Long Exposures


The Milky Way Long Exposure

Long exposures are normally used to capture changing conditions or movement in a scene, such as  Star Trails, Moving water, Sunsets & Sunrises. This tutorial will primarily discuss long exposures beyond the automatic capabilities of most cameras, or 30 seconds plus.

Required Equipment:

  • Sturdy Tripod.
  • Good weather, No wind or changing light conditions.
  • Camera with ability to use a “Bulb” setting.
  • External Shutter release cable, Electronic or Manual.
  • Chemical Heat Pack for long exposures in cold temperatures.
  • A method to calculate exposure times, download the Exposure Computer
  • ND filters (if required).
  • Extra Batteries (Battery Grip) for really long exposures.
  • A torch to check manual lens settings in low light.

During this tutorial I will only refer to “full” f-stops as it’s easier to begin with.

Before attempting a long exposure the first thing to understand is the relationship between ISO, Aperture and Shutter speeds.

In basic terms:

ISO:
ISO in basic terms is the sensitivity of the sensor or film.

Aperture:
Aperture is the amount of light the lens is allowing through to the sensor / film. This is normally referred to as an f-stop. for example: f2 will allow twice as much light in as f2.8 and f2.8 will allow twice as much light as f4.

The complete f-stop sequence halving the light as it increases is: f2, f2.8, f4, f5.6, f8, f11, f16, f22. Thus f2 lets in 4 times as much light as f4. It is important to understand that some cameras have 1/3 or 1/2 f-stops as well, but to keep this tutorial simple we will only use full f-stops as it makes the calculations easier.

Shutter Speed:
The Shutter Speed is how long the light will be allowed to exposure the sensor for, or how long the shutter on the camera is opened for.

The Relationship:
The most important thing to remember is that all 3 are related and if you reduce one you must increase another, Some basic examples are:

Assuming the same ISO:
A 2 second exposure at f-stop f22 is the same as a 1 second exposure at f16, by reducing the time the shutter is open I must allow more light in to have the same exposure.

Assuming the same ISO:
A 1/30sec exposure at f8 is the same as a 1/120sec at f4, by reducing the shutter speed twice (halving twice), I must increase the light by increasing the light twice (reducing f-stop), OR 1/4 the original time means 4 times the light allowed in for the same exposure.

Assuming f-stop is f8:
A 1 second exposure at ISO 100 is the same a 2 second exposure at ISO 50, by increasing the time the shutter is open I have to make the sensor less sensitive to the light for the same exposure.

More Complex:
A 1 second exposure at ISO200 / f8 is the same as a 1/2 second exposure at ISO100 / f4. This example is more complex as it involves changing all 3 settings. However working through it logically:
First we reduce the ISO to 100 giving us (2 seconds at ISO100/f8), we then change the f8 to f4 and decrease the exposure to 1/2sec because we are letting more light in, giving us the final figures.

It is really important to understand the concepts above and be very familiar with f-stops, ISO and shutter speeds. If you camera has Manual mode, then I recommend you use it and play around to understand the relationships more,

Because the exposures we are talking about are very long, waiting around for an hour to find your shot was incorrectly exposed will not make you happy. Remember the Rules: halve and double, and experiment, experiment & experiment.

Where to Start:

  • Setup camera with tripod, connect shutter release cable, required filters or hardware and a heat pack if required. Make sure the Camera is sturdy and won’t move.
  • Assuming your in a low light situation use a high ISO to get a exposure level on your camera and test. Check this shot for good exposure it will be your reference point. It will give you a good indication of what the final shot will look like. For example you may have to use ISO 3200 @ f2.8 for 2 seconds to get the correct exposure.
  • Adjust your camera to your desired ISO (preferably 100), calculate the number of steps between the start and end ISO, for example 3200->half->1600->half->800->half->400->half->200->half->100. 5 steps of halving in ISO.
  • Double the shutter speed by the number of reductions in ISO, example: 2->double->4->double->8->double->16->double->32->double->64, (5 steps of doubling) final time: 64 seconds.
  • You could now set your timer to the calculated shutter speed and shoot, or you can further adjust the f-stop (recommended)
  • So far in our example we have a shot consisting of: ISO100, 64sec @ f2.8, if you wish to increase the f-stop and allow less light to the sensor we will further need to increase the exposure time. example: target is f8, f2.8->decrease (light)->f4->dec->f5.6->dec->f8. We have reduced the light by 3 f-stops.
  • Increase the shutter speed by the same amount of f-stops (3): 64->inc->128->inc->256->inc->512 or 8.53mins (8 mins 30 seconds)
  • Check your calculations again, you wont be happy if your exposure is incorrect.
  • Set the shutter timer to new time and take the shot.

The Exposure Computer:

Download the Exposure Computer, print and cutout the circles, then pin the smaller circle on top of the larger one through the middle, allowing it to rotate. This device allows you to look at equivalent pairs of f-stops and shutter speeds for the same exposure. example: After you have adjusted for new ISO in above example align 64seconds and f2.8. All the other values (pairs) will also work, f2 & 32 seconds, f4 @ 2m8sec etc, giving the same exposure.

You can also use it to recalculate the new values based on the higher ISO, example to reduce ISO3200 to ISO100 is 5 halvings, set the start f-stop and shutter speed pair, then rotate the top circle 5 steps CCW. you can now use any of the pairs for exposure at ISO 100.

Some things to consider:

  • For really long exposures in cold temperatures wrap your lens in a heat pack, this will stop it fogging up, you must keep the lens temperature above the air temperature.
  • Some cameras have long exposure settings to reduce “hot pixels”, be aware of this as the camera will take time, normally just as long as exposure time to calculate this information. So a 1 hour exposure can turn into a 2 hour one if you have this enabled.
  • If you are using an SLR use the viewfinder cover on the strap to reduce stray light entering in via the viewfinder to sensor.
  • Make sure you have enough battery power for the length of the shot and any camera calculations.
  • Your camera probably won’t focus well in low light, use your lenses in Manual mode.
  • Know your lens to set focal point in poor light conditions, i.e. check manual focus settings during the day and note the settings / marks on the lens.

Some examples of long exposures:

Sydney Harbour

Sydney Harbour

10mins+ reduces waves.

Lake Wakatipu - Queenstown

Lake Wakatipu – Queenstown

20mins+ swirling stars

Gum Trees

Australian Gum Trees

30sec+

Misty water

Misty water

5min+ Water turns to mist.

Tasman bridge, Hobart, Tasmania

Tasman bridge, Hobart, Tasmania

30sec+

Share on Facebook