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Canon 24-105 f/4 exposure drift on zoom

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Renny McCauley
Canon 24-105 f/4 exposure drift on zoom
on Jun 13, 2016 at 6:07:09 pm

When shooting video on the 5D Mark iii with the 24-105 f/4 lens, there is a very annoying issue. As you zoom in, the image will darken. The screen still shows the f-stop as the same setting but the image gets noticeably darker. I do a lot of documentary-style shooting and the reason I have a fixed aperture lens is so that I don't have to adjust the aperture as I zoom in and out. Right now I have a silly workaround. I try to always shoot at f/4 because the problem doesn't exist at f/4 for whatever reason. Then I'm adjusting the ISO instead of aperture for changing exposure. This is not a desired solution.

I know others have documented this problem so I know I'm not crazy. This is not user error. I've shot with 5D's and 7D's for years and know how to use them well. I'm just new to the 24-105. What I'm looking for are some solutions on how to deal with this exposure drift. Changing firmware? (I'm on the latest, but maybe downgrading). Install Magic Lantern? (Not a big fan, but if it solves the issue, I'd be up for trying it.) Or do I just need to start thinking about buying a different lens for the camera? I hate to do that because I like this lens in every other way - the zoom range, the sharpness, the colors. But this is such a ridiculously annoying problem. Anybody have suggestions?

Renny



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Warren Eig
Re: Canon 24-105 f/4 exposure drift on zoom
on Jun 14, 2016 at 4:13:11 pm

It's the lens. It's a good lens but much cheaper in the L series Canon arsenal. I'm not an expert in lenses, but I think it has to do with all the glass and as you zoom, with a small aperture you're magnifying that light loss. I don't see this with my 70-200 IS 2.8L or the 16-35 2.8L.

Warren Eig
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Renny McCauley
Re: Canon 24-105 f/4 exposure drift on zoom
on Jun 15, 2016 at 11:08:32 am

Thanks for the reply Warren. I think it has to do with more than just the construction of the lens because of it's variation in behavior (eg it doesn't have the problem at 4.0 nor in stills mode). Somebody suggested that it actually may need to be repaired. If anybody is looking for information on this in the future, here are a few threads to check out.
http://www.dpreview.com/forums/thread/4016235#forum-post-57921728
http://www.dvinfo.net/forum/canon-eos-crop-sensor-hd/489029-video-darkens-d...



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Blaise Douros
Re: Canon 24-105 f/4 exposure drift on zoom
on Jun 15, 2016 at 4:48:26 pm

Your lens is not broken--this is normal for the 24-105. It's well documented; I have two copies of the lens and they both do this.

Remember, f-stops are not T-stops. F-stops basically have to do with the ratio between the front element of the lens to the size of the aperture--it's not a guarantee of identical exposure. For example, if I put a big medium-format lens on my DSLR, f/4 might let as much light in as f/2.8 on a lens of the same focal length meant for 35mm, because the aperture would be larger to compensate for the larger front element.

T-stops, however, are meant to tell you how much light is let in through the lens; T2 will ALWAYS be the same, even if you change lenses--that's why T-stops are the standard for cinema lenses, so that exposure remains the same, regardless of how big the lens' front element is (and thus, the ratio to the aperture, which is the f number). F/4, though, changes depending on the size of the front element/entrance pupil of the lens.

In the case of the 24-105, I believe what's happening is that as you zoom, because of the narrowing FOV, less light makes it into the lens in general; the front element/entrance pupil stays the same size, as does the aperture. But because the FOV narrows, less light makes it to your sensor since a wider FOV=more light. So the f/ratio remains the same, while the equivalent t-stop changes slightly.

Someone with more experience in lens design/physics will probably have more concrete answers, but this is my understanding of the difference.


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Blaise Douros
Re: Canon 24-105 f/4 exposure drift on zoom
on Jun 15, 2016 at 5:04:31 pm

You may find DXO's measurement of the T-stop transmission of the lens to be illuminating (har).

http://www.dxomark.com/Lenses/Canon/EF24-105mm-f-4L-IS-USM-mounted-on-Canon...

Click the "Transmission" tab to see. It pretty much lines up with your observations, as well as my experience; the EV drops by about .2 when zooming from 24mm to 105mm.


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