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

Discussion in 'B&W: Film, Paper, Chemistry' started by Bill Burk, Dec 22, 2010.

  1. Bill Burk

    Bill Burk Subscriber

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    Sometimes I feel like I'm driving in the snow with all this system calibration. You turn right when you want to go left. Turn too far and if you're lucky you hit the hillside and wait for some helpful citizen to come along with a tow line to pull your radiator off. (OK, I was the helpful citizen. How was I supposed to know the brace behind the Fiero's bumper held the radiator. The driver was really nice considering).

    I've heard that instead of being calibrated to 18% gray, most meter manufacturers include a "K-factor" which is approximately a 1/3 stop deviation away from 18% gray.

    In the specifications for my meter, the Calibration Constant is 12.5 for reflected light. I have the opportunity to change this to any number I want.

    Does the K factor improve the accuracy of my meter? Does it compensate for flare? Should I try to determine my own K factor?
     
  2. Vaughn

    Vaughn Subscriber

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    IMO -- You'll be lucky if your meter is within 1/3 stop calibration one way or the other. If you are shooting B&W, just use the meter and judge from the negs where you need to go.
     
  3. RalphLambrecht

    RalphLambrecht Subscriber

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    Bill

    K-factors vary from manufacturer to manufacturer. Some use '12.5' other use '14'. You'll find more info about this on wikepedia and the rest of the web. Nevertheless, I agree with Vaughn, for all practical purposes, it's better to consider your camera, meter and exposure method as one variable. As long as you keep that consistent, your results will be sufficiently consistent to make consistently good negatives. It won't hurt to assume that your meter is calibrated to 18% reflection, even if it isn't (and most likely it won't). Everything else will make your head spin in a hurry. I know, I've been there! Important is that your film receives enough exposure for the shadows to have detail and that development is keeping your highlights under control.

    If you are, however, interested in more technical detail, check with another APUG member, Steve Benskin. I consider him to be the expert on this subject.
     
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  4. CBG

    CBG Member

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    If you are trying to match some standard meter or standard reading then by all means apply a "K" factor so you are calibrated that way. Otherwise creating your own working film speeds can wrap up all corrective factors in one step.
     
  5. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    Forget about it.

    Light meters of different designs perform differently when tested under 'ISO reference conditions' - where 'calibration' is established - and when they are used to take pictures in the real world.

    The 'K factor' is the fudge factor that makes up for this difference.

    The real test of a meter used to be "Does it give a properly exposed Kodachrome slide?" where a being off a 1/3 of a stop is noticeable. Kodachrome was the standard because processing was rigorously controlled and Kodachrome doesn't have a wide dynamic range. If a meter didn't produce a proper Kodachrome slide it got returned to the store and exchanged for one that did.

    When shooting black and white this is all irrelevant. The dynamic range of the film is huge and the processing control is close to non-existent unless you are in the movie industry.

    The value of going through the folderol of establishing a personal E.I. is that it is good training in metering and processing. The end result is always the same: "Expose Tri-X at 320." The speed dichotomy arises because the 'personal EI' methodology isn't the same as the ISO methodology. Set your meter at whatever it is that gives you the negative you want and be happy.
     
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  6. Steve Smith

    Steve Smith Member

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    I expect the variance in light meter calibration is less than the variance in the way you use it (angle held, etc.) so I wouldn't worry about it.


    Steve.
     
  7. Bill Burk

    Bill Burk Subscriber

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    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.
     
  8. Stephen Benskin

    Stephen Benskin Member

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    K is more than just a calibration constant, it’s an integral part of exposure and film speed too. It’s one side of a Trinity of constants - K, q, and P. This is a question of theory and not pragmatic usage. How can you know if your exposure is correct if you can’t define what correct is, or what film speed has to do with exposure, or the relationship between film speed and exposure? Everything is interrelated. And K explains everything.

    The best source is Connelly, D, Calibration Levels of Films and Exposure Devices, Journal of Photographic Sciences, Vol 16, 1968. I have a pdf copy of it and will send it to anyone who wants it.

    I’m still trying to work out how to approach the topic here. It can be rather complex and it’s easy to get caught up in the minutia and lose the big picture, but sorry, the minutia is necessary.
     
