Ctein also reports in his book "Postexposure" that there was no difference in printing a resolution chart between the placement of the filters.
Originally Posted by BetterSense
You are working with a single light source, either collimated or close to it. The range of light intensities in the darkroom is much narrower than outside in daylight. The colour spectrum of the light source in the darkroom is relatively narrow, and very consistent. The paper has limited sensitivity to most colours, so chromatic aberration isn't as much of a potential problem.
All working distances are in the macro range, rather than ranging from macro to infinity.
There are a lot fewer sources of high intensity off axis flare in the darkroom as compared to outside in daylight.
With a very few exceptions, in the darkroom you don't have to worry about your lens having to zoom through a wide variation of angles of view, so your filter doesn't have to be concerned with that.
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
Sounds like the difference is really not enough to worry about....
Develop, stop, fix.... wait.... where's my film?
All filters come in two types. One type type is used above the lens and another below the lens. It used to be that they were marked with identifying notation such as "CC" and "CP" which is what Kodak used.
Your filter instructions should telll you.
When using my Focomat IIA I tried putting a filter in the negative mask slot below the negative but above the lens and below the condenser above the film. I also compared prints using the same filters under lens on top of condenser. No difference with the above condenser and below the lens but I can say that the filters under or over the negative, between the condenser and lens, without a doubt caused the image to degrade.
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I've used the "above" Ilford filters under the lens. I didn't notice any degradation but additionally did a test:
- ultra high resolution negative (USAF1951 on Orthopan UR / Adox CMS) with 180lp/mm (sic!)
- enlarged to 45*65cm
- analysed with a loupe: 180lp/mm are there.
So my first hand evidence is: as long as the filter is clean and not scratched you are fine. Common sense says scratches and dirt will only lower contrast.
Having interviewed some of the best printers in the UK in my series 'Master Printers' for B&W Photography magazine, I can confirm that several of them use filters below the lens without any obvious degradation in image quality. It's especially helpful when you're split grade printing, if you haven't got a multigrade head there's less risk of knocking the enlarger as you change filter drawers if you hold the filter in your fingers beneath the lens.
Yes, there's a theoretical chance that you may degrade the image with a very scratched, dusty, filter but it remains mostly theoretical. If you've got that much dust on your filters then, in putting them into your enlarger, you're probably dumping a shed load of dust onto your negative anyway...
Do whatever suits you best.
Having used both, commercially and at home, I have to agree with Jerry and others that below the lens filters are fine, just treat with care.
There's no compromise in quality with below the lens filters, the Ilford ones are in good holders and will easily last a few years.
A quote by Ctein from Post Exposure, 2nd Edition, p144: "According to my tests, it makes no difference. I set up a high-resolution target with my 55mm Computar lens at optimum aperture and examined the projected aerial image with no filter under the lens, with modern thin filters under the lens, and with older cast plastic filters under the lens. In all cases, I could see a clean, 320 line pairs per millimeter (lp/mm) in the center of the field and more than 280 lp/mm at the corners. I could not convince myself that I saw any degradation in image quality with the filters in place, no mater how hard I looked. That surprises even me, but it's true. As long as your VC filters are not scratched enough to create serious flare (see Chapter 5) I can see no reason for avoiding below the lens filters."