Friday, February 4, 2011

The Quick Exposure Quiz

A few days ago I posted a quick quiz on my page on Facebook and I promised a blog post to explain it for those who might not have been sure of the answer. So, below are the three exposures (sensitivity - aperture - shutter speed) I provided and the question was: if you took a photo of the same scene, would the pics look identical or different?

A. ISO 100 - f/5.6 - 1/60 sec.

B. ISO 400 - f/8 - 1/125 sec.

C. ISO 3200 - f/11 - 1/500 sec.

Strictly Exposure
In regard to exposure, A, B and C are identical. In fact, you could put your camera into manual mode and take three photos of the same scene with these settings and the shots should look very much the same (in the section below I discuss some other effects that occur).

In making up this quiz, I decided to stick with standard values for sensitivity, shutter speed and aperture. Therefore, you can use full-stops to figure out any differences between the exposures. In case you're not familiar with the concept of "stops" in photography, I'll give a brief explanation here and I've added a few links to the resources section below on the topic.

First off, a stop can be applied to sensitivity, shutter speed or aperture, as it basically represents a doubling or halving of the intensity of light. So in regard to:

Sensitivity: The standard sensitivity (ISO) scale includes, but is not limited to: ...100, 200, 400, 800, 1600, 3200... Each of these values is one stop apart with each successive value being twice as sensitive to light as the value before it. As an example, 400 ISO is twice as sensitive to light as 200 ISO. 100 ISO is half as sensitive to light as 200 ISO. 400 ISO is four times as sensitive to light as 100 ISO.

Shutter Speed: The standard shutter speed scale includes, but again is not limited to (values in seconds): ...1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1... As you've probably guess, these values are a stop apart, with each successive time being twice as long as the previous; in terms of shutter speed, the shutter would be open for twice as long, thus allowing twice as much light to fall on the focal plane. Of course the reverse is true, each value preceding the next is half as long, thus a shutter speed of 1/60 of a second opens the curtains for half as long as a shutter speed of 1/30 of a second.

Aperture: Lastly, the standard aperture scale includes, but is not limited to (from small to large aperture): ...f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.4, f/1... You guess it, each value here is one stop apart, and each successive value allows twice as much light through as the one preceding it. For example, f/2.8 allows twice as much light through as f/4 does. f/4 allows half as much light through as f/2.8 does. f/2.8 allows four times as much light through as f/5.6 does.

Before moving on, you may have noticed that you have a lot more options available to you on your camera than just these standard numbers. Those "in-between" values, like f/3.5 or f/4.5 and other shutter and ISO values not shown here, are generally a third apart. Unlike the good old manual cameras many decades ago, these silicon brainy ones of today can easily accommodate/calculate such a variation of settings, thus providing photographers with a finer degree of control if need be.

Now I still briefly want to go through the quiz, and as you'll see, using the concept of stops will make this task quite simple. I'll be relating both "B" and "C" to "A". By the way and for those unfamiliar with this, positive stop values mean you are moving to a higher ISO, widening the aperture or using slower shutter speeds (in a way, methods of increasing light), whereas negative stop values mean you are moving to a lower ISO, closing the aperture or using faster shutter speeds (in a way, methods of reducing light).


B and A: "B" has an ISO difference compared to "A" of positive two stops, one from ISO 100 to 200 and another from 200 to 400. But "B" has a negative one stop (or one stop down) for aperture and the same negative one stop difference for the shutter speed. Thus, no change: +2 + (-1) + (-1) = 0

C and A: "C" has an ISO difference compared to "A" of positive five stops: 100 to 200, 200 to 400, 400 to 800, 800 to 1600 and 1600 to 3200. "C" is two stops down in regard to the aperture: f/5.6 to f/8 and f/8 to f/11. "C" has a negative three stop difference compared to "A" in regard to shutter speed: 1/60 to 1/125, 1/125 to 1/250 and 1/250 to 1/500. Thus, no change: +5 + (-2) + (-3) = 0


For photog newbies, this may also make it clearer why a negative EV compensation (like -0.7) will darken the photo whereas positive compensations (like +0.7) lighten the image. Depending on the mode you're in, the camera manipulates the aperture or shutter speed to under or over-expose the photo, respectively.

Other Considerations
In my quiz, I was not specific in regard to other image artifacts that would occur due to those exposure settings, but thanks to some eager folks, those things did not go unnoticed. One of those things is depth-of-field (DOF). "A" has an f-number of 5.6, which would produce less DOF than f/8 (B) or f/11 (C), which for example could mean a blurrier background behind the subject in image "A" or conversely, an overall sharper image for "B" and "C". Next, the amount of noise in each photo would vary. There might not be much of a difference between "A" and "B", but "C" at 3200 ISO would surely produce more digital grain; add in the effects of noise reduction and the image might also be softer/less detailed than the other two. And lastly, also thanks to a fan on my page, the difference between the shutter speeds could play a role, especially if the subject is in motion. Time would be more "frozen" in "C", but a 1/60 of a second shutter speed in exposure "A" might result in that subject being blurred (or the background if following the subject; whatever the case).

I really enjoyed creating this quiz and I believe those involved did too, so I think I'll be coming up with more of these in the future. Plus, I also had a vote going to determine which episode I should produce next and the "winner" is ACTION PHOTOGRAPHY (ouu caps lock for excitement)! For those who wanted time-lapse photography, don't fret as I'll be doing that one in the future too.

Web Resources
http://www.digital-photography-school.com/reversing-the-inverse-square-law
http://www.geofflawrence.com/photography_tutorial_inverse_square_law.htm
http://www.uscoles.com/technical.html
http://www.uscoles.com/fstop.htm
http://www.photonhead.com/beginners/stops.php
http://en.wikipedia.org/wiki/Shutter_speed
http://en.wikipedia.org/wiki/F-number
http://en.wikipedia.org/wiki/Film_speed

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