How to calculate a minimum shutter speed to yield an adequately sharp image of a moving subject

How to calculate a minimum shutter speed to yield an adequately sharp image of a moving subject

Photographers often face situations where they have to shoot a moving subject and yield a sharp image free of subject motion blur. The minimum shutter speed required to freeze motion is dependent on several factors.

  1. The speed of the subject.
  2. The movement of the subject across from the sensor plane, 45 degrees to the sensor plane, or toward or from the sensor plane.
  3. The focal length of the lens used.
  4. The distance of the subject from the photographer.

One option is to choose a very high shutter speed like 1/500th second. The problem with guessing is that as you move up in shutter speed, you lose light and have to compensate with a higher ISO so you get noise. It’s better to start with the right value so you don’t waste light. Another option is to test images and look for sensor blur but there are literally tens of thousands of combination for those four factors and it’s impractical to test them all.

Well someone at some point figure it out and applied it to a formula. I don’t know if it was done by trial and error, advanced calculus, or divine intervention. All I know is that a formula exist so we don’t have to reinvent the wheel.

This looks complicated but I guarantee it will be easy. Once it’s done, you won’t even need a calculator to apply it.

This is a given:

Directional Factor:

  • Motion across field:  1
  • 45 degrees to field:   2
  • Toward or away:       4

Step 1: Calculate (LM) Lens Magnification

Quite simply, it’s the focal length divided by the normal focal length of the format. So a 50mm lens on a full frame 35mm body will be 50/50 or 1. A 200 mm lens will be 200/50 or 4.

Step 2: Calculate the exposure time in seconds.

(Distance X DF) divided by ((875 x speed) X LM)

Plug that into a spreadsheet and you get a decimal value which has to be multiplied by 0.6 to convert it to fractional seconds. From there, the decimal value has to be converted to a fraction since that’s what cameras use. There is a complex series of steps to do this but I found it easier to build a lookup table. Just find the closest matching decimal value and convert it to the camera shutter speed. For those who want to build their own charts using different Len Magnification or Directional Factors, download this spreadsheet to use as a base.

I’ve done all the hard work for you and put it in to a chart that assumes the following. Don’t worry. You can easily adjust this to your shooting situation or make your own chart.

  • Subject motion is across the field so the Directional Factor is equal to 1
  • Lens used is a 50mm lens on a full frame body so the LM is 50/50 or 1
  • The top row shows selected subject distance
  • The left column shows selected variable subject movement in mph
How to calculate a minimum shutter speed to yield an adequately sharp image of a moving subject

All you have to do to adjust from this is to move up or down to the next calculated value. Note that as speed of movement increases so too must the shutter speed to freeze movement. Note also that closer subjects require faster shutter speeds to freeze motion. If you don’t understand why, run your hand across your eye 2 inches away vs 2 feet away and you’ll see which is more blurry.

The above chart is based on subject moving across the sensor field. If the your subject is moving 45% to the sensor, you use the next slower shutter speed. For subjects moving toward or away from the photographer, you move down two stops. Some may notice the shutter speed does not change from 60 mph to 125 mph. In fact it does but most cameras cannot go faster than 1/8,000 with only a few capable of 1/16,000 so use the fastest speed available.

Doubling the focal length increases the shutter speed by one stop. So add one stop for a 100 mm and two stops for a 200mm while a 25mm will slow down the shutter speed by one stop. Most photographers should know how to adjust by stops so it’s all very simple. I suggest you create a chart or charts that closely match your typical shooting scenario for motion photography so you have to make less adjustments.

  • MAFM

    What happens to the above computation if I have a 1.5 crop sensor ?

    • http://blog.patyuen.com Pat Yuen

      Multiply your calculations by 1.5. A cropped image enlarges the image thereby magnifying any blur.