Can an achromatic doublet be used as an enlarger lens?

[ProfessorC1983]

More of a theoretical question perhaps, but I've been pondering designing my own DIY enlarger and wondering, what's the simplest optical arrangement one could get away with for "decent-looking" prints? Obviously there are cheap triplet enlarger lenses which aren't nearly as well-regarded as six-element lenses... but what exactly are the trade-offs? What kind of distortions would one expect from using an even simpler optical system, say an achromatic doublet, on an enlarger?

 

[ic-racer]

The Anthony landscape lens on my 8x10 is very sharp. No good for 'airy' portrait. That lens is a doublet. There is a raytrace worksheet out there somewhere to aid in doublet design.

 

[Donald Qualls]

Achromatic doublets are the standard for small refracting telescopes, including being in service as the objective in all or nearly all binoculars, field glasses, monoculars, and spotting scopes.

Add to that the fact that unless you're making WAY TOO BIG a print, you'll have a hard time telling the performance of one lens from another (you have to look at the grain, not the image, of an 8x10 from 35mm to tell a six-element asymmetric double Gauss from a Cook triplet), I don't know any good reason a suitably specified (focal length and aperture) doublet shouldn't work just fine.

The main limitation is that if you install a lens faster than f/8, you'll probably see blurring in the corners due to chromatic aberration -- what you'd see as color fringing in a telescope will print as blur. Make sure you have enough light (or patience) to stop down to f/8 or preferably f/11 when you print, and be aware of focus shift as you stop down, and you should get very acceptable prints.

 

[Maris]

An achromatic doublet can be used but the results will be far different from using any actual enlarging lens.
Most readily available achromats are corrected for visual work, telescopes, binoculars and the like, and match the axial focal length for the red and green wavelengths.
The blue rays to which the eye is least sensitive are left largely uncorrected but it is the blue rays that enlarging paper is most sensitive. This secondary spectrum will limit sharpness in enlarging work.
Telescopes and binoculars use rays that are on the optical axis or very near to it. Enlarging an image over a significant area will employ off-axis rays and the following off-axis aberrations will dominate your image:
Lateral chromatic aberration
Spherical Aberration
Coma
Astigmatism
Field curvature
Distortion

Good luck.