Please help.. 17-55 /w XTi, underexposed?

Just tried 17-55 f/2.8IS, snapshot of my light switch at my dark room.
Tried to compare it with Sigma 30mm at the same f/2.8.. but found out the 17-55 underexposing about 1 stop.

Here are the full picture, Both of them shot using Manual mode, at f/2.8, 1/60, sigma at 30mm, canon at 31mm (according to exif), ISO1600.

Shot at manual, auto WB (did several shots, the result was the same), one shot, center focus. The vignette is more result of insufficient lighting of my room.
Here are the pics

First, Canon 17-55 f/2.8

Do either of them have a filter on? If so, try without it.

both of them have UV filter on. I'll post pictures without filter soon.

okay, here are both without filters. The same exposure,
1/60, f/2.8, ISO1600, sigma is at 30mm, canon at 31mm

First, Canon

probably mostly due to the overall contrast of the lens and light fall off... you can see the 17-55 has alot of vignetting.

If you look at some of the tests that Lightrules has done involving the 30 1.4, you'll notice that it seems to expose brighter than several Canon lenses.

http://www.pbase.com/l​ightrules/lenstests

seems to underexpose, maybe by a 1/3 stop or so to my eyes.

Thanks for the help guys!

Bacchanal,
I saw the Lightrules test and it had the same phenomenon.

I guess it is just how it renders the light maybe? with the same exposure, one lens is brighter than another. I wonder if non EF-S lens will give out a brighter image than EF-S lenses.

I would presume that as the 30mm has less glass in it, then less light is actually lost overall.

mcmadkat, I think that is really possible. As the Lightrules test on
http://www.pbase.com/l​ightrules/image/604365​62

it also looks darker than the 17-85 IS and 18-50EX.

It is interesting at the same exposure, two lens can have different brightness.. or in a way, 2 different exposures. Maybe it's good because the 17-55 IS will take care of the highlights better (maybe?) and keeping the detail.

Interesting....I'm curious if you use you're cameras built in meter to meter the exact setup, will it give you different exposures?

thedjo wrote in post #3934801
okay, here are both without filters. The same exposure,
1/60, f/2.8, ISO1600, sigma is at 30mm, canon at 31mm

First, Canon


and then sigma


Canon 17-55 still underexpose, or 30mm overexpose(?). (btw i think my 30mm front focus.. i aimed at the 'e' of the 'cue')

I think that the 17-55 pic is far better, on my monitor at least.

Take note on the USB plug - in the 17-55 shot you can see the detail on this plug, but nothing on the 30.

And yes, the focus does seem out on the 30.

I don't question it's sharpness. it's excellent!

I shot them with Manual mode, at EV0, which one do you find has the correct exposure?
maybe there is none.. but I am more used to what 30mm give because I had it longer than 17-55.