Know your lenses.
Pentax makes high-quality cameras and lenses for the amateur and professional market. However, as with other brands, it can be difficult to determine the best lens for a specific situation or for the shooter on a budget to choose which lens will be best for his collection. Although the lenses may seem similar, they have a few critical differences.
Aperture
The first significant difference is the aperture range. Although both lenses will stop down all the way to f/22, they have slightly different minimum f-stops. The 85mm features an f-stop of 1.4, while the 77mm features an f-stop of 1.8. This may seem like a subtle difference, but it will allow the 85mm to perform slightly better in lower-light situations than the 77mm. Otherwise, they have standard f-stops up to f/22.
Focusing Distance and Picture Angle
They are also different in minimum focusing distance and picture angle. The 77mm features a 27.6-inch minimum focusing distance, with a picture angle of 31.5 degrees. The 85mm, however, features a minimum focusing distance of 33.6 inches and a picture angle of 28.5 degrees. This means that the 77mm can obtain a slightly wider and closer picture than the 85mm, while the 85mm is slightly better for more distant subjects.
Groups and Elements
Another factor to consider is groups and elements, the parts of the lens and how they're put together in the barrel. The fewer parts there are, the easier the lens is to repair, and the fewer elements there are, the more light reaches the lens. The 77mm has seven elements in six groups, while the 85mm has eight elements in seven groups. The 77mm will be the easier to fix and will catch more light from the subject.
Size and Weight
When traveling, size and weight are especially important. The 77mm is 1.9 inches long and weighs less than a pound, at 9.5 oz., while the 85mm weighs just over a pound, at 17.7 oz., and is slightly longer at 2.8 inches. Both are designed to be portable and lightweight, but the 77mm is slightly better on size due to the fewer lens elements it needs.
Use in the Field
The 77mm is best for quick shooters who want to take their photographs and move on to the next subject. It is lighter, has a wider picture angle and a closer focusing distance and will work well under natural light. The 85mm is more suited to art photographers, as it needs a little lens light and caters to photographers who want to take their time or are taking pictures of more distant subjects, especially natural ones.
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