Camera adjusts aperture while zooming even in manual mode

[saofaihp]

This was a problem pre-magic lantern as well, but whenever I take video and move it to a lighter or darker area while zooming it sounds like the aperture is being adjusted and the image becomes lighter or darker like the camera is adjusting for the exposure.
I'm using a T3i, have move exposure set to manual, auto lighting optimizer disabled, and I set the aperture, shutter, and ISO manually with magic lantern (or with Canon's menu) but I still have this problem. And this is only when I'm zooming using the lens.
On the bottom of the screen where it shows the aperture number it will change as I zoom in and out enough, but sometimes it won't change and I'm thinking it will only show the standard aperture amounts and not any stops in between.

[dmilligan]

When you change the zoom you change the focal ratio, if the aperture stays the same size. The only way not to change the effective f-ratio when changing zoom is to actually change the aperture.

See: http://en.wikipedia.org/wiki/F-number

[NedB]

@saofaihp: Apologies if you already know this, but some zoom lenses do not have a constant maximum (i.e., smallest F-number) aperture throughout their zoom range. For instance, with my 550D (T2i), the kit lens is an 18-55mm lens with a maximum aperture of 3.5 (at the 18mm setting) to 5.6 (at the 55mm setting). So if you are using anything more open than an f5.6, and you're zooming from 18mm to 55mm, your aperture will change, even though all your setting are set to manual. The lens cannot help it, its maximum aperture is only 5.6 at 55mm.

That's why sometimes you don't see it happening: if you are using an aperture of 5.6 or smaller, you can zoom in and out at will and your aperture won't change.

Cheers!

[saofaihp]

@saofaihp: Apologies if you already know this, but some zoom lenses do not have a constant maximum (i.e., smallest F-number) aperture throughout their zoom range

I thought maybe that was it but it doesn't seem to help. Even at f8, f16, etc it still makes this adjustment.

[NedB]

@saofaihp: Hmmm. I tried it on my 550D with the kit lens, and I see what you mean. You hear a clicking sound, like the aperture is changing, and the LV does seem to change its brightness subtly, perhaps in both directions. But to test if this has something to do with the exposure, I did the same test with the lens cap on, and I still hear the clicking. I have not noticed the aperture value in the ML display changing in either of these tests. Not sure what is going on, but perhaps it does in fact have something to do with the lens. Since the LV (when the lens cap isn't on) does change a bit, it seems like the aperture is changing (because of the three variables ISO, shutter speed and aperture, the only one which involves a mechanical change should be the aperture's blades moving in or out) but apparently not enough to cause the displayed f/stop number to change in the display.

Anyone else want to weigh in on this? Is this what one gets for using cheap lenses? Anyone with L-series glass having the same symptoms? Cheers!

[dmilligan]

Read what I said.  There is a difference between absolute aperture and F number or focal ratio. The absolute aperture is the diameter of the entrance pupil (or if there is no entrance pupil, i.e. wide open, it's the diameter of the lens). The F number is the ratio of the aperture (entrance pupil size) to the focal length. In photography we usually express the aperture in terms of F number, this makes it easy to calculate other parameters no matter what the focal length is, but F number and aperture are not exactly the same thing.

When you zoom you change the focal length. Therefore, the only way to keep the same F number or focal ratio when you change the focal length, is to change the size of the entrance pupil. Since you can change the focal length smoothly, but the entrance pupil size (i.e. aperture diaphragm blades) steps discretely, as you change the focal length, the camera is going to skip to the next aperture size in order to hold the F ratio constant. The since the real F number can be anything, b/c you can have any focal length in the range of a zoom, the F number the camera displays is actually rounded to the nearest number on the standard 'F number scale' (i.e. 1.8, 2, 2.8, 3.5, 4, etc)

On fancier lenses that have a constant minimum aperture throughout the zoom range I don't think this is a problem, because the lens design allows smooth change of the entrance pupil size, but I'm not totally sure about that (I don't own any such lenses).

Basically your problem is a limitation of the lens design.

[saofaihp]

@dmilligan
Ahh, thanks

[lktyrrell]

@NedB  Hmm I seem to have the same problem. I understand completely that the aperture is going to change when zooming in and out to compensate but that is NOT mu problem and it sounds like it is not yours either. My lens was working fine before installing ML. When I zoom in even slightly the aperture goes crazy - I have uploaded a video to show you what I mean: http://www.youtube.com/watch?v=PjZuSE6_mkA