Pentax K-5 and the DA* 300mm F4 lens
The Pentax DA* 300mm F4.0 lens was introduced in 2008. It is designed for digital cameras, and as such cannot be used on film cameras as it does not cover the full-frame image circle. When people see the lens, the usual question is: “what's the equivalent focal length?” Because the Pentax APS-C sensor is 1.5x smaller than a 35mm image, the DA* 300mm's field of view is equal to that of a 450mm on film. The DA* 300mm, like all Star lenses, is built to tight tolerances, featuring a durable metal exterior and weather sealing.
The focus ring is located in the middle of the lens and it requires 2/3 of a turn from the minimum focus distance of 55 inches to infinity. The focus ring is very large, a little less than two inches wide, and does not turn when you use autofocus. The lens, like most DA* lenses, does allow "Quick-Shift" focusing, which permits you to manually focus the lens at any time (without having to flip any additional switches).
The lens also has a removable tripod mount. The mount has an adjustable collar that allows the lens to be rotated while mounted. The collar has notches every 90 degrees that allow you to quickly adjust the lens to take vertical pictures without removing the lens from the tripod mount. The tripod mount can be removed easily with a coin or other flat object. Once the mount has been removed, the only indications that the lens has a mount are two small hollows on the bottom of the lens. Below is a picture of the mount and the bottom of the lens.
The lens has a small sealed window at the top of the lens that displays the focused distance in feet and meters. Because its internal focusing, the lens does not increase or decrease in size and the front element does not rotate when focusing. This is advantageous, as this lens's weather sealing will obviously be more effective than that of one with moving external parts. The lens also has an auto-focus/manual-focus selector switch on the side.
The removable lens hood can be inverted over the lens to decrease the storage size of the lens. The front element uses a 77mm filter, which is consistent with several other large Pentax lenses. The lens is about seven inches long and the length increases to about 10 inches when the lens hood is attached.
One of the reasons why this lens is so exceptional is that it handles very well. The focusing is smooth with no play and no creep. The lens is not light (weighing in at just over 2.2 pounds, or 1070g), but it is very well balanced. The focusing ring feels good in your hand and doesn't get turned by the AF system, making it a great place for you to hold the lens. The quick-shift system lens you conveniently adjust the focus manually at any time simply by turning the focusing ring.