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

[Bill Burk]

Every day I wake up with my mind refreshed and ready to learn everything I ever knew all over again from scratch...

 

[Bill Burk]

Currently having fun browsing the Tone Reproduction chapter of this book...

https://books.google.com/books?id=SyW3Pb9qqrkC

 

[BrianShaw]

T

Every day I wake up with my mind refreshed and ready to learn everything I ever knew all over again from scratch...

hat happens to me three times per year: the day after New Years Eve, St Paddys Day, and Cinco de Mayo. Constantly rebuilding brain cells and neural connections is jus too much effort to be doing every day!

 

[Bill Burk]

I graphed the chart of K and C values given at http://dpanswers.com/content/tech_kfactor.php and added some pertinent labels.

I verified that when I set the Sekonic L-758DR exposure compensation in reflected light mode for -0.6 EV compensation... lumisphere and reflected light readings from gray card read the same as each other a few times. This suggests that if I wanted to make the incident meter lumisphere agree with 18% gray card, -0.6 EV compensation would do the trick. If I wanted to match the flat disk to the 18% gray card, I would set reflected light mode for -0.1 EV or -0.2 EV compensation.

I have not yet decided I really want to cancel this standard calibration to fit the idea that a reflected light reading of 18% gray card should agree with incident readings, so I reverted to 0.0 EV compensation.

 

[Chan Tran]

With the flat disc your meter would read perfect incident and reflected with a 15.6% gray card because your meter has K=12.5. Which is --0.16 EV and that's why you sometimes get -0.1 and sometimes -0.2.

 

[Diapositivo]

Whatever shade of grey the light meters are calibrated to, the K factor is a huge red herring.

In another thread I think it was Benskin who clearly stated that the K factor is a numeric factor to take into account things like flare or response of light meter to different wavelenght, in the process of converting a certain luminance to a certain exposure value given a certain film speed.
K = 12.5 doesn't mean the light meter is calibrated to a 12.5% grey or K = 14 doesn't mean the light meter is calibrated to 14% grey.
Benskin gave some tons of mathematics and added some ANSI standard for good measure. Not that I read or understood it. I just took it on his word, which is confirmed by other sources.

The question of which exactly shade of grey is meant to be "matched" by a light meter is probably still open for debate, although I think that pretty conclusive evidence was reached in favour of good, old, plain 18% (Minolta, Pentax) or just slighly less (Sekonic).

K factor should be dismissed as a huge red herring and forever banned from any photographic forum. The problem is that Saint Ansel from San Francisco made a polemic about it (misunderstaing its meaning) and now that became gospel.

 

[Diapositivo]

Regarding disc or sphere, I understood this:

The disc gives the correct answer for plane subject. The disc is the only incident method that can be compared with a grey card. The disc is the only method that gives correct exposure for flat subjects, such as documents or stamps.
The sphere is a smart "collector" of light coming from different directions. It's like a funnel.
The comparison of disc and sphere is never possible. The sphere might, in a certain light condition, capture more light than the disc, or less light than the disc.
The sphere is an intelligent attempt to give one measure that is an average of two light sources.
If your subject has the sun at 45°, and one cheek more in light than the other, one cheek being lighted only by the sky, the sphere gives you an average of the lights on both cheeks, it collects the sun light on one side, and it collects the sky light on the other side, and mixes them into one average.
IMHO averaging two readings with the lumisphere defeats the real reason why a lumisphere exists. The lumisphere is already an averaging device, and works very well when the contrast of illumination is not dramatic.
When things begin being complicated only a spot light meter (or two incident measures with the disc pointed at the light sources, in order to have a precise appreciation of the contrast of illumination) will give you firm ground. Averaging two averages will not give you a firm ground to know whether you really are protecting your highlights or your shadows.
That's all IMHO.

 

[Bill Burk]

The question of which exactly shade of grey is meant to be "matched" by a light meter is probably still open for debate

The graph that I drew illustrates exactly which shade of gray card correlates to three example values of C.

Also graphed is the EV correction from 18% gray.

At this point, I tentatively believe the deviation due to what Ansel Adams called K... amounts to -0.16 EV.

And anyone who has trouble getting an incident meter to agree with a reflected light meter should use the flat disk to compare the two modes... because I tentatively believe that using the hemispheric dome is just not the right thing to do when you want to compare to a flat gray card.

 

[Bill Burk]

Regarding disc or sphere, I understood this:

The disc gives the correct answer for plane subject. The disc is the only incident method that can be compared with a grey card. The disc is the only method that gives correct exposure for flat subjects, such as documents or stamps.

This is what I think too. Actually your whole post makes perfect sense.

 

[Arklatexian]

Thanks all, lots of great thoughts. Like I got to get that package off to Dwayne's pretty quick.

I've got a good grasp on process control thanks to my background in film-based prepress. I can pretty much call my CI for each sheet I process.

I'd like to be able to spot on a shadow and if I step down 4 zones, have it land on 0.1 (though I would normallly step down 3 zones for an extra stop of detail).

I just don't want it to turn into a fudge system where I lose another third stop for K factor and another full stop for safety factor and another stop for flare. I just want to nail down the facts and place my exposure on the film where I want it to be.

Then if I am off by one whole stop I won't care because I'll be covered.

I can't answer your question and I would like to ask you a question. I think that I am correct when I say you are famaliar with Fred Picker's way to find your own EI based on your equipment and technique. If you use that system, wouldn't the EI that you arrive at take in consideration all of these variables? And would you not be relying on any other EIi that relys on the film to correct any mistakes. The old timers who helped teach me photography always seem to say that there is only one correct film speed and anything else was, as you said "fudging".......Regards!

