Today's episode is for Leticia in Portland, Oregon. She asked a great question: How do you decide which neutral density filter to use in any given situation? Honestly, the answer is pretty straightforward, but I get this question a lot. So she's definitely not alone. ND filters only do one thing, and that is to reduce the amount of light entering your camera. So if you're reaching for one, it means you've got too much light and you're trying to reduce it.
Usually to allow for a slower shutter speed, although there are those who are using it so that they can open their aperture wider in open sunlight. So how do you choose which ND you need? Well, here's an example. Say you're photographing a waterfall in the middle of the day. The sun's out, no clouds, no shade. In that situation, almost guaranteed, your base exposure is going to be f/16, ISO 100, and 1/125th of a second.
If you didn't catch that, I'll say it again and you need to write it down. Pause now and get a piece of paper. Your base exposure is going to be f/16, ISO 100, and 1/125th of a second. That's one over 125. But to make the water look silky, you need a much longer shutter speed, somewhere around two seconds. So how do you get from 1/125th to two seconds? First of all, you count the stops.
A stop is an increment of light. And I got into that in the previous episode. So if you want to better understand stops, give that a listen. All right, so here we go: 1/125th goes to 1/60th, 1/60th goes to 1/30th, 1/30th goes to 1/15th, 1/15th goes to 1/8th, 1/8th goes to 1/4th, 1/4th goes to 1/2 second, 1/2 second goes to 1 second, and 1 second goes to 2 seconds. If you're counting along, that's eight stops of light.
So how do we reduce the light by eight stops? Since you're already at ISO 100, you can't go any lower. Maybe you can squeeze one more stop by going from f/16 to f/22, but that still leaves you at seven stops. And you can get that from an ND filter. Most ND filters come in fixed values—most commonly, five or ten stops. There are one- and two-stop filters out there, and you can stack them on top of your five-stop filter.
But stacking multiple filters isn't always ideal, and I don't know anyone who has all of them. On the other hand, if you have a variable ND, you can just dial in the seven stops you need. Easy. But let's say you only have a five-stop filter, making you two stops short. So now what? An option would be to shorten the shutter speed to a half second instead of two seconds. This gives you your two stops. It's not perfect, but you'll be close enough for most purposes.
Notice we're not touching ISO because, again, you're already at 100 and that's usually as low as you can go. And we are also at f/22, and that's usually the max for most lenses. Now, hang on—you could actually add a polarizing filter, which would give you one or two stops of light loss as well. Let's go to the other side. What if you have too much ND and you only have a 10-stop filter? That's three stops too many.
This is actually not a bad place to be. You have several options you can deploy. You can increase your time by three stops, going from two seconds to 16 seconds. If you didn't catch that: two goes to four, four goes to eight, eight goes to 16. That's three stops. Another option is to open your aperture three stops: f/22 goes to f/16, f/11, f/8. That's three stops. And personally, this would be my preference.
A third option is to make up the three stops via ISO, which would put you at 800 ISO. Or you can do a mix: maybe go to f/11, extend the shutter speed to four seconds, and move the ISO to 200. The decision on how you want to make up the missing stops of light depends on what you want in the results. There's not really a right answer to that.
So Leticia, the key is knowing where you're starting and where you want to end up. Count your stops, then match the difference with your gear—whether it's a single filter or a variable ND. That's it. Thanks again for the question, Letitia, and thank you for listening. For the rest of you out there, if you've got a question, feel free to send it in: [email protected]. And as always, keep on shooting.
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