HD Pentax-D FA 150-450mm F4.5-5.6 (APS-C)

Focusing

Pentax engineers have gone to great lengths to implement cutting-edge focusing features in the 150-450mm, which leads to an interesting discussion.  You may wish to read the lens manual for additional background information on some of its features.

AF Buttons

Four dedicated autofocus buttons have been placed around the middle of the lens barrel at 90-degree intervals.  These will activate the camera's AF when used with a compatible body (i.e. the Pentax K-3 and newer). The buttons are a first for Pentax and are certainly a rarity industry-wide.  We did not make much use of them during our shoots.

Note that the K-S1, K-S2, and K-3 all require a firmware update to activate these buttons.

AF button (center), preset button, limiter, AF switch

The other features pictured above are discussed in the following sections.

Preset Button

The preset button can be used to quickly snap the focus to a user-defined point.  It can also be used to cancel autofocus operation.  The preset function can be a life-saver / time-saver during manual focus, as the focusing ring has a very long throw.

Limiter

The 150-450mm features a focus limiter with three different settings: full, 2-6m, and 6m-infinity.  The latter two settings will restrict the autofocus operation to the given range.  The restriction is purely electronic, so the limiter does not introduce any mechanical points of failure.  As a result of this design, manual focusing beyond the limiter's range is of course possible, which is a good thing.

Using the limiter can increase the focusing speed by nearly a second when hunting is involved, i.e. when tracking a moving object.  We strongly recommend keeping it set to the 6m-infinity range by default.

Manual Focusing and AF Modes

The 150-450mm's focusing ring is better suited for manual focusing than even film-era lenses!  It has a 300-degree throw and its gearing allows for precise fine-tuning, which is extremely desirable when shooting distant subjects.  The lens focuses internally, of course.  The ring itself is well-dampened and has soft stops at either end of the distance scale, but otherwise turns indefinitely.  The distance scale on the back of the lens is large and easy to read.

A downside of such a long throw is that more dramatic changes in focus require extra time. This is alleviated in part by the preset button.

Full-time manual focus overriding is possible thanks to the Pentax Quick Shift function.  When set to QFS/A, the camera will continue focusing shortly after one stops making manual adjustments as long as the AF button is held down.  In QFS/M, AF is disabled as soon as the focus is manually adjusted; autofocus will continue after the AF button is released and pressed again.  The M (manual focus) mode is thus rendered redundant except for users who have focusing enabled upon a half-press of the shutter button.  These three modes are fairly standard within the industry for professional telephoto lenses.

AF Adjustments

Pentax cameras allow for AF fine-tuning via the custom function menu.  It is not uncommon for fine adjustments to be necessary due to manufacturing tolerances.

Since telephoto lenses have a limited depth of field, autofocus precision is of utmost importance and we recommend all owners to perform calibrations using a quality alignment tool such as the LensAlign.  Our test copy required a +5 adjustment (again, not considered a defect).

AF Speed

Overall, the Pentax 150-450mm is a stellar performer when it comes to the speed of its DC autofocus motor.  While the DC motor is not necessarily as swift as ring motors in competing systems, the difference has little practical significance. The issues we experienced while focusing with the lens can almost entirely be attributed to the camera body, i.e. the Pentax AF algorithms in general.

From a reliability point of view, the DC motor has had an exceptional track record (unlike SDM).  Failures of other Pentax DC lenses are few and far between; this gives us reasonable peace of mind with respect to the 150-450mm, but due a relatively short testing period, we cannot draw any definitive conclusions of course.

When pairing the 150-450mm with the Pentax K-3, we found the overall autofocus performance to be good but not exceptional.  We mainly had trouble in AF-C (continuous) mode while shooting birds and animals, as the camera did not trigger fine-adjustments frequently enough for moving subjects.  This was an issue since small changes in focus can have a significant impact when working with a shallow depth of field (i.e. at long focal lengths and/or wide apertures).  The critical nature of the focus setting, especially when shooting at F5.6, calls for improvements in the way the camera focuses.  We had no problems with AF accuracy in AF-S mode, though in this mode, the camera liked to fine-tune the AF after a preliminary focus lock.

Although on paper, the Pentax K-3 II promises to bring undisclosed improvements to AF-C performance, we did not observe any evidence of faster focusing, lower latency, or quicker target re-acquisition with the K-3 II.

In live view, the 150-450mm fares surprisingly well for a telephoto lens.  It focuses especially quickly in daylight in multi-point mode, though this might not always deliver the precision of spot or select mode.  The latter two modes are considerably slower in low light or low contrast scenarios, but their performance is still commendable.

When it came to AF accuracy and consistency, both live view CDAF and viewfinder PDAF (when properly calibrated) delivered good results.  However, nothing beats using live view with manual focus, image magnification, and focus peaking for precisely focusing on stationary subjects.  As we stated earlier, this lens is a joy for manual focusing due to the long focus throw, which makes it unusually easy to notice when the focus peaking outlines peak.

AF Demonstration

The video below demonstrates the speed and sound of the DC motor.

AF Speed Tests

We compared the AF speed of the Pentax 150-450mm on a K-3 to that of the Nikon 80-400mm VR II on a D810.  Times were measured using frame-by-frame video analysis.  The results are tabulated below; tests 2 through 6 only measured the speed of the AF motors, whereas tests 1 and 7 measured practical performance.

Pentax 150-450
w/ K-3
Nikon 80-400
w/ D810
1 AF start latency 0.3s <0.1s
2 Infinity-min 1.0s 0.4s
3 Min-infinity 1.0s 1.0s
4 Infinity-min-infinity 2.0s 1.5s
5 Infinity-limit-infinity 1.3s 0.9s
6 Infinity-limit 0.4s 0.3s
7 Infinity-target (9m) 0.4s / 1.2s* 0.3s

AF speed test results

An interesting finding, as evidenced above, is that the Nikon lens moved considerably faster in one direction than when backtracking. This implementation has the potential to increase AF speed without compromising accuracy in critical situations.  Overall, the Pentax motor runs at a slower maximum speed but in typical scenarios, this will only slow real-world performance by around 0.1s— a negligible amount.  To put things in perspective, we measured a 0.3s latency between the pressing of the AF button and activation of the 150-450mm's motor.  We suspect that this latency can be attributed to the camera as a similar delay occurs when the lens reaches one end of the focusing range, prior to backtracking.

*We performed one other test: having the lens focus on a target approximately 9m away in daylight.  The Pentax lens took 1.2s total to confirm focus, though 0.8s of this time was spent solely on micro-adjustments.  The Nikon lens focused in 0.3s total.

In a nutshell, the Pentax setup gets the job done relatively well and the lens focuses quickly, but it requires the photographer to understand the nature and quirks of the system and intervene as needed.  There's certainly a learning curve involved. We look forward to higher-performance Pentax AF implementations in future cameras, as the 150-450mm lens itself has everything needed for cutting-edge focusing capabilities. 


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