Review of variable Infra Red filter

I must start with a confession. I have seen variable infra red filters on Ebay and been intrigued by them.

How does a variable  infra red filter work?

The filter is a variable ND filter, which is made up of 2 polarising filters. Added to the top layer is  a long pass red filter. The idea being they can vary the IR light transmitted from 590 to 720nm by rotating their outer ring. This sounds a great idea if you don’t want to be changing filters whilst out and about.

We were kindly loaned one recently, so we took the opportunity to try it for ourselves.

For this review we used a Protech converted Panasonic G2 camera, fitted with a full spectrum internal filter and equipped with a standard 14-42 mm lens, zoomed out to 42m to reduce the chances of vignetting. The camera was mounted on a tripod to prevent camera shake and keep accurate framing. We used Zomei 590nm and 760nm IR screw in filters for our control images, our test filter being a Fotga 590-760nm variable IR screw in filter. The variable filter has makings provided to ensure the adjustment ring is not rotated too far, causing strange effects, the ring itself is smooth enough. Curiously, whilst the filter to lens thread is 52mm the front thread appears to be 55mm, not sure of the reason for this.

 

 

 

 

 

Fotga filter with case

 

 

 

 

 

 

 

 

Fotga filter IR markings

We created custom white balances for all the filters, (590 nm on white card and 760nm on grass).
Out of interest we also used the fixed filter custom balances for the variable filter and visa versa.

What conclusions did we draw from our test and whilst a variable filter seems a good idea, how did it perform in practice?

590mn comparison

 

760nm comparison

 

Strange banding seen far left and right

Our control images taken with the Zomei fixed filters and using their respective white balance settings provided clean sharp images. Using the Fotga variable filter with the same white balance settings gave a noticeable colour difference proving it is not as accurate as it should be. Things improved marginally when we used the dedicated custom white balances for the Fotga variable filter, but still not as true as the fixed filter the 590nm setting proved less accurate. More worrying though was the strange dark bands (seen in the sky). Using the same white balance settings for the fixed filters again gave a much better result at 720nm than at 590nm. Overall sharpness was not as good as that of the fixed filters.

Conclusion

A variable Neutral Density filter can be problematic at the best of times due to inherent softness, in addition to this it cannot be used with very wide angle lenses due to the filter causing banding. Now add in the fact that the variable IR filter is filtering visible and infra red light. All this is certainly born out in our experience of the sample we tested.
Would we buy a variable IR filter? The resultant strange banding, softness of image and inaccuracy of filtering, makes the answer simple, an emphatic NO!

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