ND filters and shutter speeds ⋆ Mike Muizebelt Photography

ND Filters and shutter speeds

ND filters and how to get the most out of your long exposures

The advantages of using ND filters (Neutral Density) to reduce your depth of field are of course well known. On bright, sunny days, depending on your artistic intentions, ND filters are almost always necessary to maintain a beautiful bokeh.

But the other main reason to get started with ND filters is the ability to work with long exposures.
You use them if, for example, you want to blur the movement of the water, "blur" the clouds in the sky or make the moving cars and people in a scene invisible. This application is very suitable for capturing busy tourist locations. The use of ND filters can make a busy and chaotic scene look surreal in this way.

But how do you use these ND filters for long exposures? Unfortunately, there is no golden rule for this and you will have to make a new assessment every time. The right filter depends on the shutter speed you want to achieve, the lighting conditions at the time you take your photo and your camera settings such as aperture and ISO value.
One of the first considerations you have to make is the so-called diffraction limit of your lens. All lenses have a certain maximum aperture value at which the lens is at its sharpest. Setting the aperture beyond this point means that you no longer achieve maximum sharpness. For most lenses, this diffraction limit is around f9 to f11.
If you use your lens around this value, you are assured of the sharpest possible image.
In addition, you must set your camera to the basic ISO value of your camera. Some models have an extended ISO range where you can go up to ISO 80 (or even lower), for example. If you set an extended ISO value on your camera, you reduce the dynamic range of your camera. It is therefore better to shoot at your basic ISO value.

Now that we know what aperture and what ISO to use, let's focus on shutter speeds. First we will determine what shutter speed we need. This mainly depends on the artistic effect we want to use. As an example I will use an extreme situation in which we could find ourselves.
We're on a sunny beach and we want to take a picture where the waves break against the rocks in a blur. According to the “Sunny 16” rule, our base exposure would then be: f16 – ISO 100 – 1/125s. If we use a derivative of this because we want to be around our diffraction limit, we need at f11 – ISO 100 – 1/250s.
We thus have maximum sharpness (aperture), maximum dynamic range (ISO) and the least noise (ISO). But the 1/250s is nowhere near suitable for blurring the water. We then need to get out at least around 2 seconds to get this blur.
Which ND filter do we need to go from 1/250s to 2 seconds? So we're going to calculate how many stops (EVs or f-stops) there are between these two exposures.

F-stop difference	0	1	2	3	4	5	6	7	8	9	10
Exposure time	1/250s	1/125s	1/60s	1/30s	1/15s	1/8s	1/4s	1/2s	1s	2s	4s

As the table above shows, we need a 9 stop ND filter for 2 seconds of blur! Or, for an extra boost of this effect, we need a 10-stop ND filter at 4 seconds. So we need a relatively heavy ND filter for long exposures on a sunny beach.

Every filter manufacturer gives its filters different names to indicate its strength. It is therefore important that you know what each label means. For example, Sirui uses the ND32x marking on its filters. That means that when using an ND32x filter, your shutter speed will be 32x slower.
let's use our previous example in combination with the table above. To go from 1/250s to 4 seconds we need an ND1000 filter. Namely, from 125s to 4 seconds, 1000x is the original exposure of 1/250s.
However, a filter can also be labeled with its so-called optical density. (e.g. 0.6, 1.2, 2.7, etc.). In this case, simply divide this number by 0.3. The result of this division indicates how many f-stops of difference that filter gives. You can see this again in the table below.

optical density	0.3	0.6	0.9	1.2	1.5	1.8	2.1	2.4	2.7	3.0
exposure factors	2x	4x	8x	16x	32x	64x	125x	250x	500x	1000x
F-Stop Reduction	1	2	3	4	5	6	7	8	9	10

Now that we have unraveled the name of the ND filters, we can see which shutter speeds are possible. In the table below we see all the possibilities, based on you original, unfiltered settings

