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05-24-2011, 07:10 PM   #1
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Discovery: TAv Mode can directly measure SBR (Subject Brightness Range)

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, set 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.
05-24-2011, 11:30 PM   #2
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Sorry for my ignorance but how do you use that information? What does SBR = 3200/100 = 32 mean? It's not f-stop as from 100 to 3200 would be 5 f-stops. Please explain.

Regards,

Peter
05-25-2011, 11:23 AM   #3
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You're right, it's a 5 EV contrast range. I use it to give me an idea of the contrast in the scene and therefore how much can be captured by my DSLR. Generally, you can capture about 5 EV of good detail, and 7 EV with hints of detail. Beyond that you have to decide whether highlight or shadow detail is more important, then bias your exposure in that direction with the consequence being that you lose info at the opposite end of the tonal range.
Others have given detailed descriptions about it, here's some links(google):
Subject Brightness Range (SBR) explanation and illustration
Photography & exposure: Subject brightness range (SBR) | The D-Photo or Photography & exposure: Subject brightness range (SBR)
Advanced Ansel Adams Zone System
05-25-2011, 05:41 PM   #4
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Thanks Bruce. After reading the Ansel Adam Zone System, I think I understand it better now.

Sincerely,

Peter

05-25-2011, 06:23 PM   #5
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Given the K-5 capture 14 EV's @ ISO80, wouldn't that mean a 7EV scene is no problem?
05-26-2011, 02:28 AM   #6
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Can't you do the same in every mode? Tv or Av. What's special about TAv ?
05-26-2011, 04:06 AM   #7
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QuoteOriginally posted by yusuf Quote
What's special about TAv ?
I guess because it isolates the variation in sensitivity.
05-26-2011, 11:42 AM   #8
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QuoteOriginally posted by twitch Quote
Given the K-5 capture 14 EV's @ ISO80, wouldn't that mean a 7EV scene is no problem?
Good question. I don't know the answer to that and I wasn't aware that the K-5 has a 14EV dynamic range. That's an incredible dynamic range I didn't know the current digital sensors were capable of performing at yet. Are you sure that's correct for image capture??? That exceeds b/w film when using push/pull tricks. I guess the ever improving digital technology is shifting old paradigms and this may be another one???
I suppose you'd have to devise a series of tests with image capture in high-bit RAW format and also upsample to 16bpc for more analysis.

Bruce
EDIT: I wonder if you're confusing 14bit RAW capture with dynamic range???
EDIT2: After giving you time to reply, I checked DP Review. K-5 dynamic range plots show about 9EV which is more believable than 14EV. I don't know where you found your info that dynamic range is 14EV????


Last edited by BB_Zone28; 05-28-2011 at 11:53 AM.
05-26-2011, 11:46 AM   #9
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QuoteOriginally posted by dadipentak Quote
Can't you do the same in every mode? Tv or Av. What's special about TAv ?


I guess because it isolates the variation in sensitivity.
That's correct. The only exposure parameter that changes in TAv mode is ISO sensitivity. Plus, presetting it to display ISO 100 simplifies the reading of the result.
05-26-2011, 12:18 PM   #10
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I don't get it either, for instance measuring darkest and lightest parts of the scene with Av mode, exposure times difference will give you same EV range simply, isn't it? Why TAv mode with formulas?
05-27-2011, 10:35 AM   #11
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QuoteOriginally posted by cbaytan Quote
I don't get it either, for instance measuring darkest and lightest parts of the scene with Av mode, exposure times difference will give you same EV range simply, isn't it? Why TAv mode with formulas?
Carefully note that I described using TAv mode to directly read SBR.
Sure, you can measure the light & dark in any mode using any parameter, but then you have to count the number of EVs and then calculate what that ratio represents, e.g. 5 EV is equivalent to a 32:1 ratio. However, if you use TAv mode and set the brightest reading to display ISO 100, then the darkest reading would be 3200 (for this same example), so basically you have a direct reading and just have to ignore the two zeros (or call it dividing by 100). I don't know how to explain it any clearer than that!
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