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Is the Sigma 18-35 1.8 really 1.8?


Jonesy Jones

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So, I just got a hold of the Sigma 18-35 for the first time. Lovely lens. I'm trying to do some tests to make sure I want to keep it. Decided to compare it with another lens that I own which is a Leica 19mm 2.8. I thought I'd run them both through a few tests. Just fiddling around I shot the keyboard on my desk with both lenses wide open... and to be honest I don't see too much of a difference. Granted the 1.8 is slightly brighter, but not by much. Definitely not a stop +, at least to my eyes. Both lenses are using adapters to get them on my BMPCC. The Sigma lens is using the Metabones (non speedbooster). I've looked at the aperture blades of the Sigma/adapter and they appear to be opening up all the way. The Leica is using a cheap (non speedbooster) adapter. 

You guys are smarter than me. What am I doing wrong? Thoughts?

EDIT: Just added the sample photos. Can you tell which is which?

EDIT #2: Added a couple more examples. I also compared the Sigma to an old 24mm 2.8 Nikon I have (this is the 3 pair of photos), with the same results. I am going to try a couple more tests, the suggestion of shooting a candle seems like a great idea. However, at this point, it is becoming obvious that the Sigma 18-35 is, regarding exposure, closer to a 2.5 than 1.8. 

Blackmagic_Pocket_Cinema_Camera_1_2015-0

Blackmagic_Pocket_Cinema_Camera_1_2015-0

Blackmagic_Pocket_Cinema_Camera_1_2015-0

Blackmagic Pocket Cinema Camera_1_2015-01-25_0444_C0000_4100164.png

Blackmagic_Pocket_Cinema_Camera_1_2015-0Blackmagic_Pocket_Cinema_Camera_1_2015-0

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Host 'em at imgur, then attach 'em via 'Insert other media' > 'Insert image from URL', use the 'Direct image link' from your imgur upload.

All given f-stops are in fact the correct aperture f-stop of a particular lens. Unfortunately, that doesn't mean much. Therefor T-stops should actually be given for any lens out there. Can't imagine the Sigma being off, the cine mod is T2.0 and just overall the Sigma is brilliant though.

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Yes, it is really 1.8
You can look at the measured T stops on DxO: http://www.dxomark.com/Lenses/Sigma/Sigma-18-35mm-F18-DC-HSM-A-Canon---Measurements

When comparing it to your Leica, you don't have to 'guesstimate', you could test (and confirm the aperture) by shooting in manual mode. Shoot with both lenses wide open, with the Leica use 1/50s, ISO 800 for example, and set the Sigma to 1/50s ISO 400. Brightness should be almost equal with those settings.

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Hey guys, thanks for your help. I just uploaded some sample pics to illustrate what I am talking about. Yes, you can sort of see a difference. But no where near the difference a stop + should represent.

​What are the exact settings of each shot?
Did you try to reproduce it?

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You must make sure that you:

  1. use the same camera for both shots;
  2. have the scene lit by nothing but artificial light (no day light/sun light interference) which remains at the same brightness;
  3. keep identical shutter and ISO settings on the camera through the comparative shot;
  4. make sure that when developing the raw images, your raw converter uses identical settings and applies no automatic exposure/gain adjustment.

 

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You must make sure that you:

  1. use the same camera for both shots;
  2. have the scene lit by nothing but artificial light (no day light/sun light interference) which remains at the same brightness;
  3. keep identical shutter and ISO settings on the camera through the comparative shot;
  4. make sure that when developing the raw images, your raw converter uses identical settings and applies no automatic exposure/gain adjustment.

 

  1. ​Check
  2. Check
  3. Check
  4. Shooting ProRes Lt (same settings on single frame export)
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f stops and t stops are not the same.  the sigma may be f1.8, but probably has 3 times as many optical surfaces than the leica meaning (assuming both lenses use the same efficiency coatings) if you set the sigma to f2.8 it won;t be as bright as the leica.  dof should be the same, but there will be a difference in exposure.  It shouldnt be a full stop difference though!

 

 

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For depth of field, I think it may be easier to tell if there was a greater space between the subject and the background. As it stands now, the keyboard and the desk are basically adjacent with not so much space in between. For exposure, there should have been a more significant difference, so I'm not sure what happened.

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Well looking at the Dxomark results, at 19mm Sigma is T1.9.

Leica 19mm f2.8 could very well be T2.7 or even lower, I do not know that as a fact, just guessing. So the difference shrinks very quickly.

Just a thought, do you have metabone adapter with aperture control? If yes, have you simply tried switching adapters just to see if the adapter is not messing with the aperture pin on the Sigma.

Also I would try to photograph or make a video of a something much darker, like a candle on a table with it's surroundings. The difference should be much more obvious then.

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Added some more examples. There is just no way this Sigma is even close to a 1.8. 

Also I would try to photograph or make a video of a something much darker, like a candle on a table with it's surroundings. The difference should be much more obvious then.

​This is a great idea. I am going to try this next. 

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