Why incedent metering is very primitive.

Discussion in 'Exposure Discussion' started by Quinten, May 30, 2005.

  1. Quinten

    Quinten Member

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    I was about to buy a flash meter that can only read in incedent mode, but thinking of it that can never be precise in case the main light source isn't behind the camera. How would you meter with a lot of back or side light, you can't just pick something out and expose for that... Aiming the meter at the light source would not be a solution neither since the light the subject will refelct will be different.

    But are there any cheap spot/reflective meters with a flash metering mode? The gossen or minolta thingies seem a bit expensive in that catogary.

    Would the digipro f from gossen be something?
     
  2. Nick Zentena

    Nick Zentena Member

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    Incident meters measure the light falling on the subject. That means you don't worry about reflection. A black cat will meter just like a white one.
     
  3. NikoSperi

    NikoSperi Member

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    You are right - Nick has misunderstood the issue it seems. The problem of where to "point" the dome arises in backlit flash, just like it does with sunrises/sunsets. You can do both: meter facing the flash (that'll give you your highlights exposure) and then point the dome to the camera and meter for your shadows.

    Sekonic has several flash meters, although one would have to define "cheap" rather liberally. L508, 558, 608 I believe all have flash spot metering (along with a whole other bunch of bells and whistles).
     
  4. Nick Zentena

    Nick Zentena Member

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    "Aiming the meter at the light source would not be a solution neither since the light the subject will refelct will be different."

    Maybe I misunderstood-) But he seems worried about reflection.
     
  5. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    In the studio, you can do a variety of things with an incident meter.

    One is to determine the overall exposure (usually the main light plus the fill), which I do by holding the meter in front of the subject and pointing it toward the lens. Backlight and accent lights in general don't affect this reading (unless you are using an accent light on the subject's nose for some peculiar reason--perhaps you are photographing an actor playing Cyrano de Bergerac).

    The other is to determine the contrast ratio. For that I usually switch from the dome to a flat diffuser and point the meter toward the light I want to measure, but you can also do this by shielding the light with your hand or turning on one light at a time. So for most subjects you'll want a ratio of 1:2 or 1:3, occasionally 1:4, between the main (key) and the fill. If you are using a soft-focus lens, you might want a relatively high contrast ratio to get specular highlights that glow. You might want the backlight or accent lights to be one half to two stops over the main, depending on how dramatic an effect you want. For a Hurrell-style Hollywood portrait, you might use only backlight and a low power or reflector fill, so the backlight could be three stops over the fill, with no real "main" light (sometimes, he would really blow out the hair, but that was the effect).

    I use a Minolta Flashmeter III. These aren't too costly these days, and there is a good range of accessories for them. An inexpensive and very compact new flashmeter is the Gossen Digiflash. I have the Digisix (which doesn't measure flash), and really like it as a field meter.
     
  6. modafoto

    modafoto Subscriber

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    I use Sekonic FLASHMATE L-308 which is GREAT and quite small. Great for flash and ambient. It handles both reflected and incident readings.
     
  7. Quinten

    Quinten Member

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    This question came up after taking some pics with a borrowed flash meter that didn't seem to have a reflective mode. (the owner knew really nothing about it so I wonder why I can't keep it).
    I was shooting a friend while she looked up at a window in a fairly dark room with the sun shining truw that window, the contrast was big so I used a flash from the other side of the camera to cope with the contrast (actually only the second time I used it) but from another angel than the window. So I measured the window light while aming the incedent meter at the window and than the flas aiming at the flash. The contrast seemed okay (about 2.5 stops.) But on the picture the contast was close to 0.
    I also took some pictures with another reading from both flash and window light toghetter in one measurement while aming the meter at the camera. The pictures where underexposed (like I expected them to be.)

    With a reflected reading on the light part of the face I think the exposure would have fit (after all that was what I should expose for)

    I think the problem occured because of the different angles of light and little coming from the camera side. Most incedent light was reflected in other directions I asume...
     
  8. Quinten

    Quinten Member

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    Thanks that one seems to have the tools, and not too expensive. I'll look at some second hand minolta's as well BTW
     
  9. Bob F.

    Bob F. Member

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    But note that it is not a spot meter. I have one too - a very useful little thing.

    Bob.
     
