Variable gray filter - the strength of the effect (extending the exposure time) can be adjusted by turning the filter. Variable gray filter - the strength of the effect (extending the exposure time) can be adjusted by turning the filter.

Testing a variable gray filter

I finished last week this report from Gunther became aware of a very inexpensive variable gray filter. I've only ever seen something like this for around double the price at enjoyyourcamera and for a multiple of the price at "Original" by Singh-Ray. In a previous blog post I described and showed what you You can do this with a strong gray filter.

The one now tested variable gray filter is not as strong as in the blog post shown, but you can infinitely adjust the strength of the exposure time extension. The extension factor specified by the manufacturer is 2 to 400 - i.e. a doubling of the exposure time in the weakest level and an extension of 8,5 f-stops in the strongest level. For comparison: the gray filter used in the Rome images had an extension factor of 1000, i.e. 10 stops. Yes, I know, the names are confusing. This has something to do with the logarithm...

How the filter works is relatively simple: two polarization filters are twisted against each other, allowing different amounts of light to reach the sensor.

variable gray filter – the strength of the effect (extending the exposure time) is adjusted by turning the filter.

The positive first: the design of the filter means that there are no unwanted vignetting, even with wide-angle shots (I was able to test 24mm on the full format). In addition, at the lowest setting, the viewfinder is still bright enough to be able to easily control the image section and focus - this is unimaginable with a pure 10x ND filter, which eliminates the hassle of screwing the filter on and off. The color shift that is usual with gray filters remains pleasingly small. This also shows what is probably the most boring series of pictures you have ever seen here. But the pictures also show the negative side of the filter. All shots with 24mm on full format - by level I mean the points that you can also see on the picture of the filter above - 0 at minimum, 10 at maximum.

Recording without gray filter at 1/4000s
Minimum level of the gray filter – 1/1250s (probably just light clouds in front of the sun)
Level 2 of the gray filter – 1/1000s
Level 4 of the gray filter – 1/800s
Level 6 – 1/400s
Level 8 – 1/100s
Level 10 – 1/5s

Since it wasn't sunny all the time, slight fluctuations in the exposure times are possible - so this is not a scientific test track. But it becomes quite clear that the filter works relatively linearly at the beginning, only to quickly become much stronger than expected after level 6. This makes it difficult to adjust the filter effect to your liking - you have to rely on the camera's automatic exposure.

I was shocked at the last picture, the result of the maximum level. Although the factor 400 is pretty good (as I said: sun, clouds, etc.), the brightness distribution in the image makes the filter unusable in this setting.

Good, I thought to myself - it's probably because of the wide angle. So: same experimental setup with 70mm focal length. I'll spare you the intermediate steps now, otherwise you'll fall asleep for me with this blog post. I hope you also believe me that the filter works quite well up to level 8. But here it comes!

Gray filter at the lowest level – 1/500s
Gray filter at level 8 - 1/20s - corresponds to six two-thirds f-stops - a slight color shift through the filter can be seen.
Gray filter at maximum level - 1/5 second exposure time - measured by the camera's automatic system and this also corresponds to the expected exposure value if the filter is to be an ND400

Well – uh, huh? The camera's automatic exposure system measures something and, according to Adam Riese, it does it well and afterwards everything is black? Yes, that's how it is with this filter. I tried several different shooting situations. Different lighting situations, different lenses, different cameras and different automatic measuring systems – the result is always the same. At maximum strength, the camera's automatic exposure system gets out of step.

I have had the best experiences when the camera automatic is set to +5EV (i.e. +5 f-stops) at maximum setting. Then the exposure values ​​are approximately correct again, but the brightness distribution remains very unpleasant.

Gray filter at maximum strength - camera automatic at +5EV - 2,5 second exposure time

Conclusion: the Vario-ND filter ND2 to ND400 is really easy to use from ND2 to around ND7. This will be sufficient in the usual applications - especially given the price. If the exposure time is extended more strongly, it counteracts this good dense gray filters not on.

  1. Hello Stefan, interesting test. However, I would have wished that you had chosen a motif that would have made it easier to recognize the inhomogeneity that comes from the filter and what clouds, shadows, etc. are. So I find it very difficult to judge.
    To be honest, I'm not surprised that the camera's automatic system fails to measure the exposure, as you have 2 polarizing filters in front of the lens. This is known to confuse autofocus and camera automation.
    When I work with gray filters, I always use manual mode and take test shots with control over histogram/image playback. After 2-3 shots I usually found the right exposure.
    It would definitely be interesting to compare it with one of the significantly more expensive Vario filters. Since I believe that the effects you describe come from the design of the polarizing filters, I can't actually imagine that the more expensive filters would react any differently. Maybe one of your readers has already had experience with this and would like to share it with us. I would be very happy.
    In summary, you have to consider whether you can live with the limitations or whether you want to invest in a set of ND filters of different strengths (at least for the video sector), which of course means constant changing and a financial investment that should not be underestimated.
    Regards
    Gunther

    1. Hello Gunther,

      If you only knew how many white walls I have photographed... 🙂 I can absolutely understand your objection or wish.
      However, the distribution of brightness varies depending on the focal length. I think this is also clear from the two examples shown. In one case you see a dark cross lying over the image, in the other example it is completely dark.

      Because of this difference and focal length dependency, I have omitted an even less exciting example. I didn’t want to bore my readers even more… 🙂

      Regards
      Stefan

  2. Hi Stefan,

    thank you for this nice report. Another question: on which lenses did you use the filter and did you ever have problems with the autofocus. With a Cokin ND8, the focus completely failed for me on a D3 with various lenses, even though the lighting conditions were very good.

    collegial greetings

    1. Hello Thomas,

      That's the nice thing about the filter: you can use it at the weakest setting to focus and then turn it to "close". This way I had no problems setting the focus correctly even with manual focus.

      Regards
      Stefan

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  6. Why on earth are you taking comparison pictures of a flower pot that couldn't be more static?

    This is so completely meaningless!

  7. Hello, everything is put into perspective if you only use these filters for filming, like I do! So if you want to ensure that you can always work with the same aperture. e.g. 25p/1:50/iso 100. I set the values ​​mentioned and turn the filter until the exposure is correct. I'm so glad that I have an adjustable filter just like that, because if the lighting conditions suddenly change etc. I have exactly one filter that I can adjust by adjusting it. Unfortunately, with my other lens I only have 2 filters, one is often too weak and the other too strong! Of course you can buy everything that is missing, but the constant changing is, to be honest, annoying.

    This is where the adjustable one was used: https://vimeo.com/65006696

    Mfg Maik

  8. I had to experience the same thing with mine. A few days ago I bought a comparable ND filter for very little money. The first tests shocked me a little. The next tests will definitely come but as I see I shouldn't expect much 🙁
    a happy New Year 2014

  9. I find the article very interesting and will continue to explore the topic. Just a little tip - the mathematician was called Adam Ries and not Riese.

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