Apex System:

A system introduced by the American National Standard Association Inc.
Shutterspeed and aperture were simply related to filmspeed(sensivity) and exposure value. All neccessary parameters were calculated to Log2.

See chart III - Aperture(A) + ShutterSpeed (T)= FilmSpeed(S) + Luminance(B)
Thus knowing the S(Filmspeed) and B(Luminance), one can simply add the Log values and place this value in Chart I and read the corresponding aperture/Shutterspeed combination.

A value is given to a single exposure, it's Exposure Value or EV. This is the combination of the film sensitivity (or ISO), aperture and shutter speed. Thus a single "exposure" could be ISO 100, f/2.4, 1/60s.

In a true photographic sense, Exposure Value (EV) is a single number which refers to the amount of light for a given exposure, this value is then used to calculate the correct combination of aperture and shutter speed (at a given sensitivity), the table below represents the relationship between shutter speed, aperture and EV at ISO 100.


EV chart Shutter   Speed                          
Aperture   f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64
  1 s 0 1 2 3 4 5 6 7 8 9 10 11 12
  1/2 s 1 2 3 4 5 6 7 8 9 10 11 12 13
  1/4 s 2 3 4 5 6 7 8 9 10 11 12 13 14
  1/8 s 3 4 5 6 7 8 9 10 11 12 13 14 15
  1/15 s 4 5 6 7 8 9 10 11 12 13 14 15 16
  1/30 s 5 6 7 8 9 10 11 12 13 14 15 16 17
  1/60 s 6 7 8 9 10 11 12 13 14 15 16 17 18
  1/125 s 7 8 9 10 11 12 13 14 15 16 17 18 19
  1/250 s 8 9 10 11 12 13 14 15 16 17 18 19 20
  1/500 s 9 10 11 12 13 14 15 16 17 18 19 20 21
  1/1000 s 10 11 12 13 14 15 16 17 18 19 20 21 22
  1/2000 s 11 12 13 14 15 16 17 18 19 20 21 22 23
  1/4000 s 12 13 14 15 16 17 18 19 20 21 22 23 24



EV = log2(aperture2 x (1/shutter speed) x (ISO sensitivity/100))


Apex? What is it?    Click here for a pdf file on this subject.

After reading and understanding the basics, use tables below....

ASA ASA ° ASA ASA ° ASA ASA °
3 0 100 5 3200 10
6 1 200 6 6400 11
12 2 400 7 12500 12
25 3 800 8 25000 13
50 4 1600 9    

Aperture(A) ASA ° ShutterSpeed (T) ASA ° Luminance (B)(cd/m2) ASA °
f/1 0 1 0 3  .25   cd/ft2 0
f/1,4 1 1/2 1 6  .5   1
f/2 2 1/4 2 12 1 2
f/2,8 3 1/8 3 25  2 3
f/4 4 1/15 4 50  4   4
f/5,6 5 1/30 5 100  8   5
f/8 6 1/60 6 200  16   6
f/11 7 1/125 7 400  35   7
f/16 8 1/250 8 800  70   8
f/22 9 1/500 9 1600  150   9
f/32 10 1/1000 10 3200  300   10
f/45 11 1/2000 11 6400  600   11
f/64 12 1/4000 12 12500  1200   12
        25000  2400   13
        50000 14
        100000 15

NOTE!Colors in red are cd/ft2 for Weston Master


How to use these tables?

  • In the chart below look for Asa(Iso) speed and note the corresponding Log value(ASA°)
  • Now write down the Log value(ASA°) for your light measurement.
  • Add these numbers! Let's assume we're using a filmspeed of 400 the log value is 7 Our lightmeter tells 200 having a Log value of 5 both numbers added and we get 12
  •  
  • we want to use f8. Look for the Log value 6
  • Deduct this number from the 12 we had! 6 is the result.
  • To find the corresponding shutterspeed look under shutterspeed on the same row as Log value 6.
  • We will find:1/60sec.
  •  
  • Resume: We will use 1/60sec @ f8
  • We could also use Chart I with our earlier found number (Asa and luminance) 5 and 7 =12
  • Choose your aperture f8 (in our case) go down this column till the number 12
  • On the far left in this row we will find 1/60sec for our Shuttertime!

Conversion from Candelas per Meter2

 (candelas = candles)
  • to Ft-Lamberts : cdm2 * .29188558
  • to Stilb : cdm2 * 10^-4 = 1 cd/cm2
  • to Apostilb : cdm2 * 3.142
  • to Lambert : cdm2 * 3.142 x 10^-4
  • to Millilambert : cdm2 * .3142
  • to Candela/ft2 : cdm2 * .09290
  • to Candela/in2 : cdm2 * 6.452 x 10^-4


1 Candela /ft2 = 10.9576608 Candela / m2
Conversion to other units luminance.
1 Candela / m2 = 0.0912604 Candela / ft2
Conversion to other units illuminance.


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