Unfiltered shutter speed (correctly exposed)	0.3 OD ND2x f stop 1	0.6 OD ND4x f stop 2	0.9 OD ND8x f stop 3	1.2 OD ND16x f stop 4	1.5 OD ND32x f stop 5	1.8 OD ND64x f stop 6	2.1 OD ND128x f stop 7	2.4 OD ND256x f stop 8	2.7 OD ND512x f stop 9	3.0 OD ND1000x f stop 10
1/8000s	1/4000s	1/2000s	1/1000s	1/500s	1/250s	1/125s	1/60s	1/30s	1/15s	1/8s
1/4000s	1/2000s	1/1000s	1/500s	1/250s	1/125s	1/60s	1/30s	1/15s	1/8s	1/4s
1/2000s	1/1000s	1/500s	1/250s	1/125s	1/60s	1/30s	1/15s	1/8s	1/4s	1/2s
1/1000s	1/500s	1/250s	1/125s	1/60s	1/30s	1/15s	1/8s	1/4s	1/2s	1s
1/500s	1/250s	1/125s	1/60s	1/30s	1/15s	1/8s	1/4s	1/2s	1s	2s
1/250s	1/125s	1/60s	1/30s	1/15s	1/8s	1/4s	1/2s	1s	2s	4s
1/125s	1/60s	1/30s	1/15s	1/8s	1/4s	1/2s	1s	2s	4s	8s
1/60s	1/30s	1/15s	1/8s	1/4s	1/2s	1s	2s	4s	8s	15s
1/30s	1/15s	1/8s	1/4s	1/2s	1s	2s	4s	8s	15s	30s
1/15s	1/8s	1/4s	1/2s	1s	2s	4s	8s	15s	30s	1 minute
1/8s	1/4s	1/2s	1s	2s	4s	8s	15s	30s	1 minute	2 mins
1/4s	1/2s	1s	2s	4s	8s	15s	30s	1 minute	2 mins	4 mins
1/2s	1s	2s	4s	8s	15s	30s	1 minute	2 mins	4 mins	8 mins
1s	2s	4s	8s	15s	30s	1 minute	2 mins	4 mins	8 mins	15 mins
2s	4s	8s	15s	20s	1 minute	2 mins	4 mins	8 mins	15 mins	30 mins
4s	8s	15s	30s	1 minute	2 mins	4 mins	8 mins	15 mins	30 mins	1 hour
8s	15s	30s	1 minute	2 mins	4 mins	8 mins	15 mins	30 mins	1 hour	2 hours

Of course you don't have to go out right away to buy an ND256X if you already have an ND32X and an ND8X, for example. Using these two filters together gives you the same effect as an ND256X filter. In other words, you can use them together and the effect will be cumulative. Your 5 f-stop ND32X filter plus your 3-stop ND8X filter, gives you 8 f-stops in total.
The other calculation method gives you 8 x 32 = ND256X. You would like the different manufacturers to sit down to agree on 1 standard…
Please note that you do not get vignetting when combining screw filters in particular. This can especially happen with extreme wide-angle lenses where the viewing angle is so large that the metal ring of your filter falls just within this angle. Ultra narrow filters can offer a solution here.

Buy ND Filters

Which filter is now the best to buy? As you may have already seen, there are so many possibilities and situations that it is impossible to give a good answer. Your base shutter speed, your desired result, and your own camera all vary. Some photographers choose to purchase a so-called variable ND filter. By turning this filter you change the power of this filter. Since I own it 100mm NiSi filter system I have no experience using this.
As a basic set I would start with a combination of two filters. An ND8X and an ND64X give you a nice spread of 3, 6 and combined 9 stops. You actually get 3 filters for the price of 2! The extra flexibility this gives you can come in handy.

TIP: If you are an android user you can download a handy app from the Google store: ND Filter Calc (DSLR), here is a handy timer! Otherwise you can do this ND Filters Shutter Speeds Table print it out and take it with you in your bag.

With these long exposures now within reach, the possibilities are only limited by your own creativity. There are so many different things you can do with this right now. And that is not limited to clouds, rivers, sea, light and mesen. Also consider painting with light (light-painting), this is a technique in which you make a drawing in your photo with light. You probably know how to draw letters with stars, the person fades away, but the letters remain visible. But you can also light up parts of your scene by coloring them with a strong flashlight…
Just go out and experiment!

I myself regularly give workshops in which we use filters and/or creative techniques with long shutter speeds. Have a look at my workshops to see if there's anything for you