Learning Photography - Depth of Field

One of my aims on this blog is educating.  Whether it is sharing knowledge about the history of this state or other things.  I don't know everything about photography but I do know quite a bit and I figured I could share some of that knowledge with you.

One of the first things you will learn as you dive deeper into photography is that there is a vocabulary.  If you find yourself with an SLR camera, you will run across terms like f-stop, shutter speed and ISO.  As you go a little deeper, you will run across a term called depth of field.

A camera is a pretty simple device when you get down to it.  There is some sort of opening to let light in and there is a medium that reacts to light.  The simplest camera is what is known as a pinhole camera.  Typically you would have some sort of box with a pinhole and either film or photopaper (I forget which because I've never done one).  The pinhole acts as both your light source and lens (this is due to some physics principles).

One of the disadvantages of a pinhole camera is that it is hard to focus.  You kind have to figure out where to put it so that your subject will be in focus on the medium that reacts to light.  So the next step up with be a box with some sort of adjustable lens.  This would let you focus a little better.

But the problem with that setup is that you can only use it in certain degrees of light.  Plus you would have to do something to keep the light out of the film once you've taken your subject.  So the next step up would be a camera with a shutter where you can press a button.  You can activate the shutter, expose your film and close the shutter.

However, to use a camera like that, you would have to know the light condition and how long you would need to expose the film in that condition.  If you guess badly, your film could be either over or underexposed.  So the next step would be adding a light meter to your camera.  This way you can measure the intensity of the light and get a better idea of how long to expose your film.   Eventually, you would add enough pieces that the camera could do all that figuring for you.

As I said, you have a medium that reacts with light.  In the old days, this would be some sort of film.  In modern times, it is an array that detects the light.  Now in the above example, you would want to know how long you would have to expose that film depending on the type of light.  This is where ISO comes in.  In the film days, ISO would be how sensitive to light your film was.  A low ISO number is less sensitve to light than a high number.  Because these numbers were standard, you could use rules of thumb if you had to.  But unfortunately there is a trade off, higher ISO numbers produce higher amounts of grain (in the film days) or noise (in digital days).  I think this is mostly due to the way light behaves.

Going back to the simple camera, you only have one hole size.  In photography, you want some degree of control of how focused your overall picture is going to be.  There are certain optical properties that tie the size of your hole to how far your picture is going to be focused.  A larger hole means less will be in focus than a smaller hole.  In photography, this hole is called an f-stop.  However, because of a reason that escapes me at the moment, a low f-stop number means a larger hole and vice verse.  So if you only want your subject focused and the background out of focus, you would use a higher f-stop.

So here are some examples.

The pictures below are from a figure that was based on a Navy recruiting poster.

 This picture was shot at f 5.6.  As you can see, the sailor is in focus and the ship isn't.  If I had a lens with a wider aperture, the ship would be even less in focus.  Typically you would use a setting like this for a portrait or if you want to emphasize something in the foreground and de-emphasize something in the background.

 The next picture was shot at f 11.  As you can see the ship is a little more in focus.  Typically you would use a setting like this if you want to draw some attention to something in the background but not that much.

And the last picture was shot at f 20.  The ship is mostly in focus as well as the sailor.  Typically you would use a setting like this for a landscape or something where you want everything in focus.  One other phenomenon with this is that pretty much everything behind your subject would be in focus.

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