My Dominant Hemisphere

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Understanding Reflective Light-Metering And The 18% Reflectance Phenomenon

with 6 comments

[I took this shot with the macro setting on my camera. The shot was taken in a semi-dark room with a pink candle (seen in reflection) by the right side. This is my very first flickr macro shot! The screwdriver was put through hell when I tried unscrewing some of the tight fitting screws on my tripod :) . If you pay close attention to the way the hair are arranged at the tip, you'll realize that they are actually aligned along the magnetic field lines that developed there due to static electricity! Neat :) .]

Howdy all! I’ve been studying digital photography lately and totally enjoying it. Today, I’ll share with you folks essentially what had me in one of those ‘aha! so that’s it!’ moments.

Most digital cameras have an in-built light meter whose job it is to discern the brightness of incident light falling on an object, so that the camera can adjust its aperture, shutter speed or ISO settings to allow into the camera, just the right level of light to yield a proper exposure. These light meters usually are of the reflective category, i.e. they try to predict the incident light’s brightness based on the
assumption that reflective light equals 18% of incident light.

In other words, if

x = incident light
y = reflective light

then,

0.18x = y or x = y/0.18

It so happens that most objects satisfy this empirical rule and the camera judges incident light correctly.

In the case of white objects such as snow, etc. the actual equation is different. Assuming white reflects 90% of incident light:

0.90x = y or x = y/0.90

Because the camera calculates x as y/0.18 rather than as y/0.90 , it perceives incident light as being brighter than it actually is. The camera thus tries to decrease the amount of light falling on the film/digital sensor and as a result, underexposes the object. White objects turn into gray. And by extension, all highly reflective objects end up underexposed.

With black objects, the opposite happens. Assuming black reflects 4% of incident light:

0.04x = y or x = y/0.04

Because the camera calculates x as y/0.18 rather than as y/0.04 , it perceives incident light to be dimmer than it actually is. It thus tries to increase the amount of light falling on the film/digital sensor and as a result, overexposes the object. Black objects turn into grey. And by extension, all low reflective objects end up overexposed.

A great discussion about this topic and ways to solve this problem can be found on this website.

Any thoughts or tips of your own? I’m waiting to hear them! Adios for now :) !

Copyright © Firas MR. All Rights Reserved.

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Written by Firas MR

August 17, 2008 at 8:51 am

6 Responses

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  1. Dear Sir:
    Interesting. I’d just like to point out a small problem in Physics you’ve got there. The small oriented filings on the top of your Phillips screwdriver are oriented due to the magnetic field as you say, but have nothing to to with static electricity.
    The screwdriver is made of an iron alloy which being handled can acquire and possess a magnetization, whose effect is strongest at the tip.
    Fine blog!
    Wido

    Wido Schreiner

    January 17, 2010 at 5:06 pm

    • Thanks for the heads up! :) Looking forward to more comments…

      Firas MR

      January 22, 2010 at 5:21 pm

  2. Thank you very much, Sir, for elucidating me on this point in a notably coherent, comprehensive, yet simple manner. This has been the only element of photography about which I have still been somehwat befuddled with until now. This topic is one I have encountered in books, read articles about on other websites, and tried to fully understand. However, you tied it all together for me, and I FINALLY GET IT! Wow, that feels good, because everything else I have studied and experimented with concerning photography has been effortless to comprehend except this point. Thanks!

    Ryan T.

    August 19, 2010 at 5:13 pm

    • Thank you so much for the kind words! I’m also doing my best to learn every bit of photography, it’s such an enchanting mix of science and art. I’m glad that I helped a fellow student :-) !

      Firas MR

      August 19, 2010 at 5:22 pm

  3. [...] once in a while, I receive feedback from readers as to how much they appreciate some of my writing on non-clinical/non-medical subjects. Sometimes, the subject matter concerns books or web resources that I’ve recently read. [...]

  4. [...] could extend this concept of seeking out phenomenal truths in everyday things to many other fields. As a photography buff, I can tell you that ordinary and boring objects can really start to get interesting when viewed up [...]


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