What's the difference between f/3.5 and f/4???

[NathanO]

I'm torn between 2 lenses.
Sigma 10-20mm f/3.5 EX DC HSM ELD SLD
Sigma 10-20mm f/4-5.6 EX DC HSM

My biggest concern is the f/3.5 versus the f/4. The astrophotographer in me says go for the 3.5. But at the widest part of the lens on each...the aperture is still a very small diameter. Since the difference in cost is around $200....my question is....
How much of a difference is the 3.5 than the 4? Is the 3.5 really worth the extra money? Better quality lens? Sharper at the edge of the frame? I don't really get it.

-nathan

[IABoomer]

The f/3.5 is a fixed aperture over the entire zoom range. The f/4-f/5.6 is one where the aperture varies throughout the zoom range, being f/4 at the 10mm and f/5.6 at the 20mm length.

Thus, the 3.5 is faster at the long end, which is why you pay a bit more for it.

[OsmosisStudios]

Funnily enough, the f/4-5.6 is actually sharper in the corners.

[oldwolf]

Originally Posted by NathanO

How much of a difference is the 3.5 than the 4?

About 0.5.

Sorry, I had to say it. hehe

I second what IABoomer is saying. If you're going to use the lens for anything but shooting at night on a tripod you've got a little bit of an edge with the 3.5 because it's through out the whole focal range so you can shoot with a faster shutter speed or lower ISO from 10-20mm.

[kirbinster]

I have the older cheaper one and love it. It is very sharp, sharper than the newere one, and if you plan to use it outside for landscape work it will serve you very well.