Finding the optimum enlarger lens f stop.

[fidget]

hi, I have a few lenses which I could use for my 6x6 and 6x4.5 MF work and would like to check them against each other at a selection of apertures. They are componon -s 80 f4, componon -s 100 f5.6, rodagon 80 f5.6 and a Minolta E Rokkor 75 f 4.5. (although I believe that not too much may come of the Rokkor, not 6 element?). The largest print I expect to make is 12x16. Anyway, I have a small problem before I get started, I'm not confident that my focusing by scope is good enough, or if it is that critical. I noticed that my large scope and small scope give different focus points, although each could be said to be adjusted OK. There seems to be a small margin where the wire is in focus. I can adjust them to be the same, but the doubts have set in. How can the focus be checked and the scopes set up properly?
Regards, Dave.

 

[BruceN]

The wire (or other pattern) on your grain focuser is for adjusting the focuser to your eyesight. Simply adjust it so the lines are in the sharpest focus, you can do this under a room light or whatever - it doesn't have to be under the enlarger. Once that is done it should provide the correct focus under the enlarger.

Bruce

 

[srs5694]

If the two grain focusers give different focus points, then I'd recommend making several small prints (no point in wasting a whole sheet of paper on each) from the centers of photos (where sharpness is likely to be best) at high enlargement (to magnify differences in focus) and at a large aperture (to minimize depth of field). The goal is to get test images that will highlight any deviations from correct focus obtained when using the two grain focusers. In theory, just two prints should do the trick (one for each grain focuser), but in practice, random human error in focusing could lead you to incorrect conclusions if you do this, so I recommend doing at least two or three prints from each grain focuser.

When you make your prints, mark them on the back so you know which grain focuser you used for each. Shuffle them up (or at least the pairs, if you enlarge different negatives) and judge them side-by-side without looking at the back. You should then be able to rate them as to proper focus. Flip them over to determine which grain focuser you used for each one. If one focuser is superior to the other, that should become obvious at this point. If the difference is subtle, you might get inconsistent results that trend one way. If they're equally good, you should see neither grain focuser "winning" in a clear way. If you're anal and have the mathematical skills, you could apply statistical tests to the results, but that's probably overkill.

 

[fidget]

Thanks for your suggestions.
I am amazed at how the depth of focus (if that's the right term for this?) is so shallow at f4 on a 80mm lens. It is quite difficult to adjust for good grain focus seen through the scope. It makes me wonder if the leeway is only about as much as the thickness of the wire at this aperture. It is clearly much wider at small apertures. If I am near the right focus point and expose at f8 or f11 won't the depth of focus swamp scope induced focus errors? I'm sure that some sheets of paper are curved enough to stand off the easel by an amount much greater than the thickness of the wire.
I ran off a few small prints from the centre of a well exposed print, using at least one aperture of each of my MF lenses. I can understand that I'm not trying to compare the sharpness of the picture, but the sharpness of the grain. I can't really see much of a difference in grain sharpness, even on what I would have thought is the poorer lens, and so am concerned that my process is poor. When I look at the images created, the 100mm componon-s at around f8 gives more bite, but this could be due to lucky focusing or contrast differences. Anyway, an interesting feature to check on more deeply when I've got my head round what is happening.

 

[davetravis]

Well you own good quality lens, I have the same ones and found that 3 stops was optimum for detail and minimum falloff at the edges.
DT

 

[Dave Miller]

The wire in the grain focuser is, as Bruce said, only there to help set-up the instrument, it has no other use. The depth of field you refer to obeys the same laws as those that govern your camera’s taking lens, there remains one plane of focus at all apertures, but it is harder to see with a reduced stop, and therefore the latitude of acceptable focus increases either side of the focus plane as the lens is stopped down, perhaps to several inches. The degree of enlargement will also be a big factor. As with all things in this game it will pay you to spend a little time testing your lens, and enlarger set-up to determine it’s optimum settings. Once you are comfortable with those you will be better placed to forget the technicals and get on with picture making.