Author Topic: Understanding and Controlling Exposure  (Read 1459 times)

keithsnell

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Understanding and Controlling Exposure
« on: March 13, 2008, 03:00:35 PM »
I've updated the "Understanding and Controlling Exposure" tutorial and posted it under the Recent Articles link on the HOME page.  You can access it directly here:  http://community.spiritofphotography.com/index.php?page=23.

Please let me know if you have any questions or comments on the article.

Keith

werikblack

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Re: Understanding and Controlling Exposure
« Reply #1 on: April 24, 2008, 12:58:41 PM »
Would you run me through the 11-21 rule again (or whatever it was, I'm forgetting) for daylight exposure? I'd be interested in some general rules for sunny day photography (and others) so I have some general rules about places to start from on exposure and shutter speed. Should make things quicker when I'm messing around since I'm still a beginner on the technical end. :) Thanks a ton! I'm slow picking some of this up, but it will seep into my brain eventually, and I appreciate the extensive tutorials.

keithsnell

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Sunny Sixteen Rule (was Understanding and Controlling Exposure)
« Reply #2 on: April 24, 2008, 05:35:52 PM »
Erik,

Thanks for asking the question.  The "rule" you are asking about is called the "sunny sixteen" rule, which is a general guideline for achieving decent exposures in midday sun.  It originated BEFORE the time of cameras with built in exposure meters, but is still a very handy reference, especially for high contrast lighting situations that can easily "fool" your exposure meter.  

Many people will question the need for understanding this "sunny sixteen" rule in this day of super advanced camera meters and automatic exposure technology.  Unfortunately, as camera meters get more sophisticated, they are becoming MORE prone to being fooled by high-contrast scenes.  Most camera meters today use a type of "matrix" metering that is significantly influenced by the area under the active autofocus point.  (After all, they assume that you are focusing on the "primary" subject, and therefore want to optimize exposure on this part of the scene.)  Unfortunately this can result in images that are significantly over or under exposed if the area under the autofocus point is significantly darker or brighter than the rest of the scene.  Switching over to manual metering and setting the exposure based on "sunny sixteen" allows me to quickly nail the exposure almost every time.  It also serves as a quick "sanity check" of the exposure values the camera's metering is recommending.  If the recommended settings differ significantly from "sunny sixteen," this will alert me that I should double check my exposure on the histogram and adjust if needed.

In its simplest form, the "rule" states that the proper exposure for front-lit subjects in midday sun is an exposure value equivalent to setting your aperture to f16 and your shutter speed to the reciprocal of your ISO.  For example, if your ISO is 200, your exposure settings should be equivalent to an aperture of f16 and a shutter speed of 1/200th of a second.  

Even if you don't necessarily want to use an aperture of f16, or a shutter speed of 1/200, you can still adapt the rule to achieve the proper exposure by understanding the relationships between ISO, shutterspeeds and aperture, and how changes in these settings effect your overall exposure values.  

Doubling the ISO value (i.e. from 200 to 400) doubles the sensitivity of the film or the amplification of the sensor output , making the light appear twice as bright.  This "doubling" of light intensity is equal to a +1 "stop" (or +1 EV) increase in exposure, whereas halving the ISO value is a -1 stop decrease in exposure.  With shutter speeds, doubling the amount of time the shutter remains open (i.e. 1/100th of a second instead of 1/200th of a second) would result in twice as much light hitting the sensor, or a +1 stop increase in exposure.  

Changes in aperture values can be a little harder to understand because the numbers aren't "double" or "half," instead they are numbers that represent a doubling or halving of the area of the lens opening (diaphragm).  After a while it becomes second nature to remember that the progression in aperture settings from f16 to f11, f8, f5.6, f4, f2.8, f2.0, and f1.4 each let in +1 more "stop" of light than the prior setting.  (You can reference the "Understanding and Controlling Exposure" tutorial for more discussion on aperture values.)

This is easier to understand by looking at a chart of sample "equivalent" values.  All the settings in the chart below will result in the same exposure or light intensity in the image.

ISO    Shutter Speed    Aperture          Exposure Value
                                                      (measure of light intensity)
100            1/100               f16                     15
200            1/200               f16                     15
200            1/400               f11                     15
200            1/800               f8                       15
100            1/1600             f4                       15

But what if you aren't photographing in bright midday sun?  Using "sunny sixteen" as an anchor point, you can adjust your exposure values for the following conditions:

Bright or hazy sun, "average" subject, shadows are sharp and distinct = sunny sixteen
Bright or hazy sun, "bright" (white) subject or background = sunny sixteen -1 stop
Weak, hazy sun, shadows are soft but readily apparent = sunny sixteen +1 stop
Cloudy Bright, sun is behind "bright" cloud, no shadows = sunny sixteen +2 stops
Heavy Overcast, no extremely dark storm clouds = sunny sixteen +3 stops
Open shade, subject is in shade under large open sky = sunny sixteen +3 stops

Hopefully this will give you a "place to start from" when you are experimenting with different exposure settings, and a good "sanity check" for evaluating your camera's recommended exposure settings.  I find that when I am being lazy and relying on the camera's auto exposure settings, I often get significantly worse results than if I consciously set a manual exposure based on "sunny sixteen," especially in high contrast situations.

Please let me know if you have any other questions, or if I can clarify anything in the explanation above.  Thanks again for asking the question.

Keith
« Last Edit: June 29, 2009, 01:17:33 PM by keithsnell »

werikblack

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Re: Understanding and Controlling Exposure
« Reply #3 on: April 28, 2008, 03:51:28 PM »
Thanks, that helps a lot! I need to start bringing a pad of paper with me when I shoot with you just to keep track of this stuff. :) I swear I think I'm 36 going on 95 some days with my memory. I really appreciate all the time you've spent on tutorials. I'm reading them multiple times and absorbing things gradually.