In the spirit of Pentax's "BE" idioms, here's a "BE RESOURCEFUL" idea
I stumbled on this recently after being inspired to consider other possible uses for the TAv mode after reading a few threads discussing some other uses for it on this site.
It has its limits, but it is another use for the TAv mode!!!!
What I discovered is that
TAv Mode can be used to directly measure SBR (Subject Brightness Range) in a scene for use in conjunction with the Zone System for evaluating and setting exposure. Measuring SBR is nothing new, it's just a measure of the dynamic range (or contrast range) in a scene. Usually measured using a spot meter aimed at the brightest and the darkest part of the scene and taking the ratio of the two values.
1) To begin, s
et your Pentax DSLR to TAv mode and change your camera's built-in light meter to the spot metering pattern. I'm using a Pentax K20D.
2) Next,
aim the spot meter at the brightest part of the scene and adjust the shutter (T) and aperture (A) values so the combination yields ISO 100 in the viewfinder display. It's overcast today and I found aiming at the sky yielded ISO 100 for T=1/60 and A=f/8.0.
3) Then
aim the spot meter at the darkest part of the scene and use this formula:
SBR = ISO / 100.
For my overcast day example, some landscaping using railroad ties under a group of small trees was the darkest. It made the K20D viewfinder display an ISO value of 3200.
Using the formula, SBR = 3200 / 100 = 32 .........ISN"T THAT SIMPLE!!!!!!!
Small correction: Technically, SBR should be shown as the ratio 32:1, but 32 is all you really need to know.
Limitations: I know this is a small SBR, but it is an overcast day, so I could get away with an ISO 3200 reading on my K20D. On really brighter days, my K20D doesn't have enough ISO range to work. HOWEVER, all you fortunate
owners of a PENTAX K5 with it's wonderful extended ISO range can be used up to SBR = 128 and
SBR = 512 (via custom function) which isn't the brightest possible day, but its close to typical day (not extremely bright), but way closer than SBR=64 measurable on the K20D will ever be.
That's about it!
Bruce
BTW, I discovered this while working on some class material for a B/W Digital Photo class I teach at Front Range Community College in the NW Denver, CO area.
Last edited by BB_Zone28; 05-24-2011 at 07:25 PM.