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As a caveat to the so called "fast lens" variety, such as those in the f2.8 and less variety:
These lenses are great and I wish I had a hand full, but keep in mind that aperture - AKA f-stop, is what controls "depth of field." The lower the number the narrower the depth of field.
When you take a picture of your friend standing in front of the mountain range you might wish to select an "f" number of 11 or greater (usually to a max of 22). This small hole in the lens will give the greatest depth of field and bring both your friend and the distant mountains into focus. When you select "landscape" on the program dial the camera will do it's best to push the f-stop to the upper end. When you select "Portrait" on the program dial the opposite will be true.
Most zoom lenses of the moderate price variety will not go down this far. They usually stop out at the f3.5 range. Usually, this is enough depth of field to bring the entire head into focus.
At the lowest end of the f-stop range (the biggest hole) is the so called fast lens. When you select this lower number f-stop you will get all the attributes that accompany (ample light at higher shutter speed). One of those is a very narrow depth of field. This may mean that having focused on the end of the nose, the eyes may be slightly out of focus. If you correctly focus on the eyes, the ear may be blurred. At any rate, the full depth of the head will probably not be in sharp focus.
This very narrow depth of field is no tragedy, but it can be very annoying if you are not paying attention to this detail.
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Mike McCarty
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