Re: ef 100-400mm f/4.5-5.6l is usm

[ectobuilder]

Re: ef 100-400mm f/4.5-5.6l is usm

I've rented the ef 100-400mm f/4.5-5.6l is usm lens and everytime I zoom in and out the lens seems to change exposure and you can hear the motor running.

I've turned off Image Stabilization and Auto Focus. I am also filming in manual exposure. So why do I hear a motor running each time I zoom in or out?

I should note as well that I've fixed the aperture to F5.6

 

[ectobuilder]

OK, I'm beginning to think that it is the natural mechanics of this lens. That a "constant aperture" still requires the aperture gears to turn in order to make it stay "constant".

Whereas the EF 70-200mm F2.8 is a true constant aperture mechanically, where it doesn't need to move gears in order to make the aperture stay constant.

 

[deltoidjohn]

My understanding is that the iris needs to adjust in order to keep the aperture the same - because aperture is a mathematical ratio of focal length to iris diameter. Hence to keep the aperture ratio the same, the iris diameter must be adjusted in proportion to the focal length.

When you say the exposure changes, do you mean that it becomes darker and stays that way, or just that it flickers and then settles back to the same exposure?

 

[ectobuilder]

My understanding is that the iris needs to adjust in order to keep the aperture the same - because aperture is a mathematical ratio of focal length to iris diameter. Hence to keep the aperture ratio the same, the iris diameter must be adjusted in proportion to the focal length.

When you say the exposure changes, do you mean that it becomes darker and stays that way, or just that it flickers and then settles back to the same exposure?

It flickers. I'm assuming it is because the aperture changes size slightly due to the mechanics even though I've set the aperture to a constant F5.6.

On the other hand, my EF 70-200 F2.8 stays truly constant without flicker, meaning the mechanics represents a true constant aperture?

 

[maarek]

There is a bug in the mechanics of Canon DSLR's. For some reason they want to flicker the aperture when using a variable aperture lens from Canon. I hate it. If you can find a manual (no electronics) f4-5.6 it will not flicker, just dim a bit as it should.

Constant apertures always work, no bug there.

I have no idea why these things do that. Haven't tried the newest 7d firmware to see if it's fixed but probably not.

A f4-5.6 SHOULD NOT FLICKER when you zoom, it should just dim. The same as constant apertures do (without the dimming). If you are in photomode and just look through the mirror and zoom in and out, then there is no flicker. When you set it to movie mode and the mirror jumps up...then it starts.