Monday, January 31, 2011

Let it snow, depth of field explanation, 31/365

We made it back from Iowa City just in time today and beat the snow.  Well, we didn't quite beat all the snow, but at least we made it back before the roads got bad this afternoon.  I wanted to catch the snow in a photo.  There are a few challenges to shooting snow.  Like my lighting talk, I'll wait to go into the technical details below the picture.  That way, all of you who don't really care, can just look at the picture.  :-)

One of the first problems that people usually run into when shooting snow is by using their flash.  If the flash is on, it will light up the snow closest to the camera brightly, even though it's probably not going to be in focus.  So, no flash, but it does take a fast shutter speed to actually stop the snow mid-air in a photo.  You can also clearly view the effect of depth of field in this photo.  See how the green bird feeder is the only one in focus?  The sharpest snowflakes are on the same plane as that bird feeder.  The larger, blurry flakes are in front of the area of acceptable sharpness, and the tiny, blurry flakes are behind the area of acceptable sharpness.  

Depth of field... what exactly does it mean?  Well, a pictures depth of field (DOF) is basically the area in front of and behind the subject focused on, which appear to be acceptably sharp.  Now, I'm not going to say, "in focus", because technically only the exact plane focused on is "in focus".  However, the larger the aperture you used to take the picture, the less depth of field.  Slightly more will appear to be acceptably sharp behind the plane of focus than in front of it.  I mentioned this with my picture of Rachael a couple days ago, when I intentionally threw the garage door out of focus.  

This is easily controlled with an SLR and a fast lens (with a large maximum aperture, like f/2.8).  On compact cameras, it's another story.  The smaller the sensor, the greater depth of field you will have.  Meaning, if you want to isolate your subject with a compact camera, that is generally problematic.  There are some high end compacts with super fast lenses that give you a little more control over that, but they are not that common due to the cost.  Digital SLR type cameras provide a pretty good level of control for those who are not willing to let the camera make all the decisions about your images.

I am going to mention one more thing with regards to DOF, just because I find it very interesting.  My favorite photographer was Ansel Adams.  He created gorgeous landscape images, mostly with large format cameras taking film 4" x 5", 8" x 10" or larger.  Remember how I mentioned that the smaller the sensor the more depth of field, and the larger the sensor, the less depth of field?  Well, then how could Ansel Adams create images with both rocks a few feet from him as well as mountains miles away, all sharp, with a large format camera?  On a large format camera, you can tilt the lens, which actually tilts that plane of focus.  So, then it's not parallel with the film/sensor anymore.  That is how he cold have both the rock by his feet and the mountain miles away both sharp.  There are a few tilt/shift lenses and lens accessories for modern SLR's which provide similar control.

Now that I've bored most of you to sleep, I better put an end to this.  It is a somewhat difficult concept.  If anyone has questions or is interested in any clarification, please feel free to ask.

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