The Cure for Motion Blur: Auto ISO

Whenever we shoot while hand-holding the camera, it’s impossible to keep the camera from moving around.  No matter how steady the photographer’s hands are…no human can hold a camera perfectly still.

This usually isn’t a problem while shooting outdoors during the day. Enough ambient light is available from the sun, even under thick clouds, to keep the shutter speed well above the “motion blur due to camera movement” threshold.

But, when we shoot at dusk or dawn, at night, or indoors without a powerful flash, the shutter speed slows down significantly (assuming a constant aperture and ISO).  And in these situations, we inevitably have to ask the following question:

How slow can shutter speed go without creating a blurry image?

Here’s the answer: Use the reciprocal of the focal length of the lens.  Just place a “1” over your focal length, and you’re set.  Any slower, and even the slightest movement of the camera becomes an issue.

If f= lens focal length in millimeters, the slowest shutter speed is: 1/f

For example…if you’re shooting at 50mm…the slowest shutter speed you can get away with is roughly 1/50s.  If you’re using a 300mm lens, you should stay above 1/300s.  If you’re using a 15mm ultra-wide, you don’t want to go any slower than 1/15s.  And so on…

In the image at the top of this article, the picture on the left was shot with a 50mm lens at 1/15s.  Even though I tried to hold the camera as still as possible in my hands, the shutter speed was obviously too slow.  The picture on the right was shot with a 50mm lens at 1/50s.  Clearly, this shutter speed was quick enough to fully mitigate motion blur from camera movement.

If you set your shot up and notice a shutter speed slower than 1/f, you’ll need to open up the aperture and/or manually bump up ISO.

However, if you don’t want to worry about this issue at all, you can put the camera in “Auto-ISO” mode.

Auto ISO Mode:

In this mode, the camera will choose ISO automatically to keep your shutter speed at a pre-set value.

For example, I can go into the options menu in my camera, and under “Auto-ISO”, select “1/60” as the slowest shutter speed.  If the exposure requires a slower shutter speed than 1/60, it will automatically bump up ISO to keep the shutter speed at 1/60.  You will still have control over aperture (assuming you’re in aperture priority mode or fully manual mode).

The image below displays the Auto-ISO option in the menu of a Nikon D810:

Auto ISO menu option on a Nikon D810

Remember what we said about ISO in the article on The Exposure Triangle: The higher your ISO value goes up, the more noise your photograph will display.  Keep this in mind when setting maximum sensitivity in the Auto-ISO menu.  If you set maximum sensitivity too high in very dark situations, the camera may automatically choose an ISO value which creates too much noise in the image.

On my D810, I usually set maximum sensitivity at 3200.  The D810 tends to pick up a prohibitive amount of noise at ISO higher than 3200.

Every camera renders ISO differently, so be sure to test out your camera by taking pictures at various ISO values.

Please note: This article only addresses motion blur caused by camera movement.  Eliminating blur caused by fast-moving objects in the composition is a different issue.

Happy Shooting!


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