|
I assume you are talking in terms of astronomy wide field photography. I don't have a lot of experience in astronomy terms but in general photography I can offer an opinion.
If you are questioning the expenditure and the 2.8 is twice the f4 then get the f4. The difference is not quite half the light. In dim light normal photography the 2.8 would make critical focusing easier but not much and nothing in terms of astronomical focusing. I doubt it makes much difference in terms of astronomical exposures.
Not sure what the lens is made of, but weight might be an issue depending on your mount, then again the money saved may allow you to get a better mount that might add more to your astrophotography than a 2.8 would.
Roger
|