Pros: Potent performance in most light. Solid build, smooth controls, easy to use interface.
Cons: Cost, autofocus difficulty, poor grip of lens cap.
The Pentax 16-50mm f/2.8 lens with ultrasonic motor focusing turns the Pentax K-7 camera into a stealth indoor candid photography workhorse. Combined with the unusually quiet camera itself, the 16-50mm lens' ultrasonic motor focusing makes the act of photography nearly imperceptible in even a quiet setting. The Pentax Star series lenses are somewhat expensive, and at $750, the Pentax 16mm-50mm f/2.8 lens is one of the more expensive optics I have ever purchased. In return, it has been a daily workhorse, and the one lens I install if I want to photograph indoors without using a flash.
One of the major limitations of indoor flash photography is while it can illuminate the area and people to clearly show them, it does so in a context separate from the event with a very different appearance than the lighting the people in the scene saw. At the same time, the brilliant flash changes how people behave, with children trying to avoid it and adults donning forced smiles on faces hardened to attempt to get a photograph with their eyes open despite the impending painful brilliance of a flash.
I have personally gained an interest in recording how things are as we see them, rather than recording their presence. What I mean is I want to get the feel of being in a situation and location as it was experienced at the time, since we will not pass that way again. As a result, the need for low light sensitivity is actually part of a desire to photograph in the same light I saw the scene in, and to record it as I and others around me experienced it.
I am using this lens to record scenes as they exist. I photograph people without the threat of a flash's sudden brilliance and spots. It means I can use a photographic remote to shoot a stream of images without the need for people to recover from the flash before taking the next.
The mechanism behind this is the f/number of the lens. The f/number is the ratio of the focal length to the aperture. The lower this number is, the larger the diameter of the lens is compared to its focal length. This is actually a measure of how wide the light cone going through the lens is, and this translates into how much light is hitting the detector. Since the area of the aperture increases with the square of the diameter, the actual amount of light getting into the camera also increases with the square of the change in f/number. So f/4 is four times brighter than f/8, and f/2.8 is twice as bright as f/4.
But getting to f/2.8 means the aperture will have to be large at a focal length of 50mm, since the iris controlling the aperture is towards the back, so the front lens has to be big. Conventional zooms have a bottom end around f/3.5 to f/4, which yields a very compact design for an 18mm- 55mm zoom, for example. The similar range of zoom in the 16mm-50mm zoom results in a much larger lens primarily to gather enough light to be at f/2.8. The return for this the ability to function in half the light the common 18mm-55mm f/4 kit lens can.
Description and Usage
In the box, the Pentax 16mm-50mm zoom is inside a protective nylon bag with a fold-down sealing top and a velcro enclosure. It is a large lens, with an 77mm lens thread, cap, and a light shield which gives it a definite trumpet bell flared shape moving away from the camera. As part of the Pentax Star series lenses, it has a prominent gold band near the front of the lens. The large glare shield reverses for storage and has a slide-out window in it which can be removed to let the photographer turn a circular polarizing filter with the glare shield in place.
The focus mechanism is electrically driven, so the lens does not operate off of the camera's mechanical drive motor. Instead, the lens has its own built-in motor, and only has options for autofocus on or off with a switch on the left side. The focus also has an unusually large focus ring for an autofocus camera lens which allows the photographer to turn the focus at any time. The ring has an internal clutch, so when it hits the end of focus travel, it doesn't jam, but rather smoothly slides over the mechanism without risking damage. The zoom ring is full sized and has satisfyingly deep ribs on the rubber exterior for a good grip.
In practice, the Pentax 16mm-50mm zoom is very easy to control, though it has some behaviors which require the zoom ring to resolve. In autofocus mode, the lens is unusually quiet. Unless there is complete silence in the room, it is nearly impossible to hear. Which means it can do a focus run, and if, like me, you have grown used to hearing the motor movement as an indication focus has happened, you can miss the response to pressing the shutter release entirely. This is good for not alerting the photograph subjects, but not so good for knowing when focus is complete.
In full automatic mode (the Green P on the dial), the camera will not use the full possible aperture range of the lens. Instead, it bottoms out at f/4. To get stops between f/4 and f/2.8 with automatic features, you have to put the camera in the adjustable program mode (White P on the dial), here you can use the thumb roller to command a lower f/number, and the camera will set the shutter speed and ISO value to match. While this is very enabling to get bright images, note the camera has considerably more difficulty with automatic focus in very low light.
One of the effects of going to a lower f/number is the depth of field gets shallower, so only a thin plane is in focus, and objects in front and behind it are blurred. This means you have to carefully look at the image in the viewfinder to determine if the camera has focused on what you want. It also means autofocus traps where the camera sees a situation it can't resolve are fairly common (the contrast is low). I have periodically found autofocus yields a slightly soft focus result as well. Because of this, the manual focusing ring is a necessary and extremely useful feature for this lens since it lets the photographer resolve focus in two modes:
(1) When the camera can't reach focus, the photographer can override (the lens lets you do this even with the lens in autofocus mode).
(2) The lens will quickly convert to manual focus with the switch on the ring.
Especially at f/numbers below f/4, this becomes the better way to make sure you have a fine focus on what you want. Make sure you are inspecting and enlarging photos on the screen periodically so if you are making a focusing error, you catch it before you lose a large set of photos. Embracing the manual focus is a necessary part of using the Pentax 16mm-50mm zoom.
The manual focus ring also has an important role to play for shooting movies, where the camera has a behavior where it sets up once for the start of the movie, then stops autofocus. The Pentax 16mm-50mm zoom can overcome this limitation if the photographer puts it in manual mode. This way, you can change zoom and adjust focus in real time. Personally, I have found this feature enabling, though it shows how a shot where you will need to adjust zoom needs to be thought out beforehand in order to come out smoothly.
Perhaps the only unresolved problem I have had with the Pentax 16mm-50mm zoom is it has been very difficult to get the lens cap to stay on. I installed a Hoya skylight UV filter, and afterwards had little thread for the edges of the lens cap to grab on to. As a result, the cap is often already off when the camera comes out of the bag. And in practice, I just put it in my pocket rather than risk losing it.
While the Pentax 16mm-50mm zoom is enabling for portrait and indoor shots with low light, its use should not be limited to those scenarios. It performs extremely well outdoors and in bright light. The conventional lens zoom range is as useful as it is for any other zoom in its range, but the additional control for focus makes effects possible which are otherwise difficult to achieve. For example, a flower can be focused with a depth of focus where it is the only resolved object in the photograph all the way to where the entire plant is resolved.
Its large light grasp also means when in bright scenes, it can support the fastest shutter speeds without using the noisier ISO settings from the sensor (3200 and 6400, for example). Because of this range of abilities, the Pentax 16mm-50mm zoom has become my most used lens, and is the one I leave attached to the camera.
The Pentax 16mm-50mm was clearly built knowing it would be used a lot. The construction is superb, the controls are easy to find, and its action is remarkably smooth. It is a tool for the photographer, and doesn't argue when you take the focus ring and move the focus where you want it to go. The quiet operation and light gathering ability mean it can get photos of small children without advertising a camera is in the area for them to look at and smile for a picture. While it is a somewhat expensive lens, the cost has been forgotten as it has become my first choice, with others only considered in special circumstances.