The Savoy House, Downtown Houston
This is my web page for sundry topics in photography.
1/12/2008 Lens Quality and Chromatic aberration
Here's another photography topic: lens quality. No matter the "perfect" sensor recording the light that falls on it, the light has to go through a lens system. The perfect sensor will record all the lens's flaws faithfully. In the real world, the sensor's flaws add to the lens' and cause chromatic aberration (sometimes call "purple fringing") in the recorded image. While image processing (by in-camera software, or by post processing) techniques can help remove this, you always want the best image possible to start with.
The most severe test for any optical system is it's ability to handle white against black. Telescopic and photographic lenses may use special lens glass (called "ED" lenses in the Nikon line; Canon and Takahashi have used Fluorite) and may be said to be "apochromatic" or "APO" for short.
The shot below is tough for any lens/sensor system, it's a test shot using my Nikon 300mm f4 ED AF-S. I wanted to make sure the lens performed well on high contrast situation. The image below shows a shot of a distant building outside on a bright day. This is RGB 59-67-82 against white (RGB 255-255-255). Pixel-by-pixel examination in Photoshop CS3 shows a 3 pixel tall area of false color.
Image above 1/4000 at f4, Nikon D200, Nikon 300mm f4 ED AF-S. Here's another shot with that lens. I also bought this lens to use for bird pictures.
I'm starting a web page that will focus on the topics related to photography. Maybe this should be photography 301?
A spot meter provides the ability to measure the proper exposure for a very small area. This is very important in unusual lighting situations. The first photograph below shows a "proper" exposure in a standard "center-weighted" mode. Center weighted meters vary, but they look at the middle of the image and base the exposure primarily on that portion of the frame. You will see that in this strongly back-lit scene, the child's face is very dark. This is because the meter is reading some of the strong light source behind the subject.
The next photograph shows the results when the exposure is based on the spot-meter value of Brett's face. Isn't he cute? Both images shot using a Nikon D100 camera, re-sized, with no other processing. The lower image could possibly be improved with additional processing to "lighten up" the face a bit, but is perfectly serviceable as-is.
All images cropped, resized, and lightly tweaked in Photoshop.
Next Topic: I've been asked to try to fix people's photographs on several occasions. Aside from things like poor focus, the main problem I'm seeing is a lack of detail in the origin image. You need to Shoot at your camera's highest resolution setting!
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