# Chan's exposure formula

Discussion in 'Exposure Discussion' started by Chan Tran, Jun 8, 2011.

1. ### Chan TranMember

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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...............................

2. ### Christopher WalrathSubscriber

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Sound a lot like the standard EV index that I use mentally, regularly. And for those who wish to use it, there are a couple of \'keys\' you can use to get there quick so you don\'t need to memorize the whole thing. F/5.6 equals 5. 1/60 equals 6. 1/1000 equals 10. If you need the aperture for an AV of 7, open up two stops from 5 (5.6 to f/11). If you need to figure the TV of 4, then decrease the shutter speed by two increments from 6 (1/60 to 1/15). Assuming ISO/ASA 100, of course.

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3. ### DiapositivoSubscriber

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I'm not so scientific and I found this post very interesting. I do differently as my main "problem" is to visualise, and sometimes peg into memory, the light condition.

I set in my mind that EV 15 is the "sunny f/16 rule".
Keeping 1/125 fixed, and ISO 100 fixed, is:

EV 16 = 22
EV 15 = 16
EV 14 = 11
EV 13 = 8
EV 12 = 5.6

Normally in my activity I am confronted most with values from EV12 to EV 15, outdoor photography. It's the old same few cases of film instructions: shade, veiled sun - diffused light, sun with with clear shadow on the ground, sun with very neat shadow on the ground.
In taking readings I usually always leave my light meter set at ISO 100 also when the film is faster.
The reading corresponds first to a couple in my mind. EV 12.5 = 125@5.6 + half stop.

Also when I use 400 ISO, I want my light meter to tell me EV 12.

From there I adjust by "ticks". Some "ticks" for film if different from ISO 100, and some "ticks" in both direction for reciprocity.
Let's say that I have a 400 ISO and I want to use 1/1000, that's:
Two "ticks" bring me to 1/500, "even" for 400 ISO. Another "tick" brings me to 1/1000 that I reciprocate with a tick down on the diaphragm, f/4.

If I had wanted to use f/4, I would have added one "tick" from f/5.6 to f/4, and then three "ticks" (one for reciprocity, and two added for the ISO 400) from 1/125 to 1/1000.

When I have to span across many EV of difference, I very stupidly count ticks straight on the camera. EV 6 is "six ticks down" from canonical 1/125@f/5.6. So let me see... I want to use f/8, I count seven ticks down from 1/125 to 1/2 on the shutter speed dial.

What I find interesting of EV (LV actually) is mainly memorising. I enter a church and I read the incident light and let's say on that moment it is EV 6. "6" it's easy a number to remember for the future. I can then slowly work out that I can make this shoot with f/2.8, 1/8" and a bean bag. Or maybe f/8 and 1" and a bean bag, hopefully with self-timer. Five minutes after having walked out of the church I will have forgotten which exposure I took, or could have used. Was it 1/8@f/2.8 or 1"@f/2.8? It's too easy to forget, it's too many numbers. EV 6 is easy to remember.

By the same token, I always think in terms of "EV at 100 ISO" (or LV). That church was EV 6 (LV 6) regardless of the film I am going to use in it. One day I can decide to go round with a 800 ISO film taking free-hand church interiors. What was it? EV 6. I might need some time to compute the possible time / aperture couple in my mind with all this "ticking", but I will have remembered "6" which is the important data.

Lit monuments are normally around EV4. That's 4"@f/8, or 1" @f/4, or 1/4@f/2. At what ISO was that? It's easy to mix numbers. LV4 is easy.

Remembering is important to me for a number of reasons, and centering my reading on LV helps me remembering and also helps learning to figure out exposure before measuring it with the instrument.

4. ### Steve SmithMember

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When I am not using a lightmeter I also think in terms of ISO 100 then make the compensation for different film speeds afterwards.

If I am using a meter and I have e.g. IS0 400 film and a filter which gives three stops light reduction, I will set the meter to ISO 50 so I get a direct reading already compensating for the filter.

Steve.

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