Chan's exposure formula
After reading the other thread about Ansel's exposure formula I would like to post here the way I do it just for the fun of it. To some it can be boring so please excuse me.
Basically it's a way to arrive at aperture and shutter speed value with the meter reading in LV rather than cd/ft^2 as most meter readouts are in LV and not cd/ft^2. And it should be easy enough (may be not) to do it in my head instead of using a calculator dial (knowing that most meters do come with the dial). However, even when I don't use a meter I would estimate a scene brightness in LV so it works for me.
First the hard part, I assign every aperture, shutter speed and ISO value a number. It's really not hard to remember them by heart.
f/1.0 = 0 1 sec = 0 ISO 100 = 0
f/1.4 = 1 1/2 s = 1 ISO 200 = 1
f/2.0 = 2 1/4 s = 2 ISO 400 = 2
f/2.8 = 3 1/8 s = 3 ISO 800 = 3
f/4.0 = 4 1/15 s = 4 ISO 1600 = 4
f/5.6 = 5 1/30 s = 5 ISO 3200 = 5
f/8.0 = 6 1/60 s = 6 ISO 6400 = 6
f/11 = 7 1/125 = 7 and so on
f/16 = 8 1/250 = 8
f/22 = 9 1/500 = 9
f/32 = 10 1/1000 = 10
f/45 = 11 1/2000 = 11
f/64 = 12 1/4000 = 12
and so on.
in the case of f/0.7 it's -1, 2 sec = -1 and ISO 50 = -1 I hope you get it.
Now when making the measurement or measurements in LV I would figure out the LV I want to use for the exposure. For example if I measure LV 12 and want to place it as zone III then the LV I use would be LV 15. Or if I make 2 readings of LV 16 and 10 then I decide to set the exposure in between it would be LV 13.
Now take the first example that I want to exposure for the LV 12 it would be EV 12 if I use ISO 100 film but I am using ISO 400 film so the EV would be 12+2= EV14. Now I pick either an aperture or shutter speed I want to use and let pick the shutter speed in this case of 1/250 which has the value of 8. EV14-8= 6 leaves me the value of 6 for the aperture which is f/8.
It also makes it easier for me to do it in 1/3 stop increments. For example if I decide to expose for LV 12-2/3 and I am rating my tri-x at ISO 320 (which has the value of 1-2/3) the EV= 12-2/3 + 1-2/3 = 14-1/3. Picking my shutter speed of 1/500 (value of 9) then the value for my aperture is 14-1/3 - 9 = 5-1/3. 5=f/5.6 and another 1/3 makes it f/6.3.
Any way it may not make any sense to any of you...............................