Focus and Exposure

Tonight our emphasis was getting to know the camera and getting it off the "green zone", the automatic settings.  Shooting manually allows you to control shutter speed, aperture and ISO.  These are the three components of exposure of a photograph.

Aperture is an opening created by metal leaves in the lens.  The size of the aperture opening is expressed by numbers ranging from 1 (wide open) to 32.  The aperture number itself is actually the lower half of a fraction that expresses what fraction of the lens opening is allowing light in.  1/2.0 (F2.0) allows one half of the light in, where 1/16 (F16) allows 1/16th of the light into the sensor.

Aperture not only controls how much light hits the sensor, it also controls how much of the subject is in focus. In photography, there are always trade offs.  If you want a wide open aperture for maximum light (F2.8, for example), the tradeoff is a narrow depth of field.  In other words, only just a few inches front and behind your focus point will actually be in focus.  Using a large aperture (the word "large" referring to the size of the opening, not the number), results in blurry backgrounds.  This can be handy if you are trying to isolate your subject from blurry surroundings.
      2.8             Large opening                     Shallow depth of field
      5.6             Smaller opening                  Deeper depth of field
     11               Much smaller opening         Much deeper depth of field. . . . on so forth

For a more "in depth"discussion of depth of field, go here


Shutter speed is the amount of time the shutter curtains allow light through to the sensor.  These are expressed
in fractions of a second. . .1/2, 1/15, 1/30, 1/60, 1/125, 1/250, and so forth.  Slower shutter speeds allow movement to be recorded as a blur on the sensor, while faster shutter speeds, say 1/250 and above, freeze the action.  People tend to shoot sports at 1/250 or faster to freeze the action.  Some things are actually more interesting with a bit of blur, such as waterfalls and rivers  Practice taking pictures of the low water dam at the riverwalk at different shutter speeds to gauge the different effects.

Camera shake is movement of the camera by the photographer.  Very experienced photographers can hold the camera very still, but for most of us, we move and jiggle a bit.  To avoid camera shake, shoot at as fast a shutter speed as you can to achieve the desired effect.  What shutter speed can you shoot at and avoid camera shake?  This is determined by the length of the lens you are using.  Most of the lenses used today are zoom lenses, that can zoom from 30-300mm.

                       Speed to avoid camera shake =  1/focal length (or faster)

for a photographer shooting zoomed at 100mm, shoot at 1/100 second or faster.


ISO is the sensitivity of the camera sensor to light.  If you don't have enough light to properly expose a scene, sometimes you need to increase the ISO setting of the camera.  Again, there are trade offs.  As you increase ISO, you increase the likelihood of digital noise, especially in darker regions of the photo.  When there is more digital noise, then often detail is lost from the photo.

It may be tempting to shoot at a lower ISO and just under expose the picture.  Don't do it.  Your best bet for a quality picture lies in getting the best exposure (i.e. best looking histogram) at the necessary ISO.

Histograms are your best bet for quickly determining if you have the proper exposure.  A histogram is a graph of all the shades in your picture, from dark (left side) to light (right side).  The camera sensor can collect detail from about 5 levels ("stops") of light.  Below that, areas will appear black, above that, areas will appear "blown out" or white.  Your camera has a highlight warning that will blink if overexposed.  This is affectionately referred to by digital photographers as "the blinkies".  Avoid the blinkies in your photographs.

Remember the assignment:  post your pictures to www.flickr.com,

log in under flinthillsimages
password kansas

Upload your three best pictures of the riverwalk.  Include in your description your camera settings and name or initials.

Harold is still in the process of moving, so instead of him talking about light and composition, we will talk about a few more elements of digital photography:  white balance and file handling.

Several people on there comment sheets mentioned that they would like to do a better job with sports photography.  I attended a workshop last year with the team photographer of the University of Kansas. We will go over his keys to successful sports photography.  We may try to run over and catch some softball or baseball for practice.

See you next week!!
Joel