Thursday, February 9, 2012

Exposure Triangle pt.1 - ISO

   Recently I was at a high school basketball game to take pictures of my daughters who were part of a pre-game dance routine.  It was Homecoming night and several members of the Homecoming court were seated in the stands, dressed in their gowns, awaiting the big half-time ceremony.  The father of one of the girls in front of us, toting a Canon Rebel asked if I could give him some advice.  Seemed in the dimly lit gymnasium all of his photos were looking very blurry.  I explained that this is a very common problem.  Gymnasiums usually appear pretty bright to our eyes, but actually even the nicer newer gyms are typically very poorly lit.  Using his cameras default settings, the dim lighting was driving his shutter speed down so slow that all of his shots were blurred.
     This is where the Exposure triangle comes in......  at least one corner of it - ISO.  Photography is all about recording light.  When you press the shutter button, light travels into the lens, through the Aperture opening, bounces around in the top of the camera a bit, passes through the open Shutter and finally strikes the cameras sensor.  Three main elements here comprise what is commonly referred to as the Exposure Triangle - 1. Aperture (the size of the lens opening), 2. Shutter speed (how long the shutter remains open), and finally 3. ISO.  I S Who?? What?   ISO is simply the sensitivity level of the cameras sensor.  (remember the ASA number on the film boxes?  Same thing)   The higher the ISO number, the more sensitive the sensor becomes, and therefore less light is required to make a good exposure.  A very low ISO number such as 100, will typically be used outdoors on a sunny day.  In the very bright sunlight the sensor need not be very sensitive to record ample light.  Inside a high school gymnasium however, is a very different scene.
     So getting back the Homecoming ceremony, how do we fix Dad's burry photo issue?  First, I had him do the unthinkable - take the mode dial out of the green Auto mode!!  It has to happen sometime!  "P" mode is a good place to start.  "P" stands for Program Mode.  With the mode dial selected to Program mode, the camera will still automatically set your Aperture and Shutter speed for you, but you have the freedom to change other settings such as White Balance, Focus Points, and of course ISO.  So next we accessed the ISO selection, (on Canon DSLR's look for an ISO button on the top of the camera) and we cranked the ISO number way up.  In this case the maximum ISO for his camera was 1600, so that's what we selected.  some cameras today will go much higher, but 1600 - 3200 is usually sufficient for gym light.  So that's it!  With his ISO set to 1600 the sensor is much more light sensitive.  The camera will still set the shutter speed automatically, but it can now use a much faster shutter speed due to the increased sensitivity.  The faster shutter speed means less blur caused by camera shake and moving subjects. We can see a little bit of how each point on the Exposure Triangle affects the other points.  In other words- ISO affects Shutter Speed affects Aperture affects ISO affects Aperture affects Shutter Speed..............  and so on.
     Easy enough right?  Just select "P" on your mode selector, then adjust your ISO up to around 1600.  Your shutter speed will get faster and your blurry photos will get sharper.  BUT.... there is a catch :(  Everything in photography is a compromise.  Here it is - The higher your ISO, the more grainy (or noisy) the image will be.  The amount of noise in the photo varies with different camera models.  Some of the newest and greatest DSLR's produce amazingly clean images even at incredibly high ISO's.  Generally speaking, a noisy image is better than a blurry image.  This is especially true considering most images will only be viewed on a computer screen or in small prints, and will likely never be blown up to large size.  When viewing at typical size, even moderate noise is barely noticeable to most folks.  So don't get too overly concerned with noise.  Good exposure is priority one.


photo jabber blog by tim wyler
simple digital photography tips and techniques

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