How do I use spot metering to make a black subject render as black?
Asked 6/5/2011
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If I spot meter from a black object, the camera will try to place it around middle gray. Can I deliberately underexpose by a set number of stops so that the object appears black instead of gray? If so, roughly how many stops below the spot-meter reading should I use, and what affects that choice?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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Yes. Spot metering gives you 18% grey. Each stop below that halves the reflectance.
The number of stops to dial-in depends on how close to black you want the result and the dynamic range of your camera.
With a perfect noiseless exposure, -1 EV would give you 9%, -2 would give 4.5%, -3 would be 2.25%, etc. As you can imagine, most cameras are not perfect and there is such thing as a noise-floor. If you exposure below this, then all details get drowned by noise.
The best thing to do it to experiment and figure out the value of your camera. Unless you are shooting RAW, the number of stops depend on image parameters, mostly Contrast and High/Low key if your camera has them. For fixed image parameters and ISO, your that value will be fixed for your camera.
Most Olympus DSRLs and SLDs have a metering mode called Spot Shadow which is designed to exposure something very dark while retaining details. It is equivalent to normal Spot metering -3 EV. Doing the same using EC with your camera is probably what you want.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
15y ago
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Yes. A spot meter reading is typically intended to place the metered area near middle gray, so if you meter from a black subject you usually need negative exposure compensation to make it look black.
As a rough guide:
- about -2 EV: dark black with some detail/texture retained
- about -3 EV: very deep black, often close to no detail
Each stop down roughly halves the recorded brightness from that metered gray placement. The exact amount depends on how black you want it, your camera or film’s dynamic range/contrast, noise floor, and—on digital—how the file is processed. RAW capture gives you more flexibility afterward.
So the method is valid, but there isn’t one universal fixed number for every camera and scene. The practical advice is to test your camera and preferred settings, then learn what compensation gives you the result you want.
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