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I have a small darkroom where I've just started making B&W prints from 4x5 negatives. Even when the negative is very in focus, and scans nicely, when I print 8x10 with my enlarger the details aren't very sharp. I use a smaller enlarging lens aperture (f11 - f22) and a grain focuser. The paper is Ilford MG RC Glossy, the developer is ilford multigrade developer. Paper exposure tends to be at least 16 seconds, time in developer about 2 minutes. Enlarging lens is a Wollensack 135mm f/4.5.

What could be the cause of this loss of sharpness?

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With a grain focuser, compose with the lens wide open. Now stop down to the aperture you will be using. Now use the grain focuser. It is likely that you are focusing wide open, it is likely that the focus is shifting as you stop down. This is not uncommon. Also, check and see which is sharper, grain focuser on easel, no paper on the easel or, sacrificial scrap of the same paper under the grain focuser. Some are calibrated, no paper, some must be on paper the same thickness as you are using. Place the grain focuser half-way center to corner. This placement optimized an improper focus due to curvature of field of non-flat field lens.

  • If you're in focus wide open, you should be in focus across the entire range of apertures for any lens. I don't know what would cause the focus to shift while stopping down, unless the OP is unknowingly unscrewing the lens, bumping the enlarger, etc. while adjusting stops. Still, they should check. – bvy Jun 7 '17 at 12:23
  • A focus shift often occurs - you can see this for yourself if you are using a grain focus aid. This is actually a slight change in focal length. This has nothing to do with bumping or unscrewing. It is due to imperfect figure (curve of lens) which is revealed when making critical demands. Most moderate priced enlarger lenses exhibit curvature of field, if you focus based on the axis (center) the corners will be less sharp, best focus 1/2 distance corner to center. – Alan Marcus Jun 7 '17 at 13:52
  • I've only heard of that with very wide aperture lenses. I doubt that is the OP's problem here. Good advice though about placing the grain focuser off center. – bvy Jun 7 '17 at 14:23
  • First hand experience with exactly this same condition and the Wollensack enlarger lens, tells me differently. This is exactly what is happening. There is a slight possibility that the 4X5 negatives are popping - heat will cause them to warp and thus become non-uniform as to their flatness -- happens if the negative carrier is glassless. – Alan Marcus Jun 7 '17 at 15:02
  • @bvy can confirm after testing this. Stopping down caused a focus shift, and I was focusing wide open. This was exactly the problem. I confirmed my grain focuser was adjusted for paper thickness, and the best focus comes when the grain focuser sits directly on the easel. Thank you @AlanMarcus! – steel Jul 29 '17 at 16:05
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Your aperture (f 11 - f 22) seems rather narrow. It is possible you are seeing diffraction artefacts.

Try using f 8 at about 8 seconds (half of your stated 16 seconds). I am not familiar with your lens or its reputation, but this should be the upper range of the optimal aperture (usually stated as "a stop ot two from wide open").

Other warnings, such as using a grain focus tool, having a stable enlarger column and taking care of light leaks - loss of contrast due to fog can look like loss of sharpness - apply.

I do not think your paper and developer choice affect image sharpness - and Ilford papers are pretty much "the standard" now; I use Ilford RC Pearl finish in Agfa Neutol and get tack sharp 8×10 (or rather 18×24, I live in the metric area) prints from medium format negatives.

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