I use my K20D as a
de facto light meter. I do this afterwards using the EXIF info to work out the Light Value (LV) that produced the exposure, but to some extent, you can so it in the field too.
First off, consider the "Sunny 16" rule:
The basic rule is:
"On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting]." Sunny 16 rule - Wikipedia, the free encyclopedia
For example:
1/100s,
f/16, ISO100
1/125s,
f/16, ISO125
1/200s,
f/16, ISO200
A Light Value calculation shows that these EXIF values indicate a 14.6 LV level (a sunny day). See
here for my LV calculator.
A link explaining EV (Exposure Value) is
here. Look at Table 2.
EV is related to LV, but EV is for ISO100, aka EV
100, whereas LV incorporates ISO changes.
1 EV = 1 LV = 1 Stop.
At the beach I've found the light maxs out at 15.6 LV. Using a
f/16 aperture (I don't usually stop done this far due to diffraction softening, but I'm showing it here to relate it to the Sunny-16 example), some examples of the shutter speed & ISO to reach 15.6 LV are:
1/200s,
f/16, ISO100
1/400s,
f/16, ISO200
1/800s,
f/16, ISO400
or in the example in the mentioned Wikipedia link:
1/100s,
f/22, ISO100, since
f/16->
f/22 is 1 stop less aperture area/less exposure. So to produce the same exposure level on the sensor or film, the light level must have been 1 stop/EV/LV higher.
So you could say 15.6 LV brightness requires the application of the "Very Sunny 22" rule.
Say, you're the sort of person who likes to walk around with the manual settings of
1/125s,
f/8, ISO100. That's 13 LV (cloudy bright - no shadows)
While 1/125s,
f/5.6, ISO100 is 12 LV (heavily overcast or in the shade on a sunny day), because,
f/8 ->
f/5.6 is 1 stop greater aperture area.
So, if you can remember these two:
1/125s,
f/8, ISO100 = 13 LV
1/100s,
f/16, ISO100 = 14.6 LV
it's not hard to work out ball park LVs in the field using just mental arithmetic, at least with full-stop steps in ISO, shutter speed & ISO sensitivity. With .33 or .5 stop steps it becomes more difficult and that's where a LV calculator comes in handy.
Note: the camera-as-LV-meter method assumes that the shot is well exposed.
Dan.
Last edited by dosdan; 10-15-2010 at 05:14 AM.