I wanted to shoot some close-ups of some early Spring blooms at
high magnification (1:1). But I was in a public garden and a tripod
was not allowed.
Great, I get to handhold to get the right composition, then use
X-sync flash to freeze any motion.
That’s when I discovered the DREADFUL TRUTH. At macro distances, DOF is so narrow that autofocus becomes unreliable, and an exercise in frustration.
A TRAP focus function would come in handy. With the lens pre-focused, the camera would trip the shutter when something is detected by an autofocus sensor. This is used by wildlife photographers to capture pictures of incredibly shy creatures, so it could be used to help me take macro pictures of elusive bugs and flowers.
Unfortunately, my Elan 7E is incapable of TRAP focus. The EOS 1 series has an optional TRAP focus, but this feature needs to be programmed using a computer running the EOS Link accessory software package. Is there any alternative, without having to spend big money?
Almost Trap Focus
Of course! This method makes use of certain features of the EOS system, such as focus priority, and full-time manual (FTM) focus override.
You will need a lens with the FTM feature, such as the EF 100mm f/2.8 USM Macro.
- Choose an appropriate camera mode, such as MACRO mode 🙂
- Set the camera to SINGLE FRAME advance, and ONE-SHOT focus (macro mode). Custom function 4 should be disabled.
- Set the lens to automatic focus, and the focus-limiter to full range.
- Cover the lens with the lens cap or your hand.
- Push AND HOLD the shutter release button (or use an RS60-E3).
- The camera will try to find focus, and MUST fail.
At this point, the viewfinder FOCUS indicator should be blinking at 2 Hz. If not, repeat the previous step.
- Without switching out of AUTOFOCUS and without letting go of the shutter button, use FTM to set the lens for maximum magnification.
- Remove the lens cap.
Your camera is now in Almost TRAP focus mode. Why Almost? Because the autofocus system will attempt to refocus on the next subject it can detect. In true trap focus, the lens will not refocus, waiting quietly for a subject to appear .
- Point your lens at the subject in such a way that it remains OUT-OF-FOCUS. This is easy with macro subjects, since DOF is so shallow.
- Now, SLOWLY move the whole camera towards, then away from the subject to find correct focus.
- The camera should fire AS SOON AS focus is detected.
It is best if the focusing-motion is AWAY from the subject, otherwise the camera might autofocus on an unintended subject.
Voila! Almost-trap focus!
How it works
The method relies on a couple of EOS-specific features to overcome the EOS-specific deficiency. Firstly, the camera has to rendered “unable to focus”. This is usually considered an error condition, but it is put to good use here.
Since the lens usually comes to rest at infinity, the focus has to be adjusted so that it is ready for a macro shot. Switching to manual focus will release the shutter without anything in focus, so the FTM (full-time manual) focus override feature of the EOS system is required.
Finally, the camera has to be set to ONE-SHOT, which is a focus-priority condition (the shutter will open only when focus is achieved). Almost-Trap focus will not work if the camera is set to SERVO AF, a shutter-priority state, since the shutter will open on the first shot whether or not focus is achieved.
This method was devised for use with the EF 100 mm f/2.8 USM Macro lens on the Elan 7E, and has also been tested on an EOS 1000FN. As usual, practice with your own camera without film before committing to a shoot.