variance in camera light meters

Discussion in 'Exposure Discussion' started by David Lyga, Jan 15, 2014.

  1. David Lyga

    David Lyga Subscriber

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    I have a Canon AE-1 and an Olympus XA. Both have working meters. Both deliver identical and accurate readings for daylight situations with, of course, the same film speed. But, oddly, the XA shows two to three stops less exposure needed under incandescent lighting than the Canon requires.

    Now, most of us know that color film requires about two stops less exposure under incandescent lighting (without the blue filter) than the same speed of traditional B&W film does under that same incandescent situation. This leads me to come to a possible (but erroneous?) conclusion that the Canon metering is optimized for traditional B&W negative exposure and that the XA is optimized for color (slide?) film. Comments? The XA metering is from a CDS cell and, I believe that the AE-1's cell is silicon blue photodiode. - David Lyga
     
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  2. michael_r

    michael_r Subscriber

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    In addition to possible calibration differences and the use of different standards by manufacturers, different types of cells vary in their spectral response.

    Differences could also arise from how much of the field the meter is measuring, which also means the test scene or setup is an important variable, which introduces other potential variables like flare.

    You haven't explained the test conditions beyond (outside/incandescent). What did you meter outside? A neutral grey card? What did you meter inside? Did the test cards fill the frame? What lenses were used? Light sources? Etc. Etc. Etc.
     
  3. David Lyga

    David Lyga Subscriber

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    No, Michael, I closed all options for variance other than the actual meter responses in question. I metered 'normal' scenes in both cases: outdoors, a park scene and, indoors, a room lit by a ceiling tungsten light. In all cases the field of view was virtually identical, obviating the possibilities of different reflectance values coming into the equation. I could not come to another conclusion than the one I posited. But, your assessment that different types of cells deliver different spectral responses is something that should be discussed more fully, as the manufacturers have not done that, ever, as far as I can see. - David Lyga
     
  4. michael_r

    michael_r Subscriber

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    Some spot meter manuals give some information with respect to how they were calibrated (k factor) etc. Spectral response has mostly to do with the type of cell, which means it can depend on how old one meter is relative to another, as the technology changed over the years.

    If you want to delve into meter calibration standards, linearity, theory etc., it is interesting stuff - be warned however - it is complicated. Richard Henry did some interesting testing in his book, and had some help from a very reliable insider at the time, but even that book only scratches the surface. Stephen Benskin has posted some interesting threads here.
     
  5. David Lyga

    David Lyga Subscriber

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    Yeah, but don't you think that the manufacturers should have offered this information freely? Amazingly, the 'public' seems to have never voiced concern with these spectral differences. It just seems that it is sufficiently important an issue to have been parsed and discussed at length, and, herefofore, seems to have been not even 'noticed'! Thank you Michael. - David Lyga
     
  6. michael_r

    michael_r Subscriber

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    What I'd say is that this is just another of the sensitometric theoretical issues (along with things like exposure theory, tone reproduction theory, etc.), which very few practicing photographers have any real knowledge of, but which ends up mattering relatively little to anybody from a practical perspecitve because of two things:

    1) The materials and processes we use offer so much latitude and flexibility that with practice we produce high quality outcomes by effectively working around all the differences between what we think we're getting and what we're actually getting from our metering, exposure, development decisions - without realizing it.

    2) There are so many variables (some beyond our control) piled up in the end to end photographic process that often some of them cancel out other ones, reducing the net "error" to manageable levels we can work with.

    How else can you reconcile the fact almost everybody is wrong about what is going on, and yet people can still make great prints!
     
  7. BrianShaw

    BrianShaw Member

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    These data (spectral response characteristics of metering cell technologies) are freely available. The "problem" is that they exist in supplier specifications and engineering journals. One must not only know how to access them, but know how to read and interpret that level of knowledge. I have seen such discussion in books on photographic metering too. As for the manufacturers, as long as their product meets their customer requirements (a decent exposure most of the time) then they have no need in educating on the underlaying engineering... escpeciall with regard to how it compares to a competetors. This is true of all consumer products. It is even mroe true when we are talking historical products. :smile:
     
  8. Chan Tran

    Chan Tran Member

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    If anything I have to increase exposure for incandescent lighting with color film unless it's balanced for tungsten. Because the blue layer is underexposed color correction is not possible. Overexposure ensure adequate density in the blue layer. Extra density in the red layer doesn't matter much it can be filtered out during printing.
    I seriously think the XA is wrong and yet when the XA meter is wrong it could still deliver correct exposure because (like the OM-2) it has a meter circuit just for displaying and another totally independent circuit to do auto exposure control.
     
  9. bernard_L

    bernard_L Subscriber

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    Is it the spectral response (against different spectral contents of incandescent vs daylight) or is it that one of the two meters is deviating from true response at low light levels (artificial light is typically much weaker than sunlight)??

    To sort this out you can try
    (1) with both cameras, meter in sunlight a piece of red paper, fabric, etc... Do they still agree?
    (2) with both cameras, meter incandescent lighht at high intensity, e.g., staring into bare bulb at same close distance. Do they still disagree (as much)??

    Would be interesting if you report the results here...
     
