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Neutral Density Filters - the math

E

Empty_Quarter

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Dec 23, 2007
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Frak, wrong sub-forum, mods please move to photography sub

I'm looking to try out some extended long exposures without doing the whole multiple exposure thing. I do have a few ND filters and I love using them to extend time and darkening highlights. What I am looking to try however are exposures longer than the standard 30sec exposure set by the camera, this means I'll have to calculate the exposure time and control that via bulb.

Let's say I use an ND8 filter. Which has 3 stops of light reduction or 1/8. Does this mean my exposure time will have to be 8x longer?

So, let's say without a filter, I have an exposure time of 5 seconds. With an ND8 Filter, that time has now increased to 40 (5*8)?

Am I getting this right, it's that simple, right? So following that logic, an ND100 filter should yeild 100x longer exposures.



I *could* do this via trial and error.... but really, I'm just looking to save time. It's cold outside.
 
ND1 = 100% of light. 5 seconds. This would be a bare lens and your starting exposure.
ND2 = 50% of light. 10 second exposure.
ND4 = 25% of light 20 second exposure
ND8 = 12.5% of light 40 second exposure. Another way to look at it is 5/.125 = 40

To confirm: 40/5 = 8 times. So the ND8 would take an exposure 8 time the base.

ND100 = 1/100th or 1% of light. So 5/.01=500 seconds.

To confirm: 500/5 = 100. So the ND100 would take 100 times the base exposure.

As an aside, I'd recommend against the variable NDs, for photo taking. You never mentioned them, but they normally get brought up in the conversation as a flexible alternative. They get an X pattern showing up in the photo when you crank them down to block more light.
 
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Thank you for the response & confirmation! fantastic! That should save me quite a bit of time of figuring it out while I'm out there freezing my nuts off.

I don't have a variable ND, but did consider it. It wasn't until I saw the results in the pics thread that I backed off from it (were those your pics?). I just have an ND400 (that is 400x I assume, damn, that's "too" long for what I want), and an ND8/16. I use a polarizer if I want to get an effect similar to an ND2/4.
 
If your subject matter allows it you can always stop the lens down a bit to stretch out the time. I know that you know that though...

ND400 = 0.0025% of the light. The 5 second base would turn into 2000 seconds. Yes on the 400x. It is a bit between a 8 and 9 stop light reduction.

I don't own the variable ND. I have the 10stop B+W ND filter. Last time I used that was with the Venus transit and the Solar Eclipse.
 
^I think you're refering to my craptastic example. It's a Singh-Ray filter, btw. They do make some pretty awesome stuff, but I guess the 10-stop isn't their strongest suit. I'm in the market for a new filter, but at the same time, I don't really have the time to do the things I want to do with it, so I'll hold off on the purchase.
 
bingo, got it to work.


Urban by Empty Quarter, on Flickr

It didnt exactly turn out the way I had hoped as the clouds werent "varied" enough, and it wasn't windy enough to blow the city steam far enough to create long trails. I'll try this again when it's even colder.

BUT.... the math worked just fine. f/8 @ 8 seconds, and with an ND16 filter, it was extended to 128seconds.
 
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following up on this: If I were to stack filters, for example, two ND8s (hypothetically speaking, I know I could use an ND16).

Would 2 ND8s = 1 ND16, or 1 ND64?
 
I'm not sure if you're using the screw-on filters or the Cokin type, but if it's the former, look out for vignetting. Hypothetically, you could pick up three ND filters with 1, 2, and 3 stops each, and have a nice range on them all the way up to 6 stops by stacking them all together; but by doing so, you introduce a very visible vignette (this obliviously will depend on the focal length used).
 
Screw on, only cokin types I have are GNDs. Vignetting is indeed a problem, even with just 1 filter, especially since I'm almost always on an ultra-wide. My only solution is to simply shoot, and crop later.
 
Not to be persistant about this, but I'm confused.

If I were to stack two ND8 filters. Doesn't that mean that 1/8th of the light passing through the first one gets cut down by another 1/8th? So 1/8th of 1/8 = 1/64 total, no?

I dont see how stacking two ND8 filters will total 1/16 of light reduction o_O
 
Your feelings on the math not working out to 1/16 is right.

An ND8 is 3 stops of light reduction. So you'll get 6 stops when stacked. A stop is doubling of the time.

5 second example
10 seconds
20 seconds
40 seconds - 3 stops
80 seconds
160 seconds
320 seconds - 6 stops - 320/5 = 64 times the base exposure - And also confirms your 5/320 = 1/64 the light getting through.


Edit (Corrected the following to be more clear): You can confirm all this by pointing your camera at a wall. Mount a prime lens on there, Stop down 6 stops from the widest aperture, take a photo. Then mount the ND filters on there and take a photo at the widest aperture. Lastly compare.
 
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Excellent! Thanks, I knew I wasn't going mad :p

Imagine the possibilities! An ND8 or ND16 stacked onto an ND400/1000!
 
^Or you can just get a pinhole with a ridiculous f stop, and not worry about the ND filters at all ;) A friend of mine has one, with an f/231 (or something ridiculous like that). It's pretty... nifty :D
 
For these night landscapes where we are focusing on the exhaust trails, I'm presuming that you want full depth of field, so stopping down to at least F/11 would be good. I know with my 16-85, F/11 is sharpest it is going to get before diffraction starts kicking in. You could really drag the time out by going to F/16 and F/22+ and you wouldn't suffer from vignetting and the effects of adding extra glass.

The extra glass might cause some odd reflections. I know when I take long exposures of house lit with Christmas lights, I'll get ghosting of the bright light sources from the corners down to the opposite corner (reflecting off the sensor to the back element, then back to the sensor). I can't imagine what tossing filters on the front would do to the quality.
 
Pinholes are actually the next thing I'm willing to try :p

I am indeed trying to keep the aperture fixed at f/8, or at least a minimum of f/11. Would much rather keep that fixed at f/8, and deal with the longer shutter speeds as I'm using a tripod regardless. I am also not a fan of the "stars" the lenses create when using such a small aperture (some love it, but I find it distracting - sometimes... they can look good though).

For night shots, 1 filter is plenty (ND16 slapped on was more than enough time). However for day shots, I've found that my ND400 doesn't really drag on as much as I would have liked. On a bright sunny day, it would be hard for me to drag on the time for longer than a couple of seconds. Looking back in retrospect, I should have gotten the B+W 110 (which is in my cart right now), but even then, on a bright day, I'll probably get ~10-15 seconds, which isnt enough to smoothen water. Before I do that, I'm going to try out an ND8 over an ND400 which *should* in theory give me at least 30 seconds in the day without altering the aperture. But... It looks like I'll get the 110 regardless :p, because ND filters are simply magic :D

I have experienced the reflections before with multiple filters, and they can get bad without a doubt, but do you think I'll have this issue during the day?
 
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Regarding pinholes, they aren't the best at night, unless you are willing to stand out there for hours, or use a much higher ISO.

Just looked at one of my old night pictures in Saint Paul, MN of the skyline under the high bridge. Was taken at 38 seconds at F/8. Doing the math out that is anywhere from 4.5 to 5.5 hours with my pinhole F/135 camera. That doesn't even include the film reciprocity failure, so essentially when that figures in, I should be leaving it out there all night.
 
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