Correctly Exposing Long Exposures

August 15, 2017  •  Leave a Comment

Correctly Exposing Long Exposures 

Long exposure photography can create a very different mood and atmosphere in a landscape image compared to a ‘normal’ exposure. Extending exposure time from fractions of a second to multiples of a second can completely change the feel of a scene.

108 second exposure to blur the clouds and smooth out the sea

 

However, when you start to get to multi-minute exposures then getting the exposure right first time can be really important.

Let me explain what I mean.

You have arrived nice and early for a dawn shoot, or sunset shoot. There is some nice cloud and wind and a long exposure to blur the cloud movement is the choice you want to go with.

Long exposure images, like any other, still need ‘good light’ and light is transitory. Beautiful light on a scene may only last for a few minutes.

Of course with a ‘normal’ exposure time, or even a long exposure of a few seconds, the chances are you can take a shot, check the exposure, correct if necessary, and still get your image while the light is good. However, let's assume that the effect you want is going to need an exposure that's a bit longer than that.

Let’s say you are looking for a long exposure of somewhere around 2 to 3 minutes to really get the blurring effect that you want in the final image. Maybe you’re using an ND filter to achieve it or, perhaps the ambient light is low enough that you don't need any filters. Either way the time you have to get that shot is measured in minutes, after that the beautiful light will be gone

You set up, calculate your exposure time, manual exposure, bulb mode, click the shutter release…. and wait.

When the two or three minutes (or whatever your exposure time is) is over you check the histogram display on your beautiful image. 

And now the dismay. The image is underexposed and all of the shadow areas are far too dark. Or, even worse, the image is overexposed and the highlights are completely blown out.

(As an aside, the second scenario is normally worse because it is normally easier to recover shadow detail from a RAW file than it is to recover highlights but, if the image is seriously underexposed, then even shadow recovery will be at the penalty of increased noise and reduced overall quality.)

By the time you have done all this the chances are that the beautiful light has gone and you either have to try to make the best of the badly exposed image you have managed to get, or go away with nothing. Either way, not an ideal situation.

So, how can you check that you have the correct exposure before you start?

Here’s the tip (and it’s what I do).

My camera has a base ISO of 100 and this is what I would normally choose to shoot at. The specific details in this tip are based on shooting at ISO 100. If you have a different base ISO (ISO 200 or maybe ISO 64 if you have something like a Nikon D810) then you will need to adjust the details, don't worry I’ll cover that at the end) 

Step 1.

I set up my composition, focus, add any required filters, and set my exposure mode to manual, and dial in the aperture that I want to use .

Step 2

I then adjust my ISO from 100 all the way up to 6400.

I then use the built in exposure (light meter) display to calculate recommended exposure at this high ISO. 

Of course, in theory, if the light meter says it’s correct then it should be. The truth is, however, that the light meter is only a guide and the amount of contrast in the scene, and how the light and shadow areas are distributed, and the metering mode you are using, all have an effect and you can’t completely trust the meter.

OK. I check my exposure reading on the meter and, let’s say it comes out at 1 second. I take a test shot, it only takes 1 second so I have plenty of time, and check the histogram. If the exposure looks correct then I am ready to go to the next step. If the image is underexposed then I will increase the exposure time and try again. If the image is overexposed then I will decrease the exposure time and try again. 

After the adjustment I check the histogram again, Even if I have doubled the exposure time to overcome an underexposed image I am still only looking at a 2 second exposure. 

Once I have the correct exposure I take note of the number of seconds. Let’s say I ended up with a 3 second exposure.

Step 3

I now dial my ISO back to 100.

I then simply substitute the number of minutes for the number of seconds. So a 3 second exposure at ISO 6400 becomes 3 minutes at ISO 100. 1 second becomes 1 minute, 2 seconds becomes 2 minutes.

Really easy isn’t it?

But what about if your test shot at ISO 6400 comes up with an exposure time of less than a second or if it works out as somewhere between, like 1.5 seconds?

If you end up with a 1/2 second exposure at ISO 6400 then you can use 1/2 a minute (30 seconds) at ISO 100 and so on. Basically you can use fractions of a second to correspond to fractions of a minute. Of course the usefulness of this technique reduces with shorter exposures and, once I get below 30 seconds I just take the shot at ISO 100 and then correct and shoot again if necessary.

If you end up with, say, 1.5 seconds then simply go for 1.5 minutes (90 seconds).

I have used this technique successfully with exposures of up to 4 minutes ‘in the field’ and up to 8 minutes as a test and it has always worked out for me. I suspect, although I have never tried it, that the technique will become progressively less accurate if you start to extend exposure up to 10 minutes and beyond but I cannot think of any reason why I would ever go that extreme (although that could be famous last words I suppose)

The only warning to give on this is to consider how the light is changing and factor this in. 

For example, shooting early in the morning when the ambient light can be increasing quite quickly as the sunrise approaches can mean that the scene gets quite a lot brighter in the space of 3 or 4 minutes and what was the correct timing at the start of a long exposure is no longer correct at the end. If the light is changing a lot I will normally adjust my exposure time slightly. If it is getting brighter, like at dawn, then I will shorten my exposure by a small amount, so a 4 minute exposure may end up being more like 3 minutes and 40 seconds. I will always err on the side of underexposing slightly rather than overexposing.

What if your cameras base ISO isn't 100?

It’s not a problem. This technique works because ISO 6400 is 6 stops more sensitive than ISO 100. 

If your camera has a base ISO of 200 (one stop more sensitive then ISO 100) then you need to do your test shot with the ISO 6 stops higher which is ISO 12800.

If you have a camera with a base ISO of 64 (and assuming you don’t want to go to ISO 100 for the shot, and why would you?) then you need to go ISO 4096 (or the nearest ISO you can get to it, I’m not familiar with the ISO adjustments on any such cameras) for your test shot and the calculation works the same way.

I hope you find this a useful tip. Of course there may be other ways to ensure the correct exposure and I would love to hear if you have a different technique.

The ISO 6400 time for the test shot was 2 seconds so I intended to expose for 2 minutes. I notice partway through the exposure that the sky was getting quite a bit brighter so ended the exposure early at 106 seconds
 


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