Are you stopping? ND filter strengths in photography.

In a post a few days ago, I mentioned the idea of Stops in the context of the Aperture triangle. When you use neutral density (ND) filters, the idea of Stops becomes a central issue. You use filters to reduce light entering the sensor in situations when using the shutter speed setting alone doesn’t fit the bill. ND filters are available in a range of strengths – these can be called by different names, but the easiest one to remember is the number of Stops of light which the filter offers. You have to choose, by looking at the situation, how strong a filter you want to use. If you are photographing a waterfall in bright light, perhaps 3 stops will do. If you are photographing waves then probably 10 stops would be your minimum – and you may want to stack a 10 stop plus a 5 stop. You are the judge. What you then have to do is to take the shutter speed that your camera recommends without a filter, and look it up in a table – such as that below, or by using a phone app – to find out what shutter speed you will need when you have a filter installed.

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So, if you’ve taken a test shot without a filter and the preview tells you that your exposure is OK, have a look at the shutter speed that you used, say 1/125 seconds. Now, let’s say you want to use a 6 stop filter, the table tells you that, with the filter installed, your shutter speed will need to be 1/2 seconds. At this point, you need to lock your focus, remember your settings and switch to Manual mode. Enter those settings – ISO, Aperture – and your new shutter speed (the 1/2 seconds one in this case), pop in your filter and you’re almost ready to go (I’ll say more about that tomorrow.

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I said above that filters are known by different names by different manufacturers – for example as a 6 stop filter, a 1.8 optical depth filter or as an ND 64 filter. The table above provides the equivalent types of naming for each strength. The table also shows the equivalent fraction of light that is admitted. So, a 6 stop filter admits only 1/64 of the light that would have been admitted without a filter. Think back to the first table. Without a filter you would have opened the shutter for 1/125 of a second. With the 6 stop, ND64 filter you will open the shutter for 1/2 second – ie 60/125 or roughly 64 times longer.

The table examples above are just to illustrate the principle. There are excellent phone apps, such as ND expert, where you just enter the base shutter time and the filter type and which also provide a timer.

Today’s featured image is of the lighthouse at Penmon Point, Anglesey, North Wales with Puffin Island to the right of the image.

This photograph was taken only slightly later with a 10 stop filter plus a 2 stop polariser. The light had also changed. The base shutter speed was 1/200 seconds, the filtered time was 10 seconds ie 2000/200 equalling a factor of 1000 (roughly 1024)

Tomorrow, I’ll provide a detailed checklist of everything you need to do to prepare for your long exposure outing.

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