Capturing the Night, an Astro-Landscape Photography Workshop at Acadia National Park
Ander Naturalist Photography Gallery
 

UNDERSTANDING EXPOSURE

I can not stress the point enough; proper exposure is essential for a fine photograph. If a photographer must modify an image's exposure in post processing, then a mistake was made when the camera went click, pure and simple. Post processing is a way to enhance an image, not to repair it, though you certainly can do so. At the moment your camera goes "click" everything should be as perfect as possible. So why would you rely on an automatic exposure meter system to set your exposure for you? Such a system will provide a generic mid-range exposure, set for generic situations, and may or may not give you the results you are seeking.

At night, setting exposure is a bit more chalanging, not too many light meters can read star light , so the exposures must be set manualy. The lens focus must also be set manualy, see Tips Page for more about focusing on the stars.

It might help if you understood the exposure system of your camera a little better. So, here goes:

A light meter senses light level and provides a single number exposure value to any particular reading, known as EV. Here is a good example of typical light exposure values:

Lens Calibration
 

 

Converting EV
So how does this EV number affect the exposure of any particular photograph? Uh Oh, here comes the math!

There are three settings on the camera that affect exposure: ISO, the light sensitivity of the camera sensor (or film); Shutter speed, how long the shutter is open to expose the sensor (or film) to light; Aperture f/Stop, adjust the amount of light hitting the sensor (or film), while affecting depth of field.

Here is an example of an exposure calculator chart:

 
 
Exposure Calculator

 

How it works.
Let's take a small section of the above chart and try a few examples of how to calculate exposure.

 

It's a bright sunny day, and the light meter in the camera is reading EV 15, so it automatically sets the camera to ISO 400 at f/2.0 for 1/30000 of a second. This is a random exposure setting based on EV 15, but may not be ideal for the subject. It might be great if you're shooting a fast car and want to freeze frame it, but would have next to no depth of field at f/2.0 and will be a bit grainy at ISO 400.

Looking at the above chart you will see the ISO settings in the upper left of the chart, and right below that is the EV numbers. The f/Stops are across the top with the coinciding shutter speeds below. Simply look at the intersecting lines to determine each set of values.

 
exposure example
 

Still at EV 15, we could now choose a little lower ISO for less grain, say 200 and add a bit of depth of field by closing the aperture to f/4 which will require a speed setting of 1/4000. ISO 200 is a great setting for a bright sunny day, not much grain while providing enough speed for most circumstances.

exposure exampe
 

For a bit more depth of field, let's take it to f/8 without changing the ISO and we see that the shutter speed will be 1/1000 of a second, still an easy hand held speed. You can hand hold a camera without built in stabilization down to 1/60th of a second before hand shake becomes a problem. . . break out the tripod at anything below 1/30th of a second, personally I use a tripod as often as possible, even at much higher speeds.

Night EV
Now let's take a look at setting exposure at night. For this example our goal will be to capture the Milky Way Galaxy. As per the Exposure Values chart at the top of this page, the proper EV is -7. This has to be discovered by experimentation, for each camera and lens combination will give different results. This is not a hard and fast number, you may discover for your camera, you prefer to use EV -8 and tweak the results in post processing.

6400 ISO night sky exposure
 

Some cameras will not go past ISO 6400 or have lenses with a minimum f/Stop of 2.8, so let's start there. Assuming we are using EV -7, then there is only one number left. . . the shutter speed; 15 seconds is great for most wide lenses to prevent star streaking. ISO 6400 is grainy and a bit noisy but will produce decent night sky images.

 

ISO 1600 has far less grain and noise and is a good baseline setting for capturing the Milky Way with a 24mm f/1.4 lens. This is a fast lens and will allow twice as much light to hit the sensor than at f/2.8.

 

Now let's take a look at the other end of the calculator chart and push the ISO to 25600. With a f/1.4 lens, we are able to take a portrait of the galaxy at a 1 second exposure. Sure it will be grainy and very noisy, however we will be able to examine the resulting image in one second rather than waiting 15 seconds. This is a handy test shot method to check exposure, focus and composition before committing to longer exposures. Remember, you're doing all this in the dark, so focus and composition are somewhat hit or miss and need to be tested. Often trial and error is your best tool at night, for the camera can record much lower light levels than your eye can.

Night Exposure Chart Handout
Here is a chart you can download and print:

Night Exposure Chart Handout

Once printed, record the various lenses you might use at night in the "Lens" row above their respective maximum apertures at the top of the chart, as in the example below:

Night Sky Exposure Chart Example

 

In this example we have four lenses in the camera bag; two fast fixed lenses at f1.4, a 24mm and a 50mm; two zoom lenses are listed above the f2.8 and f4 columns respectively. Say you are going to use the 24mm lens to capture the Milky Way, then your fStop will be set at f1.4, your fastest setting for that particular lens. With the EV fixed at -7 for the Milky Way stars, you now have two of your exposure variables locked. . . all you need to determine is the ISO and shutter speed.

The 600 Rule
The most common method used to calculate the slowest shutter speed to prevent star streaking for any particular lens you can use the 600 rule: Divide 600 by the lens focal length. For example, with a 24mm lens (600/24= 25 seconds). So , with the 24mm lens any shutter speed slower than 25 seconds will begin to show streaking of the stars caused by the Earth's rotation. Some folks use the alternate 500 Rule or even the 400 Rule.

Why the discrepancy? I'm glad you asked. These sets of rules were based on film and really do not apply to today's modern digital cameras and should only be used as a rule of thumb calculation. Factors such as sensor size and mega pixel resolution must also be considered in any such formula. I recommend running your own tests with each lens you intend to use, that way you know the limitations of each lens in your bag. Personally, with my 24mm lens, I tend to shoot a bit faster at 15 seconds, to insure minimal star streaking, 10 seconds is even sharper but raises the ISO. There is an outstanding article about calculating the precise formula here: NPF Rule

The chart shows that at f1.4, 15 seconds, EV -7, the ISO will be 1600, a very clean ISO for night photography.

So now we have all our variables for the example:

24mm lens at f1.4 for 15 seconds at ISO 1600

I hope this clarifies the workings of exposure at night. . .

Now, go capture some stars. Happy shooting.

Tips Page