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Field Tests

Like other DA Limited lenses I tested recently, the smc DA 70mm f/2.4 has a mechanically coupled AF, meaning that there's really no AF motor in the lens itself and all focusing isi done by the camera, using a mechanical rotation of the screw that locks into a slot at the base of the mount of the lens. Despite this rather outdated focusing mechanism, the focusing itself is pretty fast, albeit quite noisy. The focusing ring travels for about 90 degrees from the infinity to the closeup, so obviously this helps improve the focusing speed to some extent. The focusing itself though is a hit and miss, which I think is more of a problem of the camera itself, although the mechanical lens element movements have a precision limitation as well. As with other pencake Limited lenses, the focusing ring is quite narrow and is located in the front of the lens, so be careful not to end up with your fingers in the picture frame when manual focusing the lens. The lens also offers a DOF scale, which can be used to preset the lens when using it in a fully manual focusing mode.

Image quality wise, Pentax smc DA 70mm f/2.4 Limited delivered very good and consistent results. The lens showed very good performance with vert even across the frame results - literally, there were no significant loss in visual detail whether you shoot at f/2.4 or f/8! Not bad for a pancake and the lens certainly exceeded my personal expectations here (Why you might ask? Because any pancake lens design tradeoffs something for that very compact design. In many cases that something is resolution.) Bottom line is that as long as you manage to accurately focus the lens, the results should not disappoint you. There is no field curvature effect and there is no degradation except for the diffraction which starts to kick in at f/16 and lower apertures.

Considering that the lens will be used on APS-C cameras (at least until Pentax comes up with a full frame digital body), the field of view of the lens would turn from 70mm into 105mm - still a medium telephoto, and quite suitable for portrature type work (among other things obviously). And since the lens offers a moderately fast aperture, let's take a look at what it can deliver up close and personal, that is what the DOF will look like at close to minimum focusing distances.

 

ISO 200, 1/90, f/2.4, 70mm (Pentax K2000)

 

The image above, which was taken at ~45cm with the camera pointed at ~45 degrees to the surface, simulates what you can expect from the DOF at this distance. While not razor thin  like for example with the smc DA* 55mm f/1.4, it is thin enough to give you enough separation between the foreground and background. This can be observed in the two shots below (both shots were also taken at ~45cm focusing distance). As can be seen from the shots, at f/2.4, the lens blurs out of focus areas quite nicely, creating pretty smooth transitions between high-contrast areas such as bright red background flowers and greyish building and green grass. There is very little definition to any background object and I woudn't even be able to distinguish that these are actually red flowers in the background, if I did not observe that while taking the picture. Obviously your DOF 'mileage' will vary, depending on many things, but most notably the focusing distance to the subject. Move back by a few feet and you'd actually see a noticeable difference in the amount of definition you'd get in the background. But at the minimum focusing distances, even small apertures give you a very decent blurring of OOF areas, as can be seen from the f/8 shot below. Yes, you can see more detail around the objects that are closer to the point of focus, but even then it is still more of an unidentifyiable blurb of color then anything else.

 

ISO 100, 1/1500, f/2.4, 70mm (Pentax K2000)
ISO 100, 1/125, f/8, 70mm (Pentax K2000)

 

So far, so good. Let's just wrap up with the DOF review by taking a look at the final example of how the lens behaves at close to minimum focusing distances. If it was not obvious from the shots above, the image below, also taken at f/2.4, actually shows that the lens delivers somewhat lower levels of contrast across the frame, not just in the OOF areas. This is rather disappointing, since the image looks as if it was drained of 'life' and you would need to add a little bit of contrast and saturation to correct that. This seems to be persistent only at closeup distances though, and at long/infinity distances images looked just fine. On the brighter side, the OOF highlights were round and uniformly lit, with mostly neutral looking edges and there was no sign of double-edging around the OOF objects.

 

ISO 200, 1/180, f/2.4, 70mm (Pentax K2000)

 

As one would expect from a moderately fast medium telephoto, the smc DA 70mm f/2.4 Limited showed very minimal amount of color falloff. The lens obviously benefits from the smaller, APS-C sized sensor, which does not cover the entire imaging circle of the lens - at f/2.4 the lens shows traces of vignetting in extreme corners, but stop is down to f/4 and there's basically no sign of color falloff at all.

 

Vignetting - APS-C

 

And while vignetting control was quite good, the lens did not fare as well in handling flare. The two shots below demonstrate this (as always, keep in mind that we're trying to stress the lens and find the points where it breaks, ughhmm, I mean fails to behave correctly - this does not mean that you will be able to consistently see this type of behavior under normal shooting conditions). The sun in both shots was positioned right above the picture frame, in the right hand corner and the sun rays were hitting the lens at about 70 degrees. You can see a very pronounced purple/magenta color shift, which exists throughout the aperture range. There's a minor sign of ghosting at smaller apertures, plus you get the usual lower contrast and obviously reduced resolution, which are quite typical in most lenses. The shallow lens hood seemed to help a little bit, but you'd probably be better off just re-composing your images  so that the sun is not near the picture frame.

 

ISO 100, 1/1500, f/2.4, 70mm (Pentax K2000)
ISO 100, 1/125, f/8, 70mm (Pentax K2000)

 

The lens showed very decent color reproduction capabilities, although as can be seen from the image below, chromatic aberration would show up here and there, particularly in high contrast areas. Signs of both lateral as well as axial aberration can also be observed in the sample images below. Otherwise, color palette remained mostly neatral, with good amount of saturation. Images carried sufficient amount of contrast throughout the aperture range (except at the closeup distances, as mentioned above), and as long as you shoot under regular lightning conditions, your images should have a well distributed tonal histogram.

 

ISO 100, 1/4000, f/2.4, 70mm (Pentax K2000)