My Approach to Long Exposure

Long exposures are achieved by reducing the amount of light hitting the camera’s sensor, the less light the longer the shutter can remain open without over-exposing your image.  There are several ways of controlling this – aperture, shutter speed, ISO, filters, time of day or a combination of any or all of these.

Aperture – measured in stops and referred to as f numbers.  The smaller the aperture the less light hits the sensor, the larger the aperture the more light hits the sensor.  It’s important to remember the lower the number the larger the aperture, the more light, so f4 will let more light in than f16 resulting in a faster shutter speed/exposure.

Shutter speed – the longer the shutter is open the more light will hit the sensor.  In the correct conditions you can have very long shutter speeds.  I have some of up to 20mins (taken at night by moonlight) and I have seen images with much longer exposures.

Filters – there are many different types of filters, coloured, neutral density (ND’s), grads, soft focus, polarisers etc..  The main ones I use are ND’s, polariser and my favourite a 10 stop filter.  ND filters come in different strengths – the most frequently used are .3 = 1 stop, .6 = 2 stops, .9 = 3 stops.  I should explain that I haven’t referred to my 10 stop as an ND because it isn’t.  The 10 stop will leave a slight colour cast on your images which can easily be corrected with a little post production. This is because it is designed for industrial and scientific photography of extremely bright subjects; the glass is so dark you can not see through it.  This is also the reason why you need to ‘set-up’ your shot before you put the filter on your lens, explanation below under Daytime Shooting.

Time of Day – early morning (pre-dawn through sunrise), late evening (sunset through dusk) or night all have low levels of light and will allow for long exposures.

ISO – this controls how sensitive the camera sensor is to light, the higher the ISO number the more sensitive the sensor is to light.  The ISO speed works together with the shutter speed and aperture to give you the correct exposure for your image.

It’s worth remembering that shooting with a high ISO can introduce noise to your image.  A basic rule of thumb is that the smaller your sensor the greater the noise for the same number of megapixels, e.g. 8 megapixels on an APS (half frame) sensor will show more noise shot at ISO 800 than 8 megapixels on a full frame sensor image shot at ISO 800.  This is because to get the same number of pixels onto a smaller sensor the individual pixels are smaller and receive less light creating more image noise.

Once you have chosen your scene and effect your after you’ll need to decide how your going to achieve your desired result.

Daytime shooting with a 10 stop filter - to get a long exposure in daylight you will definitely need to use filters to reduce the light getting to the sensor.  I frequently use my 10 stop.  During bright sunshine this can give exposure times of 20 seconds or longer shooting at ISO 100, F10.  Before you attach the 10 stop to your lens compose your shot, focus, set your aperture and make a note of the exposure time.

If your camera isn’t already in Manual mode you will need to set it to manual, this includes switching off auto focus on your lens.  If your exposure time is going to be greater than 30 seconds remember to switch to the Bulb setting on your camera.  Check your composition again and ensure all the settings are correct.  When your happy that you have the correct settings you an attach your 10 stop, taking extra care not to move the lens or camera.  Open the shutter for the require time.  Check the result not forgetting to look at the exposure graph on the cameras LCD.  Fine tune your settings and re-shoot if required.

Calculate exposure required for use with a 10 stop filter
Using the table below (reproduced from Wikipedia)

1- Select f number – e.g. f11
2 – Check exposure time either with a light meter or on your camera
3 – Under the f number column locate the exposure time – e.g. 1s
4 – Check the EV number in the far left column – e.g. 7
5 – Subtract 10 from the EV number (because this is for a 10 stop filter, change number to subtract as required) e.g. 7 – 10 = -3
6 – This will give you your new EV number e.g. -3
7 – Locate your new EV number and move across to the required f number (f11 for this example) to find your exposure time e.g. 16m

If you want a quick guide you can use the timings shown below

• 1/500 – 2s
• 1/250 – 4s
• 1/125 – 8s
• 1/60 – 16s
• 1/30 – 32s
• 1/15 – 64s
• 1/8 – 2m
• 1/4 – 4m
• 1/2 – 8m
• 1s – 16m

For long exposures I would also recommend using a cable release or similar and don’t forget your tripod.

I hope you find this article helpful.