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Dimitris,
One simple way to find out what camera lenses are like is to put them on a tripod aimed at an artificial star (a distant bright reflection of the sun off something small will do nicely) and hold up a short fl eyepiece (4...8 mm is needed, longer fl won't show you much) to take a look at the image.
Most camera lenses are very far from diffraction limited, ie. the image won't be a dot.
While it may be OK for photography generally, the reason this matters is that by smearing the incoming light across a wider area the image of a point source is dimmer than it would be if concentrated in a diffraction-limited image. This is why a diffraction-limited telescope objective will outperform most long telephoto camera lenses.
IMHO the Zenit lenses were pretty awful, poor copies of Takumar lenses from the 1970's; optical design has advanced significantly since then. At least choose something more decent for astronomy like the MTO 1000mm f/10.
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