Pentax 645 smc A 75mm f/2.8

 

Introduction

Pentax 645 smc A 75mm f/2.8 is a classical medium format manual lens for the now defunct Pentax 645 system. The lens was often included in the Pentax 645 starter kits as a default prime, but was eventually discontinued and replaced by a smc FA f/2.8 variant with auto-focusing capabilities. The lens is commonly available on used markets with good copies selling for ~US$150 (as of March 2008).

The optical construction of the lens consists of 6 elements in 5 groups. The build quality is pretty good - all metal barrel, focus and aperture rings. The focus ring is pretty smooth and the aperture ring is snappy. Speaking of the aperture ring. The lens supports automatic diaphragm operation, which can be activated by rotating the aperture ring to the A position (all the way past f/22). The ring locks in this position, so if you want to switch the lens back into manual mode, you need to press the push-in knob on the side of the aperture ring and at the same time rotate the ring into one of the f-positions.

The lens is pretty compact and light (although it does look wider then your typical medium telephoto lens), measuring 38 x 65mm (1.49 x 2.55in) and weighing 240g (8.46oz). The minimum focusing distance  is 60cm (1.96ft) and the minimum supported aperture is f/22 (aperture moves in half f-stop increments in the f/4-f/16 range and one full f-stop increments through the rest of the aperture range). The lens accepts 58mm screw-in type filters.

Image

Pentax 645 lenses can be easily adopted to a variety of 35mm film and digital cameras, both APS-C as well as FF. However, all medium format lenses have imaging circles that are larger then those of conventional lenses designed for 35mm camera systems. As such, a 35mm camera sensor is not going to completely cover the imaging circle of a medium format lens (this of this as if using a FF lens on an APS-C body). This might actually be considered beneficial by some, since the camera sensor will be utilizing only the central part of the lens (the benefit here rests on the assumption that most lenses produce better resolution in and around the center then around extreme borders). Another side benefit of the larger imaging circle is the ability to combine a medium format lens with a tilt/shift adapter to get a potentially inexpensive tilt/shift lens. Within the scope of this (and other medium format lenses in general) lens review, I do not have any intention of testing tilt/shift capabilities and will rely on generic adapter.

When used on a native 645 medium format camera, the lens has field of view of 50mm, which turns out to be an equivalent of 50mm field of view on a traditional 35mm camera (including FF digital cameras). On an APS-C body the field of view resembles that of a 80mm lens on full frame body.

 

Summary
Lens Composition          
6 elements in 5 groups
Angular Field ~50 degrees (35mm EFL: 50mm)
Minimum Focus 60cm/1.96ft
Focusing Action MF
f-stop Scale f/2.8-f/22, manual
Filter Size 58mm
Lens Hood N/A
Weight 240g/8.46oz
Dimensions 38x65mm/1.49x2.55"
Lens Case N/A

 

Field Testing

Pentax 645 smc A 75mm f/2.8 showed pretty good overall performance in the field. Images remained sharp on both APS-C as well as FF cameras with no major visible degradation in quality across the aperture range.

 

Vignetting @ f/2.8 - full frame vs 1.6x crop (75mm)
Vignetting @ f/2.8 - full frame vs 1.6x crop (75mm)

The lens showed negligible amount of vignetting on a full frame body at f/2.8 and practically no vignetting once stopped down to f/4 or so. Vignetting on an APS-C body was basically non-existent across the testes aperture range. All this is not very surprising, considering the fact that sensors of traditional 35mm digital SLRs do not fully cover imaging circles of medium format lenses. The lens did not exhibit major color fringing on either APS-C or FF body, although it fell prone to very mild axial CA at f/2.8 (notice haloes around flowers in a tree shot below). Color reproduction was pretty decent, with images carrying enough contrast across the aperture range. Flare was mostly under control, as was barrel distortion.

 

ISO 100, 1/4000, f/2.8, 75mm (100% crop)
ISO 100, 1/4000, f/2.8, 75mm (100% crop)

 

Lab Testing

Please note that MTF50 results for APS-C and Full-Frame cameras are not cross-comparable despite the same normalized [0:1] range used to report results for both types of cameras.

 

Pentax 645: At this time I do not have plans to start testing any medium format system.