  10. gr82bart

    gr82bart Subscriber

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    Quinten,

    What you're describing is a confusion between ambient light and flash light. Try this next time. Point the incident meter at the window, make sure the dome is retracted or block all other light from the dome except the window light. Take a reading.

    Say this is 1/125s at f8. This is your ambient reading.

    Now you can play with the flash. Set your flash so that it outputs at f8 to start. Take a meter reading with the dome pointed only at the flash as close to the model as possible to ensure the flash is set at f8. Take a pic at 125s. This pic will be balanced and kind of flat since the output of the flash matches the ambient backlight of the window.

    To get a nice 'halo' backlight around the model. Reduce the flash output to f5.6. Take the pic at 1/125s at f5.6. You're going to get a nice rim highlight around the subject.

    Now set the flash back at f8. This time take the pic at 1/250s at f8. This will 'dim' the window light. In fact you 'blacken' the window light by taking a pic at 1/30s at f8.

    The bottom line is that the shutter speed controls the amount of ambient light and the f-stop controls the amount of flash coming on to your film. You can now play with the settings to get whatever contrast you want.

    Attached are two pics. One of Amanda where I metered for the window light mostly and the other of Apryll where I balanced the window and flash light to get the details in the shadow.

    Just a note: I only use incident metering when my subject is 'human' size.

    Hope that helps a bit.

    Regards, Art.
     

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  11. NikoSperi

    NikoSperi Member

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    Eh... somewhat. Not in the traditional sense of the black cat on the pile of coal and reflected reading. The problem with incident readings (flash or not) arises in backlit situations. Typically, one points the dome in the direction of the lens to take a reading that accounts for main light/fill lights etc that are roughly on the camera side of the subject. With the backlit (sunset, say), the incident reading becomes very tricky - where do you point the dome? Camera or light? That will make a huge difference, and is why I believe people invented spot metering flash meters! :wink:

    In short, I don't think the trouble was with the reflectance of the subject, but how to measure the light falling on it.

    PS: Oh, forgot... the Sekonic 308 is a great and very portable little meter. But the "spot" for reflected readings is something like 15 degrees, so don't expect it to do what it wasn't designed for. (Sold mine, and now might buy one back for portability reasons :rolleyes: )
     
  12. rbarker

    rbarker Member

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    To paraphrase the old saw regarding guns, incident meters don't kill good exposures, people do. :wink:

    As with the use of a reflective meter, the use of an incident meter requires both an understanding of what it's doing, and a (tested) method of interpreting the reading. Part of the interpretation is considering the lighting arrangement, and thinking about what portion of the subject illuminated by each light source is going to be seen by the lens. Then, adjustments can be made to the actual exposure to balance between the light sources (or, that process may suggest that the lighting needs to be changed or supplemented).

    When using an incident meter, I point the dome at the light source(s) from the subject position. Each of those readings will accurately render the portion of the subject illuminated by that source in it's true tonality - if that light source is treated as the main light. The "main" is usually the light source producing the strongest reading, but that can be varied depending on what you wish to create, what the ratios are, etc. As the angle between the lens axis and the light source increases, increasingly less of the illuminated area of the subject will be seen by the lens. Thus, adjustments to the exposure suggested by the meter reading may be desireable.

    Because the meter's dome may be seeing light from multiple sources, and will average between them, some care (or, shielding) is needed to discriminate between the multiple light sources. When all of the primary light sources are at less than 90° to the axis of the lens, i.e. on the "camera side" of the subject, it is that averaging process that prompts the usual recommendation to point the dome at the camera. For me, metering the light source, and then thinking about the lighting, produces better, more predictable results.

    Back lighting is obviously a special circumstance, as the portion of the subject illuminated by a light behind them won't be seen by the camera - only hair highlights, and such, that are really transmitted light from the lens's perspective.

    When mixing ambient light with flash, balance between them by using the f-stop to determine the contribution of the flash to the total exposure, and the shutter speed (at that aperture) to determine the contribution of the ambient light - within the limits of the shutter's sync speed, of course.

    Note, too, that other aspects of the overall exposure, development, and printing process also affect what is the optimal exposure for any given image. Thus, people who use different procedures in development or printing may also use different metering techniques. The trick is to establish a consistent metering procedure that works for you and your overall process and "vision".
     
  13. TPPhotog

    TPPhotog Member

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    Another vote here for the Sekonic L-308B, fits bootifully in my pocket and easy to use without my glasses.
     
