Automatic Iris Change While Zooming (In MANUAL Iris)
 

While fiddling around with my new T3i in MOVIE mode, I've noticed on a wide shot, the exposure is fine where I've set it. Mind you, I'm in MANUAL for my iris settings, yet, when I zoom in to the end of the lens (the 18-55mm), the iris automatically closes down and the image darkens. What gives? Anything in the menu that I'm not setting correctly or should do? Will Magic Lantern correct this? Not that I'm a huge fan of zooming while shooting but say I was doing a dolly in to a subject, would the camera do this same thing? One would think, if you have your camera in manual iris mode, it would be completely MANUAL.

Re: Automatic Iris Change While Zooming (In MANUAL Iris)
 

Hi Mike,

The 18-55 isn't a constant aperture lens. If you have it set to 3.5 on the wide end it will shift to 5.6 as you zoom in since 5.6 is as big as the lens gets at 55mm.

You shouldn't see much difference if you set the lens to 5.6 on the wide end then zoom in.

Re: Automatic Iris Change While Zooming (In MANUAL Iris)
 

This is ramping, its normal. Your lens has a maximum aperture of f3.5 at the wide end, this rises to 5.6 at the tele end. Constant aperture zoom lenses hold the aperture, ie, 2.8 throughout the entire zoom range. But they obviously cost a lot more.

Re: Automatic Iris Change While Zooming (In MANUAL Iris)
 

You can always tell by looking at a zoom lens whether it has a constant or variable maximum aperture.
The lens will have its maximum aperture value marked on the barrel. If there are two aperture values,
then it's a variable aperture lens. The larger aperture value (the lower f/number) is the max. ap. at the
wide end, while the smaller aperture value (the higher f/number) is the max. ap. at the telephoto end.

Meanwhile, if the zoom lens is marked with only one aperture value, then it's a constant aperture lens.
See the photos below for examples. Hope this helps,

Re: Automatic Iris Change While Zooming (In MANUAL Iris)
 

Fantastic info everyone! Thanks a million, very appreciated!

Re: Automatic Iris Change While Zooming (In MANUAL Iris)
 

I've seen it do this on the stock lens even with an aperture setting at 5.6 - which should remain consistent - and indicates that it is on the display. The weird thing is the exposure changes a second AFTER you've changed the focal length. I'm not sure what the heck is going on with it. It's definitely on manual so this shouldn't be happening. I haven't noticed any similar problem with my EFS 17-55 f/2.8 lens. So combination on lens and camera? I don't know. Very weird.