Today’s Question: I have a question of clarification on your answer to focusing with a strong neutral density filter. Can you please tell me if after you set the camera settings without the filter on do you then have to switch to manual focus on the lens or does it not matter?
Tim’s Quick Answer: Yes, if autofocus is enabled for your shutter release button, then you should disable autofocus on the lens after adding a neutral density filter for a long exposure.
More Detail: As mentioned in yesterday’s Ask Tim Grey eNewsletter, I recommend configuring all of the camera settings for your photo with the neutral density filter detached. You can then add the neutral density back to the lens, and adjust the shutter speed to increase the exposure duration based on the strength of the neutral density filter.
In other words, you should refine your composition, establish exposure settings, and set the focus with the neutral density filter removed from the lens. Make sure the exposure settings are established in manual mode, and then add the neutral density filter and adjust the shutter speed setting.
If your shutter release button is configured to activate autofocus, then pressing that button to capture the image will cause the camera to attempt to refocus on the scene. This can result in inaccurate focus, in part because of the presence of the neutral density filter.
By turning off autofocus on the lens, you’ll ensure that the camera isn’t able to autofocus when you capture the image. Of course, if you’re using back-button focus and have disabled autofocus for the shutter release button then this additional step is not necessary.
Also, be sure to re-enable autofocus when you’re finished capturing the photo.