Pentax K-5 Optics
Pentax K-5 Optics
The Pentax K-5 has an APS-C sized sensor which is smaller than 35mm film, so it's designed to work with DA lenses as well as full-frame lenses. DA lenses tend to be smaller and lighter than full-frame models with the same focal length and maximum aperture. The sub-frame sensor on the Pentax K-5 means that it has a smaller angle of view (by a factor of approximately 1/1.5x) than a full-frame camera with any given lens. While most properly called a "crop factor," the ~1.5x ratio is most commonly referred to as the "focal length multiplier," since that's how it works in practice: Any lens used on the Pentax K-5 will have the same field of view as one with a ~1.5x greater focal length will when attached to a 35mm camera. For example, a 100mm lens on the K-5 will show about the same field of view as a 150mm lens on a camera with a 35mm frame size.
The table below (courtesy of Pentax USA), details the K-5's lens compatibility with the various types of Pentax lenses.
The SAFOX IX+ AF module's optics have improved transparency, which the company says translates to improved performance in low light. The AF module's optics also have better controlled aberration, improving autofocus accuracy. Ambient temperature, says Pentax, has less of an impact on the SAFOX IX+ module. Sharp-eyed readers will note that the K-5's SAFOX module adds the "+" designation, which has only previously been featured in the 645D and K-7. This hints at one important feature that's included in the prosumer K-5, but not the mid-range K-r. Like the K-7 and 645D models before it, the Pentax K-5's AF system includes a secondary light color sensor dedicated to determining the light source type, which is then taken into account when determining focus, a capability the K-5's more affordable sibling lacks.
Perhaps most importantly, though, Pentax has added the ability to select the priority for single and continuous focus modes. For single AF, K-5 users can opt for either focus priority or shutter-release priority. Focus priority which mirrors the K-7's behavior in requiring an AF lock before the shutter can fire, while release priority starts exposure immediately that the shutter button is fully depressed, even if an AF lock hasn't been achieved. In continuous AF mode, the options are focus priority, or frame rate priority. The former requires an AF lock for each individual frame in the burst, while the latter replicates the K-7's behavior by emphasizing frame rate for each shot in the burst, even if this means a lock can't be achieved for individual frames in the burst.
When using autofocus, you can magnify the image 2x, 4x, or 6x by pressing the INFO button, and this defaults to being centered around the autofocus point. The four-way controller can be used to move the magnified area around the frame, and if set to Contrast Detection AF, the focus point moves so as to remain in the center of the view. The green button is used to immediately return the view to the center, and for Contrast Detection AF, likewise resets the AF point to the center of the frame. Unless already zoomed in to the maximum 6x, when using Contrast Detection or Face Detection AF, the view will smoothly zoom in to a 6x view centered on the point of focus during AF operation, and then return to a full image view shortly after an AF lock is achieved. This is great for static scenes, providing an intuitive view of where the point of focus lies, but unfortunately it can't be disabled, and applies even when you'd expect a moving subject -- such as when continuous autofocus is enabled -- making it quite tricky to keep moving subjects within the image frame during Contrast Detect AF operation.
When manual focus is used, an even higher degree of magnification is available, up to a maximum of 10x zoom, as an aid to focusing. This defaults to the center of the image frame, but can again be positioned anywhere within the image.
Although the AF assist lamp can be enabled or disabled through the Custom Function menu, enabling it doesn't guarantee that it will illuminate in all shooting conditions. Instead, it's left up to the camera to decide whether or not it needs help from the assist lamp to determine focus. In our experience, we found it only illuminated in relatively low light conditions.
The drawback to Pentax's Shake Reduction technology is that you can't see its effect as you look through the optical viewfinder, as you can with Canon and Nikon's lens-based stabilization systems. But thanks to the Pentax K5's Live View mode, you can indeed see the effect on the LCD, and SR seems to be pretty solid and effective. In exchange for this tradeoff, however, you get a couple of features not available on any other system. The Pentax K-5's Shake Reduction system can be used to automatically correct for tilted horizons within a range of up to +/- 2.0 degrees, regardless of whether you're shooting through the optical viewfinder or in live view mode. When in the latter mode, it can also be used to fine-tune image framing by manually adjusting the sensor position and rotation.
While dust removal systems can increase the interval between manual cleanings, it bears noting that no such system system can completely eliminate the need to occasionally clean the sensor manually. Copper Hill Images is an advertiser of ours, but we'd recommend their wet/dry cleaning system even if they weren't (it's what we use in our own lab): See the Copper Hill website for details.
Pentax K-5 Optical Test Results
The Pentax K-5 is sold body-only, or bundled with the DA 18-55mm f/3.5-5.6 AL WR kit lens. Below are the results of our optical tests on the Pentax K-5 with the optional kit lens. The studio test shots on other pages of this review (apart from the flash range shots) use a very sharp, reference prime lens (Sigma 70mm f/2.8 EX DG Macro).
