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Is the Sigma 18-35 1.8 really 1.8?


Jonesy Jones

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I have the Sigma 18-35 also. (favourite lens in the world by the way)

At f1.8 you get the DOF of a f1.8. Having said that, the light transmission is indeed not a full stop improvement over f2.5 but this might have to do with the fact that the lens suffers from vignetting at f1.8. 

To what extend this influences the actual light transmission? I don't know. 

I do know that my partner shoots with a Lumix 12-35mm f2.8. After we shoot in together in a dark venue, his shots are way to dark and unusable. Mine are miles and miles better. 

Mind you, I use a Speed Booster so my lens basicly turns into a f1.2. It might not be t1.2 due to the vignetting, but as sure as hell it's at least 1.5 times brighter than a Lumix f2.8.

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I have the Sigma 18-35 also. (favourite lens in the world by the way)

At f1.8 you get the DOF of a f1.8. Having said that, the light transmission is indeed not a full stop improvement over f2.5 but this might have to do with the fact that the lens suffers from vignetting at f1.8. 

To what extend this influences the actual light transmission? I don't know. 

I do know that my partner shoots with a Lumix 12-35mm f2.8. After we shoot in together in a dark venue, his shots are way to dark and unusable. Mine are miles and miles better. 

Mind you, I use a Speed Booster so my lens basicly turns into a f1.2. It might not be t1.2 due to the vignetting, but as sure as hell it's at least 1.5 times brighter than a Lumix f2.8.

​That's good to know. Thank you for sharing. Keep in mind that you are comparing the Sigma to another zoom... while using a speedster. Compared to a prime the Sigma is only slightly better than a 2.8. I'm guessing 2.5-2.6.

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So once again I've done another test, against both my Leica and Nikon prime 2.8. This time the test was with all the lights off and just a candle on a table, and once again the results are the same. Again, the difference of light transmission from the Sigma 1.8 and the prime's is minimal. If I were to guess t-stop, it would be 2.6 or 2.7 at best for the Sigma. 

Do you think there is something wrong with my copy of this lens? Or do you think this is consistent across all copies?

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I think I have read somewhere that the Sigma is close to a t2.2 or something. What is possible is that the Leica prime is actually closer to t2.6 and thus the difference is only half a stop. 

I don't think there is anything wrong with your lens. Do you have the Nikon mount? Then you can push the small lever on the down side of the lens to the side so it opens up the iris completely. Make sure the lens is at 35mm, look into the barrel and push the lever. If the lens opens up full and you don't see the aperture blades anymore at all, the lens is opening up fully. 

The chance that there is anything inside the lens that wrong which is blocking the light, but still gives sharp images, is very unlikely to me.

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So once again I've done another test, against both my Leica and Nikon prime 2.8. This time the test was with all the lights off and just a candle on a table, and once again the results are the same. Again, the difference of light transmission from the Sigma 1.8 and the prime's is minimal. If I were to guess t-stop, it would be 2.6 or 2.7 at best for the Sigma. 

Do you think there is something wrong with my copy of this lens? Or do you think this is consistent across all copies?

​I think there is something wrong with either your lens or your test setup. I just quickly replicated your test using

(a) a Sigma 18-35mm/f1.8

(b) a Nikon 28mm/f2 Ai-s prime

...both attached to the Metabones BMPC Speedbooster and a Blackmagic Pocket Cinema Camera mounted on a tripod, shooting a color chart in CinemaDNG raw at 24fps with a 172.5 degree shutter angle and ISO 800, in a completely dark room using only one artificial light source (a camera-mounted LED panel). The CinemaDNG files were imported into Resolve, with white balance/color correction first applied to the Sigma clip and applied with identical parameters to the two Nikon clips.

Sorry that I didn't spend time on more careful framing/focusing - it didn't matter for this test.

First picture is the Sigma at f1.8, second picture the Nikon at f2.0, third picture the Nikon at f2.8. I think that the pictures speak for themselves.

 

 

01_1.1.1.jpg

02_1.2.1.jpg

03_1.3.3.jpg.4cc74001cafb79c27737992bae9

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Thanks guys for your help. I have decided to exchange my Sigma for a new one. Partially because of the light transmission issue, which may be identical on all copies, and partially because something doesn't look right with the aperture blades when I stop all the way down. It turns into a strange oval hexagon shape. When I get my new copy I will test it again and post the results.

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