Light metering and a gray card...

Discussion in 'Beyond the Basics' started by Tennessee Landscape, Jan 26, 2008.

  1. Tennessee Landscape

    Tennessee Landscape TPF Noob!

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    Okay, got my D80...I'm starting to take some nice pics.....when should I use a gray card, oh yeah, and how do I use it in conjuction with the meetering in my DSLR.....maybe someone might post a couple pics to help their explanation....please and thank you :)
     
  2. Alpha

    Alpha Troll Extraordinaire

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    Use it always. You can use it in several ways. For starters, you can meter off it in any situation, since it's 18% gray and that's middle gray as far as your meter is concerned. Also, if you're doing studio-type work, start with a test shot that has the gray card in the scene. You can use this later to adjust either in digital post by setting it as the middle gray point, or in traditional printing as zone 5.
     
  3. Tennessee Landscape

    Tennessee Landscape TPF Noob!

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    okay, but what do I actually do with the card then? Point the camera at it before I take the shot, what happens when I reset the scene? I'm not making the connection here.....
     
  4. domromer

    domromer TPF Noob!

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    [SIZE=+3]Using A Gray Card[/SIZE]

    [​IMG] [​IMG] [​IMG]
    • [SIZE=-1]Do not tip the card up to the light like the picture on the left. Too much light is seen by the meter[/SIZE]
    • [SIZE=-1]The middle picture is correct. Position the card parallel to the front surface of your lens like this. [/SIZE]
    • [SIZE=-1]Do not tip the card down like the picture on the right. Less light is seen by the meter.[/SIZE]
    [SIZE=+1]Get the camera close enough[/SIZE]
    [SIZE=+1]so that only the gray card shows in the viewfinder.[/SIZE]
    [SIZE=+1]Once you have the meter reading, back off to frame the view as you like.[/SIZE]
    [SIZE=+1]Be aware that automatic cameras go back to their erroneous [/SIZE]
    [SIZE=+1]ways as soon as the gray card is out of view.[/SIZE]
    • [SIZE=+1]What is a gray card?[/SIZE]
    [SIZE=-1]Camera light meters see reflected light, but it is often useful to know the incident light (the light hitting your subject). With an 18 percent gray card you can get a reflected light reading which is essentially the same as reading incident light. Our gray cards are made from mat board. They have been painted to give results similar to an 18 percent reflectance Kodak gray card. This is equivalent to zone V. In the zone system, V is the middle tone half way between black and white.[/SIZE]
    [SIZE=+1]Why do we use a gray card?[/SIZE]
    [SIZE=-1]Here are several situations that benefit greatly by the use of a gray card to check the exposure.[/SIZE]
    • [SIZE=-1]Whenever there is any sort of back lighting such as a bright sky or window behind the subject.[/SIZE]
    • [SIZE=-1]When there is a light source such as bulb or light fixture in front of your camera.[/SIZE]
    • [SIZE=-1]When there is any type of back light such as a bright sky or window behind the subject.[/SIZE]
    • [SIZE=-1]When a large part of the scene has a fairly dark tone like a close up of a big black car.[/SIZE]
    • [SIZE=-1]When a large part of the scene has a fairly light tone like a snow scene.[/SIZE]
    [SIZE=-1]These are lighting situations that fool the typical camera light meter. It never hurts to check the light reading with a gray card. If you get the same reading with the card as without, you didn't need it. If the gray card gives a different reading, you should generally use the gray card recommendation. Or, you could make several exposures to see which negative comes out best.[/SIZE] Links to other web sites that explain the use of a gray card [SIZE=+2]© [/SIZE][SIZE=+1]Marvin Bartel, 1998[/SIZE] Photo Communication Home Art 315: Photography Home
    Photo Communication Course Information
    Marvin Bartel Home Goshen College Home
    updated August 22, 1998
     
  5. Joves

    Joves No longer a newbie, moving up!

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    I was reading in another forum, that some people use a coffee filter over the lense and, point it towards the general area of the light source.
     
  6. Rhys

    Rhys TPF Noob!

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    Quite honestly I have never ever used a grey card. When I use a light meter, I measure incident light - not reflected light. When I use a camera I use matrix metering then correct any flaws with RAW PP. I never have any problems.
     
