The Whys of Back-Button Focus and Focus Lock

Back-button focus is a technique where you change the focus button to on of the programmable function buttons (F1, F2, F3, etc) on the camera so that it makes the camera focus instead of the usual half-press on the shutter button.  This will let you lock the focus at the same distance from the end of the lens until you use the function button to refocus.

You then:

  • Choose a subject.
  • Push the focus button to get a focus lock.
  • Move the camera back and forth until the subject is focused the way you want it.
  • Push the shutter button for an instant picture.
  • Keep taking pictures with the same focus.

“That sounds complicated just to take a picture.  Why would you do this?”

For starters, autofocus is slow.  You half-press on the shutter button, the camera picks a piece of the picture, moves the lens in and out until the blur disappears or is the smallest that it will get, then signals that it has focus.  You then push the shutter button the rest of the way.  If you lock the focus, then for the cost of a little bit of time setting up the shot, you can take all of your pictures after that very quickly.

You get more control.  You can think of focal plane as a sheet of glass perpendicular to the lens and at a fixed distance away from the end of the lens.  As long as something is inside that sheet of glass, it’s in focus.  Now you can do like I did with the skeleton shrimp below and put eyes and “hands” in focus by angling the camera so that the those pieces of the picture are inside the focal plane.

You can focus on something that’s not the subject and then reframe.  I do this a lot with subjects that are hard to focus on.  Moving things.  Things inside holes.  Things not in the center.  Point your camera at something the same distance away from the camera lens as the subject and then focus lock on it.  You can then move your camera back to the subject and move it towards and away from the subject to get it in focus.

Macro photography is almost impossible without focus lock.  You have too many variables to consider to make a shot.  Simplify your shooting by reducing the effort of using autofocus by locking your focus.

Shooting in low-light situations is hard, even if you’re using strobes.  You can’t always use a focus light.  Crabs and shrimps look the other way when you shine white light in their eyes.  Nudibranchs feel the heat and change direction.  When you turn off the focus light, you’ll see the camera “hunt” when you try to focus: the lens moves in and out trying to find the right focus but because it’s too dark it can’t see the difference in focus distances.  So turn on the focus light, focus on the sand or coral, turn the focus light off, reframe on the subject, and keep shooting.

Most compact cameras don’t have a moveable focus point.  On most DSLRs and mirrorless, you can use the direction arrows or joystick to move the focus point around inside of the frame.  With compact cameras, you can get focus in the center of the frame, lock the focus, then reframe the subject.

“Wow, Mike, that sounds like an awesome idea that I’m really sold on, how do you set it up?”

It depends on the camera, they all do it differently across brands.  For a howto specific to your camera, try google for “<model> back button focus”.  I’ll post later on how to do this for the Olympus OMD mirrorless.

On compact cameras, programmable buttons are fairly rare.  However, they sometimes have a “focus lock” feature where you can focus on an object and then lock the focus point.  I’ll post later on how to do this for the Olympus TG4 and TG5.

Sample times when I’ve used back-button focus:

  • Fast Little Blue Fish.  They move in and around the coral too fast for you to get a good focus.  So lock your focus and take a picture when they appear in the gap between coral.  I talked about this in Little Fast Blue Fish.
  • Skeleton Shrimp.  They live usually on hydroids: cousins to coral that look like little white-brown shrubberies.  These hydroids sway gently in the current.  Back and forth, back and forth.  Too fast for your auto focus.  Next time they swing by, focus on them and lock your focus then snap each time they swing by after that.
  • “Fast-Moving” Nudibranchs.  Focus on a spot in front of their direction of movement where they crest a micro-hill and lock your focus.  When they move up on top of the terrain, get a picture when they’re more silhouetted. I describe this in this post about Bornella.
  • Critters in Tunicates. You’re taking a picture where the subject is inside a tube.  The autofocus on the camera sometimes will lock on the top of the tube.  Lock focus then move the camera forward to move the focus into the middle of the tube.
  • Hairy Shrimp and Other Teeny-Tiny Things.  When using supermacro gear and taking pictures of things smaller than 3mm across, your focal plane is extremely small.  Use focus lock to lock your focus then move the camera slightly to give you the focus that you want.

 

 

See you underwater!!

–Mike

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