  14. Quinten

    Quinten Member

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    Thanks lads, some very usefull info here.

    The thread brought up one more question, when I set the meter in reflection mode and aim for the part of the subject I want to have the exposure based on it should be okay.
    But with a 20degrees reflective meter one should get closer to the subject to take a good measurement. Will this give the same result as using a 5deg meter from the same distance as the camera is?

    Something tells me this is different from the principle used with the flash lights getting closer or further away...

    And sorry for being a bit too analitical about this, but I just want to think about this now so it will become a neutral habbit in the future with a much higher succes ratio.

    cheers!
     
  15. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    If you take a 20-deg. reflective reading up close, you should get the same result as if you take a 5-deg. reflective reading farther away of the same area. The only significant factor is the flash-to-subject distance.

    To think of a more extreme case--in general the exposure for a floodlit building at night is about 1/4 sec. at f:2 at ISO 400. On a clear night it doesn't matter if you are standing across the street and photographing with a wide lens or a mile away and using a long lens. The only issue is the distance between the lights and the building. The exposure does not change.
     
  16. photobackpacker

    photobackpacker Advertiser Advertiser

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    I used to have a client with a product that had deep, low-reflective black shelves and brushed chrome uprights. The customer wanted these on a white background for catalog use.

    Using a 1600 watt-second four light set-up shooting through 4' x 8' diffusion panels and a Minolta IV flashmeter in incident mode, I created a nightmare in the darkroom as I blasted the reflected light off of the brushed chrome blocking up the brushed chrome look or, alternatively dumped the white background into blotchy shadow that required tough up to print pure white.

    I solved this problem by going to a light-yellow seamless paper, using the 5 degree spotmeter attachment and metering off of the deepest blacks and adjusting development to hit the right level for the chrome. I then shot these using a #12 yellow filter (1.3 stop adjustment) which cleaned up the background. The negatives were an absolute dream to print!

    For me, this is the difference between incident light meters and reflective. An incident light meter will give you the "correct" exposure provided your subject is of "normal" contrast. Reflective light meters will tell you about your subject so you can chose an appropriate exposure and adjust your development (in the case of color transparencies) add or subtract light to hit the sweet-spot for contrast.
     
  17. rbarker

    rbarker Member

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    The law of inverse squares applies only to light coming from the source because of its spread. The farther the source is from the subject, the less light that falls on the subject. Once it falls on the subject, and is reflected, exposure remains constant regardless of camera (or reflective meter) distance.

    With a 20°, or even a 5° reflective meter, one needs to be aware of what it is measuring, however. In most cases, a meter like that will be measuring both highlight and shadow values, and will give an averaged reading. That may be OK, or it might be misleading, depending on the subject's contrast, lighting ratios, etc. Personally, I prefer to take reflective readings with a 1° spot meter, so I can check the values for smaller areas, and thus keep all values within the capabilities of the film (or, the contrast range required by the presentation media, such as a magazine ad).

    Another key factor to keep in mind with all of this is that metering and exposure is all relative. That is to say, the values in the scene, and thus the exposure, are relative to each other. Thus, by lighting elements in the scene separately, one can adjust or manipulate the relative values, making them fall within the desired exposure and contrast range. That's what we're doing when we add "fill" (either with a reflector or a flash) to an outdoor portrait. That's also what we're doing by lighting a background separately from the main subject. If one can control the lighting, and the relative exposure values, a light-colored background can be made to appear much darker, or a dark background appear to be much lighter.
     
  18. gr82bart

    gr82bart Subscriber

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    A light meter is like a light sabre. Trust the forces within it and you will conquer the dark side.

    Art.
     
  19. Michel Hardy-Vallée

    Michel Hardy-Vallée Membership Council Council

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    Funny, I misread the title and with the typo it gave me "Indecent Metering". Anybody practices that? :wink:
     
  20. rbarker

    rbarker Member

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    I shoot nudes, and I think Morten does fetish work, but I don't think either is indecent. :cool:
     
  21. gr82bart

    gr82bart Subscriber

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    This must be a new age sin! LOL

    Art.
     
  22. modafoto

    modafoto Subscriber

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    Hmmm...I once was metered by Scientology (some funny tool to measure what they call your engrams)...I bet they found me indecent :tongue: