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Using a hand-held light meter adds a whole new dimension to determining the correct exposure for a scene. Modern cameras all have light meters built in to them, but all are reflectedlight meters (meaning they measure the light reflected from the scene). Using an in-camera meter in "Spot Mode" can be very useful for surveying the light levels in a scene and determining if the highlights are too light, or the shadows are too dark. However, the size of the metered area varies with the focal length of the lens, and in the case of a zoom lens, the area of the "spot" varies as the lens zooms! Using a dedicated, hand-held spot meter eliminates the guess-work and lets you put the camera down for a few moments.
SPOT METERS
A Spot Meter is more properly called an reflected meter - because it measures the light reflected from the scene. All meters built into cameras are reflected meters, and almost all can be programmed to meter only a narrow "Spot", or portion of the scene, usually at the center of the viewfinder. A camera with its internal metering system set to "Spot Mode" is indeed a reflective spot meter.Spot meters can be very useful. The photographer first chooses what portion of the scene will be neutral tone (the tone of an 18% grey card). Aim the spot meter at the neutral tone (or mid-point) area of the scene and the meter will read out the correct f-stop, shutter speed, and ISO value to correctly expose that area. (All camera light meters act in this exact way). Once the neutral tone is determined, the rest of the scene from absolute white to absolute black will "fall in place" and the scene will come out fine. Or will it???What about that bright, sunlit sky? What about the shadows under those near-by trees? Will the sky be "whiter than white" and blow out? Will the shadows be "blacker than black" and disappear? Simply take the spot meter and aim it at the sky and then the shadows. If the spot meter says more than about 2.5 EV (or "stops") either way, the highlights and/or shadows are lost!Use the spot meter to determine if you need fill light for the shadows (you do have a flash, don't you?), or a Graduated Neutral Density filter (GRAD) for the bright sky. If you have a light meter that has flash metering capability, you can meter the shadows and add flash to bring them up to about -2 EV (from your neutral tone). Point the spot meter at the bright sky and place your GRAD over the lens of the meter. Does the sky fall down to about +2 EV (above your neutral tone)? Once you've filled the shadows and "toned down" the bright sky, you now have a scene that will show detail in all areas.
INCIDENT METERS
An incident light meter measures the light falling on (or arriving at) the scene/subject. It's almost impossible to use a camera as an incident meter. I've read of techniques using ultra-wide angle lenses and locking exposure settings - all with limited success. In portrait, still-life, and commercial studio photography the incident light meter is indispensible.The photographer simply puts the incident light meter sensor at the same position in the scene/subject that has a neutral tone. The incident meter then displays the appropriate f-stop, shutter speed and ISO value for that particular area. The scene can then be exposed using these settings and all the tonal values will fall into place.Just as with the Spot Meter, the incident meter can be moved around the subject/scene to determine if bright areas are too bright, and if dark areas are too dark. The lights and darks can be adjusted by adding or removing lighting - resulting in an image showing full detail from pure white to pure black.
FLASH METERS
Both incident and reflected meters can be used to calculate flash exposure values as well. The photographer needs some way of triggering the flash while metering the scene/subject. This can be done by an assistant, a remote shutter release, or in many cases a flash synch cable connected directly to the light meter. Some light meters even have built-in wireless transmitters for popular flash radio trigger systems such as the Pocketwizard.Light metering with flash is identical to the above techniques (both reflected/spot and incident). Metering simply requires the flash to fire when the light meter reading is taking place.
POOR MAN'S CAMERA LIGHT METER CALIBRATION
THE BEGINNING
l recently purchased a Sekonic L-758DR light meter that has a really cool feature. The meter enables you to calibrate up to 3 cameras and store the calibration data in the light meter. That way, you have the actual points of highlight blow-out and shadow loss in the light meter for that given camera! I knew I had to try this with my Nikon D700.Sekonic provides free software to help you calibrate your camera(s) and load the calibration data into the light meter. It looks like fine software, but you need to buy a special Sekonic test target for the software to work. You don't really need the software to enter the camera calibration data - you can enter the data manually. Given the price of the test target, I decided to find another way to get the data.....
THE SET-UP
l grabbed my trusty Kodak 18% grey card and set it up vertically in an evenly-lit area of the room. I then grabbed my Nikon D700 with a 105mm macro lens, a tripod, and got "close and personal" with the grey card. Putting the camera on manual, recording JPEGs, and setting the aperture to a fixed f4, I made a series of exposures.Starting at the camera meter (in spot mode) settings for correct exposure, I went +/- 4 full stops in 1/3 EV steps. This gave me something like 25 exposures - I lost count! Once I got the exposures on a card, I brought them into Adobe Camera Raw. You can probably use many other pieces of software to do this - just as long as the software displays the RGB values in the scene. Adobe Camera Raw displays real-time RGB values (0-255) for wherever the mouse pointer sits in the image. I used the center of the image as my measuring point and read the RGB values from the Adobe Camera Raw display. Since the image is truly neutral grey, the 3 values (RGB) should be identical! If they are not identical, adjust your white-balance to make them so....
THE NUMBERS
According to the Sekonic instructions, we are looking for five specific RGB values. From brightest to darkest they are:
245 = Clipping Point (+)
230 = Dynamic Range (+)
118 = Mid Tone Range
35 = Dynamic Range (-)
20 = Clipping Point (-)
I started at the brightest (most over exposed) images and moved closer to the properly exposed images and noted the RGB values in the center of the image. When the RGB values came close to 245/245/245, I adjusted the EXPOSURE slider in Adobe Camera Raw to "nail" the value at 245/245/245. This determined the EV of the Clipping Point (+).Continuing through the images (from over exposed to under exposed), I then chose the image that was closest to 230/230/230 and determined the Dynamic Range (+) EV. Continuing through the remaining images I determined the Mid Tone, Dynamic Range (-), and Clipping Point (-) EV values.
THE RESULTS
For my Nikon D700, the EV values are as follows:+2.5 EV = Clipping Point (+)+2.1 EV = Dynamic Range (+)-0.3 EV = Mid Tone Range-2.6 EV = Dynamic Range (-)-3.4 EV = Clipping Point (-)I tried to "err on the side of caution" with these values - these numbers are conservative. The D700 performs better than these numbers indicate (almost 6 stops is great!).After programming the values into the L-758DR, the light meter now displays the dynamic range of my D700 on the LCD display. I can now pick a MID TONE point in a scene and store it in the meter. The meter will then tell me if other areas in the scene are within the dynamic range of the camera. No more guess-work, no need to remember any more numbers!
CONCLUSION
This is a "Poor Man's" approach to calibrating a DSLR camera to a hand-held light meter. It is certainly not "scientifically accurate" and certainly will not stand up to laboratory testing! It does however give me a truly useful guide when metering a scene. I can now tell if a 1, 2, or 3-stop grad is necessary to "tone down" a bright sky. I can tell if a reflector and/or fill flash is required in the shadows. I can now balance light from electronic flash very accurately with ambient light.In short, this has taken the guess-work out of exposures for me. I can now devote time to composition and finding that "perfect shot" rather than trying to second-guess the camera's light meter.
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