How to use ND filters in Long exposure photography

Chateau Queras is placed like the old fortress it is, to guard the entrance to Queras, a beautiful area in the French. Quite a gem really, full of beautiful places and then this beautiful old castle. Photo by: Jacob Surland, www.caughtinpixels.com

Chateau Queyras in the French Alps looks fantastic lit up in the night. This is a long exposure of no less than 370 seconds.

Long Exposure Photography is fascinating. By changing the exposure time, you achieve dramatic changes in the final result. The two photos featured in this post, are shot within a 25 minutes of each other. The first one is a long exposure og 370 seconds, and the second is a normal exposure of 0,5 seconds. Even if the images are processed different, you can easily tell the difference, if you look at the clouds.

When I went on photo workshop in the French Alps, I also got the chance to shoot the beautiful Chateau Queyras. If you walk up the mountain across the castle, you get a fantastic view of the castle. Photo by: Jacob Surland, www.caughtinpixels.com

This is a normal exposure shot 25 minutes earlier than the first one. It is only 0,5 seconds, and is processed as an HDR image.

The first photo I shot 25 minutes later having attached a 2 stop ND filter on the camera. A neutral density filter, or ND filter, stops the light down, without changing the colors. A 2 stop ND filter will double the exposure time twice. This photo was the last photo I shot from that location, and it was getting pretty dark, and I wanted to be sure that the clouds were completely smooth.

The clouds were moving a lot, and sometimes I could see the top of the mountain just behind Chateau Queyras, and at other times it was hidden in the clouds. The exposure is 370 seconds or 6 minutes and 10 seconds. In long exposures like that, you can not predict what happens. I was lucky that clouds left the top of the mountain clear in most of those 6 minutes, giving me a hat on top of Chateau Queyras.

I shot it, at f/16 and ISO 100, using my 2 stop ND filter. Had I not used the ND filter, I would have had an exposure of 370 seconds diveded by 2 twice or just by 4, that is 370/4 = 107.5 seconds. A 107.5 seconds is still a long time, and might have yielded a similar result.

I find that to smooth everything out 100% and leaving no textures what so ever, does require more time, than a minute. I wanted to be on the safe side in this case and put on a 2 stop ND filter.

You can also use ND filters during more bright times of day. Had I used the 2 stop ND filter on the first image, I would have increased the exposure time from 0,5 seconds to 2 seconds, which wouldn’t have changed a whole lot. But if I had put on a 10 stop filter, I would have had a 512-second exposure, which would have yielded a similar smooth result.

To fine tune the necessary time, you can change ISO and f-stop values. To lower the exposure time from 512 seconds, to about 370 seconds, I could have lowered the f-stop to f/14 or f/13.

To be able to work flexible using ND filters you need a set of ND filters, to be able to match the desired exposure time. I have a 2, 3, 6 and 10 stop filters. Choosing the right filter, combined with the right ISO, Shutter speed and aperture (f-stop) is what makes your exposure, when working with ND-filters and long exposures.

How to calculate the exposure time

But how to calculate the exposure time? It does require some brain activity to calculate your ways around to get the right exposure time.

Depending on, what ND filter you have on, and how long you approximately plan on making you exposure, you can do a few things. If you plan on exposing for 60 seconds, you can increase ISO to the double value (like ISO 100 to 200). Then then you can adjust filters and f-stop to until, the camera measures a 30-second exposure. Adjust the ISO back to the original value.

Another approach is to do test shots without the filter and when you have the right exposure time, attach the filter and recalculate exposure time.

You can also use an App to help you calculate and time your exposure. While this might seem perfectly easy, I always end up finding it too cumbersome and end up doing the math myself and use the timer in my iPhone.

One thing you have to be aware of the light is changing fast, is that your will have to compensate for that. If it grows brighter, you have to take that into account, shortening the exposure time. And the opposite if the light is fading, you will have to increase the exposure time with a stop or two, depending on how fast the light fades.

The shooting

The obvious thing is; you need a tripod. You can’t do long exposures handheld.

You have to be in manual mode, and switch of auto ISO to do long exposures. You need to be in control of all three parameters, ISO, Aperture and shutter time. Most DSLR or DSLR-like cameras (including the mirrorless cameras) has a maximum of a 30-second exposure. Bulb mode is required to make exposures longer than 30 seconds. Usually, this is something, which you dial to, using the same dial as you use for changing the exposure time.

You also need a remote release. You really want to be avoiding to touch the camera, while shooting. I have a simple cable release. I bought an advanced electrical one, but it died on me, in fact right before shooting this image, and I had to borrow another cable release.

You also want to switch on long exposure noise reduction. If you don’t you get strange hot pixel noise. The sensor heats up while exposing for such a long time, and the long exposure noise reduction, will help reducing the result from that. The downside is that it takes just as long to do, as your exposure took. A 6-minute exposure will also give a 6-minute noise reduction. That is a hard one to swallow.

And that is the basics of long exposure photography.

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