The term Conventional Wisdom, coined by the economist John Kenneth Galbraith in The Affluent Society, describes an idea that has become accepted as truth simply by repeatedly saying it is so. The two lenses in this matchup, the SMC Pentax-A 1:1.2 50mm and the SMC Pentax-F 1:1.7 50mm, occupy opposite ends of the Conventional Wisdom scale in one important rendering characteristic - sharpness. Ask anyone - the Pentax-A 50/1.2 is soft. The Pentax-F 50/1.7, like all Pentax 50/1.7 lenses, is sharp. So goes the Conventional Wisdom.

Unconventional Wisdom

Sharpness

Compared at a fairly common f/4 aperture, both of these lenses are pleasantly sharp. Full-sized crops by the Pentax-A 50/1.2 here and by the Pentax-F 50/1.7 here show a virtually identical result as far as real-world resolution is concerned. The contrast may be slightly higher on the Pentax-A 50/1.2 version, but since this was a hand-held shot of a moving subject, it would be impossible to say for sure given the conditions.

Compared at f/1.7, as in this more-controlled scene here and , they produce roughly the same results. Certainly once you get down to f/1.2 with the Pentax-A 50/1.2, you are looking at a generally much softer image, but it is one that is impossible for the Pentax-F 50/1.7 to make at all. At f/1.2, the Pentax-A 50/1.2 has a razor-thin area of sharp focus, has a bit more flare and lacks overall contrast, yet I still consider it usable at that aperture. It is especially useful if the light levels mandate it, as I found out at the end of a recent bicycle race in this shot that accidentally caught someone else's flash. More surprising than the exact focus was the sharpness of the cyclist.

So, according to my samples, the Conventional Wisdom on these two lenses is only half right. If you consider the Pentax-F 50/1.7 to be sharp, then certainly so is the Pentax-A 50/1.2.

This shot, taken at f/2.8 with the Pentax-A 50/1.2, is plenty sharp. Click here for a 100% crop.

This shot by the Pentax-F 50/1.7 shows disappointing hard edges in the out-of-focus areas.

Depth of Field

The Pentax-A 50/1.2, at f/1.2, has a razor-thin area of sharp focus. For an object 1 meter away, the depth of field is a measly 1.8 centimeters, certainly outside the margin of error for a matte focus screen like the one in my K10D. So using any lens at f/1.2 (or indeed even at f/1.4 or f/1.7) can provide a 'falsely soft' look based solely on slight mis-focusing.

Bokeh

Compared side-by-side, the out-of-focus area is where the most telling difference can be identified between these two lenses. Comparing 100% crops of sushi bowls here clearly shows the different rendering characteristics of these lenses. The blur disk is much more evenly lit by the Pentax-A 50/1.2 at all apertures, with the Pentax-F 50/1.7 showing clear bright rings around highlights. This can be seen even in non-highlights as a sharper edge in comparable shots by the Pentax-F 50/1.7. Even reduced-size versions of the same subject by the Pentax-F 50/1.7 here and the Pentax-A 50/1.2 here show this clearly.

Build and Usability

The Pentax-F 50/1.7 holds one clear advantage over the Pentax-A 50/1.2, in that it is autofocus. If your shooting needs (or physical limitations) require an autofocus lens, then there is no contest between these two lenses.

For manual focus, however, there is an inverse disparity. The Pentax-A 50/1.2 has a long, smooth, and accurate focus throw and a wide rubberized grip. The Pentax-F 50/1.7 has a small, serrated plastic ring on the very end of the lens that, like most autofocus lenses, has a short throw that is more difficult for fine manual focusing.

Conclusion

So far in the Normal Lens Shootout, it has become obvious that I hold the out-of-focus rendering of higher importance than any other attribute. I find it easier to overlook (or fix) a soft image than one with a harsh bokeh - for many, the reverse is true. But in this case, where both lenses are capable of producing sharp images, I think we can all agree that a lens with a nicer blur is better than those with a harsh one. Couple the harsher bokeh with a plastic build and poor manual focus capability, and the Pentax-F 50/1.7 clearly loses to the SMC Pentax-A 1:1.2 50mm.

For those who prefer 'real' pictures, both of these lenses have collections here for the Pentax-A 50/1.2 and here for the Pentax-F 50/1.7.