[Leica] Photographic reaction time

[LRZeitlin@aol.com]

For critical sports photography, human reaction time must be added to the 
camera shutter lag to get the total time between deciding to push the button and 
when the shutter actually makes the exposure. I know everyone already knows 
this but we tend to forget that the human response latency may be as long or 
longer than the mechanical lag of the camera. My old experimental psychology 
textbooks give the simple reaction time of most people as about 200 milliseconds. 
That is, the time from seeing a visual signal to pressing a key. (Or seeing a 
ball bounce into the street and hitting the brake pedal) Simple reaction time 
time may range from as litte as 100 ms. to as much as 350 ms. but 200 ms. is 
the commonly accepted figure. Add to that the camera lag time of 100 to 500 
milliseconds, and the total delay can easily exceed half a second. If we know 
ourselves and our equipment well and have a familiarity with the sport, we can 
largely compensate for the lag but it is wise to know what delay we can expect 
in trying to catch that decisive moment.

Here is how to measure your own total lag time (human plus camera). Set up a 
meterstick against a wall. Have a friend hold a small object, say a black 
marble, at the top of the meterstick. Focus your flash equipped camera on the 
stick. Now tell your friend to drop the marble at any time during the next few 
seconds but not to give you any indication of when it will be dropped. When you 
see the marble start to move, snap the shutter. Repeat the process at least 
half a dozen times.

Now look at the prints (or just the negatives through a magnifying glass). 
Using the equations we learned in high school physics we know that the distance 
the ball will fall in a given time is:

distance = 1/2 a  * t^2

where distance is measured in centimeters, 
a is the acceleration due to gravity and is equal to 980 cm/sec/sec
t is time in seconds.

This equation can be rewritten as:     time = sq. rt. (distance/490)

If the average distance the ball travels is 20 cm, the total latency is 202 
milliseconds.
If the average distance is 30 cm, the latency is 247 milliseconds
If the distance is 40 cm, the latency 286 milliseconds
And 50 cm = 319 ms.; 60 cm = 350 ms.; 70 cm = 378 ms.; 80 cm = 404 ms.; 90 cm 
= 429 ms.; 100 cm = 452 ms.

If the ball bounces off the floor before the camera clicks, either get a new 
camera or forgoe sports photography. You will miss too many pictures.

Larry Z
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