Interesting find on OOF: 35 lux pre- vs. asph comparison @ f/1.7

[lutz]

</CENTER>Hi folks,<P>Just back from the darkroom. Interesting speculations here. Just one thing, first: When I do a comparison, be assured I shoot same stops at same shutter speeds... ;o) BTW, I guess it was 1/500th, as about a tenth of the 50Hz TV screen is visible. <P>

As the slides are equally dense, I assume 1.7 on the pre (yes, it does step down correctly...) equals 1.7 on the asph. As I wrote before I doubt that any difference in camera to object distance (of a maximum of maybe 3 cm) can be held responsible for the effect in question, IMHO. At this point I must add for the record, that a very valid contribution to this topic has been made off-line by Samuel Dilworth, whom I heartily invite to post here!<P>As for the glow I was looking for - I found some of it in the b&w negs that I just developed. I'm posting a scan of one of them in order for all of you to see what the glow-nuts are talking about...;o) BTW, I never experienced a similar koma like in Greg's pic with any of the three pre-asphs used so far... <P>Roberto, I do not know of any correction been made to this lens, the serial # is 2803324, Canada. Cheers.<P><P>

<CENTER><IMG SRC="http://www.konermann.net/leaves.dusk.jpeg"><P>

35 lux pre-asph, f/1.7, 1/60th, Delta 400</CENTER>

[fotografz]

Nice photo Lutz.

[samuel_dilworth]

Here are the relevant parts of the email I sent Lutz. It's rather longwinded; apologies.

 

I saw your bokeh comparison of the pre-ASPH 35mm Summilux with the current ASPH version. I've noticed this phenomenon before, most clearly when comparing shots from my Nikkor primes to those from the Leica equivalents. The Summilux ASPH that seems to have poor bokeh in your example actually produces a much more cohesive background than the Nikkor AI 35mm f/1.4 at identical apertures and distances. I'm sure you know that bokeh refers to the quality of the out-of-focus areas rather than simply the extent to which they are out of focus. Personally, I like the lens to render the background and foreground areas in a style that is smoothly out of focus while maintaining clear legibility. The most obvious differences appear in shots where text is in the background (advertising slogans, for example). The Nikkor renders the text unreadable in situations (aperture and distance) where the Summilux ASPH allows the viewer to quite easily make out the lettering. Clearly the pre-ASPH would provide an even greater improvement over the Nikkor in this regard.

 

From my experience, it is clear that these differences are not due to a discrepancy between the indicated f/stop (f/1.4) and the actual f/stop. The pre-ASPH is a genuine f/1.4 (at least at low image heights, i.e., on axis), and this lens is no slower than the ASPH, which in turn is no slower than the Nikkor. They all produce genuine f/1.4 image brightness at the centre (with artificial vignetting and natural light falloff evident to varying degrees at greater image heights). Rather, it is the design of the lenses that causes these differences, and perhaps the position of the aperture stop relative to the lens elements is one influencing factor (among many others that certainly affect bokeh). Some designs result in the physical size of the aperture stop being somewhat different to what the f/stop would indicate (e.g., a 35mm lens at f/1.4 may not have a physical aperture size of 25mm).

 

Your sample photos do show evidence to support these facts. The JPEGs are a little small for us to read too much into them, but observe the large difference between the rendition of the picture frame on the right (and the upward-running text on the poster which is only readable in the pre-ASPH image). One might expect the difference to be even greater at farther distances, such as infinity, but in fact the difference remains the same or perhaps even decreases slightly. This indicates that the largest factor responsible for the bokeh difference is the degree to which the lenses corrupt lines or point sources of light in the out-of-focus areas, rather than the actual degree to which they render the image out of focus. Clearly the ASPH causes more corruption of the bokeh.

 

By the way, depth of field is computed solely from the numerical specifications of a lens. It would be incorrect to deduce from these images that the pre-ASPH has more depth of field than the ASPH at the same f/stop (they have identical depth of field). In fact the depth of field of a lens is quite discrete from its bokeh characteristics.

 

It's interesting to note the huge difference in contrast between the two lenses (at f/1.7, of course). Look how much darker the vertical window frame is in the ASPH image! A silhouette against a brighter background is a critical test of flare resistance, and the pre-ASPH is clearly very much more susceptible to veiling flare than the ASPH. This flare spills over into the music sheets also (to a lesser degree), though the differences are almost undetectable at the area around her shoulder. So although the pre-ASPH does flare seriously, it contains it within a relatively localised area. The apparent slight difference in exposure is probably caused by veiling glare. I see practically no difference in focus point that Jack Flesher comments on, and even if it were true, bokeh is not nearly as sensitive to focal distance as is commonly assumed. Besides, you say you see the effect in other shots. I think it's the extra contrast that makes the ASPH image seem sharper; it's hard to believe one could see a sharpness difference at this image size, though the ASPH image certainly appears a good bit sharper to me.