To zoom with a prime lens, you must position yourself closer to the subject.
A Canon 85mm lens is a fixed-length lens. This type of lens, also called a "prime" lens, cannot zoom in or out, but generally produces higher quality images. These lenses are most often used by studio photographers or users who need excellent images for printing. Evaluating lens quality requires taking a few test shots, then looking at the result for common lens flaws. Ideally, a lens will produce exactly what the eye sees. While some lenses come close, no lens can produce an identical image.
Instructions
1. Attach the lens to a Canon-compatible camera and take a few photos with diverse subject matter, including landscape images or images from a high angle.
2. Load the images onto your computer so you can view them at a higher resolution. View the image at actual size for your evaluations.
3. Inspect the image for any vignettes that may appear around the edges. A vignette is a radial area of increasingly darker tones caused by the lens' inability to retain light on the fringes. While some lenses have vignette naturally, most modern lenses should be able to capture an image without it.
4. Inspect the image for blurriness. If the subject was in focus, the amount of blurriness still present is due to glass imperfections. Cheaper lenses will have more blur, while some professional lenses produce sharp images and cost thousands of dollars.
5. Inspect the image for chromatic aberration, which can be measured by the misalignment of colors. A poor lens will cause different wavelengths of light, such as red or blue, to refract slightly differently than other colors. This results in the subject having outlines of color untrue to their real appearance.
6. Inspect the image for any distortion. This can appear either as the subject being stretched or shrunk in a radial fashion. Most modern lenses have very little distortion.
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