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I wanna shoot video on a sunny day with a 1/50 shutter speed and f/1.8 (for a bokeh effect). I'm wondering what ND filter rating should I get in order to not over or underexpose the video. Could you guys help me out?

I'm gonna be shooting with a Canon T3i and the Canon 50mm 1.8 lens.

Thank you.

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    \$\begingroup\$ Because it's hard to exactly determine what your definition of a sunny day is you could go out on a day that has shooting conditions similar to what you desire, meter at 1/50 and f/1.8 in manual mode and see how many stops you're overexposing. Now you have to get a ND filter that roughly darkens it by the amount of stops you were overexposing. \$\endgroup\$
    – Saaru Lindestøkke
    Jul 10, 2014 at 17:00

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Consider the Sunny 16 Rule. i.e. on a sunny day with Aperture of f/16 and ISO of 100, you need to set shutter speed of 1/100 s to get proper exposure. So in your case, compared to the Sunny 16 exposure settings, your exposure is:

  • 1/50 s shutter speed which is 1 stop more exposure compared to 1/100 s.
  • f/1.8 which is approximately 6 1/2 stops more exposure from f/16 (f/2, 2.8, 4, 5.6, 8, 11, 16) (more precisely, 2 * log2(16/1.8) ≈ 6.3.)

So your shutter speed and aperture have you overexposed by 7 1/2 stop compared to the Sunny 16 Rule settings. You need at least a 7 1/2 stop ND filer to compensate (or more, since you can compensate with ISO).

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  • \$\begingroup\$ Remember that filter naming convention is differernt from F-Stops. For example ND2,4,8 are 1,2,3 f-stops only. You need something that has appx 8 1/2 f-Stops... Not sure what number that is. \$\endgroup\$
    – Viv
    Jul 10, 2014 at 19:25
  • \$\begingroup\$ @Vivek on that scale, 8 stops is 256, and 8.5 is approximately 362. But not all ND filters are sold that way. \$\endgroup\$
    – mattdm
    Jul 11, 2014 at 10:31
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Take still images with the correct exposure and look at the settings. Then count the stops down to the 1/50s F1.8. It is simplest if you can achieve the correct exposure at F1.8 with e.g. 1/4000s. Then you just divide the 4000 by 2 every stop until you hit 50. Then you also see how the bokeh will look. It it is still overexposed stop aperture down the amount of stops you need to expose right.

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It kind of depends on what the lighting conditions are exactly. The usual haunts (incl. eBay) sell cost-effective filter sets with several ND filters (for example 2, 4, 8 stops).

Might be worth buying one of those with a diameter matching the kit lens, and an adapter to fit the (larger) filters on your 50mm f/1.8 lens. Then you have several to try out before potentially getting higher quality filters.

(If ND-8 is not enough, you can stack the other ones atop, too.)

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  • \$\begingroup\$ Thanks for the answer, Cornelius, but I live in Brazil and filters here unfortunately tend to be a bit more expensive. So I can't buy several (at least not at first; I'm a neophyte). Of course it depends on the exact conditions, but I was hoping for at least a guess for a specific stop number I could get that would not overexpose the image on your average outdoor sunny day. I've read somewhere that 3 or 4 stops would be a safe bet, but I wanted to get other opinions. \$\endgroup\$
    – Luis Calil
    Jul 10, 2014 at 16:42
  • \$\begingroup\$ htp://www.ebay.co.uk/itm/321252756264 – 14gbp for an all in one filter set plus 6 pounds shipping. I'm not sure how expensive ~75 real are, though. :( I unfortunately don't have enough experience for a reliable reccommendation which single filter to get, so I would rather wait for others to chime in. \$\endgroup\$
    – Cornelius
    Jul 10, 2014 at 16:58
  • \$\begingroup\$ That's fairly cheap, but importing stuff to Brazil isn't super reliable, it can get stuck in customs, etc. But I actually might take the risk, it's a good deal. Thanks again! \$\endgroup\$
    – Luis Calil
    Jul 10, 2014 at 17:04
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As others have said you'll want about an 8 stop ND filter.

One thing to remember with stronger NDs though - they tend to have rather stronger colour casts than weaker filters. Test your filters heavily beforehand, in different environments, so you don't end up with footage that you can't colour correct to where you want.

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