Why use Incident metering?

Discussion in 'Exposure Discussion' started by Max Power, Sep 20, 2004.

  1. Max Power

    Max Power Member

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    Hi All!
    I have a newbie question about using an exposure meter. I have used the search function, but came up empty handed.

    My question, simply, is why does a photographer use incident metering? I have gone through a number of publications and web-sites which explain how to spot meter and take reflective and incident measures, but nothing tells me why incident is useful. Instinctively, reflective and spot make sense, incident metering does not.

    Thank you,
    Kent
     
  2. Ole

    Ole Moderator Staff Member Moderator

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    I have my doubts about the usefulness of reflective metering, but not incident and spot!

    If your subjct is very light or very dark, and incident reading gives better results than reflective. On second thought - strike the "very". One stop lighter or darker than "18%" is enough to give a clear advantage to incident metering. It could even be of average tone, but strongly coloured - most reflective meters have a somewhat different spectral response than film.

    With B&W and large format the spotmeter is king. For anything else, I use incident.
     
  3. Konical

    Konical Subscriber

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    Good Morning, Kent,

    Actually, for many subjects, incident metering makes more intuitive sense to me. The reading obtained simply measures light falling on the subject without taking into account the subject's degree of reflectivity. Unlike reflective metering, it involves no decisions on exactly where to aim the meter, no variables such as the color of the subject affecting the reading, etc. Naturally, a similar result can be obtained by using a reflective reading off a standard gray card. Consider that most studio use of flash meters is done in the incident mode, and that usually works out well.

    Konical
     
  4. mark

    mark Member

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    There are going to be a lot of BZTS people chiming in, at least i hope, so I won't go that route. For me incident metering is like a really big averager. I use it most with color landscape. Yes, there are times I just hold it above my head and it has not missed. I only use my spot meter when I have tricky lighting.

    In BW I seem to get consistent results with the incident meter.
     
  5. Donald Miller

    Donald Miller Member

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    Owning and having used both types of meters, I find that I have consistantly better exposures using an $85 (circa 1984) incident meter then I do with my $400.00 + Zone VI modified digital spot meter.
     
  6. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    An incident meter gives you a reliable Zone V placement with one reading, so it's particularly useful in the studio, where you can control the contrast range of the scene by adjusting the light. It can also tell you the difference in output of different light sources by comparing the reading from one source to another source. The BTZS incident method, as I understand it (and BTZS users will correct me if I'm wrong, I'm sure), uses a variation on this studio technique to measure the difference between the direct light and shadow value to measure the contrast of the scene.

    Spot reading depends on careful testing of your materials and previsualization, so that you can point the meter at an object and place that spot on a particular tone, just as if you were a painter with a palette of gray tones and the meter were a brush. The downside of this method, is that as it offers a certain amount of direct control, it's also open to a good deal of error--both in judgment and in placement when using a narrow spot. You can control two zones fairly precisely--the shadow value used to calibrate film speed and the highlight value used to calibrate development time--but zones in between or on the outside are going to fall where they may, and they might not always be where you want them to be.

    Both methods can produce fine results as well as errors. An incident method can't really account for the reflectivity of objects in the scene--say, the difference between your average landscape and your average landscape with a shaft of light illuminating a brilliant white egret in the foreground, but if one is attuned to those situations, it seems to work in many situations for those who use it.

    Personally--I usually use an incident meter for controlled lighting conditions and spot metering for landscapes.
     
  7. Helen B

    Helen B Member

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    A very similar question was asked on APUG a few days ago here. I'll restrain my urge to write more on the subject!

    Best,
    Helen
     
  8. Thomassauerwein

    Thomassauerwein Member

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    I go both ways also. Incident for studio and outdoor and reflective for out door. personally I find incident more accurate. Using a spot is easier to meter subjects further accurately I suppose but overall ambiance is ambiance everywhere within the environment your in, so a little intuition at the moment goes allot further in capturing the moment than does keeping yourself wrapped up in the tecknical. After a while you won't rely on meters very much at all. My suggestion would be to start with an ambient meter and use the spot meter inside your cameras to double check the range of exposure of your scene.
     
  9. jnanian

    jnanian Advertiser Advertiser

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    sorry to hi-jack the thread but i have a question for helen b -

    hi helen -

    i haven't read the minor white text you mention in the linked-thread, but
    why did minor white suggest to use your hand rather than a gray card?
    is it because no matter one's race the palm of the hand is 2 stops brighter than 18% gray? i sometimes use my hand because i can never remember where i put my gray card, and my hand is harder to misplace :smile:

    -john
     
  10. Helen B

    Helen B Member

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    John,

    There are two very similar threads, so it makes sense to transfer everything to one thread.

