Pentax digital spotmeter index numbers

Discussion in 'Exposure Discussion' started by Palantiri7, Apr 20, 2010.

  1. Palantiri7

    Palantiri7 Member

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    Hi everyone,

    My Gossen Starlite lightmeter died in a heavy downpour yesterday and I just purchased a Pentax digital spotmeter to replace it. After reading the manual online I observed that there is a table in there advising the user to adjust the index number based on the reflectance of the colour being measured. According to the manual, yellow has the highest reflectance and thus should be placed at '8' on the index number scale: leaving it at the standard index would result in underexposure. For purple and indigo colours the index should be set for 2 or 3. I wanted to know if anybody actually does this as I have not seen it mentioned in any thread that I looked through. I will be using the meter for Velvia as well as for b&w film exposures. Thank you everybody!
     
  2. SWphoto

    SWphoto Member

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    I hope we hear from someone who understands this :smile: I take it that yellow should be placed 3 stops off of Zone V, not on "8" on the index number scale, so you should close down three stops from a reading on yellow... But not sure this is what they are saying.
     
  3. Palantiri7

    Palantiri7 Member

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    Thanks SW. I clearly don't understand! I know my Rollei 6003 meter used to overexpose blues slightly and underexpose reds so I don't know if that's the same thing here. Out of all the posts I've read about the Pentax (a lot!) I've never read anyone making mention of oddities in the meter's readings w.r.t. colour.
     
  4. Greg Davis

    Greg Davis Member

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    Looking at the page in the manual you are reading, it appears they recommend setting your EV value that you read inside the viewfinder on "8" according to the very bottom scale, the one labeled "IRE" when reading a yellow object rather than on the big orange arrow that we normally use. That arrow is set at 3.5 on the "IRE" scale. All I can say is shoot a few test exposures using your normal method and the method described in the manual and see what you get.
     
  5. RalphLambrecht

    RalphLambrecht Subscriber

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    I've never worried about subject colors or the meter's spectral sensitivity. Anyway, since Pentax does not know what film I'm using, or how I alter its spectral sensitivity through filtration, how could they possibly offer advise.
     
  6. Palantiri7

    Palantiri7 Member

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    Thanks for the answers guys. I will test it out with some film when the meter gets here.
     
  7. Q.G.

    Q.G. Inactive

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    Different colours do reflect different amounts of light.
    Just ignore the colour for a moment, and think of them as grey cards with different reflective properties. Or look at B&W pictures (they, after all, ignore colour too) and see how tones of parts of the subject differ though shot in the same light, depending on what colour they were.

    So you do need to worry about colour when using a spot meter.
    It's one the many 'joys' of reflected light metering.

    Grassy green is close to 'middle grey'. So if grass or leaves are available, take a reading off them and you don't need to correct.
     
  8. Dan Henderson

    Dan Henderson Member

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    I have used a Pentax digital spot meter for years. I almost always measure the important shadow area of the scene and place it on Zone III to decide on the exposure, and measure an important highlight to assess the brightness range. I have one of Ralph Lambrecht's zone stickers covering the scale, so the orange triangle and other markings are covered. I never did know what the others were for anyway. My shadows are always properly exposed unless I make a significant error in translating the settings to the shutter.
     
  9. Palantiri7

    Palantiri7 Member

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    Thanks, Dan. Frances Schultz/Roger Hicks had an explanation about the IRE scale in "Medium format and large format photography". Apparently the "1" on the IRE scale corresponds to 2.667 stops underexposure w.r.t. the metered reading and the '10' represents 2.3333 stops overexposure w.r.t. the metered reading. For negative film they meter a shadow and place the EV reading against "1" on the IRE scale.
     
  10. Doremus Scudder

    Doremus Scudder Member

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    As I understand it, the IRE scale was developed as a kind of "Zone System" for television cameras, with the "1" being the lowest practical value and the "10" being the highest (100% signal). Placing colors for TV/video exposure higher or lower on the scale in order to correspond to the absolute reflectivity is exactly what we film photographers do when we "place" a color on our Zone System Dial (thanks Ralph!). Pentax's suggestions about where certain colors "fall" on the scale is the same as visualizing that, say, bright yellow, falls on Zone VII (or VIII if you like...). Pentax is just doing the thinking for you.

