my sigma 24-70 seems dim at f2.8

[Chai]

It simply shows this lens is not particularly good at wide open. Most lenses will show lack of contrast at wide open.

[goldfries]

yeah. i tested 50mm f1.8 again, same method as the above.

exposure level indicator remains same f2.8 and f4.0 as well.

at first i thought it could be aperture blade issue but if it was then the fact that exposure remains constant at wider angles already disproves the thought.

[Chai]

Sometimes, the aperture blade might not move correctly. But it shouldn't happen at wide open, because the aperture blade only closes when you use smaller aperture. Try something like F4 and F8 see if you get the same inconsistent exposure. If it doesn't, then it's simply because the lens' sweet spot is not at F2.8

[goldfries]

yup.

regardless of the zoom distance, the aperture closes the same way. f2.8 is f2.8.

now based on test, f2.8 works fine at other zoom range, exposure results same as f4.0 and / or higher.

so i think we can conclude that the aperture blades work fine at f2.8, it's just that the lens doesn't capture light too well when f2.8@70mm.

please do correct me if I'm wrong.

anyway from what i see, the "problem" is easily fixed by upping the exposure by 1/3.

[Chai]

Even the contrast improves?

[goldfries]

so far only 1 composition causes the contrast to appear. the rest like the toys and outdoor seem fine.

i have no idea how to re-create the contrast issue.

in that composition
f2.8@70mm - washed out
f4.0@70mm - fine
f2.8@24mm - fine
f4.0@24mm - fine

contrast issue aside, yeah it works fine by just upping exposure by 1/3.

[peaz]

It's just the lens performance at 2.8. I believe the glass of that lens isn't that big right? Quite compact lens in comparison to the canon's equivalent.

That's why I'm still collecting money for the Nikkor one instead. Hehe

The only way is for you to compensate via post processing and only use 2.8 when you really need it.

[Chai]

It's definitely not small, comes with 82mm filter thread.

[Adrian Wong]

Originally Posted by goldfries

so far only 1 composition causes the contrast to appear. the rest like the toys and outdoor seem fine.

i have no idea how to re-create the contrast issue.

in that composition
f2.8@70mm - washed out
f4.0@70mm - fine
f2.8@24mm - fine
f4.0@24mm - fine

contrast issue aside, yeah it works fine by just upping exposure by 1/3.

Well, upping the exposure should reduce the shutter speed, so it's not a workaround without any consequences. Better than increasing ISO though.

[goldfries]

Originally Posted by peaz

It's just the lens performance at 2.8. I believe the glass of that lens isn't that big right? Quite compact lens in comparison to the canon's equivalent.

That's why I'm still collecting money for the Nikkor one instead. Hehe

The only way is for you to compensate via post processing and only use 2.8 when you really need it.

PP can't help the contrast issue. well, it shouldn't be in the first place. color then no choice la.

the Nikkor version $$$ oh. my friend bought a Nikkor 24-70 f2.8 ...... phew....... nearly can buy the EF 70-200 F2.8 L IS USM already.

Originally Posted by Chai

It's definitely not small, comes with 82mm filter thread.

82mm yes but size still a little smaller than it's non-3rd party counter parts.

Originally Posted by Adrian Wong

Well, upping the exposure should reduce the shutter speed, so it's not a workaround without any consequences. Better than increasing ISO though.

well yeah, workaround with consequences.

the thing I'm more concerned about is the constrast issue. I have so far encountered 2 instances where it happens.

so i just send it for warranty.