Calculating Shutter Speed for Long Exposures

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I wanted to share a great trick I recently learnt, that is perfect for Calculating Shutter Speed for Long Exposures.

Nailing exposure can be tough, especially if it’s likely to be a longer exposure. 

Today, I am going to show you a super simple calculation you can use to make your life a whole lot easier. No Apps, No Charts, just your camera and a teeny tiny piece of brain power!

As with most of my tutorials here, there is a video to go with it, so have a watch of the below, or read on if you’d prefer.

Introduction – Long Exposures

What if you need to do an exposure, that could be 10 minutes.

How do you know whether it is 10, or 15 minutes.

I took this shot a couple of years ago

Shutter Speed in Photography

I knew I wanted to get star trails, but I was totally unprepared, and had no idea what shutter speed I would need to get the stars trailing. 

In fact, I had no idea of any of the settings. This photo is a 10 minute exposure.

How did I get this amateurish shot?

It wasn’t trial and error, to Live View, I actually guessed, as I had little idea what I was doing

Turn seconds into minutes.

Let me explain.

How to do it!

You basically set up your shot at ISO 6400. It must be ISO 6400. There is a reason for that.

You adjust your shutter speed so that the exposure is correct. If the Live View or Histogram cannot give you an accurate reading, just take a test shot. At ISO 6400, the test shots shouldn’t be too long. Maybe a few seconds. So not too long to wait.

Once you have the correct exposure, make a note of the shutter speed. Let’s say it comes out at 10 seconds..

All you need to do now is change the ISO to 100, and change the 10 seconds to 10 minutes.

This works out perfectly, there are 64 ISO 100’s in 6400 ISO, and 60 seconds in a minute. By turning the ISO down, but lengthening the shutter speed by the same factor of 60, they cancel each other out.

This means you can compose and test the exposure at ISO 6400, and if the exposure is correct at 10 seconds, you know when you change to ISO 100 and shutter of 10 minutes, the exposure will be the same.

Here are a couple of screen shots from the camera, to prove this. As you can see, the exposure is identical, but the setting very different.

Image on the left – ISO 6400 / 10 Second Exposure……..Image on the right – ISO 100 / 10 Minute Exposure

Calculating Shutter Speed for Long Exposures – Bullet Points

  • Compose your shot, and include any filters that you may want to use
  • Dial in your ISO to 6400
  • Take a test shot at ISO 6400
  • Once you have that perfect exposure, take note of the exposure length.
  • Go back to ISO 100
  • However many seconds exposure at ISO 6400 now equal minutes at ISO 100

That is it. Simple really.

This is going to really help me next time I am out, and want to try and get those star trails, or smooth and dreamy water shots without spending ages taking test shots.

Thank you for reading, and leave me a comment down below if you want to add anything, or sign up to the mailing list if you fancy more tutorials, straight to your inbox!!

Dont forget to check out the YouTube Channel for more videos.

And the only thing for me to do is to say thank you for again, and I will see you next time.