from Ilford photo site
Diffuser vs Condenser Enlargers
The difference between condenser and diffuser enlargers lies in the manner in which the light from the illumination source is brought to provide uniform illumination over the negative being printed. As the names suggest, condenser enlargers achieve this using condenser lenses, whereas diffuser enlargers use a diffuser, normally a ground-glass screen, in the light path.
ILFORD black & white photographic papers are designed for use with both condenser and diffuser enlargers.
Enlargers using directed light produce more contrasty prints from a black and white silver negative than those using diffuse light. The effect results from the scattering of light by the silver grains in a conventional negative: it does not arise with the non-scattering dye images from ILFORD XP2 SUPER film or ordinary colour negatives.
In practice, a silver negative on a fast grainy material may require to be printed on paper that is harder by one contrast grade using a diffuser enlarger than it would using a condenser enlarger to produce an equivalent print. ILFORD MULTIGRADE paper with choice of the appropriate filter value is clearly an easy way to achieve equivalent contrast.
The difference is not only an issue when trying to produce equivalent results from the same negative using different enlargers. The same effect is true the other way around, in that the contrast aim for an ‘ideal’ negative to print with a condenser enlarger is lower than the contrast aim for printing with a diffuser enlarger. Photographers who routinely use a condenser enlarger may prefer to develop their negatives for slightly less time (10%) than standard to provide this lower contrast, possibly also at a lower exposure index.
Scratches and dust on the surface of the negative also scatter light, and therefore are much less visible on a print made with a diffuser enlarger; this effect assumes greater importance the smaller the negative. Retouched negatives should always be printed with diffused light, as the retouching can be visible when printed with a condenser enlarger.
On the other hand, it is easier to achieve a high light intensity with the condenser enlarger as there is no diffuser in the light path, and therefore this type may be preferable when large enlargements are being made as it reduces the exposure time. These effects apart, there should be no visible difference between prints printed using the two types of enlarger, given that they have the same contrast.