Long Exposures Explained

In this article I would like to talk about everything you need to know to take long and very long exposure shots. I will list all the required and recommended equipment, point out common mistakes, show differences between various exposure times and give some helpful tips.

Let’s start off with what you really need.

Required equipment

1. Tripod
2. Cable release

To be able to capture a long exposure shot you definitely need a very stable support for your camera. It’s vital, and every even the slightest movement will ruin your photo so it’s very important to have a solid and good quality tripod. In a windy weather you might want to load the tripod with additional weight like backpack.

Cable release is another required tool which allows you to expand the exposure time beyond the 30 seconds limit. You can say that 30s exposure is long enough, and you will be right, but for me the real fun with long exposures starts way after that time. But if you don’t have a cable release at the moment, you can experiment with shorter times too.

Recommended equipment

1. ND filter/filters
2. Viewfinder cover

ND filter (ND stands for Neutral Density) is a filter that reduces the amount of light coming to the camera sensor giving no changes in hue of colour rendition. This is theory and unfortunately in reality it’s not really true, because every filter introduces more or less colour cast to your photos. It depends on the length of exposure is and directly from the quality of the filter. Even the most expensive ND filters leave colour cast on the images. For the cheaper filters the colour cast might be very difficult to correct in post-process though. So again, the quality matters. If you don’t have any ND filters right now, remember that increasing the f-stop and lowering the ISO will give you the longest exposure time.

A viewfinder cover is a very useful tool used to basically cover the viewfinder in order to block any light that might reach the sensor especially when the sun is behind the camera. This little cover can prevent uneven exposure and unwanted artefacts.

Common mistakes

1. Autofocus
2. Vibration reduction
3. Camera mode

The first thing I do before I start the long exposure is to make sure I have turned off the autofocus. Long exposures are taken in dark conditions or with a dark filter in front of the lens so in most situations the autofocus sensor is not able to detect the focus properly. When you try to focus just before taking the photo, focusing system will jump in and will not be able to focus correctly. As a result you will end up with a picture which is out of focus.

Vibration reduction or image stabilisation of your lenses should be turned off anyway for the tripod use. The low amount of light coming to sensor during long exposure will only make things worse, so keep it in mind to check that setting too.

Another mistake is the use of automated camera mode like aperture priority. In this mode camera will try to calculate exposure time by itself, but again, the amount of light is so low that in many cases this calculation will be wrong, especially for longer exposures. For long exposures the right choice is only manual mode.

Examples

The following examples will show you the same scene taken with different settings. The first photo is a regular exposure taken without any filters:

The exposure time for this image is 1/4s, ISO 100 and aperture f/16.
Now what happens if we try to make the exposure a bit longer:

This image was taken with the exposure time of 15 seconds (the same ISO and f-stop as the first one). To achieve that I used ND 1.8 filter which basically reduces the light by 6 stops (1/4s, 1/2s, 1s, 2s, 4s, 8s, 15s – 6 stops).

Now let’s try a really long exposure. For that I used ND 3.0 which is 10 stop filter. The following picture was taken with the exposure of 250s, ISO 100 and f/9:

Now, the time used here created this nice effect, the clouds and the sea are really blurred.

You probably noticed that the aperture for the last shot was changed from f/16 to f/9. It’s because I used a very strong ND filter (10 stops) and I had to compensate with the f-stop so that I wouldn’t end up with the exposure of over 1 hour. Why? Let’s see how to calculate exposure times.

How to calculate exposure times with ND filter

Calculating the correct exposure time is a mathematical task. It’s the balance of light that comes to the sensor. The less light goes in, the more time is required to properly expose the image. To be able to calculate the exposure time we need to know how much the shutter time changes for every stop of light. The strength of ND filters is measured in stops, so having a 2 stop ND filter will take 2 stops from our shutter speed.

Now, step by step:

1. Put your camera to Aperture priority mode.
2. Set your ISO and aperture.
3. Read the shutter speed that camera calculated.

Let’s say it’s 1/250s. Now we attach the 6 stop ND filter, which means that we need to prolong the shutter speed by 6 stops. To do that let’s use the standardised list of shutter times:

1/8000 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 2 4 8 15 30 1m 2m 4m etc.

Take the time that camera calculated (1/250) and move 6 places to the right – 1/4s. That’s the correct shutter time with ND filter attached to the camera and will give the same “brightness” as a picture taken without a filter with 1/250s time.

There’s an app for that!

If you have a smartphone you can download apps for the exposure time calculations – Longtime Exposure Calculator for iOS and Exposure Calculator for Android.

If it doesn’t work

Now, again this is theory, and in reality this not necessarily might be true. First of all, the quality of ND filters may vary and the 6 stop filter might stop the light a little more or a little less. Another issue is changing light – during an exposure of a few minutes it is possible that the light changes, especially when you take pictures during the sunset or sunrise hours. By the time you finish your eight-minute exposure the sun might not be in the sky anymore and the calculations are no longer right. How to solve that?

Experiment

Use the calculations at the beginning to have a general idea of the correct exposure times, but try to play with them. Keep the exposure a bit longer. If the picture is too bright, try again for a shorter amount of time. After some time you will be able to predict more or less the correct shutter time without any tables and calculations.

The advantage is that with long exposures it’s harder to make a mistake in calculation of shutter speed. If your correct exposure time is 8 minutes and you expose for 7 minutes the picture will be all right. If you expose for 9 minutes – again, the difference will be very small. It always takes a double amount of time to change the exposure by 1 stop. So to make the picture 1 stop brighter you would need to expose it for 16 minutes. That’s quite a difference.

Hope this article will help you with your long exposures. If so, let me know and send a link to your work!