clarity? what is the most logical method of using enlarger's aperture, I have heard that you should close down the stops till you could see the details in the shadow areas of the negative. How far this holds true? thanx shreepad

-- shree (shreepadjoglekar@usa.net), August 27, 1999

Answers

The enlarger aperture can be varied to vary the exposure time, or keep the exposure time constant while changing the magnification. It might also be small to allow for depth of field (but you should not normally need this).

The trade-offs are similar to camera lenses: it won't perform best fully open (but the image is brighter, with less depth of field, so use this for focusing), not closed down (diffraction). It probably performs best about 2 stops down from fully open.

-- Alan Gibson (Alan.Gibson@technologist.com), August 27, 1999.

In principle, I agree with Alan's statements, but there is one warning to be issued: I have often read (although never found the effect in one of my enlarger lenses) that some lenses are prone to a shift of focus when stopped down. This means that if you focus with the aperture all open, the image will be slightly out of focus when stopped down. You can best check your lens using a focusing aid. Once you have ruled this out, focus with large aperture and print stopped down 2 to 3 stops.

To come back to your original question: I hardly ever vary my printing aperture. The only reason I could imagine is a gain in depth of field when I wish to distort an image (e.g. when correcting converging verticals in an architecture shot using an enlarger where the head and lens plate cannot be tilted).

-- Thomas Wollstein (thomas_wollstein@web.de), August 27, 1999.

An enlarging lens has one or two optimum f/stops, usually one or two stops down from maximum. Viewing and composing are fine with the lens opened all the way up, but focusing must be done at the printing aperture, and must be done with a grain focuser. Also, some people have reported focus shift with heavy filtration in the light path, but I have not experienced this personally. Check the focus with a grain focuser before each exposure, as negatives, especially medium format negs, have a tendency to "pop" out of focus when warmed by the enlarging light.

-- Peter Hughes (leonine@redshift.com), August 27, 1999.

Yes, thanks Thomas, I keep forgetting that effect. I've never seen it myself, but I have heard it can happen.

-- Alan Gibson (Alan.Gibson@technologist.com), August 27, 1999.

Question? If you focus at 5.6 and ajust bellows for sharp focus and then close down 3 stops which would be f/16 how can there be a shift in focus when you are increasing the deapth of field with each stop? If you do not change the lens height, the only thing that could cause a focus shift would be negative displacement, which should be prevented by glass neg carriers if your using a condenser light source, and something I have never had happen with my cold light using glassless neg carriers, not negative pop due to heat because it never gets that hot. Pat

-- pat j. krentz (krentz@cci-29palms.com), August 28, 1999.

Question? If you focus at 5.6 and ajust bellows for sharp focus and then close down 3 stops which would be f/16 how can there be a shift in focus when you are increasing the deapth of field with each stop? If you do not change the lens height, the only thing that could cause a focus shift would be negative displacement, which should be prevented by glass neg carriers if your using a condenser light source, and something I have never had happen with my cold light using glassless neg carriers, no negative pop due to heat because it never gets that hot. Pat

-- pat j. krentz (krentz@cci-29palms.com), August 28, 1999.

I don't know Pat, some of us with condensors would rather not have to clean the dust from 6 surfaces, so we use glassless carriers. That said, I've never had a negative "pop" with my Beseler or the Omega I used to own.

-- Sean yates (yatescats@yahoo.com), August 29, 1999.

For that matter, why does any lens, like say a triple convertible exhibit "focus shift" as you stop down? Not that I have experienced it myself with my t.c. lenses, but there it is, Ron Wisner, et. al. warning us about the compromises of using older glass.

-- Sean yates (yatescats@yahoo.com), August 29, 1999.

As good old Ansel Adams said, there are some very good enlarger lenses with a focus shift when stopping down and I see this happening all the time, using a Peak grain-focusser for *new* Schneider Componon enlarger lenses. So do not focus with the lens wide open. And pat j. krentz: depth of field is rather small at these distances, so that's an explanation. That's also why it is recommended to focus on a test-sheet of the paper to be used and not on the naked easel. As a starting point: the optimum aperture is comparable to camera-lenses. The 50mm, used mostly for 35mm, has F8 as its optimum, the 80mm used mostly for MF has F11 as its optimum and the 150mm or 165mm for 4*5 has f16 or f22 as its optimum. I tested my Componon 80mm for this rule-of-thumb and it turned out that F8/11 was as good as F11 and F8 almost as good. At F8 the overall contrast impression becomes a bit harsh, as is certainly the case with the lens wide open. So I only use F11, and F8/11 if exposure-times are getting too long.

-- Lot (lotw@wxs.nl), August 29, 1999.