EOS Documentation Project

DOF Preview facilitates FTM

by Neil Tiberius and Julian Loke

Contents

• Introduction
• Method
• Revision History
• Comments

Introduction

I'm using an EOS 300D with a Canon 28-135 IS USM lens. I want to be able to auto-focus on a subject, then tweak the focus manually by adjusting the focus ring on the lens.

The procedure for doing this so called "FTM" (Full-Time Manual) focusing, so I've read, is to press the shutter halfway on the subject--wait for the autofocus--then keep the shutter halfway depressed and adjust the focus ring. I was under the impression that so long as the release button was kept halfway depressed, you could adjust focus manually without having to switch the lens from AF to MF.

Now, I've tried this, but when I try to turn the ring, the lens "clicks" at me and resists my attempts to turn the focusing ring, instead setting it back to where it thinks it should be auto-focused. The autofocus system misinterprets the refocussing as subject movement, and switches into SERVO AF mode, thereby undoing any attempts at FTM.

Method

One solution is to autofocus first, switch to manual focus on the lens, and THEN use the focusing ring. Another solution is to buy a more expensive camera with a Custom Function (typically CF4) that transfers the AF-start function to the "*" button on the camera.

Julian suggested another zero-cost solution:

• Press and hold the shutter button halfway.
• Wait for the autofocus to lock.
• Press and hold the DOF preview button. This locks AF and AE.
• You can now use FTM on your lens

I tried this technique on my EOS 300D, and it did work for me. I was able to manually focus in Av mode even though my 28-135mm USM lens was on auto! And yes, it is a contortionist's feat, but at some point it'll probably be worth it. At least the method is free. Note that this method only applies if your camera has a DOF preview function.

Revision History

v0.1b2003Dec10 jul: clarified focus lock
v0.1 2003Nov30 nt & jul: Original draft

Comments

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