  9. Stephen Benskin

    Stephen Benskin Member

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    Sorry, flare won't allow for any certainty.
     
  10. BetterSense

    BetterSense Member

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    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.
     
  11. RalphLambrecht

    RalphLambrecht Subscriber

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    Originally Posted by Bill Burk
    I'd like to be able to spot on a shadow and if I step down 4 zones, have it land on 0.1.

    ... and especially not for Zone I. But, my question is: Why do you spot-meter for Zone I? Most people meter for Zone III or Zone IV. Zone I has no pictorial value.
     
  12. CPorter

    CPorter Member

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    Shadow placements can be made anywhere on the toe of the curve---making a Zone I placement for a certain area may prove beneficial for the visualization and in controlling the highlight end of the scale. However, the area of important shadow detail for full texture should meter at least two zones higher in EV with that shadow placement. Doesn't Zone I have pictorial value in the sense it represents "the first step above complete black, slight tonality but no texture" in the print? I've never made a shadow placement that low, but I have on several occaisions on Zone II. A few of my portfolio pictures, while not great stuff, have shadow placements as low as Zone II.
     
  13. Bill Burk

    Bill Burk Subscriber

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    I can easily change the calibration constant on the meter, and it's dedicated to one camera, one film.

    It's OK with me if flare pushes me above 0.1 density. I just don't want to go below.

    I have some leeway in my definition of quality because I use 4x5 on a tripod - I can tolerate a denser negative with higher grain than I could with 35mm. I can tolerate lower speed.

    I know the film speed when I develop to the ASA gradient. That's what I lock everything down to. With 0.1 as my speed point, my EI will be lower at N development which, due to less development time, doesn't give full film speed.

    Davis' Beyond the Zone System points out that the EI changes with different development times. I will know my film EI for different N by where the curves cross 0.1 density.

    Tell me if I am off my rocker here: I probably can account for flare by NOT changing my EI in response to it.

    Because...

    With N+ development, developing longer because of a short scale subject - there won't be much flare.

    When N- development, developing shorter to hold down the highlights - lots of flare. Flare will bring up the shadows.
     
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  15. BetterSense

    BetterSense Member

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    Why do you think that development can cause flare to change?
     
  16. Stephen Benskin

    Stephen Benskin Member

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    It's the other way around.
     
  17. CPorter

    CPorter Member

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    But most likely with the extremes of "+" or "-" development times due to increases in fog with +2 and a decrease in fog with -2 or more---not with every change in development time. More fog from "N" will mean less log exposure needed to reach the threshhold and less fog from "N" means more log exposure needed to reach the threshhold. I have found that with +1 and -1, the increase and decrease in fog is not significant enough to alter speed. I have seen a +1/3 and a -1/3 change due to fog, but instead of changing EI, I make the adjustment with the expsoure and keep the EI constant.

    I'm sure Stephen will chime in, but flare changing with development? Not sure about that one, fog density changes with development.
     
  18. Bill Burk

    Bill Burk Subscriber

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    I meant the extreme brightness of the subject that would lead to an N- development would be the kind of situation where there would be lots of flare.

    Steve, did you mean the cause versus effect as I clarified, or did you think short range subjects are more prone to flare
     
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  19. Stephen Benskin

    Stephen Benskin Member

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    Bill,

    A shorter luminance range has less flare so the reduction of the illuminance range at the film plane is less which mean it needs less development. Longer luminance ranges have highe flare resulting in shorter illuminances ranges at the film plane requiring higher CIs.

    Sorry Bill, I read your post wrong. I got the impression you were talking about development and flare. Yes, development will compensate for the effects of flare in the shadows. But nothing is ever straight forward with flare. Flare isn't just dependent on luminance range but also on tonal distributioin. A scene with a higher luminance range may have lower flare than a normal with normal luminance range. Although thinking about it, I think lower luminance rnage are less of a problem in this regard.
     
