Trying to figure our the required ND

Started 2 months ago | Discussions thread
Off The Mark Veteran Member • Posts: 6,639
Re: Trying to figure our the required ND

Sean Nelson wrote:

JohnPhotoP wrote:

I'm currently using a Full Frame and wish to shoot on apertures between 1.8-4.0.

If I understand correctly, each stop of light of an ND filter equals to 1/f of the aperture?

For example If i was filming in F/14, in order to not change shutter/ISO settings, and to film in f/4, I have to get a 10 stops ND filter, correct?

I hearken from the days of film, so I figure aperture using f/stops engraved on classic lenses:

f / 1.0 - 1.4 - 2.0 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22

Each number you move left or right is 1 stop which doubles or halves the light intensity and is compensated for by doubling or halving the shutter speed. There are about 3-1/2 stops between f/4 and f/14. Therefore you'd need a 3 or 4 stop ND filter to keep the shutter speed at f/4 close to the same as it would be at f/14.

Note that a 3-stop ND filter is sometimes called an 8X ND filter because it only allows 1/8 of the light through. Be careful when you buy and use ND filters to understand which nomenclature they're using.

Just to add on to what Sean has pointed out above...

There is the Sunny 16 Rule, which is just a guideline that to get proper exposure on a sunny day, you would shoot at ISO 100, 1/100th of a second, at f/16.

Now, you (the OP) mentioned you want to shoot at 1/50th of a second shutter speed, so you would have to close down the aperture one stop to f/22 (doubling the shutter speed doubles the light).

So to get from f/22 to f/4, you would need (theoretically) a five-stop ND filter.

To get down to f/2, you would need an additional two stops of ND, meaning, a seven-stop filter.

Sean mentioned that a three-stop ND filter is sometimes referred to as an 8X filter. They base this on 2 to the power of 3 (i.e., 2 X 2 X 2 = 8)

So a four stop filter would be 2 to the power of 4 (i.e., 2 x 2 x 2 x 2 =16), so it is a 16X filter.

A five-stop filter would be 2 to the power of five (2 x 2 x 2 x 2 x 2 = 32), so a 32X filter

One thing to keep in mind is that for outdoor shooting, you MIGHT end up using a picture profile that has a minimum ISO that is ABOVE ISO 100, depending on your camera.

Many cameras have a LOG picture profile (which has increased dynamic range over standard picture profiles) with a minimum ISO of 640 or 800. So you might even need MORE ND to open up to faster apertures like f/1.8 to f/4.

One other thing that I can mention here is that shooting video wide open can be quite  tricky and cause various issues depending on the lens. On full frame, I almost always use f/5.6 or NARROWER for more depth of field.

I understand that ultra-shallow depth of field is a look. Some people like it. Some people will tell you that ultra shallow DOF is "cinematic." I really haven't seen any films where an ultra-shallow DOF was used predominantly throughout the whole move (Barry Lyndon being the exception, and that was more for getting the most light through the lens when shooting scenes lit by candlelight).

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