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Originally Posted by [1ponders]
 Are they cheaper than canon lenses for similar or better quality? |
Yes they are cheaper because of the need to manual focus,but for astronomy doesn't matter,Check out Ebay for adapters and lenses.Also haunt camera stores that sell second hand.Quality depends on the lens, but generally the smaller the F number the better the quality,ie F2.8 to F1.8, F1.4, F1.2, under 200mm.Above 200mm don't settle for anything less than F4. and at and above 600mm F5.6.
The really unfortunate thing is you can easily pick the quality lenses as they have the highest asking price.Also check out the other thread in this forum Sigma lense for widefield
Bert