Vignetting with Variable ND

[JB Rasor]

Greetings everyone! I had a quick question about my variable ND. It seems to be vignetting like crazy at the wide end of my zoom, but clears up about 7-10mm tighter. Specifically I'm using a Canon 16-35, with a Lightcraft Workshop variable ND and a Sony A7r. At 16mm crazy vignetting. At 21 and up it completely disappears. Is this a normal phenomenon?

[Paul2660]

It's possible if the filter holder is visible to the lens. The Lee system for example mounts in a way that the filter holder is very close to being flush to the lens. You can mount (2) 2mm filters without vignetting.   No problems with the 16-35 Canon.

Not sure how yours mounts and the size of your filters.

Paul

[Bart_van_der_Wolf]

It's possible if the filter holder is visible to the lens. The Lee system for example mounts in a way that the filter holder is very close to being flush to the lens. You can mount (2) 2mm filters without vignetting.   No problems with the 16-35 Canon.

Not sure how yours mounts and the size of your filters.

I agree, with Paul. Hard to say what the exact cause is without seeing an example though. Vignetting is usually caused by a mechanical obstruction like a filter or lens hood's edge, or by the lens construction which creates a more elliptical exit pupil instead of a circular one. Light fall-off is usually caused by ray travel distance, magnification, and oblique incidence on the capture medium/sensor.

In addition, depending on the physical thickness of the filter (and variable NDs are thick), the more oblique the rays are traversing through the filter, the more they get attenuated. I also get additional light fall-off (!) with the Lee Big-Stopper, but there is no vignetting because the filter is plenty large enough.

Cheers,
Bart

[JB Rasor]

I think I've found the culprit...the variable ND itself. It's good quality, relatively slim and a reliable brand, but as the ND stops down significantly, especially on wide angle glass, it starts to vignette. I thought I had replicated the same vignetting on my 24-70, but after retesting, it appears to be ok on that lens. So it's really just an issue with max ND at very wide angle. I can live with that I suppose. Thanks for the feedback guys!

JB
 

[stamper]

These filters aren't very good wide open. You will possibly get 2 or 3 stops less of light entering the lens. They don't "work" properly till you zoom to about 70 or 80mm and then depending on the type you will get possibly 7 or 8 stops of light less entering the lens.

[EduPerez]

Just my two cents:

• Variable ND's use polarizing filters, and polarized filters tend to be problematic on wide angles.
• Moreover, ND's use cross-polarization, and thus tend to produce artifacts when stopped down too much.
• ND filters in general tend to produce vignetting on wide angles (light that traverses the filter in diagonal has to travel a longer path inside the filter).