 

[Bill Burk]

I think that I am correct when I say you are famaliar with Fred Picker's way to find your own EI based on your equipment and technique. If you use that system, wouldn't the EI that you arrive at take in consideration all of these variables? And would you not be relying on any other EI that relys on the film to correct any mistakes. The old timers who helped teach me photography always seem to say that there is only one correct film speed and anything else was, as you said "fudging".......Regards!

Yes, you're right... I've studied Fred Picker's newsletters and a book or two. In fact one of his main gripes was that people will argue a point to death but never go out and test to see what the real answer is. So if I were to honor his gripe... I'd go shoot some TMAX 100 noting the exact incident and reflected light readings... develop the film and see how far off my results are from what I expect is the standard metered point at 0.08 meter candle seconds.

I don't do film tests the way Fred Picker did, but I do find myself most comfortable shooting 400 speed black and white at 250.

 

[Stephen Benskin]

Currently having fun browsing the Tone Reproduction chapter of this book...

https://books.google.com/books?id=SyW3Pb9qqrkC

I have the full version of Photographic Materials and Processes. It was my bible for some time. In fact, I used it so much the pages were falling out of the binding. Eventually had to buy a new copy.

 

[Ashfaque]

Thanks for this fascinating thread everyone - especially Stephen and Bill. So I have started reading Connelly (TJPS, vol.16, 1968). However, page 189 is missing from the article. I couldn't find any soft copy of it. Is it possible for anyone to kindly post the missing page.

Thanks

 

[RalphLambrecht]

I recently had an opportunity to view the effect of light meter calibration constants when I was calibrating my homemade digital lightmeter. In this case, since it was an incident light meter, the constant is referred to as C and the typical values are 200-300 since the units of illuminance are lux rather than units of luminance, as is the case with reflected light meters and K. Anyway, changing the constant from 150 to 300 shifted the whole EV vs. illuminance curve up and down slightly. In other words, arriving at a custom film speed would do the same thing as changing the constant. So in your case I wouldn't adjust the constant. In my case, I will probably adjust the constant to whatever it needs to be to put the meter right on EV 15 (E.I. 100) in sunlight. Seems as good a reference point as any.

That's what I do too.the sun is a very consistent light source.EV 15 in direct sunlight works every time; at least in sunny Florida.

 

[RobC]

topic has been running for nearly six years and you're all still disucssing what a K factor really means. LOL.

 

[Stephen Benskin]

topic has been running for nearly six years and you're all still disucssing what a K factor really means. LOL.

There are always new people.

 

[Bill Burk]

Thanks for this fascinating thread everyone - especially Stephen and Bill. So I have started reading Connelly (TJPS, vol.16, 1968). However, page 189 is missing from the article. I couldn't find any soft copy of it. Is it possible for anyone to kindly post the missing page.

Thanks

Stephen has the missing page, I'll work with him to get that file replaced with a complete copy.

 

[Diapositivo]

Thanks for this fascinating thread everyone - especially Stephen and Bill. So I have started reading Connelly (TJPS, vol.16, 1968). However, page 189 is missing from the article. I couldn't find any soft copy of it. Is it possible for anyone to kindly post the missing page.

Thanks

This is the single missing page given by Stephen, in the meanwhile you can use this:

 

[Ashfaque]

Thank you all very much.

Another request: If possible, can you please scan the images in higher resolution. I printed the article (on a laser printer) but still can't understand some of the texts in those images.

Bests,
Ashfaque

 

[Stephen Benskin]

Thank you all very much.

Another request: If possible, can you please scan the images in higher resolution. I printed the article (on a laser printer) but still can't understand some of the texts in those images.

Bests,
Ashfaque

The scans are from Xeroxed copies. Most of the papers were scanned to upload onto Apug which is a balancing act between resolution and file size limitations.

Is there a particular paper you are referring to?

 

[Ashfaque]

The scans are from Xeroxed copies. Most of the papers were scanned to upload onto Apug which is a balancing act between resolution and file size limitations.

Is there a particular paper you are referring to?

Hi Stephen,

I am referring to:
Connelly, D. (1968): "Calibration Levels of Films and Exposure Devices," The Journal of Photographic Science, Vol. 16, p. 185-193

I found a copy through google hosted on APUG (the very 1st search result).

Anyway, I'm typesetting the whole thing (on LaTeX) for myself - that way I can read it without squinting my eyes on my mobile device. If anyone is interested, pm me.

Bests,
Ashfaque

 

[Xmas]

I'll have a look at my auto but they normally have a screw in toe hitch in the boot (trunk?) other wise you might get half an auto.

It is really only a hold down for transport.

The cross brace on mine would take off the rad too.

I only assume meter is good to one stop but mine are all close to one another 1/3 or so.

If it is static bracket.

 

[Ashfaque]

You'll find a freshly typeset copy of CONNELLY (TJPS, 1968) on Google drive. The text identical with a slightly different referencing technique. I was bored!

Anyway, let me know if you come across any mistake. I'll update the file accordingly.

Bests,
Ashfaque

 

[Andrew O'Neill]

"If you use that system, wouldn't the EI that you arrive at take in consideration all of these variables?"

This is my thinking, too.

 

[Stephen Benskin]

You'll find a freshly typeset copy of CONNELLY (TJPS, 1968) on Google drive. The text identical with a slightly different referencing technique. I was bored!

Anyway, let me know if you come across any mistake. I'll update the file accordingly.

Bests,
Ashfaque

Excellent job.