Control of Light (Neil Blair)

Photography means ‘writing with light’  But writing needs control.  In photography one of these controls is the combination of aperture and shutter speed:  EV or "exposure value".

Historically a problem encountered by beginners is understanding apertures and shutter speeds. Manufacturers have come to your aid by evolving automated exposure controls; aperture priority, shutter priority, program, matrix, spot, multi segment + different names for the same function by different manufacturers. The cure is now more complex than the problem.

Aperture/ shutter speeds  - ½ and third stops.

The concept of apertures and shutter speeds is fairly easy to understand. Let me draw a parallel:
Buy a camera - cost £100, but you need to save up for it.
    £50/wk for   2wks
    £  2/wk for 50wks
    £25/wk for  4wks
    £  4/wk for 25wks  or any combination in between.
This is a combination of Quantity and Time:
Quantity is aperture        
Time is shutter speed
Let’s look at them individually and take the easier one first

Shutter Speeds

1. Numbers range from at least 1 to 1000 + B (or "Bulb")
2. Fractions of a second  1/30  or 1/60  or 1/125  or 1/250  each more or less double or half its neighbour
   

Apertures

1. Numbers range from f2 to f22, eg. f4  or  f5.6  or f8  or  f11
2. Apertures are also doubles/halves, but not obvious.

Which way to move?

You will make mistakes!!

Let's say the ‘Correct exposure’ for a scene is 125th sec @ f8.
    1 stop      60th   @ f11
    1 stop      250th @ f5.6
    2 stops    500th @ f4
    2 stops      30th  @ f16
    3 stops   1000th @ f2.8
     3 stops    15th    @ f22

All these combinations will give the same exposure - but not the same effect.

ISO ratings

Scale refers to sensitivity of CCD - the higher the number the quicker the response.
Low number - 25 ISO slow
High number - 800 ISO fast

100  200  400  800 etc.    also doubles/halves.

Higher number leads to grain/digital noise

Definition - Exposure is the amount of light allowed to reach  a CCD via a combination of aperture and shutter speed.

Conclusion - Aperture and shutter speed is the way light is controlled to enable an exposure to be made.

by Neil Blair