I've been looking for information on enlarger aperture. What are the reosons and effects of using differnt apertures in the enlarging process apart from speeding up or slowing down the required exposure time. Is it best to feel it out myself? How is depth of field effected here?

Thankyou, justin mcmaster

-- Justin McMaster (justinmcm@hotmail.com), May 26, 1998

Answers

It works in the same way as camera lenses. Reducing the aperture size requires increased exposure times, increases depth of field, and reduces lens abberations. The "best" aperture is usually 2-3 stops down from maximum.

-- Alan Gibson (gibson.al@mail.dec.com), May 26, 1998.

In addition to what Alan said, if you are making large prints, such as 11x14 from 35mm or 16x20 from 6x6 negatives, stopping down will reduce vignetting in the corners of the print. Vignetting cause the corners to print too light. You can correct it by burning in the corners, but stopping down is easier.

-- Darron Spohn (sspohn@concentric.net), June 03, 1998.

The high qulaity enlarger lenses offered by Schneider, Rodenstock, Nikon and Leica are at their best optically at f5.6 - f8. The f2.8 lenses offer top resolution across the field at f5.6. At f8 diffusion begans to take affect, and at f11 and beyond it definitely reduces resolution and contrast to the extent that is visable in large prints. Depth of field with these lenses is not important since they are flat field lenses, and if the negative and the printing paper are flat, they will all be in the plain of focus. Use a focusing aid. Vignetting should not be visable with these lenses at f5.6 and at smaller apertures.

The effect of stopping down is easily demonstrated by observing fine detail in the projected negative image with a high powered grain magnifier.

-- Eilert Anders (eilert@dav.com), July 28, 1998.

Indeed, vignetting should not be a problem with a decent enlarging lens. However, it may be a problem with the light source, in which case stopping down will help.

Also, if the lens is flat field, and the negative is flat, and the negative, lens and paper are all parallel, there is no need for a depth of field. However, some (cheaper) lenses are not flat field, and stopping down will help. Another trick here, assuming the image is bowl-shaped, is to not focus on the centre of the image, nor the edge, but a point in between. This gives the DoF the greatest chance to get everything in focus.

And, at the risk of stating the obvious, I assume the previous reply referred to lenses with an aperture around f/2.8. Larger-format lenses tend to have smaller maximum apertures, around f/5.6, and correspondingly smaller "best" apertures.

-- Alan Gibson (gibson.al@mail.dec.com), July 28, 1998.

Good points, Alan, and I was referring to lenses for 35mm negatives.

I recently purchased an enlarger with a top level 100mm f4.5 lens. I havn't printed with it yet, but in playing around with a 2 1/4 negative and a high quality grain focuser, it appears that f11 is the best aperture for resolution across the field. Until I print, I can't make a judgement about vignetting.

-- Eilert Anders (eilert@dav.com), August 10, 1998.

In my response above, I stated that "At f8 diffusion begins to take effect...". It should read: "At f8 diffraction begins to take effect...". Sorry about my slip of mind.

-- Eilert Anders (eilert@dav.com), August 12, 1998.