 

Canon APS-C: Pentax 645 smc A 75mm f/2.8 produced pretty solid results on an APS-C body. Center image quality is already pretty decent at f/2.8 and only improves with stopped down aperture. In the f/4-f/11 range, center resolution is certainly as good as it gets. Border performance is a little bit lagging at f/2.8 (an emphasis here is on a little bit) and by f/4 border image resolution pretty much catches up with the center. Overall, the lens shows pretty balanced results across the aperture range, but the peak is reached in the f/4-f/8 range. Here the lens is capable of delivering excellent 19in and decent 24in prints. Conclusion? Overall results can be considered pretty good among traditional telephoto primes designed for 35mm systems, with Pentax smc A 75mm f/2.8 easily landing a position in the top quartile.

 

MTF50 (Line Width/Inch on the Print) @ 75mm
MTF50 (Line Width/Inch on the Print) @ 75mm

 

Normalized raw MTF50 @ 75mm
Normalized raw MTF50 @ 75mm

Chromatic aberration on an APS-C body did not pose major problems, with CA in the center averaging ~0.5px at f/2.8 and then dropping to ~0.2px by f/11, while CA around borders averaged ~1px at f/2.8 and gradually dropping to ~0.7px by f/11.

 

Chromatic Aberration (APS-C) @ 75mm
Chromatic Aberration (APS-C) @ 75mm

Here are 100% crops taken with an APS-C type Canon Digital Rebel, comparing images at f/2.8 and f/8.

 

Image borders @ 75mm (100% crop): f/2.8 vs f/8
Image borders @ 75mm (100% crop): f/2.8 vs f/8

 

Canon FF: The lens continued to show very respectable results even on a FF body. Performance is very well balanced across the aperture range, with practically no drop-offs in quality. Both border as well as center image quality is already pretty good at f/2.8 and only gets better with stopped down aperture. Overall performance peaks in the f/5.6-f/8 range, however 'peaks' is a relative term here since quality remains on a consistently high level in the f/4-f/11 range. Conclusion? The results are simply outstanding - no ifs and buts. Resolution-wise, the lens can rival some of the best medium telephotos available on the market!

 

Normalized raw MTF50 @ 75mm
Normalized raw MTF50 @ 75mm

The lens showed negligible amount of barrel distortion and at 0.133% it is generally not going to be noticeable in most shots.

 

Distortion (FF) @ 75mm
Distortion (FF) @ 75mm

Chromatic aberration was not much of a problem for the lens on a full frame body. Center CA is pretty low throughout the aperture range, never actually exceeding ~0.3px. Border CA is slightly higher - ~0.7px at f/2.8 and ~0.6px throughout the rest of the aperture, but still under control.

 

Chromatic Aberration (FF) @ 75mm
Chromatic Aberration (FF) @ 75mm

Here are 100% crops taken with a FF Canon 5D, comparing images at f/2.8 and f/8.

 

Image borders @ 75mm (100% crop): f/2.8 vs f/8
Image borders @ 75mm (100% crop): f/2.8 vs f/8

 

Alternatives

Assuming you are looking for an alternative for your medium format Pentax 645 system, then you might want to take a look at the newer, auto-focus FA version of 75mm f/2.8 lens. For a slightly longer reach, take a look at smc FA 150mm f/2.8. Unfortunately, there are not that many standard lenses available in Pentax 645 mount. You can try expanding your search to include macro lenses such as smc FA 120mm f/4. On the other hand, if you're looking for a medium telephoto lens for your 35mm camera system, then your choice is much wider and should naturally include not only other medium format lenses from Mamiya, Hasselblad, but also regular lenses from the usual suspects such as Carl Zeiss, Leica and many other.

 

Recommendation

Falling into the 'look ma what I can attach to my Canon' category, Pentax 645 smc A 75mm f/2.8 is a very interesting lens to consider, assuming you're adventurous enough to try a medium format lens on your beloved dSLR. The lens shows very solid performance, generally good handling of artifacts, combined with a solid build quality. All that for a measly ~US$150 or so dollars. Many of the much more expensive native lenses don't even come close (performance-wise) to this pretty old, no frills lens. One thing to keep in mind though: this review covers only APS-C and FF cameras and as such does not fully stress the lens around borders. In order to understand the full potential of this or any other medium format lens, you should test it on a native camera system or at least use a tilt/shift adapter that will allow you to check performance around extreme borders.