Explaining how to determine correct exposures for long exposure images

An acknowledged master of long-exposure photography, Cole Thompson shares his techniques for finding his “correct” exposure, and extrapolating that to dramatically longer exposure times.

Getting the correct exposure when using one or more neutral density filters can be challenging. I use up to 18 stops of ND with exposures ranging from 30 seconds to 8 minutes. At first finding the correct exposure was very frustrating and my images were often underexposed and I’d have to resort to guessing at the correct exposure, which often meant I missed the shot.

It took a while to figure everything out, but now I have a very simple system for setting the correct exposure and it usually works right the first time. Most of it is straightforward, but there are a couple of “secrets” that could make your long exposures a bit easier.

I use a Canon 5D digital camera for most of my work. While I could use a film camera, getting the right exposure would take much longer because film doesn’t have the immediate feedback that’s provided by a digital camera. I mention that I use Canon because one of my “secrets” applies only to Canon cameras.

Let me give you an idea of how much 18 stops of neutral density is. If my correct exposure for a scene on a sunny day is 1/500 at f8, then using 18 stops of ND would allow me to shoot a 4 minute exposure with that same sunny scene! That is a lot of light reduction and that means only a tiny fraction of the light is making it through the filters and onto the sensor.

But even with this small amount of light, the camera’s meter is up to the task! I have never needed to use a handheld light meter or long-exposure calculator.

To start off I set my ISO to 50 (typically the lowest ISO available on a digital camera) and set my shutter speed to 30 seconds (typically the longest exposure setting on a digital camera) and then take a meter reading of the scene.

And now for my first and most important secret:

Secret number 1: To get a correct exposure reading at these low light levels I must completely seal off the viewfinder so that no light enters the camera from the rear.

At these low light levels even the smallest amount of light entering the camera from the viewfinder will give an incorrect exposure reading. How do I seal off the eyepiece? I use an eyecup, such as this one offered by the Hoodman Corporation (see photo).

When using the eyecup I take extreme care to seal my eye completely so that no light enters the viewfinder, otherwise my image will be underexposed.

Once the eyecup is sealed, I simply match my exposure needles for a “correct” exposure and I’m all set for a perfect 30 second exposure. Now what if I want to use a longer exposure, say 1 minute? How can I calculate my exposure for 1 minute when my camera will only meter to 30 seconds? Simple!

I do everything the same as I did for the 30 second exposure, but now I set the exposure needles to under-expose by 1 f-stop so that my meter looks like this:

Now I switch my camera to Bulb mode and I expose for 60 seconds.

What did I just do here? By using the exposure compensation scale, I underexposed by 1 stop when I metered and then I made up for it by doubling my exposure time.

What is Bulb mode? In Bulb mode the shutter stays open as long as you hold down the shutter. It’s not practical for me to hold the shutter button for one minute because it would shake the camera, so I use a remote switch that allows me to lock the shutter button down for the duration of my exposure. You can use your camera’s official “branded” shutter lock, but there are “off brand” ones available at a fraction of the cost that do the job just fine.

Now for secret number 2 for Canon users: When I first used this approach, the images would sometimes turn out okay and sometimes they’d be very dark. I couldn’t figure out what was causing my calculation to be so far off. Was it some sort of reciprocity failure, a phenomenon we experienced in the old days of film that required extra exposure time beyond what the meter said? Or was it something else?

My research found nothing on reciprocity failure for digital cameras. And then one day I accidentally discovered why my longer exposures were not working out the way they were supposed to. This is due to a quirky feature of Canon cameras.

When I metered and set my aperture to f8 in Manual mode, I expected the aperture to still be at f8 when I switched to Bulb mode. It was not. It was set at f22 and that explained why my images were grossly underexposed. The aperture setting does not carry over from Manual mode into Bulb mode; you must manually set it each time.

So secret #2 is to be sure that I reset my aperture once I switch to Bulb mode, every time!

So how do I determine the correct exposure for a 2 minute exposure? I do everything the same again, except I set my meter to underexpose by two stops. My meter now looks like this:

I then switch to Bulb, re-set my aperture and expose for 2 minutes.

And for a 4 minute exposure I set my meter to underexpose by 3 stops so that my meter now looks like this:

I switch to Bulb mode, change my aperture and expose for 4 minutes.

What I am doing with this approach is to underexpose when metering, and then I compensate by increasing my exposure time. For every 1 stop I underexpose, I double my exposure time.

Here are the settings to remember:

Meter for a 30 second exposure, and then adjust as follows:

Meter for correct exposure	30 second exposure
-1 stop underexposed	1 minute exposure
-2 stop underexposed	2 minute exposure
-3 stop underexposed	4 minute exposure

At first this system seemed complex, but once I used it a few times it was very easy to remember.

When I’m going to use a very long exposure (over 30 seconds) I’ll start off by first exposing at 30 seconds to check the exposure and composition. If everything looks good, then I’ll expose at the longer exposure time. This saves a lot of wasted time when I’m doing a four minute exposure and then I discover that something wasn’t right!

When I’m using a 30-second exposure, the camera times the exposure for me. But my camera doesn’t time for over 30 seconds and I must time these exposures myself. At first I used my wristwatch, but I frequently forgot where I started from and I wasted the exposure. So now I use my iPhone timer because I always have it with me, and because I cannot ignore that obnoxious Marimba alarm! There are also various smartphone apps that can help time your exposure, some are free, others cost a couple dollars.

To create my long exposures images I use a Singh-Ray Vari-ND filter and stack a second 5-stop or 10-stop Mor-Slo fixed ND filter on top of that. There are two reasons why I use the Vari-ND.

First, I can open up the Vari-ND filter for easier composing. At 18 stops it’s almost impossible to see anything even after my eyes have adjusted. With the Vari-ND I can allow more light to enter the viewfinder so that I can compose the image, and then I stop down for the exposure.

The second Vari-ND advantage is that I can use the variable feature of the filter to adjust my exposure instead of using the aperture. This allows me to set my aperture to where I want it, to control my depth of field.

Both of these advantages are significant and I always use a Vari-ND when shooting long exposures.

Conclusion: Setting the correct exposure for long exposure photography is pretty easy, it’s all based on accurately metering at 30 seconds and extrapolating from there. Using an eyecup is a must to seal out any extraneous light and the Vari-ND makes composing easy and offers me more control over the scene.