I use a Peak focusfinder and stop down to my preferred f-stop for exposure. I used to use a blue filter on my older LPL focusfinder, as described in Gene Nocon's book, and found it actually did make focussing easier for me.
If I have to use a small fstop, I'll focus a stop or so above (so I can see the grain better) and then stop down; but these days, my exposure/processing/printing technique is pretty standard, so it's rare that I need to use this method - usually it's saved for my earlier work.
How do you focus your enlarger? -- Just like I focus a view camera!
Method for critical focus when negative is not held flat or lens is not perfectly flat field. Focus the enlarger by moving the head on the column and focus like a view camera. Focus on the corner of the image and note where the column is. Then focus on the center of the negative without touching the focus knob, by moving the enlarger head. Then set the head to the point exactly half-way between the extremes. This will optimize your depth of field at any aperture.
If you want, you can actually calculate the aperture to get it all in focus based on the focus spread (based on view camera focus equation of Hansma).
N = 20/(1+M) * square root of 'dv'
N = Aperture number
20 = user dependent constant (circle of confusion 0.15mm for me)
M = magnification
'dv' = millimeters of focal depth on the enlarger column.
Why convert a simple practical process into a mathematical formula?
I'm a computer engineer and PhD computer scientist.
Also a good bloke and a good photographer as well.
I spent a small amount of time yesterday with a blue filter, no filters and filters in the light path and the blue filter wins hands down, but probably not for the reasons people have mentioned.
Prints done using the blue filter on the focus scope were [to me] sharper than those focussed without the blue filter.
YMMV but for me at 60 my eyes are not what they were at 40 and for no other reason the blue filter provided a far superior delineation between critically sharp and acceptably sharp. When I checked the image on the baseboard there was no difference after focussing with the blue filter when I compared it to no filters or the normal dial in DeVere colour head filters for focussing. Focussing as I normally would with or without filters but not using the blue filter gave a sharp image but not as critically sharp as using the blue filter.
I tried this same experiment 20 years ago with absolutely no difference at all either way, so to see such a difference now is most likely the deterioration of my eyesight over time.
I will continue to use the blue Peak filter as a focussing aid for my aging eyes.
Filter on the focus scope, no correction filters in the light path then when focussed add the correction filters. And yes this is for Black and White multi contrast printing.
...I will continue to use the blue Peak filter as a focussing aid for my aging eyes....
I get that you are saying it is a matter of being able to see better for focusing. My eyes are 52 and not as good as they were when I was younger either. I use a wratten 47b blue filter and I'll try it but I think the "filter factor" is too much-- the image is a lot dimmer. ( Also I understand you are talking about a filter for your focus scope... I don't have anything like that... )
On another thread about grain focusers I mentioned that I would try getting some reading glasses from the drug store... I did and it makes a huge difference. Now I can see when my print is in focus... I'm happy now.
The late Canadian photographer Gene Nocon wrote a book about printing and focussing the lens on the baseboard. He explained that because B&W images are only really interested in the blue end of the spectrum the focussing should be made with a grain magnifier which has a blue filter. He has images in his book demonstrating the difference between a non-blue filter focusing image and one where a blue filter is used. It does apper quite dramatic. The focussing magnifyer is one that used to sold in UK under the Peak name but was available under different names.
I happen to have one of these grain magnifiers but sadly no blue filter, so I use the colour gead on the LPL enlarger and dial in 50Y and 50C which gives a good blue light source. And yes I can support his theory that a blue filter does improve the focus point when using B&W. Obviously with colour neg there is no use for the blue filtration.
I focus wide open using my Nikkor 2.8/50 and then stop down to f11 for normal negatives and F8 for those a little denser. Corner to corner sharpness guaranteed.
Ctein's book "Post-Exposure" says that in his tests w & w/o blue filter, you're better off w/o it. I have a 1c and a Micromega 25x focuser--focus it once, and it stays in focus indefinitely.