Acceptable performance with the Pentax 18-55mm WR kit lens at wide-angle, but poor performance at full telephoto.
|18mm @ f/8||55mm @ f/8|
As mentioned previously, the Pentax K-5 is usually sold body-only, or bundled with the Pentax 18-55mm f/3.5-5.6 AL WR kit lens. WR stands for "Weather Resistant", which is also true of the K-5 body. The Pentax 18-55mm WR lens offers a typical optical zoom range of ~3.1x for an inexpensive kit lens. Sharpness is fairly good across the frame at full wide-angle (stopped down to f/8), with low levels of coma distortion, but some strong chromatic aberration is visible in the corners. Results at full telephoto are quite soft particularly in the top-right, but with much lower levels of chromatic aberration. (Note: The non-WR "AL II" version of this lens performed quite well with the K-5, so we requested a second sample of WR lens for testing, thinking our first was one was defective. Unfortunately, the second copy was no better.)
Average sized macro area with the 18-55mm WR lens, with soft detail. Flash has trouble up close.
|Macro with 18-55mm kit lens
55mm @ f/8
|Macro with Flash
55mm @ f/8
Like zoom range, macro performance will depend entirely on the lens being used. With the Pentax 18-55mm WR lens, the K-5 captured an average sized macro area (for an SLR kit lens), measuring 2.58 x 1.71 inches (66 x 43 millimeters). Detail was a little soft in the center of the frame, and even softer in the corners. (Most lenses have some softening in the corners at macro distances.) The Pentax K-5's built-in flash didn't throttle down well for our standard macro shot in auto mode, overexposing it by quite a bit, though some negative flash exposure compensation may have helped here.
Slightly above average distortion at wide-angle, about average at telephoto with the 18-55mm WR lens.
|Barrel distortion at 18mm is 0.9 percent|
|Pincushion distortion at 55mm is 0.2 percent|
The Pentax 18-55mm WR lens produced about 0.9 percent barrel distortion at wide-angle, which is slightly above average and noticeable in some of its images. At the telephoto end, there was about 0.2% pincushion distortion which about average, but still noticeable in some shots. Geometric Distortion is the tendency for the lens to bend straight lines outward (like a barrel -- usually at wide-angle) or inward (like a pincushion -- usually at telephoto).
The Pentax K-5 offers optional Distortion Correction when using DA, DA L, and DFA lenses, as well as several of the company's FA Limited lenses.
|Distortion Correction Off: Barrel distortion at 18mm is 0.9 percent|
|Distortion Correction On: Pincushion distortion at 18mm is ~0.1 percent|
The Pentax K-5 does not apply any geometric distortion correction to JPEGs by default. There is however a menu option to turn Distortion Correction on. RAW files are not corrected, but are tagged to have the same correction applied when using a RAW converter that supports the embedded parameters. The crops above show JPEGs taken with the kit lens at wide-angle with Distortion Correction disabled and enabled. As you can see, the 0.9% barrel distortion in the uncorrected file has been eliminated, with a slight over-correction to about 0.1% complex (slightly moustache-shaped) pincushion distortion.
Chromatic Aberration and Corner Sharpness
Moderately high levels of chromatic aberration from the 18-55mm WR lens at wide-angle, though much lower at full telephoto. Soft corners at wide-angle and somewhat soft through-out the frame at telephoto.
|[email protected]/3.5: Lower right
C.A.: Moderately High
Softness: Very soft
|[email protected]/3.5: Center
C.A.: Very low
Softness: Reasonably sharp
|[email protected]/5.6: Lower right
|[email protected]/5.6: Center
C.A.: Very low
Chromatic Aberration. Chromatic aberration is moderately high and quite bright at the full wide-angle setting of the Pentax 18-55mm WR lens. At full telephoto, chromatic aberration is much lower and hardly noticeable. (This distortion is visible as a slight colored fringe around the objects at the edges of the field of view on the resolution target.) The Pentax K-5 does not apply any chromatic aberration correction to JPEGs by default, as uncorrected RAW files have similar amounts. There is however a menu option to turn Lateral Chromatic Aberration Adjustment on (see below).
Corner Sharpness. The Pentax 18-55mm WR lens produced soft corners at full wide-angle. The bottom corners were very soft and the blurring extended far into the frame. The top corners were only slightly soft, while the center was reasonably sharp. At telephoto, the entire frame was somewhat soft, with the corners slightly softer. As mentioned above, we requested a second 18-55mm WR lens for testing, thinking our first was a bad sample, but the second lens was no better.
Chromatic Aberration Reduction
The Pentax K-5 offers optional lateral chromatic aberration reduction for the same lenses that are supported for distortion correction.
|Camera JPEGs, Lateral Chromatic Aberration Reduction|
|[email protected]/3.5: Disabled||[email protected]/3.5: Enabled|
As illustrated in the crops above, the Pentax K-5 was removed much of color fringing from the 18-55mm WR kit lens with Lateral Chromatic Aberration Adjustment enabled, though it still leaves some visible chromatic aberration behind, especially along diagonal edges. It also seems to reduce resolution slightly, but that could have been caused by a minor difference in focus.
Overall, this is a poor performance even for an inexpensive kit lens. If you're interested in purchasing the Pentax K-5 with the kit lens, make sure to buy from a retailer that allows hassle-free returns and test the lens thoroughly within the exchange period.
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Pentax K-5 Photo Gallery .
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