The Pentax 645Z review, part III: SDM lenses and long exposures

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Having a bit more time with the relatively new Pentax 645Z under my belt has given me the opportunity to try a couple of things I’ve been curious about: long exposures, and a more thorough evaluation of the three SDM lenses currently available for the 645 system. The former is probably only of interest to landscapists, architectural photographers and people who have severe allergies to controlled lighting, but I feel the latter is probably a critically important topic in itself. Let’s start there.

Compared to other medium format system contenders – really only Hasselblad, Phase One and Leica – Pentax has about 13 lenses currently available; unfortunately this is somewhat misleading as most of them appear to be much older designs that date from the heyday of the film system in the late 80s or early 90s. For the most part, they are body-driven, plastic-barrelled (not the nice kind of crinkle finish plastic that Nikon uses, for instance, but the smoother, slightly cheaper/ more brittle feeling kind of older lenses) and have manual focus on sliding clutches rather than permanent override or switches. Build quality leaves something to be desired, and they were designed for the resolution of film – not the 645Z. Of the four contenders, I’d say only the Leica S glass gives me the confidence to use it at any aperture and any subject distance and be assured of consistently excellent results – the rest all have stars and dogs, caveats and cautions, even for the best of the lenses.

Pentax 645 lenses come in three flavors: A, which are manual focus; FA, which are autofocus, and D-FA, which are accompanied by built in SDM motors, weather sealing and full time manual focus overrides. SMC is super multi coating, something analogous to the various other branded coatings on other lenses; Aero Bright is their new equivalent of Nikon’s Nano Coating – as far as I can tell. Again, I believe this is only present on the D-FA lenses. There are a plethora of A (discontinued) and FA (still available new, and recently in the USA too) lenses; from a 35mm wide (28mm-e on the digital bodies) to a 400mm telephoto. D-FA lenses are only three: the 25/4 (19mm-e), 55/2.8 (43mm-e) and 90/2.8 SR Macro 1:2 (70mm-e), which is the only medium format lens with optical image stabilization. Confusingly, there’s also a DA version of the 25mm, which I believe is the same except for a slightly longer hood. Pentax’s own roadmap shows a couple of zoom updates due later this year or early next, which will presumably be D-FA, have SDM focusing, Aero Bright coatings and hopefully also optical stabilization. Leaf shutters would be nice too, but at this point I feel I might be pushing it.

My experience with most of these lenses suggests a few things: firstly, if you have patience and don’t mind possibly having to try a few samples, don’t buy them new. There are an abundance of them available second hand online and from various dealers as holdovers from when people quit the film days. There are bargains to be had – though probably not after this article is published – up to 60-70% cheaper than new glass, for excellent plus/ mint-minus grading. Second hand FA lenses start from $300 or thereabouts; on the other end of the scale, a new D-FA 25/4 or D-FA 90/2.8 SR will run you close to $5,000. Secondly, there definitely is sample variation: AF tolerances in the past might have been responsible for a number of these lenses acquiring undeserved reputations for being soft or inconsistent; the truth is helicoids can not only drift out of alignment, but body driven AF (or any non-lens-based coreless direct driven DC motor system) is going to have some backlash and free play that will result in zero movement for small changes in subject distance even if the AF system and motor in the body commands it. It would be impossible for the gear train to move without binding otherwise.

Since film emulsions have some thickness, the focal plane has some tolerance of movement before the image appears to be decidedly out of focus. This is not the case for digital, and small changes can make the difference between pixel-crisp and ‘might as well use a FF DSLR – or less’. This difference in tolerances, AF backlash etc. basically means that a lens which will not focus properly on one body even with maximum AF fine tune may well do so on another body with none whatsoever. (My partner and I actually pooled FA lenses to mix and match which would work best on which bodies – the 75/2.8 that would still backfocus on my body at maximum -10 fine tune adjustment focused just fine on his with zero fine tune. And all of our SDM lenses require +/-2 or less adjustment – go figure. I imagine it’s a lens thing, not a body thing.) Finally, not all of these lenses have weathered the ages well – there’s no easy way to tell the age of a lens without tracing the serial number as there don’t appear to have been many cosmetic changes throughout the lifetime of most of the optics, so guessing age this way can be tricky. Sticky or stiff actions are probably not a good sign, to say the least. And be weary of fungus and heavy internal dust. Remember that A and FA lenses are not weather sealed if you intend to use the camera in harsh environments.

Performance-wise, I have not yet encountered any FA lenses that are outstanding performers wide open on the 645Z other than the FA 120/4 Macro 1:1; most are very good one stop down and excellent by two stops, but by that point you’re at f5.6 or f8 and very much into tripod territory or elevated ISOs. Some lenses like the FA 200/4 are frustrating: it’s possible to get surprising results even at f4, but AF backlash means that most of the time you’re at f8 on a tripod for insurance. I find there’s a slight difference in color transmission between the D-FA and FA lenses; the D-FAs are neutral and moderately saturated; the FAs skew slightly warm and have both lower contrast and a slightly odd-feeling spectral transmission. I put it down to the difference in coating technology.

