Help me calculate shutter speed with ND filters

[mjhoward]

And a CPL is 2 stops.

Most decent CPL's are 1.3 stops.

If you're using a 10 stop neutral density filter, you'll have to increase your exposure by

2^10 = 1024

That basically means that your lens will be gathering 1024 times more light with the ND filter than without.

No it doesn't. The lens will always be gathering less light with an ND filter in front of it. If you've adjusted the exposure to compensate for the filter, then the sensor will receive roughly the same amount of light.

 

[dlmvegas]

I realize it's been awhile since this thread was updated but here is a link that should help everyone in using a 10 stop ND filter. Scroll down a little to view the table they have for the 10 stop or 6 stop filters they sell. No more math.

Big Stopper and Little Stopper long exposure filters from LEE Filters.

 

[TCampbell]

Keep in mind that CPLs are variable -- so there's no one-size-fits-all answer for them. I have metered, put the filter on, tuned it, and metered again, and have had CPLs take as few as 2/3rds of 1 stop. But 2 stops is also common.

This is because a CPL doesn't just cut light equally... it cuts light based on the polarity of the light waves hitting the came as compared to the tuned angle of the polarizer. If you use an ultra-wide angle lens, put a CPL on it (which I would not normally recommend that anyone do when using an ultra-wide angle lens), shoot a photo of a blue sky and look at the result, you'll see very distinct brightness banding with a dark zone where a lot of light was cut, but this fades to a weak zone where only a little light was cut.

When I use my CPLs, I don't necessarily tune them to the highest amount of light cut I can get. I usually want subjects to offer some reflection (I don't want it to cut everything), but I don't want obnoxious reflections... so I'll tune up the CPL, then back it off a touch to offer a pleasant hint of a reflection. The light cut is all all in how you're using it.

A 10 stop ND, however is usually plenty and I would not recommend stacking a CPL on a 10 stop ND. Metering and focus have to be done prior to putting the 10-stop filter on the lens... at which point you want to *know* how much light you've just cut and not be guessing at it. When you add the CPL to the mix, you're now at the point where you're probably going to have to take several shots and guess at the exposures. I also carry a 2 stop and 3 stop ND. So if 10 stops is ever not enough... I can add more.

Suppose you're outside on a bright sunny day in the middle of the afternoon and thus get to assume that it's a "sunny 16" exposure (although you'll get better shots near sunrise and sunset during the "golden hours") ... since the point of the 10 stop ND is usually to slow the shutter speed, you're probably already using settings that slow the shutter speed as much as possible even without the ND. So... ISO 100 and f/16 probably are good choices. Now your shutter speed is 1/100th, but I'm going to round up to 1/125th because that's a "whole" stop (1/100th is 1/3rd stop down from 1/125th). As you add the 10 stop ND, you have to increase the shutter exposure by 10 stops.

1/125th -> 1/60th (1 stop)
1/60th -> 1/30th (2 stops)
1/30th -> 1/15th (3 stops)
1/15th -> 1/8th (4 stops)
1/8th -> 1/4th (5 stops)
1/4th -> 1/2nd (6 stops)
1/2nd -> 1s (7 stops)
1s -> 2s (8 stops)
2s -> 4s (9 stops)
4s -> 8s (10 stops)

Now... I did round the 1/100th sec exposure time (which is not on a full-stop boundary) to 1/125th (because that is on a a full stop boundary). But in reality 1/100th is 1/3rd stop slower... so if you want to go 1/3rd stop slower from 8 seconds you end up at 10 seconds -- which is 10 full stops slower from 1/100th.

Something else to keep in mind is that most digital cameras are set to 1/3rd stop increments each time your shutter speed wheel moves by 1 "click". That means 3 clicks of the dial is 1 full stop reduction. And THAT... means that you could just turn the shutter speed dial 30 "clicks" and that would be 10 stops.

But before you assume the 30 "clicks" is good, do make sure... set the camera at say, 1 full sec shutter speed and rotate the dial to 1/2 second exposure and make sure you had to turn it three "clicks" to arrive there. Many cameras have a setting... I can set my cameras to 1/2 stop increments if I want via the customization settings (in which case it would be 20 clicks instead of 30 clicks to change the exposure by 10 full stops.)

 

[garrywillson]

Great information... As per my experience the easy way is to multiply your shutter speed times, using a calculator to get the shutter speed with the filter on. i.e. 2 multiplied by 2, 13 times (or 2^13)

Regards,

 

[pgriz]

I use this table:



Left is the time value without the filter, right is the value with the filter.