During my recent trip to Croatia I confirmed something that I had hitherto only suspected.  My Canon 5D Mk III was not always giving me accurate exposures when used in Aperture Priority mode…. although to be strictly accurate as with most photography issues the problem wasn’t with the camera… it was with the photographer.

My confession…

I didn’t read the manual, and there was a significant change between how my 5D Mk 1 worked and how my 5D mk III worked.  In aperture priority mode, as I am sure you are aware, you set the aperture… the camera calculates the shutter speed.  The issue is WHEN does it calculate it?  On my old 5D Mk 1 it calculated the exposure at the time the photograph was taken (ie when you fully press the shutter release), or when you pressed the exposure lock button (the button with the “*” symbol on the back of the camera).  On the Mk III however, the exposure is calculated when you half press the shutter release.

So what?

Now this may not sound like a big difference but when you use “focus recompose” as I do a lot it makes all the difference in the world.

I very rarely want my subject in the centre of the image.  Usually I place the subject off to one side.  Yes I could change my focus point to be one that is over the subject, but it is far quicker to point my central focus point at the subject, focus, recompose and shoot.  By default that focusing is done by half pressing the shutter release and holding it down, on the Mk III the exposure is calculated at the same time and locked in.

Why is this a bad thing?

Assume your subject is standing near a bright light source (but not so close to it that it will be in your final composition).  If that light source appears in the viewfinder while you are moving your focus point over the subject and half press the shutter release it will be taken into account as the camera calculates the exposure.  Recompose the image and the exposure the camera uses is incorrect for the final composition.

So what is the solution?

Most cameras enable you to customise when certain functions like exposure and focusing take place.  On the 5D Mk III, however, while I can prevent exposure from being locked in on half shutter press and only calculated on full shutter press… this also disables focusing on half shutter and full shutter press too.  This means to use auto-focus it is necessary to use the AF-ON button on the back of the camera.

Back button focusing

Focusing this way is known as ‘back button’ focusing and is used by a lot of photographers – some find it more intuitive.  It is an extra step when taking photos and when you first start using it, it can take a while for it to become second nature and will lead to the inevitable missed shots.  There are times that focusing like that is a real advantage.  Sports photography for example you can pre-focus on a point and just wait for your subject to arrive there without having to keep your finger half pressed on the release.

My perspective

I used to back button focus a few years ago and then for no particular reason reverted so I have slipped back into working that way with little problem, and in fact I think it has helped my photography because I have to think about focusing rather that just letting it happen.  And as for my exposures they are much better now because the camera is calculating them based on the actual image it is creating.

Lesson to learn

  • It always pays to read the manual for your camera.  Had I done so I might have discovered this change earlier.
  • When does YOUR camera calculate it’s exposure?  Is it when you expect it to?
  • Try back button focusing if you don’t already – any change to how you work makes you think about what you are doing photographically.  You might even find you prefer to work that way.