Canon 17-40mm loses light as you zoom in?

Those of you who have video mode or live view mode, let me know if this happens to you:

Go to f8-f13 (it happened to me when I was filming within that range).

Zoom in and out while recording.

See if the image is much darker when zoomed in. Looks like I'm almost losing a half stop at 40mm compared to 17mm.

Shooting in manual?

crn3371 wrote in post #12859633
Shooting in manual?

Yes, full manual.

Check it out:

40mm image:

http://img15.imageshac​k.us/img15/2958/40mmq.​jpg

17mm image:

http://img6.imageshack​.us/img6/7382/17mmf.jp​g

17mm 100 crop:

http://img263.imagesha​ck.us/img263/2596/17mm​100crop.jpg

All I did was stand in the same place with the same settings and zoom in/out.

Here's a video of it:

http://vimeo.com/27173​912

I bought this lens from Canon refurbished a few weeks ago. Not sure if this is normal or not.

Update:

It only does it in video mode. It does not do it in "live view manual" mode.

So I think it might be a video setting....

But I can't find any video settings related to this!

Using a Canon 60D.

It looks to me that the act of zooming just changed the composition of the photo enough to change the exposure value. Zoomed out you have a lot of dark green in the shot, zoomed out you don't.

crn3371 wrote in post #12860035
It looks to me that the act of zooming just changed the composition of the photo enough to change the exposure value. Zoomed out you have a lot of dark green in the shot, zoomed out you don't.

Look at the 17mm crop vs the 40mm shot. Different exposures.

It only does this in video mode for some reason. Doesn't do it in Manual live view.

If you were to shoot that scene in Av or Tv mode you would see the exposure change slightly as you zoomed in as you're taking the dark foliage out of the scene and introducing the lighter colored brick. Shooting in manual doesn't compensate for this, thus the change in images.

crn3371 wrote in post #12860063
If you were to shoot that scene in Av or Tv mode you would see the exposure change slightly as you zoomed in as you're taking the dark foliage out of the scene and introducing the lighter colored brick. Shooting in manual doesn't compensate for this, thus the change in images.

Not Av or Tv mode. In manual mode, live view, it does not do this. When I go to video mode, it does it.

Can't figure it out.

The true aperture probably doesn't change between focal lengths but the light transmission might.

I think you're missing my point. In Av or Tv mode, in your example, the camera would expose slightly different between 17 and 40 because you changed the overall balance of the scene. In manual mode the exposure value is locked. What was correct at 17mm with a balance of dark and light is slightly overexposed at 40mm because you've removed the dark foliage from the scene without making any change to exposure settings (because you're in manual)

I think I understand the reason for the problem.

Your lens is not a constant-aperture lens. Thus, when you zoom to a longer focal length the aperture stays at the same diameter and would make a darker image. Normally, the camera controls the aperture when making an exposure so that a setting of f/8 would be controlled properly regardless of the chosen focal length. This is done each time an ordinary photo exposure is made, as the lens is wide open during the time prior to pressing the shutter release button.

My assumption is that, when making a video, the camera controls the aperture at the time that the video is started but does not correct for the changing f-stop value when zooming as the video is progressing.

To verify my theory, try the same test with another consumer-grade zoom lens and also try it with a constant-aperture zoom lens such as the 24-70 f/2.8L. I suspect you won't see the change with the 24-70 but will with any zoom lens that has two f-stop values shown on the front of the lens.

crn3371 wrote in post #12860374
I think you're missing my point. In Av or Tv mode, in your example, the camera would expose slightly different between 17 and 40 because you changed the overall balance of the scene. In manual mode the exposure value is locked. What was correct at 17mm with a balance of dark and light is slightly overexposed at 40mm because you've removed the dark foliage from the scene without making any change to exposure settings (because you're in manual)

nop... 1/50 f8 at 17 and 1/50 f8 at 40, the brick should be at the same level of exposure...

SkipD wrote in post #12860465
I think I understand the reason for the problem.

You may be correct. However that depends on how/whether a lens controls the aperture while zooming mechanically or whether it's a change in what the aperture blades do at the time of stopping down. I would've thought it would be a mechanical control as you zoom (i.e. you'd see if change when zooming the lens as you look through it, not mounted to a body).

SkipD wrote in post #12860465
I think I understand the reason for the problem.

Your lens is not a constant-aperture lens. Thus, when you zoom to a longer focal length the aperture stays at the same diameter and would make a darker image. Normally, the camera controls the aperture when making an exposure so that a setting of f/8 would be controlled properly regardless of the chosen focal length. This is done each time an ordinary photo exposure is made, as the lens is wide open during the time prior to pressing the shutter release button.

My assumption is that, when making a video, the camera controls the aperture at the time that the video is started but does not correct for the changing f-stop value when zooming as the video is progressing.

To verify my theory, try the same test with another consumer-grade zoom lens and also try it with a constant-aperture zoom lens such as the 24-70 f/2.8L. I suspect you won't see the change with the 24-70 but will with any zoom lens that has two f-stop values shown on the front of the lens.

the 17-40 L is supposed to be a constant f4 aperture.

I found some other people talking about the issue:

http://www.dvinfo.net …-fixed-aperture-lens.html

http://forum.planet5d.​com …o-exposure-on-video-mode/

HansSteinert wrote in post #12860990
the 17-40 L is supposed to be a constant f4 aperture.

I had not picked up on exactly what lens was being used. That blows my theory outta the water.