HD Pentax-D FA 150-450mm F4.5-5.6
The 150-450mm, like almost all recent Pentax lenses, takes advantage of DC (Direct Current) focusing. This Pentax-developed system is much more reliable than the older SDM, as well as near-silent and generally regarded as faster. In-lens focusing means that the lens won't autofocus with cameras older than the K10D (2006), which are quite dated by modem standards anyway. On the other hand, the newer AF systems are better adapted for contrast focusing using Live View.
AF button (center), preset button, limiter, AF switch
The AF is quiet, but not perfectly silent. In noisy environments there won't be any audible sounds, however when all is silent the amount of glass that must be moved means the lens does emit a subtle noise. It is many times quieter than even a small screw-drive lens, and no problem at all in use. The lens keeps the same external length whatever the focus distance: all focusing is internal.
The following video, taken with the lens on a K-3, illustrates the AF operation in relatively good light.
The 150-450mm is a mixed bag regarding autofocus. It is clearly intended to be used in good light : under the sun or next to a hockey rink for instance. In those circumstances, the AF is fast and decisive, getting to the desired point as quickly as anyone could wish. On the other hand, when light levels get low the AF slows down to a crawl. In these cases the AF moves very quickly near the desired point, but then spends a ludicrous amount of time performing micro-adjustments.
In all cases the accuracy of the autofocus is near-perfect. We have experienced no cases of missed focus except those attributed to user error.
There is no provision for older camera bodies (before the K10D), no screw-drive option like what is found on several DA* lenses. Only the internal DC focus motor is available.
The 150-450mm yields surprising results regarding AF with live view. In our experience, it is the first time we meet a lens which appears significantly faster with live view than with phase detection AF. It is clearly the case in low light, where live view AF is not slowed down by micro-adjustments. Because of this, it is much faster, as will be measured below. In good light the difference is less obvious but live view AF appears to keep its edge.
Again, there were no cases of missed focus when using live view.
Considering the likely use cases of such a lens, manual focus probably won't be as important as, say, on a landscape lens. It is still expected to perform well.
The focus ring, placed hear the camera body, is better than many film-era lenses. It has an extremely long throw, at 300°, and allows for precise fine-tuning of the focus point. This is desirable when shooting distant subjects. 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.
Like most lenses since the first DA product, the 150-450mm allows for anytime override of the autofocus system, a system called quick-shift. While on most lenses quick-shift is a simple operation performed by turning the focus ring, on the 150-450mm it involves more controls and options.
Just like on the D FA* 70-200mm, there are two options for quick-shift. In QFS/M (the "regular" quick-shift mode, a manual-focus priority mode), AF is disabled as soon as the focus is manually adjusted; autofocus will resume if the AF button or shutter is released and pressed again. MF is thus accessible at any time. QFS/A is an autofocus-priority mode, which allows the user to switch to manual-focusing operation only when autofocus operation is completed. This allows the user to make fine adjustments after the AF operation, but prevents unintentional disabling of the AF system. The latter, "A" mode is considered the default by Pentax, and is printed in green.
Preset Button and AF buttons
There are four focus buttons, placed at 90° all around the lens shaft. These buttons can be used to activate AF at any time, replacing the AF button on the camera body, when the "Preset" switch is set to "AF".
The user can record and recall a defined AF position. In order to program it, one must press and hold the Preset button for two seconds. The camera will beep to confirm the adjustment (except if set to silent). Afterwards, pressing any of the AF buttons with the switch set to "Preset mode" will return the lens to the pre-selected AF distance. The preset function can be a life-saver / time-saver during manual focus, as the focusing ring has a very long throw.
When the Preset switch is set to "AF Cancel", AF will be disabled on the camera when holding any of the AF buttons on the lens. AF will operate normally otherwise.
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.
Note that many comparable lenses only include the "full" and X to infinity" modes by default, the "2-6m" mode on this lens is a useful inclusion.
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.
We tested the AF speed with a Pentax K-1, using both live view (CDAF) and the standard PDAF (Phase Detection AF, through the viewfinder). The subject was a black cross on a white background, about 3 meters in front of the camera. We used the central focus point. Considering the long throw of the lens, we tested it by setting the focus both to infinity and to 2m before each test. Three measurements were averaged for each data point. Measurements were performed by recording the AF noise, at various levels of ambient light.
For reference, the Pentax K-1's autofocus sensor is rated for ambient light levels as low as -3 EV.
There appears to be to operation regimes for the 150-450mm. This is likely caused by its relatively slow maximum aperture. In good light AF is fast, below one second in most cases. When the light gets low, things change for the worst. Using live view yields excellent results in all cases, but using the viewfinder show abysmal speeds. This is caused by the micro-adjustments that we mentioned earlier. For instance, at around 2 EV the camera took approximately 6.5 seconds to lock focus, but a good 4 of those seconds were spent on tiny iterative steps towards the focus point.
For reference, a heavily overcast day typically has an EV value of approximately 12. Bright conditions will be even higher, and the lens will not struggle to lock focus in those cases.
Another interesting observation concerns the differences caused by the starting point. with a subject close to the camera, there was a visible gain when starting at 2m. This illustrates that proper use of the focus limiter will greatly improve AF speed in actual use.
We experienced no occurrences of hunting with the 150-450mm during our tests. This must be distinguished from the micro-adjustments, which did occur often in low light.
The 150-450mm clearly is a lens designed to be used in good light. When combined with the K-1, at least, it can slow down to a crawl when ambient levels drop below 4 or 5 EV. In bright conditions however, the speed is blazingly fast. Going from infinity to 3 meters takes less than a second at "only" 8 EV.
In all cases, live view CDAF performs better than the viewfinder's PDAF. It is suggested to use live view in dim light, and to properly adjust the limiter whenever possible.