[DanWeso]

I'm a little hesitant on posting this cause I have more testing to do, but was curious of peoples responses.

Yesterday I was profiling my lenses to find or validate what others have said to be the sweet spots. The results were very good. However, when I was comparing the 18-20mm local range of a Sigma 10-20 lens the 18-20mm range of a DAL-18-55 kit lens, I was sure the Sigma was going to show me why I need to upgrade the kit lens. This was not the case, the 18-55 seemed sharper or as sharp as the Sigma. I have to go back and retest before I come to any conclusions, and I might be comparing apples to oranges. I have no complaints about the Sigma lens. I does not have FF/BF problems and picture are not blurry, but it now make me wonder if it could/should be sharper.

How does a fairly new dSLR user know if they have a good copy of a lens when you don't really have anything to compare with it?

[NeverSatisfied]

I thinks it's like you said, apples to oranges. That Sigma is all about ultra-wide use, although it can go to 20mm, that's not where it shines. Same with my Pentax DA12-24, that lens is stellar, but I hardly ever use it at 24mm, and its IQ is best at the wide end anyway. I doubt there's any problem with the Sigma. Depending on circumstances you'll probably carry both lenses anyway, and, now knowing the characteristics of both lenses, you can choose accordingly. Sounds like you're certainly gaining a useful familiarity with your equipment, as well. BF/FF issues on a zoom can be a bugger, so I'm glad to hear that's not a problem for you.

I was in your shoes a few years ago, with a brand new K10D and DA*16-50. That lens was de-centered and it didn't take long to figure it out. Certainly some lenses perform better than others, and you have to have realistic expectations, but I think you'll know if there's something really wrong with a lens.

[paperbag846]

Don't worry about copy variation unless you can visually see something wrong with either the lens or the product. For the most part, slight variation in quality is undetectable, and unimportant regardless.

How does a fairly new dSLR user know if they have a good copy of a lens when you don't really have anything to compare with it?

They don't really. If you buy the lens new, you can exchange and exchange until you are happy... but there is no assurance that you will end up with a better product.

I would suggest you don't even think about it unless the pictures you are getting make you unhappy. Sounds to me like you are satisfied with the sigma and the kit lens, which make a good pair.

I think it is better to think but lenses in their utility, rather than in IQ alone, because it is a subjective measure for most users, where MTF charts are very theoretical in their application to a photo.

[aleonx3]

Same with my Pentax DA12-24, that lens is stellar, but I hardly ever use it at 24mm, and its IQ is best at the wide end anyway.

I opted for the 12-24mm because of the long end instead of the 10-20mm. I used the 12-24mm as a street photography lens rather than a super wide; therefore in my case, 24mm is mostly used. However, YMMV depending on what you use the lens for.

[Jewelltrail]

DanWeso: However, when I was comparing the 18-20mm local range of a Sigma 10-20 lens the 18-20mm range of a DAL-18-55 kit lens, I was sure the Sigma was going to show me why I need to upgrade the kit lens. This was not the case, the 18-55 seemed sharper or as sharp as the Sigma.

Dan, it is true the Sigma is weakest in MTF scores @ the long end. At these focal lengths, the two lenses are virtually tied, for sharpness. However, @ 18mm the Kit is more prone to vignetting, and the Sigma is at its best for distortions. It is worth noting that pcture IQ is judged with more variables than sharpness.

An upgrade would be a lens at comparable focal lengths, with faster, constant apertures--For example, the Sigma 17-50mm 2.8, the Tamron 17-50mm 2.8, or the Da 16-50mm 2.8 would be upgrades. I haven't met anyone yet who bought the Sigma 10-20mm for its 18mm focal range.

[SpecialK]

How does a fairly new dSLR user know if they have a good copy of a lens when you don't really have anything to compare with it?

You compare your results to those of others using the same lens. When yours are technically as good or better (focus/sharpness, mainly) than others you see, then you are there.