I don't think I understand incident metering after all

Discussion in 'Exposure Discussion' started by ntenny, Jul 23, 2011.

  1. ntenny

    ntenny Member

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    So I was out in the field yesterday shooting, and I was messing around with both incident and reflective metering. My understanding is that, if I point the meter in reflective mode at an 18% grey card, and then take an incident reading in the same lighting, I should get the same reading.

    However, it just doesn't seem to work that way: almost invariably, when I feel like I'm pointing the reflective meter at an "average" scene, putting the dome on in the same lighting drops the reading (i.e., calls for more exposure) by one to two stops.

    I can think of three explanations:
    1) my estimate of "average" is one to two stops off;
    2) something is peculiar about my meter, causing either me to operate it wrong or it to read wrong; or
    3) I don't understand how incident metering works after all.

    The meter is a Luna-Pro SBC and I'm pretty sure it's in good repair---it's served me admirably as a reflective meter. So I think something is wrong with me, be it in my sense of what makes a "correct" reflective reading (#1), operating my meter (#2), or understanding the whole point (#3). Help?

    -NT
     
  2. 2F/2F

    2F/2F Member

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    It is simple if you think about it like this: Reflected meters place whatever they are pointed at as a middle gray. Incident meters place middle gray as middle gray. That means that dark things end up dark, middle-toned things end up middle toned, and light things end up light. With a reflected meter, whatever it is pointed at ends up as a middle tone, regardless of how dark or light it is in reality.

    The dome needs to be pointed at the right thing, of course.

    And no; assuming the same value is used for middle gray with both meters, a gray card reading should not match an incident reading. You should have to open up 1/2 to 2/3 of a stop from a gray card reading to match an incident reading. And the gray card takes the place of the dome, so should be placed where you would place the dome.

    Assuming both meters are calibrated properly, I would trust the incident. It is far more likely that the reflected meter is telling you to underexpose than it is likely that the incident meter is telling you to overexpose.
     
    Last edited by a moderator: Jul 23, 2011
  3. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    First off, are you pointing the dome in the right direction, which is the opposite of the direction for taking the reflective reading without the dome? In other words, the same light falling on the grey card should be falling on the dome for making an incident reading.
     
  4. Rick A

    Rick A Subscriber

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    Hmm... Lets see, when you take a reflective reading, you must decide which zone you want that reading to be in, and set as necessary. You may want to take several readings and average then to get shadows and highlites to both be included as much as possible. You can also take readings of thnge that are similar in reflectance to a gray card and use that setting.

    When you take an incident reading, you stand in front of the subject and point the light dome toward the camera lens, and thats the setting you will use. If it isn't convenient to do that, place the meter in approximatly same lighting and take a reading. If your main subject is in shadow, creat a shadow that resembles the one surrounding subject and take the reading. It really doesn't get any easier than that.

    Anyway, thats how I do it.
     
  5. 2F/2F

    2F/2F Member

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    I would add that you don't have to use the reading on the incident meter to make the shot. If you know that the scene is high in brightness range, and you want to fit it onto your printing paper more easily, you can overexpose and underdevelop. You can also do the opposite in the opposite situation. With an incident meter, you will get best results if you pay close attention to brightness range and make these changes as needed. So, they are not completely idiot proof...but definitely much more so than reflected meters.
     
  6. Chan Tran

    Chan Tran Member

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    My guess is that you pointed the meter in the wrong direction when making the incident readings. Your 1 to 2 stops less light than a reflective reading tells me that.
     
  7. ntenny

    ntenny Member

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    I'm uncertain whether this means "towards the camera" or "towards the sky" (in outdoor full sun)---I can see an argument for both. In this setting it didn't make too much difference; the sun was *extremely* bright ("sunny 22" conditions) and there was a lot of reflection from a light-colored landscape, so as long as I didn't throw an actual shadow on the dome I got pretty consistent readings in all directions.

    I don't understand this part. Isn't the idea that the dome looks to the meter like a gray card?

    Thanks

    -NT
     
  8. Rick A

    Rick A Subscriber

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    You point the dome at the camera lens for an incident reading.
     
  9. 2F/2F

    2F/2F Member

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    This is a can of worms. Not sure if you really want the discussion going on in this post. Check the archives. Points have recently been made arguing a variety of methods.

    The skinny of it: The worst you can do by always pointing the dome at the camera is to overexpose in cases in which the lighting is uneven. Most people do not notice this with negative film. But it definitely happens, and can be fatal to the shot with transparency film.

    Personally, in ratio lighting in which I want to preserve the ratio as lit, I always point the dome at the source of light for the brighter side of the subject.
     
  10. tkamiya

    tkamiya Member

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    Here's my understanding, and my reading agrees with this method.

