# Help with calculating light intensity with f-stops and shutter speed?

Question:

You take a picture with a f-number of f/4 and a shutter duration of 1/60 seconds. You change the f-number to f/2.8 and the shutter duration to 1/240 seconds. The light intensity reaching the sensor has changed by what factor?

Relevance
• Sumi
Lv 7
4 weeks ago

In full stops, f/stops go from f/2, f/2.8, f/4.  In this example, you have opened up the lens by 1 full stop.

In full stops, shutter speeds go from 1/60th, 1/125, 1/250th.  In this example, you have decreased the exposure by 2 full stops.

Decrease of exposure via the shutter speed by -2 stops plus an increase of the exposure via the aperture by 1 stop, equals a reduction in exposure by -1 stop.

• qrk
Lv 7
4 weeks ago

Get comfortable with the term "stops". 1 stop is doubles/halves the light energy. For shutter speed, doubling the shutter speed is 1-stop dimmer.

For aperture, 1-stop is a factor of 1.414. Thus, going from f/4 to f/2.8 is 1-stop brighter.

With the above info you can figure out the answer.