Mike's Super Long-Exposure Cheat Sheet

A composite from two separate images with noisy
ISO 12,800 at f/4 for 8 seconds on the left and creamy
ISO 200 at f/4 for 8 minutes on the right.
Captured on Loveland Pass, Colorado

There are lots of different ways to shoot night photography.  We have the relatively short duration exposure (usually 30 seconds or less) where we get to see details in the night sky as points of light.  Then we have the star trail images where we shoot for a little longer and we stack multiple exposures so we get the stars to appear as trails in a way that's effective at reducing noise.  Both of these approaches open the shutter for a period of time that doesn't usually exceed 2 to 3 minutes - often less.  Part of the theory behind this is that multiple exposures are more effective at reducing noise than a single super long-exposure - even with long-exposure noise reduction enabled.

The Super Long-Exposure

ISO 6400, f/2.8 for 30 seconds on the left and
ISO 800, f/2.8 for 6 minutes on the right.
Captured at Sheep Rock in Arches National Park, Utah.

In this article, I'm not letting the issue of sensor heat stop me from trying a really long exposure.  I'll make use of some long-exposure noise reduction to help address that issue.  I want to see what it looks like when I expose my shutter to light for 16 minutes or even 32 minutes.  The human eye is limited to ultra-short exposure times so we can't see much color at night.  But when the shutter is opened for half an hour, the results can be very interesting.

The Problem
The problem came when I tried to accurately predict the settings I needed for a super long-exposure.  I thought I went into it with a good guess on the settings but I usually had to add more light.  So I shot for 30 minutes and would add more light and shoot again and again and again.  Before I knew it, twilight came and then I had plenty of light but no stars!  :-)

I really wanted a way to get this right on the first try.

High ISO Test Shot
Years ago, I learned about a neat concept from Harold Davis (now with Star Circle Academy) called the High ISO Test Shot.  It provided a lot of the answers I was looking for in that I wanted a way to use my higher ISO settings to help anticipate the settings I needed.  My cheat sheet in this is article is essentially an advancement of this same idea.

Theory Behind What I'm Doing

  • There are three components of light - aperture, shutter speed, and ISO as shown in the Exposure Triangle.
  • At night, we'll assume that we'll shoot with aperture wide open (or close to it on an f/1.4 lens) and we won't change that too much.  In this exercise, we'll only change our shutter speed and ISO when using approach #1.  Approach #2 provides more flexibility with changes to aperture as well.
  • Raising your shutter speed by one stop and lowering your ISO by one stop results in the same effective light.
  • The foundation of the high ISO test shot where you shoot once with a high ISO and a short exposure time so you can determine your exposure time for a low ISO shot.


Approach #1: Counting Stops

ISO 800, f/2.8 for 8 minutes on the left and ISO 6400, f/2.8 for 15 seconds on the right.  Captured at Crystal Lake on Red Mountain Pass near Ouray, Colorado.

Count the number of f-stops you extend your exposure time and reduce your ISO by the same number of stops.  This approach works well for small adjustments.

  • Example: Let's say you just shot stars as points of light at ISO 3200, f/2.8 for 30 seconds.
  • Question: If I extended my exposure time to 4 minutes, what ISO setting would I need?
  • Answer: 3 stops would bring my ISO down to 400.

How did I do that?
I found that the exposure time increased by 3 stops in that I doubled it 3 times to get to 4 minutes.  I then cut my ISO by 3 stops by cutting it in half 3 times to get to 400.

Approach #2: Mike's Super Long-Exposure Cheat Sheet

ISO 6400, f/2.8 for 30 seconds on the left and
ISO 800, f/2.8 for 12 minutes on the right.
Captured at Park Avenue in Arches National Park, Utah.

Mike's chart provide an initial guess on settings, an easy way to tweak that initial setting by moving over to another column, and settings for 4, 8, 16, and 32 minute exposure times.  I give a laminated copy to my workshop students so they'll have it ready to use out in the field.  You can do the same by printing the acrobat copy below, trimming it to size, and laminating.

How To Use Mike's Super Long-Exposure Cheat Sheet

  • Start shooting with settings on the High ISO row in the Start Here column.
  • Using the histogram, decide if your image is too light or too dark.  If you need to, shoot again with settings from a lighter or darker column until you get a result that looks properly exposed.
  • Then staying in the same column in the table, shoot with the long-exposure settings from one of the rows below - either 4, 8, 16 or 32 minutes.

Download Mike's Super Long-Exposure Cheat Sheet (Acrobat PDF)

Long-Exposure Noise Reduction

In general, I'm not a fan of long-exposure noise reduction but I find this to be a good place for it.  It's designed to reduce the noise that appears as a result of the buildup of heat in your sensor that comes from shooting long exposures with your sensor active for extended periods of time.  And since your sensor will likely experience this buildup of heat in a 30 minute exposure, I think the extra time it takes is well worth it.  Expect that shooting with long-exposure noise reduction enabled will effective keep your camera busy for twice the exposure time.

I often leave it disabled until I know I'm shooting my last image for a location.  Then, I enable the long-exposure noise reduction and shoot my last shot.  When my exposure has completed, I let the camera complete the long-exposure noise reduction while I'm heading to my next location.

Summary
In the end, I've really enjoyed having a good solution for the problem described above.  With the cheat sheet, I can quickly determine settings for most any long-exposure on the first try.  And while I wouldn't use a super high-iso photograph for printing, I thought it was really neat to see the two in sequence - one really noisy image followed by another with the same exposure only smooth and creamy.

This approach allows me to target a long-exposure image and then move on to something else - without taking all night to do the long-exposure.  I really like that!  I hope you'll try either high-iso or medium-iso captures as a starting point for your long-exposures and let me know how it works for you.