GH4 Sigma 18-35mm is brighter at 35mm... Normal?

S

Stab

Well-known member
Today I noticed that the brightness increases when I zoom in on the Sigma 18-35mm f1.8. When I get to around 28-35mm I suddenly get like a stop increase in ilumination.
I thought this was a constant aperture lens. This means that brightness shouldn't change when zooming, right?

My buddy's Panny 12-35mm doesn't do this.

Could someone please check / reply if your Sigma does this also? Or is it a defect or does it have something to do with the Speed Booster which I'm using with it?

Thanks!
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Edit:
I made a video which shows the problem:

You can see the problem clearly at F4. At F1.8 it is almost non existent...



(at f4)



(at f1.8)
 
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Constant aperture doesn't mean constant transmission. Severe light fallout for instance can contribute to this.
 
yslee said:
Constant aperture doesn't mean constant transmission. Severe light fallout for instance can contribute to this.
Click to expand...

You're right. But I'm curious if other people's Sigma's 18-35 have the same...
I don't really need to do any tests. It's pretty obvious to the naked eye as it is.
 
Just tested mine, and yep, it's all light falloff/vignetting. The actual centre value on my C100 waveform shifts up and down a little, but stays at pretty much the same level.

Stop down to about f/4, and it all but goes away. Still a bit of unnoticeable shifting, but that's unavoidable.
 
wgtwo said:
Just tested mine, and yep, it's all light falloff/vignetting. The actual centre value on my C100 waveform shifts up and down a little, but stays at pretty much the same level.

Stop down to about f/4, and it all but goes away. Still a bit of unnoticeable shifting, but that's unavoidable.
Click to expand...

Alright, thanks. So the centre of the frame stays almost the same, but you do see the light falloff with the naked eyes when zooming in, right?
 
I just tested some more.


The difference between brightness at different focal lengths is actually less visible wide open and becomes more apparent when stopping down a bit.
The lens is most bright at 28mm with a pretty large difference with 20-24mm and gets slightly less bright at 35mm again.


Someone with an Sigma 18-35mm who can look how theirs is doing?
 
Stab said:
I just tested some more.


The difference between brightness at different focal lengths is actually less visible wide open and becomes more apparent when stopping down a bit.
The lens is most bright at 28mm with a pretty large difference with 20-24mm and gets slightly less bright at 35mm again.


Someone with an Sigma 18-35mm who can look how theirs is doing?
Click to expand...


Actually, that's exactly how the vignetting works in the 18-35:

http://www.photozone.de/canon-eos/872-sigma1835f18_canon?start=1

vignetting.jpg
 
I really start to think that this is just how this lens is build. Although I found 1-2 people reporting the same 'problem', I've heard more people say that theirs doesn't do this.


I just pointed the lens at me, and looked at the lens opening while turning the zoom ring. When zooming in, the 'hole' naturally becomes larger. A longer focal lenght, needs a bigger aperture opening so that's normal.
But when I get to arround 28mm, the opening seems to 'exponetially' become larger. Because not only does the 'inner ring' shift, but at that focal length the outer aperture also opens further. This is visible at all aperture settings, but moreso at F3.5 and onwards.


I'm pretty sure this is what is causing the extra light to come in starting at +- 28mm.


Now the only question that remains is... Is this normal for this lens? Or is it a 'defect' in mine and I do I receive too few light at focal lenghts under 28mm....
 
Stab said:
I just pointed the lens at me, and looked at the lens opening while turning the zoom ring. When zooming in, the 'hole' naturally becomes larger. A longer focal lenght, needs a bigger aperture opening so that's normal.
But when I get to arround 28mm, the opening seems to 'exponetially' become larger. Because not only does the 'inner ring' shift, but at that focal length the outer aperture also opens further. This is visible at all aperture settings, but moreso at F3.5 and onwards.
Click to expand...

Sounds like part of the design IMHO....seems like a dumb thing to do on Sigma's part though :-P
 
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