    Let's say you have a camera and you are pointing your camera at a gray card at some distance.
    1) Set your meter at reflective mode. Stand at where your camera is, and point your meter to the gray card.
    2) Set your meter at incident mode. Stand at where your gray card is, and point your meter to the camera.

    You should get the same reading. When you do this, be VERY careful your own shadow does not obscure the sensor on the meter. I am also assuming you have a SPOT meter with just enough coverage to JUST read your gray card. If it reads more, your bets are off. You'll have to get closer to the card.
     
  11. 2F/2F

    2F/2F Member

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    I'm not capable of explaining it in technical detail myself from memory, but it is basically because most meters are not calibrated to reproduce 18 percent gray, which is what a gray card represents. Read the first post in this link for all the technical details: http://www.ohio.edu/people/schneidw/vico222/gray_card_musings.html.

    It is a large part of the reason why Zone System speed testing usually results in lower working EIs than box speed, yet box speed usually works fine with incident meters.
     
    Last edited by a moderator: Jul 23, 2011
  12. Chan Tran

    Chan Tran Member

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    I understand what 2F/2F was saying and it's a valid argument. But if you simply point the dome toward the camera from the subject position and point the reflective meter toward the scene from the camera position it's not often difference by 2 stops.
     
  13. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    If you think about it, the dome and the card are not seeing the same light, because the card is flat, and the dome is curved to read the light as it falls on a three dimensional object with a highlight and shadow side. If your meter has the option of a flat diffuser, it should match the gray card, and the flat diffuser is what you use, say, in copy work to make sure a flat object is evenly illuminated.

    In a typical portrait or still life setting, where you can control the lighting ratio, I point the incident dome toward the camera.

    Now the question is--how do you get the same reading with a gray card? You might presume the incident reading is correct and use it to fine tune your gray card technique. To photograph a three-dimensional object, you often have to angle the card toward the main light, maybe 30 degrees from the lens axis, to get a reliable average of the main and the fill. Experiment with the angle of the card to match it to the incident reading.

    Take a look at the diagrams here for Kodak's instructions for gray card use in cinematography--

    http://motion.kodak.com/US/en/motion/Products/Lab_And_Post_Production/Gray_Card/cinematographer.htm
     
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  15. 2F/2F

    2F/2F Member

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    It certainly can be in bright light with bright objects in the frame...which is the light in which many (most?) people tend to shoot, and the light the OP described for his situation.

    There aren't many situations that are what the OP describes as "sunny 22," so I am guessing that at least one stop, and maybe more, of the difference he is seeing between the two readings is due to the brightness of what the reflected meter was pointed at.

    Incident readings simply should not match gray card readings, no matter what diffuser attachment is on the incident meter, for the reasons mentioned above. If reflected meters were designed to produce 18 percent gray, then they would match. But they are not designed to do this. Gray cards will cause underexposure unless you open up a bit from the reading they give you.
     
    Last edited by a moderator: Jul 23, 2011
  16. Chan Tran

    Chan Tran Member

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    Yeah I never run into sunny 22 where I live. It's sunny 14.
    Under even lighting and wih the flat diffuser my meter read exactly the same to within 1/10 stop. and yes my meter is K14 which is 18% reflectance.
     
  17. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    "Sunny 22" is bright mid-day on a white sand beach.
     
  18. ntenny

    ntenny Member

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    That's pretty close. I was in a light-colored section of fairly sandy desert near Yuma, AZ on a very bright (and very hot) midday---lots of reflection from the ground and the mountainsides in addition to the direct sunlight.

    Thanks for all the information. I've never checked the calibration on my meter more precisely than "the slides look good", so that may account for some of the difference, and directional uncertainty for the rest. I was shooting negative film, so I don't expect anything to have been ruined; I'm just trying to understand why I wasn't seeing the behavior I expected.

    -NT
     
  19. Leigh B

    Leigh B Member

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    The dome has nothing to do with the gray card.

    Your distant scene is illuminated from many different angles, including reflections from the area in front of it, like grass, dirt, sand, or water.

    The dome integrates light from all different directions and includes that information in the meter reading. It should be pointed directly at the camera lens, not up or down or sideways, from the subject position, or from a position that mimics the illumination environment of the subject.

    I normally just hold it above my head, pointed straight back. Works fine in most situations. I know when the subject and lighting are such that a correction may be needed.

    Regarding the gray card... Very few people use these correctly.
    It should be angled halfway between the camera line of sight and the illumination source. Then you take a reflected reading of the card from the camera position. As an example, if you're shooting a landscape at noon, with the sun directly overhead, the card should be at a 45° angle.

    - Leigh
     
    Last edited by a moderator: Jul 23, 2011
  20. Chan Tran

    Chan Tran Member

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    While the reflected meter will read the white sand beach, the incident meter is not influenced by the white sand beach. My meter in incident mode under sunny condition almost always read EV14.7@ISO100.
     
