When zooming on a Canon T3i, the video gets darker and then brighter at f/5.6

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I stumbled upon this about a year ago but never really got the chance to ask, so here it is.

I was using a Canon T3i/600D with the 18-55mm f/3.5-5.6 II IS kit lens and I know the lens has variable apertures based on what focal length you're at. When I zoom in it gets darker because it does to f/5.6 automatically but when I set the aperture at f/5.6 (to not have a dark image when I zoom in and then a bright image when I zoom out) and zoom in it gets darker and then brighter. It seems to be some sort of automatic thing. The shutter speed and ISO has already been set. When I zoom out, it get's brighter and then darker. The aperture doesn't change. I've tried this at lower apertures and this still happens.

What's going on here?

Thaks
 
Everything should be manual. Shutter, iso, iris.
try setting it to 5.6 when zoomed out, not when zoomed in.

Dont have, nor ever owned a zoom lens with different aperatures, but something has to be set to auto instead of manual.
 
It's because Canon APS-C cameras have a strong issue with zooming using variable lenses. If the lens has electronic contacts, the camera will always adjust the exposure when zooming - for some strange reason - and nothing can be done about it. Except use constant aperture lenses or variable lenses without any electronics. Which don't probably exist.
 
the easiest way to work around this issue is covering the contacts between the body and the lens with a piece of paper or sticky tape, which will disable the camera to do that aperture shift. i used to cut out a ring of paper, which only covered the contacts, and was easy to remove if no zoom was needed. just set your aperture when the camera is in liveview, take the lens of (camera still in LV) and cover the contacts, so that the aperture will stay the same. Thats the only solution which helped me with a lens without constant aperture...
 
This doesnt sound right, Ive never had so mess with the contacts when using a zoom. Are you sure the Auto Lighting Optimizer is not causing the exposure change? If you are in manual mode with a constant ISO the exposure should not change, ever.
 
JoeThePro said:
This doesnt sound right, Ive never had so mess with the contacts when using a zoom. Are you sure the Auto Lighting Optimizer is not causing the exposure change? If you are in manual mode with a constant ISO the exposure should not change, ever.
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Yes it doesnt sound right, but that is how it is, unfortunately...
It works fine only with constant aperture lenses as maarek said...
 
DDirector said:
Everything should be manual. Shutter, iso, iris.
try setting it to 5.6 when zoomed out, not when zoomed in.

Dont have, nor ever owned a zoom lens with different aperatures, but something has to be set to auto instead of manual.
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Yup, everything is on manual, I've even set it to lower apertures when zoomed out (like f/8, etc.), when I zoom in, what I said in the OP still happens. The aperture, etc, didn't change but the exposure gets darker then brighter. Tried this at different ISO values, indoors and outdoors. This is so odd.
 
Loads of photography lenses do this because of their design. Zooming through the zoom range creates different amount of light output as the relationship between the internal lenses changes. In stills it doesn't matter, the camera copes, but in video you see it on some lenses. Others manage it better - but none were really designed to do video, were they?
 
paulears said:
Loads of photography lenses do this because of their design. Zooming through the zoom range creates different amount of light output as the relationship between the internal lenses changes. In stills it doesn't matter, the camera copes, but in video you see it on some lenses. Others manage it better - but none were really designed to do video, were they?
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This is probably the reason why and plus I was using a crappy kit lens.
 
paulears said:
Loads of photography lenses do this because of their design. Zooming through the zoom range creates different amount of light output as the relationship between the internal lenses changes. In stills it doesn't matter, the camera copes, but in video you see it on some lenses. Others manage it better - but none were really designed to do video, were they?
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EXCEPT that the Canon's also pump the exposure up and down. Variable lenses close down when zooming in yes. But they don't close down when already set for example at f5.6 (is 5.6 is for both close and open values). Still the Canon DSLR's change the exposure around when zooming with variable lenses. This is a bug in the firmware or something odd that the canons like to do in videomode. They actually don't do this while in still mode and looking through the viewfinder.
 
