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Thread: Is the K factor relevant to me or should I cancel it out?

1. What makes all the math and numbers more than just math and numbers is how they connect it all together: exposure, film speed, and the meter. There are a few numbers to keep in mind because they keep coming up.

Eg = 8
Lg = 297
q = 0.65

And the camera exposure equation:

E = q * L / A^2

2. There are a few interesting concepts that can already be teased out of the information provided in part one. At first some of it might appear like circular reasoning but more proof will be available in the next few parts.

3. Defining K - Part 2

Part 2 introduces the constant K and shows how similar it's function is to the constant q.

4. I like the way q and K stand in for so many variables.

I see C = 1.03 Camera Flare Correction factor

Does that mean if I shoot a black tin can and find my own Camera Flare is different than 1.03... then for me, the value of K would effectively change?

I hope this doesn't mean I have to get my lens calibrated in T-stops.

5. Bill,

As luck would have it, the next part answers your question about C. T-stops are a bit extreme for still work, but they are more accurate.

Part 3a begins to define the variables in the K equation. These cover the basic assumption on the optical system. Part 3b will look at the variables r, p, and R which have a more direct connection to the exposure meter and I believe the variance in K factors.

BTW, this is the first time I've made an attempt at a coherent explanation for the constants. I'm sure I have to missing steps, mistakenly assuming prior knowledge, or simply not explaining it well. Please keep up the questions.

6. Steve

When you're done, you should publish a single corrected document here on APUG. And, we should talk to Sean to open some kind of an APUG knowledge base library and add more articles like this to it. Maybe something like a photo wiki. I wish, I knew how to set up something like that.

7. Originally Posted by RalphLambrecht
Steve

When you're done, you should publish a single corrected document here on APUG. And, we should talk to Sean to open some kind of an APUG knowledge base library and add more articles like this to it. Maybe something like a photo wiki. I wish, I knew how to set up something like that.

8. Originally Posted by Q.G.
I had not. Thanks for pointing it out. Let's see where it goes.

9. This is getting bigger and more involved than I had originally intended. Part 3b defines, as much as possible, the variable R. As I point out in this part, R, q, and p come under the domain of the meter manufacturers, and manufacturers tend to be very secretive.

I've yet to find only a few values for q, but haven't found anything on what the values for R and p are considered outside of the standard's values which all equal 1. Now this might represent the average conditions, or it could be zeroing the variables out of the overall equation of K in order to produce a universal version of K as these three variables tend to be specific to each individual meter's design. So, while it's possible to define the variables R, r, and p, we can only speculate what influence they have on the value of K.

10. Sounds like R is a weighted average factor for a regular reflected light meter.

It makes sense that a spotmeter might set R = 1.

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