D-FA 25/4 (and presumably DA also)
Beware sample variation on this one. If you get a good sample, it can be quite spectacular, even into the corners. I’ve handled four lenses: the first wouldn’t focus to infinity with or without the drop in filter (new lenses include special thin CPOL and skylight filters) in place and was very astigmatic; the second demo unit was slightly astigmatic, and the third and fourth units were fine. I suspect alignment of elements in this lens are very sensitive to impacts and the first two may well have been dropped or bumped; #3 and 4 were new lenses. Wide open, center resolution is excellent; you get a bump in microcontrast and edge performance one stop down, and the corners are not far off the center by f8, but there is some lateral CA. There’s surprisingly little distortion for a lens this wide. Enabling the default ACR correction profile cleans this up nicely and restores corner performance. What I find most impressive about this lens is the way it renders: it simply doesn’t feel anywhere near as wide as it actually is; the Zeiss 2.8/21 Distagon on my D800E has more geometric distortion/ stretching towards the edges as a wide ‘signature’ than 25/4, even though the 25/4 is a 19mm equivalent. Advantages of medium format, I suppose.

D-FA 55/2.8
The standard kit lens for both 645D and Z; it’s easy to see why. 55mm is a very natural feeling focal length for the 33x44mm format; much in the same way 80mm feels and renders on 6×6. I suspect it’s got something to do with this ‘natural’ diagonal of 43mm-e; Pentax has made a number of ‘limited’ lenses for their 35mm cameras around this range. Central sharpness and resolution is excellent at f2.8; there’s definitely some field curvature on this lens as focus and recompose too far off axis requires some compensation. The edges and corners require f5.6 to come up to central standards. Bokeh is smooth and it doesn’t quite have the microcontrast bite of the 90, but it’s not far off. All in all, much to like here: light, small, reasonably affordable – and probably the one must-have, in my book.

D-FA 90/2.8 SR Macro
In my opinion, only the D-FA lenses perform at maximum aperture on the 645Z; the 90/2.8 SR Macro is probably the best of the three, with an unqualified shooting envelope – any aperture, any distance, subject anywhere in the frame. There is some slight field curvature as there’s a clear difference in sharpness wide open between focusing with an edge AF point and recomposing slightly, or using live view (even with AF fine tune perfect). This goes away by f5.6. Resolution and microcontrast are excellent at every aperture, with almost no noticeable lateral CA and only trace amounts of longitudinal CA. Both are easily cleaned up by the lens’ default ACR profile. Bokeh is smooth and vignetting minimal. This is perhaps the best of all of the lenses available for the 645 system, and an excellent optic by any standard. On a relative basis, I’d put it somewhere between the Zeiss 2/100 Makro-Planar and 2/135 APO-Sonnar. A note on the optical stabilization technology: it seems to be not quite as effective as Nikon or Canon’s latest and is slightly noisy in operation, but it definitely welcome. It clearly reduces the ‘danger zone’ speeds by about 1.5 stops (as compared to about 2-2.5 on Nikon VRII, regardless of what the manufacturers claim). We can only hope this is incorporated into future lenses.

Long exposure performance
I have to admit, I was quite blown away by the 645Z’s long exposure behaviour; with dark frame subtraction (‘long exposure noise reduction’) set to auto, it almost never uses a second frame below 4-5 minutes. 2-3 minutes at base ISO appears to have no penalty to the noise floor or dynamic range; there’s no amp noise at the edges, either. You start to see a bit of luminance speckling in the lowest couple of stops of dynamic range up to the 5 minute range, and frankly, I haven’t found anything dark enough to require much more than that.

This image is remarkable for the amount of dynamic range it retained at two minutes, and above base ISO – nothing is clipped at either end in the original, from the deepest shadows in the upper balcony, to the brightest highlights in the lower one.

Cranking the ISO a couple of stops on longer exposures appears to have no more penalty to noise and dynamic range than increasing for a shot of more normal duration. This is one of the most impressive performances I’ve seen; even my D800E requires some cleanup above 30 seconds. Coupled with the wireless triggering app and automatic mirror lockup with self timer, it makes for very fluid and easy long exposure work. Battery life is also equally impressive – after 50 test exposures of several minutes, using live view – as a sort of torture test for both battery life and thermal-induced noise – performance at the start and end of the test was the same, and the battery indicator hadn’t budged. Using a rough approximation by recharging time, I’d say I’d used somewhere between a quarter and a third of its capacity. Again: impressive.

My next installment in this series will come after a few months, trips and assignments and using the 645Z as a documentary/ available light camera; this will be the mid term report. MT

The Pentax 645Z is available here from B&H, and the respective lenses are here: 25/4, 55/2.8, 90/2.8 SR, older legacy lenses.

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