In stills mode, don't they stay open for focus, then stop down for the shot - making the problem less obvious?
 
paulears said:
In stills mode, don't they stay open for focus, then stop down for the shot - making the problem less obvious?
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Yes. And no.

Forget about taking a shot, just look through the viewfinder with a variable lens. It keeps it open at f3.5 when zoomed out and closes to about f5.6 when zoomed in. But there is no pumping around. It's smooth as expected.
 
This was why, it was how zoom lenses are made.

M. Gilden said:
The simple answer is that it was designed to work that way, whereas the zoom barrels on your DSLR is not.

The more complicated answer:
Your camcorder's zoom is not manual like a DSLR, but rather attached to a servo motor that allows it to smoothly keep track of what focal distance you are at. It is closely tied to the electronics of the camera, and therefore can make micro adjustments along the way to keep your exposure looking constant.
Contrast this with a DSLR zoom, in which you manually control it with your hands. The camera receives a signal from the lens regarding what focal length it is currently at (I'm under the impression that the body actually pings the lens at regular intervals), however that signal doesn't necessarily get processed in real time. As soon as the camera realizes it is at a new distance, it quickly has to determine where the iris blades should be for the set aperture, and send a signal back to the lens where the motors there must respond in kind. This whole process happens very quickly, but it can't anticipate where your hands will end up or that you are still moving the lens, and therefore updates everything all-at-once instead of smoothly adjusting as you go. You must remember that this system was designed for still photography where it is a perfectly acceptable way of doing things, not for smooth adjustments while zooming in real time for video.

Also worth noting, the aperture of a common camcorder on the wide vs telephoto end has a much smaller gap than a DSLR zoom traditionally. For example, I believe the DVX100 was 1.6 at the wide end, and 2.8 at the far. I've seen other camcorders that range from something like 1.4-1.9, which means that you are dealing with fractions of a stop to adjust for (less room for visible errors).
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Zooming and changing exposure is a lens problem: Canon says this

Zooming and changing exposure is a lens problem: Canon says this

This problem of exposure change even at the smallest aperture when zooming is a lens problems. The Canon EOS M manual says this: exposure will change while zooming at all apertures except for the following two lenses: the EF M 18-55mm IS STM and the EF S 18-135mm IS STM. There may be more lenses now that do not have this issue. It is a feature to inquire about when choosing lenses for video.
 
markr041 said:
This problem of exposure change even at the smallest aperture when zooming is a lens problems. The Canon EOS M manual says this: exposure will change while zooming at all apertures except for the following two lenses: the EF M 18-55mm IS STM and the EF S 18-135mm IS STM. There may be more lenses now that do not have this issue. It is a feature to inquire about when choosing lenses for video.
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Except if you cover the contacts.

So Canon is definitely doing something wonky with the lenses deliberately or not.
 
No, Canon is not doing anything wonky. Remember that f-stop is defined as focal length over pupil diameter; so as focal length changes, pupil diameter must also change in order to maintain the same f-stop. e.g. 80mm focal length divided by a 20mm pupil diameter gives an aperture of f/4.0. Similarly, 40mm FL / 10mm PD = f/4.0. In other words, as one zooms in from 40mm to 80mm the pupil diameter must also go from 10mm to 20mm. By interrupting the electronic interface and not allowing the pupil diameter to change, the lens will essentially stop down from f/4.0 at 40mm to f/8.0 at 80mm.

I would also be very surprised if there wasn't some rounding going on, at least at the millimeter level. Say, 77mm FL and 19mm PD being considered f/4.0, then a jump/click at 78mm FL to 20mm PD which stays at 20mm until, say, 83mm FL when the PD again jumps/clicks to 21mm. Furthermore, since photo cameras generally only adjust exposure in 1/3 stop increments, it would make sense that the rounding would actually be to the nearest 1/3 stop, which would lengthen the focal length range to something like 75mm to 85mm. In other words, as one zoomed in from 75mm, the image would gradually darken, until 85mm when the aperture would jump/click up and the image would suddenly brighten. And that is the exact behavior noted by the OP.
 
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