Focusing with Neutral Density

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Today’s Question: Is there any reason not to use autofocus when using a solid neutral density for a long exposure?

Tim’s Quick Answer: In some cases autofocus may be difficult or impossible to achieve when a strong solid neutral density filter is attached to the lens. Therefore, as a general rule I recommend a workflow that involves establishing focus before attaching the neutral density filter to the lens.

More Detail: Whether or not you will be able to use autofocus when a solid neutral density filter is attached to the lens depends on a variety of factors. That includes the strength (or overall density) of the filter, the type of autofocus your camera employs, and other factors.

In my experience it is often possible to achieve autofocus with a relatively strong neutral density filter. However, I’ve also found that in some cases the performance can be slow or the results can be inaccurate. In addition, it can be difficult to otherwise establish the overall composition and capture settings when a strong neutral density filter is attached to the lens.

In some cases the Live View display combined with exposure simulation can provide an adequate solution, but this too can be challenging. For example, the exposure simulation feature may result in a very noisy preview image, making it difficult to confirm accurate focus.

For these and various other reasons, I recommend configuring your shot without the neutral density filter attached to the lens, and then attach the filter and adjust the exposure settings.

The general process here involves first configuring the overall composition with your camera firmly mounted on a tripod. You can then use whatever method you prefer to establish exposure settings based on a photo without the use of the neutral density filter. If you used one of the semi-automatic modes (such as Aperture Priority mode) to determine the exposure settings without the neutral density filter attached, then you’ll want to switch to the manual exposure mode and dial in those same settings.

After everything is configured in manual mode, you can add the neutral density filter to the lens and adjust the shutter speed (increasing the exposure duration) based on the number of stops of light the neutral density filter will reduce your exposure by.

This overall approach makes it easier to configure the overall shot. Once you have established the camera settings based on not using the neutral density filter, you can add the filter and adjust the exposure time accordingly.