- Filter Factor: The factor used for exposure triangle calculations involving the ND filter
- Optical Transmission: The percentage of light allowed through the ND filter.
- F-Stop Reduction: How many f-stops or stops of exposure the ND filter reduces.
- Optical Density: A precise logarithmic figure used for ND filter calculators.
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Neutral density filters, commonly called ND filters, are a mainstay of modern digital photography. The numbers for ND filters can be very difficult to figure out, though. ND filter numbers don’t just confuse newcomers or beginner digital photographers, many advanced and professional photographers have a hard time with them
What Do ND Filters Do?
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In simplest terms, ND filters cut down on the amount of light that reaches the imaging sensor. Alternatively, you could say that ND filters attenuate light transmission, subtract exposure value, or you could even say they add darkness. As long as you know what they do, I really don’t think it matters too much how we word it.
In general, you add a ND filter when you want to place the exposure into a range where you can do certain things with it.
And that’s where the ND filter numbers come in. By understanding the numbers, you will know exactly what effect the filter will have on your exposure. With the ND filter numbers and an ND filter calculator, you can figure it all out before you take the image.
So, regardless of how you say to yourself what ND filters do, we can all be on the same page when discussing these tools or when making purchasing and use decisions.
Prepare Yourself For Math
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Thinking in mathematical terms isn’t new to many photographers. The Exposure Triangle, Depth of Field, Rule of Thirds, are all math concepts used for photography. Once you get used to them and see how they have a visible effect, the math gets easier and makes more sense.
For ND filters, the numbers used as labels, in descriptions, and for exposure calculations are filter factors, optical transmission, f-stop reduction, and optical density. At any given time, you may see all of these listed on a label or in a discussion.
You might see a filter listed with any or all of these. To put it into a little perspective, let’s look at an ND filter you might already be using. I’ll use my H&Y ND1000 filter that I use in the H&Y K-Series Filter Holder.
In Practical Use
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Using ND filters is a whole lot of fun, but those numbers for ND filters might intimidate you at first. So, let’s look at them.
The H&Y ND1000 has a filter factor of 1024, listed as ND1000 for easier labeling and discussion. The optical density of this ND filter is 3.0 and it transmits 0.098% of the light entering the optical path. Which all adds up to a f-stop reduction of 10 full stops.
Say we are shooting a scene with ISO 400 and the exposure we calculated is 1/2000th of a second at f/8. Yes, I’m pretty much using the Sunny 16 Rule for this example. Shooting RAW most likely, too, since we will be using our post processing program to fine tune the final image.
Now, with the addition of the ND1000 ND filter into the light path, we change 10 whole stops of exposure, which most of us could do in our head. You could also type in the filter factor or optical density into an online or smartphone app ND filter calculator.
You get a shutter speed of 1 full second at ISO 400 and f/8.0 in broad daylight. That is a pretty significant change! Adjust the ISO down to ISO 50 and stop the lens down to f/22 and you could have an exposure time of 64 seconds, a minute and change, in a Sunny 16 Rule scene.
In Golden Hour or Blue Hour photography or on an overcast day, you could conceivably have exposure times of an hour or more. Think of what you could do with that!
Math Or No Math
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Whether you decide to get all into the math of the ND filter numbers or simply make use of the printed numbers for ND filters and type them into an ND filter calculator, at least you know what the numbers mean and that they're consistent, just like all of our math problems used in photography.
The end result will be great images and the joy of creating those images!