Camera Math

It’s back to school and time for math class.  Don’t worry, I won’t be asking you to solve quadratic equations (remember FOIL?); we’ll be doing camera math. So pay attention, this WILL be on the test!

F stop: Officially, it’s the focal length divided by the diameter of the aperture; in practice, this is your aperture setting.  In low light situations, your pupils dilate to allow in extra light.  In bright light, they close (think of walking out of a movie in the daytime).  The aperture on a camera lens can be set to allow in a lot of light (wide aperture) or a little light (narrow aperture).  Here’s the tricky part: because of the way they are calculated, the f stop settings are the opposite of what we might think.  A low number – f/2.8 – is a very wide aperture that lets in a lot of light.  A high number – f/22 – is a very narrow setting that allows in much less light.

Shutter Speed: Shutter speeds are expressed in fractions of a second.  A setting of 1/400 means the shutter will be open for 1/400th of a second.  A shutter that is open for a short time is said to be “fast” and one that is open for a long time is said to be “slow”.  So the smaller the fraction, the larger the denominator and the faster the shutter speed (or the larger the fraction, the smaller the denominator and the slower the shutter speed).  Remember, 1/400 is smaller than 1/60 even though 400 is larger than 60.

ISO:  ISO stands for the International Organization for Standardization and why it isn’t IOS is way too boring of a story.  It’s a logarithmic scale used to represent how sensitive a film (or sensor) is to light. This one is straightforward:  the higher the number, the faster the film and the more sensitive to light.  So, a high speed film, say 800, is better in lower light or with shorter shutter speeds.

It’s almost time for the bell to ring.  Next class we will discuss the relationship between f stop, shutter speed and ISO and which settings to use in different situations.

Part II

Welcome back!  In our last class we defined F stop, shutter speed and ISO.  These three elements combine to form the exposure triangle. Understanding their relationship is the key to taking quality photos.

Think of exposure as a number line with perfect exposure at zero.  The further away from zero, the worse your pictures are.  If your photos come out dark, they’re underexposed (negative side).  To fix this you can open the aperture by using a smaller f stop, decrease the shutter speed or increase the ISO setting.  If your photos come out too bright and lack detail, they’re overexposed (positive side).  To fix this you can close the aperture by using a higher f stop, increase the shutter speed or decrease the ISO setting.  Often, changing one side of the triangle is enough to fix exposure problems; unfortunately, “which side” and “how much” to change depend on the situation.

ISO:  A higher ISO setting will let the camera make the best use of the available light, but set it too high and that image will be very grainy.   The lower the speed, the better the photo and most consumer cameras don’t do well beyond 400 ISO.  Change this side of the triangle when you need just a little more or less light.  Use it as a first step; it might be all you need.  Settings between 100 and 400 are low enough not to be grainy.

F Stop:  A full open aperture will take in a lot of light, but produces a very shallow depth of field.  This means that the camera will only focus on the items close to you and blur the background.  If you want the entire photo to be clear you must use a small aperture that takes in less light (higher number).   Adjust this side of the triangle when depth of field doesn’t matter and a fast shutter speed is more important, as in action shots.

Shutter Speed:  A slower shutter speed will take in more light, but too slow and the photo will be blurry.  In general, you can’t hold a camera and shoot below 1/60 without camera shake.  Change this side when you need to have the background in focus, such as landscapes or group photos (the mountains won’t move, but tell the people to stay still).

We’ll finish our discussion next class and review for the test!

Part III

OK class, settle down.  We have one new topic to cover before our exam – megapixels.  A pixel is short for picture element and refers to the number of tiny squares of storage, arranged in a grid, on your camera’s sensor; it’s also called the resolution. One megapixel equals one million pixels.  Total number of megapixels (MP) is calculated by multiplying width by height (like area); so 3,000 pixels X 2,000 pixels equal 6MP. Because this is a specific number and a simple calculation, the advertising world has latched on to the concept of MP. They have bombarded us with the idea that a higher MP is always better and the only indicator of picture quality; this is what we call “the megapixel trap.”

I have two main cameras: Canon’s Digital Rebel XTI and Canon’s 5D.  The 5D is a 12.8MP camera and the Rebel is a 10.1MP. Under identical conditions, the 5D will capture a superior image.  Why? Not because of the extra 2.7 million pixels, because it is a better camera.  One of the most important differences is the sensor size.  My 5D has a full-frame image sensor (about 36mm X 24mm) and my Rebel has a cropped image sensor (around 24mm X 16mm); the larger the sensor, the more light it “sees” and the better the picture.  Phones and compact cameras have even smaller sensors. When the manufacturers increase the MP count, they are not increasing the sensor size, just dividing it up into smaller pieces.  The other important factor in image quality is the lens used; the better the lens, the better the image (with everything else equal).  Phones and compact cameras have simple lenses, often with limited focusing abilities.  Their photos will always be inferior because they are from inferior equipment, not because of the pixel count.  That’s not to say you can’t get a good photo from them; they are fine for standard print or web photos and nothing beats the convenience.  So does pixel count matter at all?  Yes, but pixel count has to be quadrupled for any obvious improvement in quality.  This explains why the early increases in MP were significant (say 1MP to 4MP), but the more recent ones are not.

In summary, don’t brag to me about the 8MPcamera in your smartphone; I will not be impressed (at least, not in the way you intended).

In addition to megapixels, students should review the definitions of ISO, f stop and shutter speed and understand the relationship between the three. The quiz can be found at:  (and you probably thought I was joking!).

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