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...............................
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.
Last edited by Christopher Walrath; 06-08-2011 at 10:24 PM. Click to view previous post history.
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 = firstname.lastname@example.org + 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.
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.
I go this way without meter: each film has a 'speed' (for which I use an EV) for daylight (K4000 to 10000) and tungsten (K2500 to 3400). Here is how I simplify matters and end up with essentially what Ansel did with his unduly intricate "ZONE" system.
For example, Tri-X has the following 'speed' in my mind: Daylignt: EV 15 to 2 and Tungsten: EV 8 to 2. What this means is this: for me, Tri-X uses a minimum exposure outdoors of EV 15 (I KNOW that this is overexposing for most but I develop less and love shadow detail; besides David Vestal originated the legitimacy of overexposing Tri-X in his 'Craft' book) and a DIM (almost dark, past dusk) EV of 2. For indoor tungsten (or fluorescent) lighting a very brightly lit room (Walmart's restrooms with all the white tile example) uses an EV of 8 and, again, the DIM tungsten (almost dark) uses the same EV of 2 that was employed for daylight.
In this way I know the outer parameters for exposing with any film. For T Max 100 simply use Daylight EV of 13 to 0 and Tungsten EV of 6 to 0. For Pan F it's Daylight EV 11 to -2 and Tungsten EV 4 to -2. You quickly develop the ability to judge the current lighting situation when you have these outer parameters firmly established. And you also have the flexibility to 'overrate' film (as most will choose to expose less than I do). For example, most will judge Tri-X as Daylight EV of 17 to 4 and Tungsten EV of 10 to 4. Hope I have not been too complicated but it does work. - David Lyga
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