    Although the IRE scale can be used as a kind of Zone System for film photography, it seems a lot more fiddley and less linear than the classic Zone System or any of the newer systems based on sensitometry for B&W films. Rory's post above, however, seems to indicate that the scale might be very useful for slide films... I don't know.

    Bottom line: If you shoot B&W don't worry about the IRE scale unless you need to meter for video cameras. If you use the Zone System, get a Zone Dial and tape it over the IRE scale :smile:

    Best,

    Doremus Scudder
    www.DoremusScudder.com
     
  11. Galah

    Galah Member

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    This whole lightmeter thing is far more complex than would at first appear.:tongue:

    I am no expert, but from what I've read, it seems that many lightmeters respond differentially to different wavelengths.:sad:

    For example, some cells (e.g., Selenium) over-respond to infrared "light" (i.e., heat), so, in low light conditions including a hot object (e.g. sun baked masonry), the reading would be inflated by the heat coming from the background (suggesting a "mid-gray object in bright light" situation) giving an underexposure of the nominal subject. So, the extra red end sensitivity could also result in underexposing red subjects, e.g. roses?

    At the same time, the concomitant lack of sensitivity to blue, would result in an overexposure of blue subject matter (remembering that the meter tries to render what it "sees" as middle-gray, so it registers the blue object as being a "mid-gray object in a low light" situation and indicates a greater exposure setting to make up for it).

    It's all a bit disconcerting, really, to those of us seeking some certainty in life:mad:.
     
  12. Q.G.

    Q.G. Inactive

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    We have (still) two different thingies running in this thread,

    First is the colour sensitivity of the 'sensors' used in meters. Get a blue filtered silicon cell, and no more worries.

    The second is not to do with the colour sensitivity of filters, but is about the fact that different colours represent different tones. Differently coloured objects will reflect different amounts of light, even when all other properties, besides colour, of the material they are made of are the same. Meters will register different amounts of light coming from differently coloured objects, even when they have an absolutely flat spectral response curve.
    And that's what the Pentax manual is on about.
     
  13. Ronald Moravec

    Ronald Moravec Member

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    Of course I do. It is like any other reflective meter, the meter assumes middle grey target.

    If you don`t want to mess around, measure the brightest and darkest areas of subjectand expose in the middle.

    That or learn where common colors fall, like cross lit snow, +1.5 or 2, green spring grass= 0, tree bark is 0 depending, blue sky, really blue, not overcast thin blue, is +1.

    I copied the colors ofsets from the book, reduced and and copied, reduced and copied again, then pasted it on the top of the meter. Works fine.
     
  14. L Gebhardt

    L Gebhardt Subscriber

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    Print out a zone dial sticker. Ralph Lambrecht has one at http://www.darkroomagic.com/DarkroomMagic/Camera.html. There are others on the web, or you can make your own. Meter the highlights place around zone 7 to 8. Bracket a bit on the first shots and you should get a feel for the meter and Velvia.
     
  15. Q.G.

    Q.G. Inactive

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    A zone dial (sticker), and a reminder of what can act as a reference and how, are not the same thing.
     
  16. Palantiri7

    Palantiri7 Member

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    Thank you so much everybody! Yes, I printed out the zone sticker and installed it in place. I have done some initial tests using the Pentax exposure readings against my 5D Mk II just to get a ballpark idea about the meter's behaviour. This meter seems very accurate indeed. So far I haven't noticed any oddities w.r.t. different colours. It seems to expose consistently .5 stop over the 5D, which always tends to underexpose a bit. The 5D's meter is, however, far more sensitive in low light conditions. The Pentax seems to be about the same as the Gossen Starlite's (now defunct) spotmeter in terms of low light sensitivity, which is to say, not too great. Anyway, many thanks for the input. Q.G. has it down, thanks!