General T4i Mystery

K

kellyboy

Well-known member
Does anyone else have this issue? I am shooting video with a T4i and a Canon 18-135 stm lens. All is on manual. Everything. Therefore exposure should remain pretty constant during a zoom. But when shooting and I do a fast zoom out...the image darkens after a half second. And it's not just on the lcd...it's on the file on the shot.
??? Am I doing something wrong?
 
Thanks Samuel. I see that I am not the only one with an exposure pumping lens camera combo. Guess it's a bug in the firmware or something.
 
Don't know if STM makes any difference, but shooting a 60D with the stock 18-135mm f/3.5-5.6 IS lens, the aperture is automatically switching when zooming in, but does not goes back to f/3.5 when zooming out. Logical to me.
 
kellyboy said:
Thanks Samuel. I see that I am not the only one with an exposure pumping lens camera combo. Guess it's a bug in the firmware or something.
Click to expand...

Its not a bug, as other's have pointed out, that sounds like exactly how aperture-changing zooms work. If you are outdoors and shooting at F6, for example, the iris blades need to close more at the wide end to be at F6 then they do at the far end. So as you zoom along the barrel, the camera has to determine what physical size your aperture is at the current zoom length and adjust the iris blades correspondingly. It doesn't happen in real time, and when it does it is very sudden and physically visible on your camera and footage.
As you backed, the aperture of your lens got wider (and your footage brighter). Once you finished and the camera caught up, it stopped down your iris blades which you visibly saw as the footage getting darker.
This will happen on just about any zoom lens, and only way to avoid this is to use one with a constant aperture.
 
Last edited:
Thanks Gilden! That makes sense for sure. But why doesn't that happen when I zoom with an actual video camera?

K
 
kellyboy said:
Thanks Gilden! That makes sense for sure. But why doesn't that happen when I zoom with an actual video camera?

K
Click to expand...

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).
 
You must log in or register to reply here.
Back
Top