In my opinion above the lens sets are preferable, but the below lens set is neat and quicker especially if you are split grade printing, its also less liable to damage. Wet fingers and filters do not mix.
As an FYI we have value promo item that should still be in stock in retailers in the USA which was a set of 6 x 6 filters and Multigrade paper in the same box for a very good price.
Simon ILFORD Photo / HARMAN technology Limited :
I have both above and below sets.
The below sets will not affect print quality. i.e. they are just as good as above the lens filters. But, and its a big but, the unit that fixes to your lens does just that and depending on your lens it may obstruct the open up / close down lever. It can make changing aperture difficult, The unit will not fit on larger diameter lens such as those for 4x5 unless the max aperture is really small and hence the lens barrel is small diameter. Then if those under the lens filters get scratched or finger marks (chemicals) or dusty then they will affect image quality. None of these things are potential problems on above the lens filters on diffusion enlargers where the filter is above the mixing box. Scratches could be a problem on condenser enlargers depending where the filter draw sits. Also if you are printing 35mm on a 4x5 enlarger and are using a recessed lens board, the unit may not fit.
If your enlarger has a filter draw I would use above the lens filters for the above reasons. But if no filter draw then below works fine. Just take very good care of the filters. And check the unit will fit your lens before you buy it.
One more opinion - I would use them above, then not to worry about perfect condition over time. They aren't that hard to scratch.
Also, I bought an extra filter drawer for my Aristo unit (V54 lamp) on an Omega D2 (6" filters), so I can swap out the filters during split printing more quickly and easily.
Simon is right about the deal, it is a good one if you can find it.
I've never seen an enlarger with a filter drawer between the lens and the negative. Every one I've ever seen has the filter drawer above the negative.
Originally Posted by radiantdarkroom
My preference is to use the filters above the lens. While the filters will not degrade the image, use will degrade the filters. My Ilford set is full of scratches after a few years of normal use. I print using split-grade filtration. That means shuffling the filters around in the dark and that means they get handled a lot. I wouldn't want to put them in front of my lens but they're still fine for using in the filter drawer.
I'll be in the minority to say below the lens. I have two sets; above and below the lens. I used both, and I can say with confidence that there is absolutely no difference between the two. I prefer the below the lens set as it's much easier to work with. Besides, I have to use under the lens for my 4x5 enlarger. :) Scratches and dust aren't a problem - just keep the filters in the case when you're not using them.