  21. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    An incident meter actually should be influenced by the white sand beach, because there's a lot more reflected light in that situation from the sand and the water than in most sunny scenes. If you look at, say, they old exposure guide they used to print on the instruction sheet they used to include in the box with a roll of film when film always came in a box, and oh, those yellow painted metal canisters...okay, I'm getting too nostalgic--anyway, the graphic would generally recommend stopping down to "sunny 22" for a sunny day at the beach, because there really is more light there. It is like the difference between photographing in a small studio with bright white walls vs. a large studio with high ceilings and black curtains along the walls to absorb stray light.

    But in more typical lighting, yes, the advantage of an incident meter is that it isn't affected by the reflectivity of the objects in the scene. This makes it good for studio use, where you can control the contrast ratio of the light and the general level of the lighting. Incident metering in available light involves more awareness of the contrast of the light and the reflectivity of the objects in the scene, and whether it is possible to read the same light as the subject, which can be quite distant from the camera in the case of landscape photography. BTZS is an approach that adapts incident metering to the field.
     
  22. Diapositivo

    Diapositivo Subscriber

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    I agree. Grey cards have 18% reflectivity and, when angled, as per user instructions, half-way between the light-source subject line and the subject-camera line, and when they are read from the camera position - i.e. not from a line perpendicular to the grey card - theoretically they would reflect a bit less light (you only measure the diffuse reflection, and not the "glare", the "specular" reflection, or not, it's the other way round, half-angled you actually take all the glare besides the diffuse reflection, let me think about it... it should give you a "closer" reading**) and the result would come near to the middle-grey for which light meters are calibrated. Notice that "half angle" is both on the vertical and on the horizontal plane. Basically you need an assistant (or the model) to hold a grey card and you have to meter to it with a tele lens.

    After having verified once in your life that if you keep the grey card angled in a certain complicated way, which is not easy if you are alone, and without projecting any shadow on it with your arm or your lens, it gives you a result that is consistent with an incident light meter, my advice is to totally forget the grey card (for light metering purposes that is) and just use the incident light meter in all those situations where the grey card would be used*.

    The entire procedure of using a grey card with the correct angle and without projecting a shadow on it is so slow and clumsy that I don't see an use for it in an outdoor situation. Incident light meters are "cheap", work better, and are much faster.

    * It's easier to verify that the grey-card method is equivalent to the incident meter method in the shade, the typical EV12 @ ISO 100 situation. In this situation, I see that grey card and reflective metering of all kind, and incident metering, normally agree quite exactly. As soon as I go in the sun for the test I suppose the "glare" of the direct sun rays weights more and the angling of the flat surface of the grey card becomes critical, besides the additional possibility of an unnoticed shadow projection on the card, and the meters don't agree any more because of a method error.

    ** Let's say reflected-light meters are calibrated for 14% grey. Whatever you give them, they reproduce - given a certain "standard" printing method - a print which is 14% grey. If you give them an 18% grey card (lighter) they would tend to give you a "closer" reading than with a 14% grey card. But if you angle the grey card in such a way that you maximise its "glare", it's even worse :sad:

    Considering that light meters are calibrated for 14% or so, I would expect the grey card should be used in a way that minimises glare, not that maximises it! The instructions given for grey cards instead want us to maximise glare on the grey card. I'm confused :confused:

    Doesn't matter. Just ignore the grey card. Grey cards suck. Incident meters work :smile:
     
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  23. rpsawin

    rpsawin Subscriber

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  24. 2F/2F

    2F/2F Member

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    I am almost certain it is due to using a reflected meter to read a very bright subject. I am especially certain because you used the same meter for both readings, so there are definitely no mis-calibration issues going on. That is just what reflected meters do, and why I generally dislike them so much. I think they do more harm than good for anyone but the rankest of amateurs, who have no idea how to judge light yet.

    Out of curiosity, what were the two exposures suggested by the two metering methods?

    On a beach in bright light using negative film, I would probably just use sunny 16. Sand is bright. So I want it to have some healthy density on the neg. Using transparency film, I would probably close down half a stop, based on experience. Sand can blow out quite easily on transparency film. In bright snow that was covering the ground, I'd close down a whole stop with transparency film, and use sunny 16 with negative film.
     
  25. Leigh B

    Leigh B Member

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    Absolutely true. :D

    - Leigh
     
  26. ntenny

    ntenny Member

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    I honestly don't remember. It was a confusing situation---I'd been metering and shooting, and suddenly looked down and said "Why, the incident dome is in place!", then tried to figure out whether it had done any harm and concluded that it probably had.

    I've just developed the roll and it looks good to the naked eye. You're probably right that incident mode was giving a more "correct" reading---in a more normal environment the fact that I was pointing it in the wrong direction would have messed that up, but the light was uniform enough on this occasion that I think I got away with it.

    -NT