Independent of the optical quality of particular instruments, obstruction has the effect to make images less defined, less contrasty, and less sharp. As one would expect, the effects depend on the amount of obstruction. Also, there is a minor effect on image brightness, as obstruction reduces the free aperture, and thus the light-collecting area. Thierry Legault has described the effects of obstruction for central, circular obstructions (typical for secondary mirrors), nicely illustrated with Lunar and planetary images which were then "artificially obstructed" (by computer simulation) in two steps (20 and 33 % obstruction). He finds that an instrument with free (circular) aperture of diameter D and (circular) obstruction of diameter d has
Reflector diameter 33 % obstruction 300 250 225 200 150 110 Equivalent diameter of refractor about image definition 200 170 150 130 100 73 Equivalent diameter of refractor about light gathering 280 235 210 190 140 100where the first equivalence (image definition) has its main impact on binary star separation, planetary and Lunar details observing, while the second equivalence roughly determins limiting magnitudes and deepsky observing.
It should be emphasized that other effects may have much higher limiting effects on image definition/sharpness, in particulary atmospheric seeing conditions and instrumental misadjustment.
What remains obscure in Thierry's discussion of this objective, at least to the present author, is how he arrives at his conclusion No. 3, that obstruction is of minor effect on high-contrast structures, and thus his Equivalent Diameters about planetary and lunar resolution; these should IMHO identically equated to the equivalence about image definition. To the present author, this point appears to be a personal and perhaps personally biased opinion of Thierry who is a very successful observer and photographer with obstructed astronomical instruments. If there is any justification for his view, an explanation for his impression might be that the effect of aperture on image definition is not so prominent for high-contrast objects such as Lunar craters.