  10. E. von Hoegh

    E. von Hoegh Member

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    It might be more telling to meter red, blue, and green, sunlight and tungsten - be very certain you actually have lightbulbs with filaments, not lookalike CFLs. Another informative experiment would be to meter in sunlight, both direct and in open shade, with and without a strong UV filter on the cameras.
    You might be surprised at what you see.:wink:
     
  11. snapguy

    snapguy Member

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    get us

    Mechanical and electronic gizmos hate us and are out to get us. Their plot is working.
     
  12. E. von Hoegh

    E. von Hoegh Member

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    Not all of us. I get along just fine with my widgets.:smile: But then, I know how they work - that's the key.:wink:
     
  13. BMbikerider

    BMbikerider Member

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    The variation in light levels may not have anything to do with the differences. How exactly is the light getting to the metering cell? If they do NOT strike the cell at exactly the same angle, and intensity there is almost certainly going to be a difference.
     
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  15. dmb

    dmb Member

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    The shutter speed shown in the XA viewfinder is not directly linked to the actual shutter speed that is used. It is operated by a separate circuit from that controlling the electromagnetic shutter. My old XA always shows about 2 stops slower speeds than a separate meter would indicate however the actual exposures are fine. It is possible to adjust the viewfinder meter but it is NOT for the faint hearted.
     
  16. E. von Hoegh

    E. von Hoegh Member

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    I was beginning to wonder about how the cell was mounted on the XA - it's not TTL metering is it?
    A more meaningful comparison could probably be made between say an original LunaPro (cds cell) and a LunaPro SBC. Calibration, metering angle, etc should all be the same, the only significant difference would be the spectra response of the cells themselves.
     
  17. momus

    momus Member

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    I wouldn't think the Canon is optimized for B&W from my experience (that's all I shoot). I do not like/trust the Canon metering because I had exposures all over the place on every AE-1, A1, AE1P camera I ever owned, and I tested the shutter speeds on all the cameras before shooting them. I finally gave up and bought an FTb and it's been spot on w/ the exposures. I think the Canon system is good for color, but not necessarily B&W film. Of course these are old cameras and who knows how they behaved back when they were new. For what it's worth, every Nikon or Nikkormat I've owned didn't have any exposure issues. I trust the Nikon meters, not so the Canons.
     
  18. mopar_guy

    mopar_guy Subscriber

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    What kind of incandescent light?
    Regular household tungsten light bulbs will be more red than Photofloods. Must film makers that reference a tungsten speed are refering to Photoflood bulbs and not household type bulbs.
     
  19. David Lyga

    David Lyga Subscriber

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    This is facinating to read. Thank you, Chan Tran. Also, thank you dmb for, essentially the same input. - David Lyga

    mopar-guy: a couple of years ago I hoarded three hundred 100W bulbs and placed them in storage because I was terrified of the flourescent assault. I never regretted that treachery. 100W bulbs did the lighting. Towards the end of the year that they were last going to be legally saleable, they were being almost given away by stores having amazing sales. - David Lyga

    momus: apparently that AE-1 IS optimized for B&W because it reads like B&W film reads light. With color, it would overexpose under the unfiltered tungsten lighting. With the proper blue filter I am not so sure. - David Lyga

    both bernard_L and E. von Hoegh: I will try the added intensities of tungsten light and, to the best of my ability, the different colors. Will report here possibly tomorrow. - David Lyga
     
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  20. benjiboy

    benjiboy Subscriber

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    snapguy, your real name isn't Woody Allen is it ? :smile:
     
  21. David Lyga

    David Lyga Subscriber

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    OK, I did the tests. These is a congruity with higher levels of tungsten lighting: both match. The different colors read do not seem to matter, only the light intensities. Apparently that CDS cell on the XA did not respond well to low tungsten (probably low daylight also) levels.

    I don't want to hype this too much as there are profound variances to be found with meters on cameras that are decades old. But it would be interesting for all to compare their various light-meters and see what happens. One does not have to even use up film, as the readings are revealing. - David Lyga
     
  22. michael_r

    michael_r Subscriber

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    To test for linearity ideally you need a single light source of known colour temperature, which is then attenuated with neutral density. And of course it must uniformly fill the field of view.
     
  23. David Lyga

    David Lyga Subscriber

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    I guess we'll have to leave it at the 'pragmatic shortcoming', Michael, as I am hardly that sophisticated. But, you are most likely correct and maybe some will delve further into this. - David Lyga
     
  24. Chan Tran

    Chan Tran Member

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    I made comparison most of the time. I found many of the old timers camera meter response are non linear.
    I use the dichroic color head as light source. Dialing in equal amount of filters made neutral density. Monitoring the filtration tightly with a color analyzer keeps the color balance from shifting when adjusting the filters. Use a good reference spot met the to check light level.
     
  25. michael_r

    michael_r Subscriber

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    Get Richard Henry's book if you are interested in this. However he was testing 1 degree spot meters so in the end it probably isn't directly applicable, although it still makes for interesting learning.
     
  26. fotch

    fotch Member

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    There is an old saying, something like a person with two watches (clocks) will never know the exact time. Just saying......