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10-30-2008, 06:02 PM   #1
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K10 Stop Values

Please help me make sense of the stop values on the K10!! I'm enrolled in an online course on How to Meter Light (BetterPhoto.com), and last week's assignment was to upload 3 images showing equivalent exposures. Let me describe my method of shooting for this assignment, and please note that in the Menu under EV Steps, I have ˝ stop increments selected:
(1) Set up the tripod
(2) Left metering in MultiPattern, thinking the scene was all pretty mid-toned (more on that later)
(3) Used Manual to shoot all 3 images .. metered using Green button, and adjusted from there
(4) 1st shot: (as metered) ISO100, f/19, 1/8sec
(5) 2nd shot: (4 clicks of SS, 4 clicks of AP to move 2 total stops) ISO 100, f/10, 1/30sec
(6) 3rd shot: (4 clicks of SS, 4 clicks of AP to move 2 total stops) ISO 100, f/4.5, 1/125sec
No matter how many times I practiced this, the f/4.5 shot seemed darker, not sure why.. (and, I do notice they were all a bit underexposed, but that’s a whole different topic!!!!!)

When I uploaded the images, the BetterPhoto site grabbed the EXIF data, and labeled the photos a little different:
f/19 @ 1/8 (same)
f/9.5 @ 1/30 (I had f/10)
f/4.8 @ 1/125 (I had f/4.5)
I’m not sure why the website “rounded” two of the values …you'll see in a minute that my instructor commented on that.

I know that to increase/decrease a stop value, it's basic 2X math , However the k10 has the following Aperture settings: f/4, f/4.5, f/5.6, f/6.7, f/8, f/9.5, f/11, f/13, f/16, f/19, f/22, f/27
Q: What’s up with the ‘in between’ values .. I see the flexibility they provide, however they throw off my ability to count accurate stops. I would think accurate stops should be f/4, f/8, f/16, f/32 … right?

My instructor’s feedback indicated that my stop values are not exactly correct. It sounds like it has something to do w/ how aperture settings are on Pentax. Not to bore you with his whole critique (he can be wordy!), here are the relevant highlights:
"First off, I think the EXIF data with the upload is correct (slightly different from yours)- you are working in 1/2 stop increments. So the exposures below would be equivalent (all at ISO 100):
f/4.8 @ 1/125
f/9.5 @ 1/30
f/19 @ 1/8
So as far as capturing Equivalent Exposures (EE) you came close but I'm not sure if you did it (you did the right thing clicking 4 clicks to go 2 stops, but the settings don't match- mainly on the first one. If it is f/4.5 and not f/4.8, then the first exposure should be 1/6 stop brighter than the rest (barely noticeable) but since it's slightly darker, then maybe the outdoor light diminished between exposures- that's all I can think of not being there.
I'm not sure that multi-metering mode was the best in manual- you can't really alter from the exposure since it's hard to tell what and where it's metering (weighing how much on what area) and the exposures are a tad dark in the overcast light- I think it might have been better to use Spot or Partial and meter the white wall, then overexpose from the meter's recommendation by almost 2 stops (or meter the wood in the middle for middle grey)."


Q: What am I missing? Was my method to "count the clicks" not effective, based on my camera's settings? This really shouldn’t be that difficult, I realize!!

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10-30-2008, 06:27 PM   #2
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QuoteOriginally posted by deludel Quote
No matter how many times I practiced this, the f/4.5 shot seemed darker, not sure why..
Chances are the lens you used isn't *really* opening up quite as wide as f/4.5. If that's the maximum aperture for the lens (or the maximum aperture at that focal length, if a zoom), that's actually pretty typical. "Most" lenses - especially zooms are a bit optimistic in quoting their maximum aperture.

QuoteQuote:
When I uploaded the images, the BetterPhoto site grabbed the EXIF data, and labeled the photos a little different
My understanding is that aperture info is stored as a fraction in EXIF using some arcane formula. I don't know the details, but I guess it wouldn't surprise or bother me if different applications (including the camera as one application) interpreted this a little differently.

QuoteQuote:
the k10 has the following Aperture settings: f/4, f/4.5, f/5.6, f/6.7, f/8, f/9.5, f/11, f/13, f/16, f/19, f/22, f/27
Q: What’s up with the ‘in between’ values .. I see the flexibility they provide, however they throw off my ability to count accurate stops. I would think accurate stops should be f/4, f/8, f/16, f/32 … right?
No. F-stops are in ratios of the square root of two - approximately 1.4. Meaning one full stop smaller than f/4 is f/5.6. The 4.5 is the half-stop between 4 and 5.6. Or at least, that's what the camera calls it. Mathematically, it should probably be 4 times the square root of 1.4, which is more like 4.8.

QuoteQuote:
My instructor’s feedback indicated that my stop values are not exactly correct. It sounds like it has something to do w/ how aperture settings are on Pentax.
Yeah, as mentioned above, there is a sort of tradition for calling this 4.5 even though my calculation suggests it should be different. Similarly for other half-stops. No idea where that discrepancy came form, but I wouldn't worry about it. It's half way from 4 to 5.6, and that's all that really matters.

QuoteQuote:
"So as far as capturing Equivalent Exposures (EE) you came close but I'm not sure if you did it"
You can tell your instructor that's just how Pentax labels their half-stops. You did it right.
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