Why does changing the aperture cause lag in live view and in videos?

Question

On my Canon 70D there's a considerable lag during the change of aperture when pressing the depth of field preview button. This lag also occurs during video recording, which causes the video to halt shortly when the aperture changes.

This happens on the Sigma 18-35 f/1.8 lens but seemingly not on the Canon 40/2.8 STM.

Do different lenses have different ways of communicating aperture changes, or is it more likely that the latter lens changes the aperture rapidly enough not to cause any lag?

(Although this is not really an issue unless your shooting videos, I hope that the question is general enough to be on topic here.)

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Update:

I checked again, and the Canon 40/2.8 actually does behave the same in live view when pressing the DOF preview button, i.e. causes lag. But when shooting videos, the aperture changes smoothly, which is not the case for the sigma lens mentioned above and Canon 50/1.8. The latter two do produce lag during filming when the fast aperture shifting occurs.

I've seen that the Canon 40/2.8 is marketed as a lens for movie shooting because of the quieter focusing due to the step motor, but in my opinion the smoother aperture change is MUCH more important. The Sigma lens is more or less useless because of this, which is a pity considering its overall quality and well-suited zoom range for recording videos.

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Update 2:

The same lag also occurs with Canon EF 50/1.8 and Canon EF-S 55-250/4-5.6. Very annoying!

Answer 1

I think the depth-of-field preview should not be used during live recording (why would you do that anyway?).

Although I can only guess, the most probable reason is that when you press the DOF preview button, the aperture changes very quickly. (This is what is approximated with a step-function in electrical engineering.) Now, the system must respond to that change, but how should it respond? Should it gradually change amplification to arrive from a dark image to the correctly lit? Or should it try to do this as fast as possible, potentially creating an overshoot that will oscillate around the final value? The usual approach is a slower tracking of light intensity - basically you do not expect sudden light intensity changes anyway, and so the AGC, the automatic gain controller can slowly change to the new value.

This is the same as if you go out from a very dark room to bright sunlight. Your eye will get gradually used to the outside light, but it takes awhile before it can operate normally.

Now if you are in Live view, the amount of light is calculated in pure software. When it drastically changes, you need processing power for the AGC algorithm, and probably that just temporarily preempts the recording task.

You can try this easily: get a piece of black paper, and meanwhile recording, just put it before your lens. If I am correct, this will do the same phenomenon you described, proving that this is not a lens-specific problem, it is just too much change in lightness...

In general, why would you change aperture during a recording? Unless you are creating a live DOF change, it is pretty much unneeded... Take multiple takes with different aperture, that is the usual way...