Sony rate the ND filters in most of there cameras using a fractional value such as 1/4, 1/16, 1/64 etc.
These values represent the amount of light that can pass through the filter, so a 1/4 ND lets 1/4 of the light through. 1/4 is the equivalent to 2 stops ( 1 stop = half, 2 stops = 1/4, 3 stops = 1/8, 4 stops = 1/16, 5 stops = 1/32, 6 stops = 1/64, 7 stops = 1/128).
These fractional values are actually quite easy to work with in conjunction with the cameras ISO rating.
If you want to quickly figure out what ISO value to put into a light meter to discover the aperture/shutter needed when using the camera with the built in ND filters, simply take the cameras ISO rating and multiply it by the ND value. So 800 ISO with 1/4 ND becomes 800 x 1/4 = 200 (or you can do the maths as 800 ÷ 4). Put 200 in the light meter and it will tell what aperture to use for your chosen shutter speed.
If you want to figure out how much ND to use to get an equivalent overall ISO rating (camera ISO and ND combined) you take the ISO of the camera and divide by the ISO you want and this gives you a value “x” which is the fraction in 1/x. So if you want 3200 ISO then take the base of 12800 and divide by 3200 which gives 4, so you want 1/4 ND at 12800.
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Thanks again Alister . I have another question about the variable ND. What’s the story re: using an external polarizer with it. Sony sounds a little vague saying only that it “may” cause problems. I shot some tests both with linear and circular polarizers and found my results confusing. Sometimes I saw some dark and warm discoloration in the lower left corner and often I didn’t. Sometimes the linear seemed to acted differently than the circular and sometimes it seemed the same. My eyes went blurry trying to find any common pattern , though the circular did seem to act more strongly than the circular ( i used a Promaster) . Oddly both seemed to work fine for AF with or without ND which I didn’t expect.
Polarisers often cause issues with wide angle lenses, even without an ND filter as the rotation angle of the light changes across the field of view. So colour shifts will often occur with any camera. But in addition there can be interactions with other filters and these become more and more acute at wider angles as the angle of incidence becomes ever greater. We see examples of this regularly, for example a polarising filter revealing rainbow patterns in an otherwise clear piece of glass that is under stress. You will also see a very similar effect if you fly in a Boeing Dreamliner and look out of the variable tint windows while wearing sunglasses (in fact many aircraft windows will show rainbow artefacts with polarised light due t the stresses in the glass). The variable ND filter in the FX cameras is very thin and it might be that some of this is stress related. Or it may be that the when the filter is darkened something about the way it works alters the stresses in the glass. I honestly don’t know.
But whatever it is it is not surprising to see a wide variation in the interaction. FoV, angle of incidence, polarisation of the light in the scene, the design of each individual filter and the amount of ND will all give differing results.
Overall the variable ND is incredibly useful and a great tool, but it does have limitations. The Venice cameras use a dual filter wheel system with very expensive high end full spectrum ND filters in each slot to provide 1 stop ND steps as Sony acknowledge that the quality of the variable ND may not meet the quality and consistency required in a high end cinema camera. But there is no way this could be squeezed into a camera the size of the FX6 and it would certainly push the price up much higher.
Hello thx for the great articles, I think the last calculation about the nds is wron it has to be a 1/2 nd or do i miss something, thx gr SH
You are correct, the last example was wrong (I’ve corrected it now 12,800 to 3200 (was 6400) = 1/4). I wish there was a prize to give you as the first to notice as the article has been online for some time now. Thank you for pointing out the error.
no prize please, may for my math teacher, thx for all your very helpfull articles, had to ask cause can’t got my head around that but thought there may could be a possible explanation