Can a variable ND filter change its gradation over time?

[Wil E Coyote]

Hello. Hope this is the right forum for this.

About 8 years ago I bought a variable ND filter from Lightcraft Workshop for my Sony and Canon DSLRs. Back then I did a series of tests, measuring how many stops each step of the filter it had.

Recently, however, I noticed that the filter wasn't behaving as I expected it to in some outdoor shoots, so I decided to take it out and test it again, and I noticed that this time the filter seems "darker" that it used to be, by around 1/3 or 1/2 stops.

I have to admit that my first tests weren't 100% rigurous: I just pointed the camera to a static shot of my room and took note of the exposure. This time I've used a grey card. Also, back then I tested using a Canon EOS550D, while this time I am using a Sony A7IV (both in spot metering).

Neverthless, can a ND filter degrade over time to the point of changing its gradation? Or perhaps I should dig up my Canon and test it again with it to be 100% sure?

 

[Dem Bell]

Hello. Hope this is the right forum for this.

About 8 years ago I bought a variable ND filter from Lightcraft Workshop for my Sony and Canon DSLRs. Back then I did a series of tests, measuring how many stops each step of the filter it had.

Recently, however, I noticed that the filter wasn't behaving as I expected it to in some outdoor shoots, so I decided to take it out and test it again, and I noticed that this time the filter seems "darker" that it used to be, by around 1/3 or 1/2 stops.

I have to admit that my first tests weren't 100% rigurous: I just pointed the camera to a static shot of my room and took note of the exposure. This time I've used a grey card. Also, back then I tested using a Canon EOS550D, while this time I am using a Sony A7IV (both in spot metering).

Neverthless, can a ND filter degrade over time to the point of changing its gradation? Or perhaps I should dig up my Canon and test it again with it to be 100% sure?

Most likely it is the way you took the measurements. 1/3 stop is within the accuracy of a metering system. If you did not average over at least 5 independent measurements, this is in the noise.

Having said that, I puzzled over a similar problem with one of my polarisers changing its strength until I realised that the marking was on the metal rim but the glass could rotate inside the rim.

A variable ND filter is made of two polarizers. It is possible that one of the glass pieces got nudged out of its original orientation. The effect is not going to be uniform though - if you are getting an extra 1/2 stop at, say, the 2-stop ND setting, it will be noticeably more than 1/2 stop at the 6-stop ND setting.