    In the context of Minor White's method the important thing is that you use something consistently, and determine your effective film speed using that method (even more off topic, that's where he and many others differ from the 'Stouffer step wedge' method of film speed testing).

    So the palm of your hand is convenient and readily available. As you have so astutely observed, you rarely leave it behind - unless it's your left hand and you are eating. It's also conveniently close to an 18% grey card in terms of diffuse reflectance (mine's about half a stop lighter than an 18% grey card).

    If anything, I think that the question is 'why prefer your palm to an incident dome?' and, er, I can't think of an answer to that one - except the blindingly obvious reason (the sort I generally miss) that you can use you palm with a reflective meter or a camera TTL meter.

    Sorry it's a bit rushed, hopefully others will chip in. It would be very interesting to hear from Minor...

    Best,
    Helen
     
  11. jnanian

    jnanian Advertiser Advertiser

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    thanks helen!

    "now, we can go back to the regularly scheduled program"
    - john
     
  12. mikewhi

    mikewhi Member

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    I always used my incident meter in night photograhpy. I walked into the deepter shadow areas and used it to meter the light falling in that area. I always got veyr accurate measurements as compared to my Pentax 1 degree digitan (Sone VI modified) meter. You can see some hight photos in my private ind critigue gallery. You'll note that the shadows have a lot of detail in them, directly attributable to the use of the incident meter in my opinion.

    Thanks.

    -MIke
     
  13. Graeme Hird

    Graeme Hird Member

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    Try this site Mike: http://www.iespell.com/index.htm
     
  14. Max Power

    Max Power Member

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    OK,
    Let me see if I have seized this correctly. The whole point of incident metering is that it will always give you consistent results because the results are always based on a single variable: the primary light source. Therefore, as a general rule, what I need to do is take incident readings with the meter pointed toward the primary light source. Should I have more than one source, I suppose that I ought to use an average f-stop based on the square root of the two primary sources, no?

    Have I got a handle on this?

    Thanks all,
    Kent
     
  15. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    Not exactly. A single incident reading with the dome pointed toward the camera will place Zone V in a good spot on the curve usually, but it doesn't help you with the contrast range. That's why it's a good method under studio conditions where you can point the meter at each light source to determine that the contrast range is within acceptible limits, and then take an exposure reading by pointing the dome at the lens from the subject position, measuring the effect of all the lights falling on the subject (except for backlighting) without need for any calculations.

    To determine the contrast range of a scene where you can't control the lighting, as in a landscape, you can use spot readings as described in Adams's _The Negative_ or the incident method described in BTZS.
     
  16. Ed Sukach

    Ed Sukach Member

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    Back to basics ... The value of incident metering lies in the fact that the intensity of light falling on the subject is the determining factor alone, and it is independent of the reflection properties of the subject. In use, an incident "dome" (purpose: diffusion) is directed - generally - toward the camera from the position of the subject.

    Reflective metering *IS* dependent on the reflective characteristics of the subject or whatever is included in the area of coverage.

    In "incident" metering the light alone is measured: a snowball will, in the final result be white; a lump of coal will be black.

    In reflective metering, there is an assumption that whatever is included will be "average gray", reflecting ~18% of the light that falls upon it: A snowball, if it occupies the entire area of coverage will appear to be average gray; a lump of coal occupying the entire area will also, in the final result, appear to be the same "average" gray.

    Incident metering is more accurate, BUT... It requires a separation of the metering from the camera position... one would have to travel all over a landscape, averaging many readings, to obtain an accurate result. Usually in landscape photography, reflective metering is the only way, and everything will average out close to 18% gray anyway. "Automatic" exposure requires reflective - the meter cannot be separated.

    In studio work, I invariably use incident metering: not only because I can (I use a separate meter anyway) but averaging could easily NOT be the way to go: A model in a black dress in front of a dark background will appear to have a starkly white skin in a slightly darker than medium gray dress and slightly darker than medium dark background.

    BTW ... Two "primaries"? - Only if there were two precisely equal light sources. Separate metering of each light source is only useful in determinig lighting RATIOS. For the overall exposure, turn all the lights on, and take a meter reading.... Whoops!!! Zone System followers excluded..!
     
  17. sanking

    sanking Member

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    And in addition to the single incident reading not helping with the contrast range you must also be aware of where the reading is taken, and whether you are dealing with negative or positive materials. If you are working with negative material a single reading in the sun will usually result in underexposure of up to one stop, whereas a single exposure in the shade will give overexposure of about one stop. Therefore the best way to work is to take two readings, one in the shadow and another in full light, and average the two. If you must work with just one reading a quick and dirty approach is to double the EI for shadow readings and halve it for readings in full light.

    On the other hand it is best to base your exposure on a single reading in full light when exposing transparency material.

    Sandy