Aperture auto adjusting during zooms
 

I'm using the T2i with the EFS 18-135mm IS lens, and I'm having an issue that when I zoom in or out, the aperture is making noticeable adjustments. I have manual aperture on, and I understand that it is going to happen if I'm adjusting while I'm at 18mm and 3.5 aperture, moving to 135mm since it's a 3.5-5.6 lens, but even if I'm at an aperture of 5.6+ when I start my zoom, the lens is still making adjustments, presumably to keep the aperture constant.

The problem is this effectively ruins the shot if I'm recording video. Is there anything that could be done to stop this from happening, or another feature that could be causing this that I've missed setting to manual in my camera?

This is a common problem with variable aperture zoom lenses. The only thing I know of that could help is if you unlock and partially unscrew the lens from the camera body The aperture will still change some as you zoom, but it won't be so sudden.

You have to be really careful if you do this, since you could lose the lens on the floor if you aren't. Also, since you have to twist the lens to zoom it, this might be kind of tough. I would recommend upgrading to a constant aperture lens if you want to zoom while recording.

An idea...I have no idea if this will work, but it might be worth a try.

I would also have thought that since you set the aperture to f/5.6, the camera should be happy to zoom without adjusting the aperture. I am assuming this adjustment is triggered by the camera, rather than the mechanics of the lens.

If this is so, this might just work. Try holding down the DOF preview button while detaching the lens. This should guarantee that the aperture stays at f/5.6 once the lens is off. If you then re-attach it with the electrical contacts masked (careful not to leave a residue if you use tape), the lens might behave as you want it to. If the aperture still jumps about, then the changes are mechanically driven and you're screwed.

Otherwise, as an alternative to buying a fixed aperture zoom, you might consider a second hand manual zoom and an adapter. If you choose carefully and read around, you can pick up some real bargains.

You have to be careful with drifting focus with zoom lenses. I think it's a problem with many zoom lenses. Still camera zoom lenses aren't designed to hold perfect focus throughout their zoom range, especially the AF zoom models and non-Canon model zoom lenses.

Ian makes a good point. To get around this problem, you need a parfocal lens. To my knowledge there are a few parfocal lenses in the Canon EF range, probably mostly L series, but I am pretty sure the entire EF-S range is varifocal.

Therefore, the OP must know about the focus drift on his EF-S 18-135mm. The amount of focus drift is better or worse on various lenses across different areas of their zoom range. Maybe with such a large aperture as f/5.6 the drift fits inside the DOF.

You could always buy or hire a cine lens, but it would look pretty strange hooked up to a t2i.

Yeah, I'm aware of the focus drift issue, but that's not as major of a concern as the jarring aperture adjustments.

Zooming while recording isn't going to be a common thing on the project I'm currently working on, but there are a few specialty shots that the director wants to pull off that will require it, so minimizing the problem without having to buy or rent a new lens is ideal.

It turns out that the suggestion that ardrinn/james had actually seems to correct the problem, the downside being that I can't adjust the aperture at that point and the lens is locked wide open, but it's preferable to having the aperture jump all around while zooming.

Thanks for all your help!

Detaching the lens completely and using tape to mask the contacts is not necessary. Depressing and holding the DOF prevue button while twisting the lens VERY SLIGHTLY until the aperture registers as f00 on the LCD totally disables all electrical contacts between lens and camera. I think this is a design requirement to prevent possible circuit damage as unmatched contacts slide across each other as the lens is being removed or mounted.

The degree of twist required is very little (probably about 1/8" or so), and then the aperture remains at the last "physical" aperture set. If you don't hold the DOF prevue button, the aperture stays "physically" wide open.

You still will see variations in brightness as you zoom. The aperture will stay at the one "physical" set aperture but the "effective aperture" will change while you zoom and the LCD image and recorded image will change in brightness accordingly.

So what you need is a constant aperture zoom lens to do what you want.

Bruce is right...constant aperture lenses are the real way to get smooth zooms. That's why my kit lens is still in it's box!