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  20. RalphLambrecht

    RalphLambrecht Subscriber

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    I'm in favor of anything that works. However, shadow placement in the Zone System is highly subjective at best. At Zone I, it's merely a guess, because Zone I is almost black, very close to Dmax. You'll find a hint of tonality there but may need a densitometer to be sure. In my view, you need more such as some texture to have pictorial value, but more importantly, visualizing Zone III is relatively easy, visualizing Zone I, on the other hand, is close to impossible (at least for me).

    I was amazed how my negatives improved after John Sexton went even further and suggested to start shadow placement with Zone IV instead of III. The results are negatives that have density where it matters most, still allowing for plenty of shadow detail. This can be done very reliably. As I said, Zone I placement is guesswork in comparison. Well, it was in my case and there may be exceptions.
     
  21. RalphLambrecht

    RalphLambrecht Subscriber

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    Bill and Steve already addressed this, and just to put what they said into different words: Development does not change flare, but we reduce development due to lighting conditions that usually come along with high flare. Therefore, N- development is typically dealing with more flare than N+ development.
     
  22. Bill Burk

    Bill Burk Subscriber

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    Chuck,

    I can't see the fog on my graphs because I zeroed on B+F (instead of air). I'll redraw the graphs to include fog next time. But I think the .10 has to be above B+F. I don't think it would count towards film speed.

    Thanks Ralph, I know when I rated TMY-2 at 64 (vignetted camera test) and placed my shadows on II, my results were like yours: remarkable shadows. So when I get all this done I think I will place shadows on IV.

    That sounds wrong at face value but I think I know what you mean.

    I know I will develop a short luminance range subject to higher CI to expand its scale. And I will develop a longer range subject to a lower CI to avoid blocking the highlights.

    If I get what you are saying... With shorter luminance range subjects the appropriate "correction for flare" is to reduce CI. With longer luminance range the correction is towards higher CI.

    So put them together. With a long luminance range subject, you subtract time in developer primarily to keep highlights in range. Because flare raises the shadows a secondary correction is to add a little time in the developer to raise the highlights as much as flare brought up the shadows. Conversely, with a short luminance range subject, you add time in developer primarily to get a full range negative but because the shadow image is nearly ideal, you reach the full range sooner. So to correct for less flare you should subtract a little time.

    So it IS like driving in the snow. You turn the wheel the direction you want to go, but you don't turn it as far as when it's dry because you're skidding.
     
  23. CPorter

    CPorter Member

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    This I understand and which makes more sense---I processed the OP's words quite differently too. Thanks.
     
  24. CPorter

    CPorter Member

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    Before graphing my curves, I measure the fb+f density of a section of the film sheet that did not receive any exposure. I place a piece of opaque tape on the step wedge before exposing the film to it---that's how I do it anyway as it provides a larger section by which to measure the film base plus chemical fog rather than a thin area of the rebate. Consider the two as one density measurement. I typically get .04 to .05, then I would subtract that from each step density of the wedge to get a "net" step density----graph the net density. It's what I'm able to do easily and in very little time. Some people may not agree with that :tongue: though.
     
  25. Stephen Benskin

    Stephen Benskin Member

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    Bill,

    I have a nice chart that has the CIs for various developmental models at home. I think it will help illustrate the concept. Then I want to start on K.

    And while your analysis in post #21 is rather good, your conclusion is backwards. Think of flare as a buffer. It keeps you from having to too long of development times for short luminance ranges and keeps you from having too short of development times for long luminance ranges.
     

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  26. Stephen Benskin

    Stephen Benskin Member

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    Defining K - Part 1

    First DON'T PANIC.

    Before being able to discuss the merits of K, we first need to understand what it is and how it works. The only way to explain how K and the other constants function and how they interact with each other is to use math. In order to have any insight into the process, you need to see the numbers and understand where they came from. I plan to have values for each of the variables and walk through the equations so that they make sense. I'm not very good at math myself which is why I use Mathcad for the equations. I just plug in the variables and let it do all the work. I'm sure for others like me, there's some type of freeware scientific calculator or Mathcad equivalent that can be found on the web.

    So that it's not overwhelming, I plan to present it a little at a time. Part 1 is an introduction to camera exposure and the constant q, the camera exposure constant. It's attached and is a jpg file because I wanted to keep the math formatting.
     

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