Wednesday, April 23, 2008

Opening Up Aperture's Mysteries

In the last lesson we discussed the secrets behind ISO and how to adjust it to serve your purposes. One of ISO's brothers-in-arms is aperture. Aperture is represented by an f-number. You might have seen something like f5.6 or f32 on a Flickr site and wondered what it was. Well, what you were looking at is the universal way to express the camera's aperture at the time of the photograph. Aperture is the second leg of getting a proper exposure and this lesson will explore it for you.

Aperture, simply stated, is similar to a water pipe. If you have a small pipe you can move a small amount of water through it in a given time. If you have a large water pipe, you can move a larger amount of water through it in that same given time. Aperture in the camera lens works in the same manner. Have a small aperture and have a small stream of light flowing onto the sensor. Open the aperture wide open and get a flood of light flowing onto the sensor. Seems easy enough, right? Well, it is, for the most part.

As with other photographic concepts, the logic behind them is solid and useful but the vocabulary is a little convoluted. If you are new to a more advanced level of control of your camera, you could be a little confused by it all. The first concept to grasp is, as the f numbers get smaller (like f1.8) the larger the "pipeline" of light is. And likewise, the larger the f number (like f22) the smaller the "pipeline" of light is.

So what does it matter what aperture you shoot things at as long as the exposure is correct? Great question. The answer is many and varied. In addition to regulating the amount of light hitting the sensor, aperture can control many other creative aspects of a photograph.
Remember, once you master the concepts, you can begin to control them to do your bidding.

Probably the most common side effect of aperture is how in-focus the background of photograph is. This, in photographic circles is called "Bokeh".

Main Entry: bokeh
Part of Speech: n
Definition: a Japanese term for the subjective aesthetic quality of out-of-focus areas of a photographic image
Example: The bokeh, or quality of the blurred image in the photograph, was described and discussed.

How does one get a photo with that nicely blurred out background? You open up that aperture wide baby! Now, here comes the rub: Unless you're using prime lenses, (Prime lenses are lenses that do not zoom. They are a fixed focal length) zoom lenses with a very wide aperture are pretty pricey. (Check out a f2.8 Canon zoom lens' price tag sometime!) The good part of this story is, prime lenses are less expensive and usually very well made. Nikon, Canon, and the like have been making those for a long long time and have the process down, and you can reap those rewards. I think a serviceable f1.8 50mm Nikon Nikkor lens is under $100.

One trade off of this great bokeh is how much of the photo IS in focus. The wider the aperture (the smaller the f-number remember) the less of the image is going to be tack sharp. It's possible, depending on a few factors, that you could have 1 inch of focal depth (or less!) in an image with a very wide aperture.

Checking out the calculations from a very handy website:

Assuming I'm using a Nikon D200, a 50mm lens, with an aperture at 1.8:

If I stand two feet away from the subject, the acceptable focus range is .02 ft in front of and .02 feet behind the subject. That equates out to a total of 1/2 inch of focal range. 1/2 Inch isn't even enough to keep an entire face in focus. So, use that bokeh carefully!

Knowing all that, what is the flip side of using a very small aperture? Strictly speaking in terms of photographic performance, the smaller the aperture you use (remember, larger f-number) the more of the photo will be in acceptable focus. (We'll get into what is acceptable focus and all the terms for that in a later episode)

There is another side effect of a small aperture. when stopping your lens down a great deal (which is the common term for making your aperture smaller) is that your lens is forced to bend light a little more than it wants to. The result is called Chromatic Aberration, or CA for short. How you can tell if your lens is suffering from this syndrome is you will notice small color shifts at the edges of objects in photos where there is a dramatic change in light, such as the edge of a dark building on a very light sky. The aberrations will typically be red or blueish in color. Many times they aren't that noticeable sometimes they require a little adjustment in post-production.

Most lenses would be happier operating in the mid range when it comes to aperture. The highest level of sharpness will be achieved this way, and the odds of aberration will be the lowest. Typically people regard f8-f11 as the ideal. And, depending on the quality of your lens, a well above average result can be had from f2-f22. Granted these are typically on the higher end zoom lenses and your more average pro-sumer lenses will be happier with a bit smaller range, say.. f5.6 to f16.

Now, how does aperture relate to ISO and exposure? This one is cool. Remember in the last post (ISO ISO it's off to work I go) When you took a photo and it looked to be underexposed. And your solution was to raise the exposure of the photo one exposure stop, so you raised the ISO from 100 to 200? Well, you now have another option to that scenario. You could open the aperture one stop and achieve the same effect.

The table of 'full stops' as it pertains to aperture is as follows:

0.5 0.7
4 5.6

So, going back to our previous example, you were shooting a photo at f8. If you wanted to let more light into the exposure to the tune of 1 stop you would open the aperture to f5.6. This, as far as exposure goes will give you the same effect as changing your ISO to 200. This is one way to illustrate that there is more than one way to skin this exposure cat. The next lesson will connect these two concepts with shutter speed and you will have the triumvirate of exposure down!