How Avoid Vignetting using a variable ND filter

Question

My camera is Nikon D700

Lens 28-300mm Lens 24-70mm

I have a Variable ND filter 77mm size

How do I avoid vignetting around the edges of the filter?

thank you.

Answer 1

It's an angle of view issue. The light path is intercepted at the frame corners by parts of the filter blocking parts of the light path.

If you wish to use a filter at extreme settings and a standard version causes vignetting then you need either

• A wider active area of filter at a given distance from your lens front surface (which may not be possible without machining your lens front !!!, or

• To mount the filter element closer to the lens (which again may not be possible with major surgery.

  If you are using another filter between lens and ND filter it will add to vignetting effect - take it off.

You could fabricate a stop that presents the lens entering the focal length range where vignetting occurs - messy and prone to either be not used when needed or left in place when not needed.

If you are serious and desperate enough you can probably fabricate or have fabricated a custom mount that uses the ND material from the existing filter and mounts it in a ring which is either broader or shallower. However, given that many other filters do not cause vignetting at short focal lengths, chances are that either you are doing something fixable or the filter manufacturer has a bad design.

You MAY be able to modify the ND filter without changing to a custom mount.View the vignetting effect at minimum focal length. Now, remove the filter and insert a flat stick (iceblock stick about right) into the corner of the field of view and adjust position until it gives similar depth of vignetting penetration to the filter ring. This shows you how much intrusion you have to deal with. Next inspect the filter ring and see what needs to be done to remove the same amount of intrusion. It MAY be possible to machine the filter outer surface enough to fix it. Maybe not. Removing the ND material while you machine the ring "would be wise". If less venturesome you can reduce the ring thickness on a whetstone with time, water and elbow grease. BUT it should not be necessary if the manufacturer has done their job properly.

If you can find some other kind of filter than fits and that does not cause vignetting you MAY be able to swap the ND element into it. Or not.

Answer 2

My guess is that you get vignetting at 24 and 28 but not at 300mm. Vignetting from a filter is due to the fact that the filter prevent the collection of the same amount of light in the border of your image than in the center. It is just a mechanical effect in the sense that the ring holding the filter intersept part of the light. Appart not using your lense at the smalest focal length you may get another filter either bigger or which can be mount closer of the lense.