Doing crossword puzzles is an impossible task for me. I simply cannot solve puzzles. Getting nearly straight A grades in the accounting curriculum and passing the CPA Exam were easier. Figure out that craziness.
But figuring 'light' intensity, despite the changing luminary parameters, can also be a formidable task. When is shade, shade? Easier said than done but the key here is making 'shade' mean something very personal to you and not ever deviating from that perception.
Slowly learning how to harness this complex subject can, at least, be made easier by forgetting 'f stops' for a while. In fact, why are there such things as f stops at all for other than theoretical verification of that all important concept of the focal distance (to film plane, at infinity) divided by lens opening diameter ? Hollywood long ago entered the real world with their sensible 't stops' zeroing in on the ACTUAL light transmission. Much more rational. (Do I sound like the pragmatism of David Vestal yet?)
There is a certain freedom with taking an extension of your eye, a mechanical camera with adjustments available, and 'knowing' how to expose a certain film. I am still learning after decades of frustration but I am certainly better at it. It is intellectually revealing how right or wrong one can be and this exploration is perfect ammunition to circumvent, even diffuse, an overconfident ego. But there are certainly hurdles here: for example, shade at noon is not necessarily shade at noon merely a block away. Much depends upon the amount of sky allowed to illuminate the scene and whether clouds intervene. Then again, twilight is not an objective measure either: When is the sun low enough? Are buildings or mountains adding to the 'light value' confusion? But there is a kind of beginning benchmark with open, noon sunlight hitting an open space. But even here one has to look just how low that sun is in the dead of winter conflated with how high one's latitude is on planet earth.
I offer some tips that have helped me in this endeavor. First, translate 'f stops' to the less cumbersome light value system: f2 is '2', f2.8 is '3'....f16 is '8', f22 is '9. Then do the same for the time part: 1 sec is '0', 1/2 sec is '1'....1/30th sec is '5'.....1/250th sec is '8'....1/1000th sec is '10'. (You fill in the grammatical ellipses.)
Thus, we have a convenient, transferable way to express exposure at least, even if we have still not mastered light itself. To add to this convenience, rate film the same way. Consider a 400 ISO film. For me, I rate negative film about 2/3 stop less than the manufacturer says to. I would, thus, rate HP5+ or Tri-X at EI 250. Translated into light values this becomes (combined light value) LV 16. (NOT downgrading this speed from 400, this LV would be 17 if the manufacturer's recommendation were to be followed.) Similarly, I rate Pan F+ at EI 16. My 'rating' expressed in LVs is 12. What does all this mean?
Simply, this 'rating' is the amount of exposure given to a brilliant, sun lit, noontime, 'no holds barred' scene that is the epitome of what nature can provide in terms of light intensity. That is my outdoor benchmark. In other words I would expose HP5+ at 'LV 16'. This is the combination of aperture and shutter speed that I would use. For example, that would translate into f22 plus 1/125th sec, or f11 plus 1/500th sec. Both, conveniently and easily, add up to the '16' you want. In the former it would be 9 + 7, in the latter it would be 7 + 9. This is why I convert both apertures and shutter speeds into these easy numbers.
That's for brilliant sunlight. For other values: My exposure chart column is stated as follows for outdoor lighting: SUN SHD TWL DIM with each category giving four more stops (or, more correctly, steps) exposure than the preceding. This means that HP5+ starts at '16' for sun and proceeds to '12' for shade, then '8' for twilight, then '4' for dim daylight. So for MY interpenetration of 'shade' I would expose HP5+ at, say, 1/60th sec and f8, or '6' + '6' to get the wanted '12' LV. For MY interpretation of dim daylight I would expose HP5+ at, say, 1 sec and f4, or '0' + '4' to get the wanted '4'. You can see that it becomes extremely easy to change the aperture and shutter speeds to accommodate the exposure as all you have to do is add up to the proper combined LV.
The work comes with mentally anchoring the amount of light that 'sun' 'shade', 'twilight', 'dim' are supposed to represent in order to get proper exposure. For example, if the negative is too dense, you judged the light as too intense. 'Shade' does not necessarily have to be a shaded situation. It can be a level of overcast that equates with the level of proper shade. Likewise, 'TWL' does not have to be actual twilight; it can be a dark alleyway whose light intensity equates with the proper level of actual twilight. This is what really 'teaches' light, folks: refusing, on occasion, to bring your 'crutch' light meter with you. It is an exercise that forces you to come to grips with the reality of actual light intensity and correctly nullifies subjective interpretations that have little to do with this actual measure, such as assuming that shade is always of nearly the same intensity, or that twilight is 'when cars turn on their lights'. It forces you to really 'see' light in objective terms and removes the 'romantic' aspects of light interpretation that can easily get in the way.
The other side of my exposure chart is for 'tungsten light'. This side is a bit more complex, as the sensitivity of chromogenic (color or B+W) films are markedly more sensitive to the lower Kelvin (say 2800 K) than are traditional black and white films. (I assume NO filtration in all my examples). (For convenience, and usual accuracy, consider fluorescent to be the tungsten equivalent.) My side of the chart for tungsten is as follows: HIGH MED LOW DIM Each category requires two stops more exposure than the previous. For example, again using HP5+, a traditional B+W film, my rating for tungsten starts with '7' for HIGH, then '5' for MED, then '3' for LOW, then '1' for DIM. Here, importantly to understand, is that the SAME intensity exists for daylight's "DIM" and for tungsten's "DIM": the considerable difference in exposure (daylight dim = 4 while tungsten dim = only 1) is because traditional B+W film is very slow when exposed to tungsten light. The ACTUAL intensity of both 'dims' is the same. With chromogenic films this interpretation is more in line with human perceived value. With, say either a ISO 400 chromogenic B+W film or an ISO 400 color film, the tungsten chart is: '10' for HIGH, '8' for MED, '6' for LOW, '4' for DIM. Again, this assumes no 'proper' blue filtration for the chromogenic films. You can easily correct this 'lack' in the darkroom's enlarger. If you were to use the blue filtration (I think the 80A filter?) your numbers would be in line with those for traditional B+W films. Few realize just how much of a bonanza this is for available light black and white photography: an increase of three stops for using the chromogenic films without filtration.
With outdoor lighting the films' sensitivity is the same. - David Lyga