Some side-by-side shots of the two lenses which seem fated to compete with one another: the 35 Summicron ASPH and the 35 ZM Biogon. I suppose they’re compared often simply because of their maximum apertures. Not the best basis upon which to deem two lenses to be of a similar class it seems to me. For instance, there’s no reason why the Biogon shouldn’t be compared with, say, the Summilux ASPH or a CV offering. Signature, not maximum aperture, is what defines a lens. Nevertheless, these are the only two 35 lenses I have at the current time, and the debate certainly is popular here on RFF.
From what I've gleaned of comparisons between these two lenses on this and other forums, the following are the key differences: the Biogon has better flare resistance; the Biogon has more pleasing bokeh; the Summicron’s image stamp shows greater, some would say excessive, contrast.
When I carefully compare two contre-jour images on the light table using my 22-power loupe, I notice the following:
- The seam on the upright of the lighter chair (the shaded side facing us) is crisper in the Summicron image. That's the very point on which I focused both lenses. The Summicron appears to have higher acutance at f/2.0;
- There is better detail in the Summicron's redering of the Shaw Festival poster on the wall above the chairs. For example, the female standing next to the fellow in the doorway stands out significantly more in the Summicron image. Because the poster is slightly out of focus, I reckon the greater contrast of the Summicron is what's telling here;
- In the seat area of the burgundy chair, there is (surprise!) significantly greater shadow detail in the Summicron image than in the Biogon image in spite of the Summicron image being a bit darker overall. To me this is the most surprising finding, especially considering what's been noted in previous comparisons regarding the Biogon's ability to render better shadow detail.
Doubtless, you'll notice the rather pronounced flare in the Biogon image. The Summicron has rendered the highlights perfectly. The Summicron appears to show better control of specular highlights. This isn't the only Biogon image that shows this problem. I bracketed this shot; all exposures show the same such flare.
The most telling difference in bokeh is in the lamp detail. Notice how the two lenses render the rim. To me, the rim seems more integrated with the rest of the lamp in the Biogon rendering. In the Summicron rendering, the rim stands out more. I believe what we see here is the classic 35 Summicron ASPH ‘double vision’ bokeh, something I’ve seen in other shots taken with this lens.
Notice the ‘double vision’ bokeh on the right-hand tombstone in the background. Incidentally, I think this shot illustrates rather nicely a classic 35 Summicron ASPH characteristic--namely, the ability, in selective focus shots, the make a subject seem to float above the background (the effect is much more obvious when viewed on the light table).
Returning to the side-by-side images, notice the rendering of the metal brackets. In the Biogon image they seem to me to be a little less out of focus than they are in the Summicron image. Generally, when I compare bokeh between these two lenses, I note that the Biogon consistently renders its out-of-focus objects in a purely soft manner. Out-of-focus objects taken with the Biogon are simply out of focus. The 35 Summicron ASPH, on the other hand, tends to introduce a bit of distortion into out-of-focus objects, which leads to a harsher bokeh. In this particular side-by-side comparison, such differences only become obvious under high magnification.
Conclusion
I find that at f/2.0 the Summicron outperforms the Biogon. In this scene the Summicron's acutance is higher, its greater contrast aids in causing certain details to stand out (e.g., the lady in the poster), and in shadow areas more detail has been realized. In this contre-jour setting, the Summicron doesn't suffer from veiling glare any more than does the Biogon. When it comes to control of specular highlights at f/2.0, the Summicron clearly wins out over the Biogon. Perhaps the 35 Summicron ASPH is one of those great lenses which allows Leica to enjoy a reputation for unequaled wide-open performance, at least from a technical standpoint.
For my taste, bokeh, a purely subjective consideration, tends to be rendered at f/2.0 in a more pleasing manner by the Biogon. Your tastes might vary.