No, filtering removes a selection of wavelengths by absorption. Absolutely no photons have their wavelengths changed.
Originally Posted by Mainecoonmaniac
If you had a spectacularly crap lens with severe longitudinal chromatic aberration, the focus points will differ with wavelength, which means you can focus either the blue, green or red parts of the spectrum. However, a good modern enlarging lens does not suffer from such stupidity, so no filter is needed.
You can focus with a blue filter and it will be perfectly accurate, but then so will focusing with no filter. Focusing with no filter will be far easier because our eyes are much more sensitive to the yellow/green part of the spectrum because we've evolved to see by sunlight. Note also that a VC paper can see well into the green part of the spectrum, so if you had such an example lens (maybe Gene's?) then focusing with a blue filter would give a sharp image in the high contrast layer and a defocused image in the lower-contrast layers.
The issue is further confused by people hearing about "chemical focus", which is the problem wherein a lens with poor chromatic correction focuses UV light differently from visible light. So if you're shooting old UV processes with an ancient lens, being visually focused is slightly wrong. People then apply the same principle to enlarging, which is erroneous IMHO because:
- longitudinal CA is mostly caused by wavelength-dependence of refractive index, so chromatic focus shift is approx proportional to wavelength ratio
- the wavelength ratio between UV (300nm) & yellow (580nm) light is far larger than that between blue (480nm) and yellow light
- lenses are far better chromatically corrected than they were in the 19th century
- VC paper can see green too, so you need to focus the whole 380-570nm band
So quit worrying, open the lens and filters up and let your eyes see properly. Once you've stopped down enough to make the corners sharp, the DOF in the print will more than cover any errors you would have made. And while there might be a measurable focus shift on stopping down, it will be much less than the DOF gained.
Don't be confused that the grain looks softer when you stop down. It's true, it does get noticeably softer but this is a demonstration of diffraction and not focus shift.
Edit: focus shift on stopping down is an issue caused by light being focused differently in the centre of the lens compared to the edges, i.e. the lens design is such that the middle of the lens has one focal length and the edges have a slightly different focal length, so they focus on slightly different points. I've never heard of an enlarger lens suffering from that problem, it seems to be an issue with some older retrofocus (wideangle for SLR) lenses, probably also some soft-focus lenses.
Last edited by polyglot; 10-21-2013 at 05:18 PM. Click to view previous post history.