MarcoB

Hi Cinegain, maybe it isn tot much of a biggie, but it is a problem. You cannot zoom in or out. You cannot reframe without checking, but most of all Sigma 18-35 has not constant aperture as advertised.

Hi Jonpais, thanks for your reply. Seems that a lot of people has my exact same problem with Sigma 18-35: at the end of focal range (28mm) there is more luminance. Here are some links: Sigma 18-35+ speedbooster Brightness is not constant troughout the focal range [Archive] - BMCuser.com - The Online Community for BlackMagic Camera Users GH4 Sigma 18-35mm is brighter at 35mm... Normal? [Archive] - DVXuser.com -- The online community for filmmaking Sigma 18-35mm acting strange...: Third Party Lens Talk Forum: Digital Photography Review There is an in depth analysis that confirms that change at 28mm: http://www.dxomark.com/Lenses/Sigma/Sigma-18-35mm-F18-DC-HSM-A-Canon-mounted-on-Canon-EOS-700D---Measurements__870 (you have to look at "Measurement/Transmission" tab) Note that the luminance problem is increased by speedbooster and that at the min and max aperture is not relevant. It is obvious at intermediate apertures. I have done a test, you can download it and watch the defect directly with your eyes (it is discernible): stare at the red couch. Zoom test with Gh4+metabones speedbooster NikonG-Micro 4/3 x0.71+Sigma 18-35, manual focus, aperture ring at 4, 4k, iso 1000. https://drive.google.com/open?id=0B_MiHNwWCmT5WTRtVUtsTDAyMFU It would be very useful to know if your Sigma has the same defect or not; can you check again trying to zooming in and out not at the maximum or minimum aperture? At 28mm you shoud note a luminance increase (if your sigma has the same defect). Thank you Marco

Hi Stab, I have the same problem. I did some research and seems that we are not alone. Sigma 18-35 has a problem, and it is not a little one, it has not a constant aperture if you record a complete zoom in (from 18-35 ) or zoom out you will have a luminance change. Andrew should add this information to his useful Gh4 guide. I bought sigma 18-35 and nobody told me about this problem: not guides, posts, reviews, video reviews.