Aperture Flicker: Why you should have a manual aperture zoom lens...

...and why electronic, variable aperture zoom lenses are a poor choice for DSLR filmmaking:

Aperture flicker is a phenomenon of zoom lenses with variable apertures at the open end. For the purposes of this explanation, let us consider the following Canon EF 28-135mm zoom lens - it will run open as low as f/3.5 when at 28mm, but will close to f/5.6 when the focal length increases to 135mm.

As the lens has an electrically controlled aperture built into the body, the lens will automatically stop down from f/3.5 to f/5.6 when zooming, causing an abrupt reduction in light as the iris is closed in individual clicks/stops.

It is the same look you'd get if you operated the iris/aperture of a manual lens that had not been de-clicked for film/cine use.

Now, at this point, you're probably saying: "Wait - what if I simply stopped down to f/5.6 for the entire shot? That makes perfect sense, doesn't it?" It certainly does, and you'd be dead wrong, despite what many DSLR photographers (as opposed to videographers) would like you to believe.

In short:  

Even if you stop down to an f-stop that works across the entire focal length of the zoom lens, IN MANUAL MODE, the electric aperture will flicker anyway.

The problem lies in the hard-coded programming of DSLR cameras, which checks at each of the critical stop-down points in the focal range to see if the aperture needs to be stopped down when you zoom, activating the iris mechanism for a split second, thereby causing the momentary darkening of the image. It will do so even if you already stopped down the lens to prevent the system from stopping it down for you.

That's right - so long as you have an variable aperture zoom lens with electronic iris/aperture control (more or less a Canon EOS and M43 issue), you cannot use the zoom on your lens without getting completely uneven light. 

For those of you who want to see this as a video, following is a direct comparison of an all-electronic Canon EF 28-135mm zoom lens  - with a variable f/3.5-5.6 max aperture - against a 1980's Tamron 28-70mm zoom. Just to make the comparison more stark, the Tamron is also a variable maximum aperture lens (f/3.5-4.5, specifically), but it adjusts mechanically - and therefore does not distract from the image.

 

Obviously, aperture flicker is not a problem for still photography, but it proves to be a severe issue for videos.
 
DSLR filmmakers are left with three options to get around aperture flicker:

1. Use zoom lenses made for your existing mount that will run a constant open f-stop across the entire focal range
• Some common zoom lenses are manufactured as described (the EF-mount Canon 24-70mm f/2.8 L - both the original and the II - and Panasonic's 35-100mm f/2.8 for M43 come to mind), but this may needlessly restrict your lens options.
• Note that if you are shooting telephoto-length lenses, using a modern, constant aperture lens is your ONLY option, due to the requirement of image stabilization and a fixed zoom ring. Admittedly, this solution is pricy, I cannot think of a better excuse to justify a 70-200mm Canon f/2.8 L IS to your significant other.
2. Use a lens made for your existing mount that has manual iris control.
• To my knowledge - short of Cinematics' customized (and pricey) cine lenses - non-existent at the prosumer level. Rokinon/Samyang's budget cinema lenses for EF mount are all primes, as are Noktor's SLR Magic series for Micro 4/3.
3. Use an older manual zoom lens with an adapter, preferably de-clicked.

Redrock Micro's LiveLens MFT

But there is a fourth way too:

Granted, this option only exists for M43 users who wish to use EF-mount lenses, but it is an option at any rate: Redrock Micro's M43-to-EF LiveLens MFT, shown at right.

Upon request, Redrock was kind enough to report that the LiveLens controller does not cause aperture flicker when used with a variable aperture lens. Specifically, the LiveLens was tested with a Canon 20-35mm f/3.5-4.5; when set at at f/8, the lens exhibited no aperture flicker when zoomed and retracted.

One last note: Obviously, photographers don't need to worry about this problem, but it hasn't prevented a number of discussions about aperture flicker to appear on photography forums. Be forewarned that a good deal of the advice written on these forums - though well-meaning - are often misleading, or do not apply to DSLR filmmaking.

Kurt K. - "Man With the 5D" 

No monetary compensation was provided for the mention of the LiveLens MFT for this article. LiveLens MFT image and data was provided by Redrock Micro upon my request.