Checking Light meter accuracy- Did I do something wrong?

Discussion in 'Beginner Questions' started by johnmikka, Sep 7, 2020.

  1. I have revisited post #10 as you mentioned.
    if you see the video, I have measured using incidence meter both towards the window as well as pointing into the room, none of the reading is less than 3 stops over.
    Certainly the camera meter is measuring both reflective light off the curtain as well as see-through light from the window behind. But as I mentioned in the video I have measured with incidence to both direction. And I thought I cleared the question myself by saying, the incidence meter ignored the reflective reading as being a white curtain. hence the 2-3 stops difference.

    But then I was just plainly shoved with comments from you "You are just not supposed to". And I have explained myself that there is a reason why you are not supposed to use a screwdriver for the nails, requesting the similiar explanation for this metering situation. And you refused to answer me in this manner.

    Personally, I don't consider any object in real life situation "theoretical" or "remote", let alone shooting a curtain with an incidence meter. Art is made when rules are broken. I don;t know what "Real world photography" you are referring to, is it a place where students of the subject are not allowed to question how instrument works and why passing a certain point?

    As I said, spot meter or incidence meter they are both equipped with a light sensor, eventually measuring light falling onto the senors. ButI have yet received any explanation as to why by mounting a dome vs lens, will determine whether I could or could not meter a curtain against a window... But I no longer have any hope in getting answers here. Everyone can just let this post sink and die like any child's innocence....
    Last edited: Sep 8, 2020
  2. I did tell you that you can't measure a backlit curtain with an incident light meter and you couldn't take that as an answer I am not so sure that you don't criticize me for giving bad answer.
    So before I attempt to answer your a, b and c questions may I ask that are you really here to learn?
  3. I'm sorry I didn't mean to direct it to you, I was just asking why "you that you can't measure a backlit curtain with an incident light meter".

    But I just got my answers in a different forum. thank you
  4. James G. Dainis

    James G. Dainis Moderator

    A hammer is a good tool to drive a nail but not if you hold the metal end and hit the nail with the handle. An incident light meter is a good tool to get a good exposure but only if you aim it back at the camera and not try to get a reading off the subject.
    William Michael likes this.
  5. Care to share that answer with us ? ... was it: "Because a dome on an incident light meter provides a diffused light for the cell in the meter to read, therefore if you point the incident meter straight at the curtain, all you get is an average diffused reading of the window light and not particularly of the curtain itself"
    Last edited: Sep 8, 2020
  6. No. Your hypothesis is wrong!
    An incident meter's dome gives an incorrect reading when used as you suggest. You need to use the meter in reflective mode, the same as the camera meter works.

    Incident light is the light falling on, or illuminating, the subject. Reflected light is the portion of that incident light that's reflected back from the subject. Two completely different things, and needing two different metering modes.

    If you can't get your head around that, then you've just wasted however much money you spent on that Sekonic L358.
    Or a complete and hopeless mess is made. Sorry, but that's just pretentious guff.
    Last edited: Sep 9, 2020
    William Michael likes this.
  7. Let's hope genuine, helpful and knowledgeable PN members don't waste any more time on this.
    William Michael likes this.
  8. AJG


    As I tell my students, Picasso could and did make portrait paintings that were conventional in every way when he was young. He was perfectly capable of perspective drawing, etc., but chose not to later in his career. But he knew how to do it if he had chosen to make paintings that way. I think the same thing applies to photography--knowing the rules allows the photographer to decide when breaking the rules in a particular way will make for a more interesting result. Not knowing them condemns the photographer to endless experimentation with multiple variables in the expectation that once in a while something good will happen. I've never liked those odds...
    rodeo_joe|1 likes this.
  9. The bottom line is that you can't break the rules unless you know the rules.
  10. The Sekonic meter I have, has the dome and also a disk (with hole) to be used for reflected metering.
    The case has a place to hold the disk, but not the dome, so the only way to store it is with the dome on.

    Incident light means "light shining on" something. It doesn't work for objects that have light other than
    the light shining on them. Curtains, stained glass windows, and projected images are some examples.

    In the beginning there were only averaging meters, which worked (and still work) surprisingly well.
    (Especially with negative film with a one stop or so tolerance.)

    Many typical scenes are close to the "standard" 18%. If the subject wears a black or white shirt,
    but the background is gray (at least in black and white) it works well.

    I first got my FM (new) after using a rangefinder with external meter. The manual exposure
    settings are especially convenient for metering. Point the camera at different parts of the subject,
    and see what the meter reads. Pick one somewhere in between. Most often, I find that the
    exposure doesn't change much pointing around the subject.

    A spot meter might read about 5% of the center of the field of view of the lens.
    An averaging meter should, ideally, average over the whole field of view and not outside.
    The usual selenium cell meter (from about 50 or 60 years ago) has a lens array in front
    to approximate the average over the scene. Some have a white plastic diffuser to
    place in front for incident reading.

    If you have a reflected light meter, and want incident readings, read the reflection off
    an 18% gray card, or something close. Some common skin colors reflect about 36%,
    so you can read that and adjust one stop.

    The important part of using a light meter is to know what it can and can't do.

    In the case of back-lit objects, with an incident or reflected meter, you have to
    use it carefully. In a common case, you photograph a person standing in front
    of a window. In this case, an incident reading at the person's face works well.
    The background will be overexposed, but the person should come out fine.

    If I did actually want to photograph a backlit curtain with an incident meter, I would
    read from behind the curtain, and then adjust based on how much light comes through
    relative to how much might be reflected from a typical (18%) object. I suspect I
    might stop down one or two stops, for darkish or lightish curtain.

    I suspect reading from outside (incident light) on a stained-glass window might
    be about right. Note that, as mentioned above, in theory you should not adjust
    exposure for the reflectance of the subject. You want dark subjects to be dark,
    and light subjects to be light. That is why incident meters are often best.

    However, theory isn't always right. Sometimes you do want to adjust for the subject
    reflectance, but not completely. You want dark gray subjects to come out dark gray,
    and light gray subjects to come out light gray.

    When using the FM with a backlit scene, when you are not specifically shooting
    the backlight, increase the exposure by a stop or two. (Some cameras have
    a backlight correction button to do this. But the manual setting of the FM makes
    it very easy.) This is part of what you "learn" when learning photography.
    When to trust the meter, when not, and how to adjust for those meter fooling scenes.
    As above, the FM is especially good for these cases. You specifically have to adjust
    the shutter speed and (more usually) aperture to center the meter. You then have
    an opportunity to adjust as needed.

    Autoexposure cameras make this more complicated. Some have exposure lock,
    where you point at something that is appropriately lit, lock the exposure. But often
    enough there isn't an appropriate object. There is, then, exposure offset where
    you can adjust relative to the meter reading. And then not forget to reset to 0
    before the next shot that doesn't need it.

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