How to know which ND filter to get, or set to?

[fxsg123]

Hi, I try Google but unable to understand or get how it works.

If I want to take video, accord to the so call shutter rules, my shutter speed should be a double of my frame rate.

So assuming my Len is f1.8 fix,
30fps - shutter speed to set at 1/60
120fps - shutter speed to set at 1/120

So, I should get ND filter with how many stops.
If I get those variable one, which value should I set to?

Thks in advance.

 

[one eye jack]

You must first understand how to get correct exposure as if you are taking a still photo,if not read through the links below:

https://petapixel.com/2017/03/25/exposure-triangle-making-sense-aperture-shutter-speed-iso/
https://www.photographytalk.com/beg...-exposure-triangle-explained-in-plain-english

After knowing what is the exposure triangle's relationships of ISO, shutter speed and aperture we can compute the exposure for
video.Eg. F1.8 and 1/60 sec.

The key thing to understand about exposure values of ISO, shutter speed and aperture system is each value either is half or double the light of the the next value.
Eg. ISO 100, 200,400,800,1600,3200,6400 these numbers equivalent are intervals of full F stops or shutter speed.There are 1/2 and 1/3 stops for the 3 variables.
Similarly for F stop like 1.8, 2, 2.8 ,4, 5.6, 8, 16, 22 or shutter speed but in this case video shutter speed is fixed at 1/60. Actually your aperture need not be at F1.8 as depth of field depends on subject distance and background.

The actual procedure: Set camera to shutter or aperture priority and measure what the correct exposure at F1.8 or 1/60 sec.at chosen ISO and mostly likely
shutter speed is very high or aperture is small so now comes ND filter to the rescue because it cuts light output depending on the ND rating.So how much or how many F stops need to cut or reduce to get to target aperture or shutter speed? See the table below:

ND filters are measured in stops of light cut:

ND Number Optical Density Stops of Light Cut
No ND 0.0 0

ND2 0.3 1

ND4 0.6 2

ND8 0.9 3

ND16 1.2 4

ND32 1.5 5

ND64 1.8 6

ND128 2.1 7

ND256 2.4 8

ND512 2.7 9

ND1024 3.0 10

4 stops of light = 0.3 x 4 Optical Density = 1.2 Optical Density

To be continued....

Edit: sorry about the formatting, I can't change it.

 

[one eye jack]

We will use the sunny 16 rule as illustration..that is on a sunny day the aperture of F16 and the shutter speed used is the inverse of the ISO. Eg. ISO200...Shutter speed is 1/200. I edited the table used from https://www.photographytalk.com/beginner-photography-tips/7183-how-to-master-the-sunny-16-rule

As you can see if you want to get to F1.8 at ISO200 its about 6 1/3 stops of ND and if at ISO100 would add another stop. To be more precise from the shutter speed
to 1/60 sec. it will be another 1 2/3 stop. All together it's about 7 2/3 stops so you can try 7 -8 stops. This is where the variable ND filter come in..
you can buy one usually from 2 -8 stops or 1.5 - 9 stops. Prices vary for branded ones or get cheap ones from china. This assumes the lighting is static never change but real life situation is not.. even when you use auto exposure unless in studio controlled environment. Below is a reference to full stop listing
of aperture, ISO and shutter speed.


https://shuttermuse.com/f-stop-chart/

https://shuttermuse.com/photography-iso-chart/

https://shuttermuse.com/camera-shutter-speed-chart/

 

[thoongeng]

I'm not a regular video shooter, just sharing what I know, maybe the videographers can give more specific advice

You only need ND filter in bright situations where your aperture and shutter speed chosen lets in too much light even at the camera's lowest ISO. Using your settings as an example, set aperture to f1.8, shutter speed 1/60s. At lowest ISO eg 100, if still overexposed, just put on variable ND filter, turn the ring till exposure is right

 

[fxsg123]

Hi, thanks both for your response especially one eye jack for the detail technical explanation.

Over the past few days, I read up more on this topic and now have a slightly better understanding. Initially, I thought ND filter stops is a hard value, able to calculate based on given aperture and shutter speed. I was wrong.

For video, usually iso is set to lowest, shutter speed fix, that leave aperture to be tune. We then tune aperture to meet correct exposure, without filter on actual scene. The aperture will likely be very low. Then calculate the stops require to reach intend aperture.

 

[one eye jack]

Hi, thanks both for your response especially one eye jack for the detail technical explanation.

Over the past few days, I read up more on this topic and now have a slightly better understanding. Initially, I thought ND filter stops is a hard value, able to calculate based on given aperture and shutter speed. I was wrong.

For video, usually iso is set to lowest, shutter speed fix, that leave aperture to be tune. We then tune aperture to meet correct exposure, without filter on actual scene. The aperture will likely be very low. Then calculate the stops require to reach intend aperture.

I'm no expert but am glad you understand exposure now. You can actually vary all 3 variables, ISO,shutter speed,aperture and that depends on what you want effect you want to see for video.For this particular instance you want to shoot at big aperture to get subject isolation.The 180 degree rule is an old rule because
of old camera technology and the frame rate associated with it gives a natural look that the eye sees.However you can creatively use framerate
to "enhance" your movie and look like a pro. The links below I think is very useful to know. Especially about frame rate, yes shoot at camera's base frame rate
gives the highest quality but then to be creative... a bit off topic but...

Do you need to follow the 180 degree shutter rule?

Video Frame Rates: What You Need to Know