  7. djrichie28

    djrichie28 TPF Noob!

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    Camera manufacturers calibrate built in meters to make sure you properly expose your shots. Since it has been established that most scenes contain 18% gray, that is what the meter will be looking for when you set your exposure or when the camera sets it itself in 'program'. The gray card comes in handy when you won't be taking a shot that contains the traditional "18% gray" scene. For example; snowy scenes (in case you travel), darker scenes etc.

    If you used your camera's meter taking a picture of snow. The snow would appear gray and not white. The meter will be looking for 18% gray in the snow so it will force an under exposure. If you held up a gray card and metered on the gray card first, and then used the same exposure your meter liked on the snow, the snow would appear white. Just make sure that you shoot in manual so when you meter off the gray card, the exposure won't change when you re-compose on your scene.

    Hope that helps an understanding a bit. Good luck
     
  8. Helen B

    Helen B TPF Noob!

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    Digital camera speeds are usually based on the highlight reading - the speed and meter calibration are set so that the exposure should not oversaturate the sensor, with a small safety factor. There are other criteria, but let's forget about them for now.

    An 18% grey card is about two and a half stops less reflective than a perfectly diffuse-reflective object. That means that if the metering system places a grey card reading two and a half stops below the level at which detail will be burnt out, nothing in the picture will be burnt out apart from specular (direct) reflections of light sources and light sources themselves.

    In reality scenes often have an average reflectance slightly lower than 18% - somewhere between 12% and 14%* is fairly common. 12% is only about half a stop darker than 18%, so allowing three stops over the meter reading takes care of that. I've measures the recordable brightness range above the metered value for a few cameras when recording to JPEG, and it is usually about three stops. (If you are shooting Raw they can be more).

    I think that it is worth finding this out for your camera: how much exposure can you give above the grey card reading before you get to a pixel value of, say, 250? You may find that with a plain, unadjusted gray card reading you are losing some of the crucial range of your sensor.

    This all means that if you do meter off a grey card that has been placed in the brightest illumination in the scene, your non-specular highlights will be safe. That's what you want for digital and reversal film. negative film is a different story, and not what we are discussing here. Later, maybe.


    What the grey card, used in that way, does not tell you is how dark the darkest part of your scene will be. That depends on the difference in illumination between the darkest and lightest parts of the scene. To do that you would need to place the grey card in the lowest level of illumination, and take a reading - the darkest parts of the scene will be about two and a half to three stops darker than the 18% grey card - few materials have a reflectance below 3%. An aside to this is that no single card placed in full illumination can show the full brightness range of a scene, if there are varying levels of illumination (even one source will cast shadows). This is also the reason why a grey card placed in the scene may not be a good reference for mid gray in the final print, in value terms, even if you are printing in a technically correct method rather than one that suits your aesthetics. It will be OK for white balance, however.

    As already mentioned, the use of a grey card turns your meter (including the one in your camera) into an incident meter. True incident meters have some advantages over a gray card, not least because the receptor isn't usually flat. It's fine to use a flat receptor (or a flat card) when photographic flat objects, but it is usually better to use a shaped receptor when photographing three-dimensional objects.

    This is why incident meters use domes - they measure light from the sides and from behind (but not directly behind) if used correctly. In technical terms a flat receptor or card has a cosine response and a dome has a cardioid response.

    Incident meters are calibrated to match reflective meters at somewhere between around 12.5% and 14% reflectance, depending on the meter manufacturer. There is no fixed value set in the ISO standard for meters.

    *One of the earliest photographic grey cards (the 'Neutrowe', 1940) was 14%, by the way.

    Best,
    Helen
     
  9. Antithesis

    Antithesis No longer a newbie, moving up!

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    I'm not a perfectionist so I always end up using my histogram and bracketing if I'm unsure of the tonal range of a scene. I think a Grey card will be a lot more important when using film as you can't physically examine your image, but with digital you can check your exposure and check for loss of detail. I'd like to expirement with one and see how close I can get to blowing out (like Helen said) without it actually happening.

    Oh, and Helen, your my hero.
     

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12.5% gray card

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a reflective light meter will read an 18% grey card as 18% grey. how will it read a dark black building?

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