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How to Use a Digital Camera as an External Light Meter
2006 KenRockwell.com
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see also

Light Meters

The Zone System

Using the Nikon Spot Meter


Why buy a separate meter when you can buy a complete camera with a built in meter for less than a separate professional meter? Yes, for the same price or less as a pro meter you can buy a digital point-and-shoot that weighs less and lets you preview the images for color and contrast.

As of October 2004 I no longer use my Pentax Spotmeter. Instead I look at the LCD screen of my Nikon D70 and copy that exposure for use with my film camera, presuming the LCD image looks as I want it. This is better than any meter; it shows me the effects of lighting and color temperatures and simulates my chromes on a light table.

Watch for these issues:

1.) Overall camera calibration. My Nikon D70 is right on, and my Canon A70 is one stop more sensitive than rated. That means that my A70 set to ISO 50 is really at ISO100, so for Velvia I have to add a stop to what the A70 at ISO 50 says. Make a few shots at various ISO variations to see which one matches your film.

2.) Filter factors. Put the same filter over the lens of the digital camera. If you have different or no filters on one camera then be sure to apply the differences in filter factors.

3.) Light Transmission. Not all lenses transmit all of the light, so you may also have to take "lens factors" into account as well as any aperture calibration variations in lenses. Zoom lenses, especially older ones, may lose as much as 2/3 of a stop compared to fixed lenses due to light lost as internal reflections. This is never a problem with TTL SLRs because TTL metering automatically corrects, however it will alter the reading if you are trying to use the reading from one camera on another.

4.) Differing ISOs. It's unlikely that your preferred digital ISO setting will just happen to match your preferred film. You'll be shooting photos with your digicam and not just using it as a meter. It is cumbersome to keep swapping ISOs between what matches your film and where you prefer to shoot the digicam. If you swap ISOs you are likely to forget and use the wrong setting and waste film. I suggest leaving the ISO of the digicam where you prefer and calculating exposure conversions in your head or use the calculator dial on a light meter. You even can use a broken old meter for this. I use the scales on my Pentax meters. Here's how:

4a.) Set the digicam's effective ISO on the Pentax meter. You figured out the effective ISO from tests in 1.) above, which may or may not be the ISO indicated on the digicam.

4b.) Set the indicated exposure from the digicam on the Pentax scale. Don't move the ISO setting.

4c.) Reset the ISO on the Pentax meter to your film's ISO. Don't move the LV ring. Now read the film exposure off the Pentax scale.

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