[Leica] Back focus distance

[henningw at archiphoto.com (Henning Wulff)]

>I came acrossed this at
>http://www.imaging-resource.com/PRODS/R1/R1A3.HTM seemed to
>be relevant to the whole M8 digital hope ....
>
>"While a live LCD viewfinder display is a nice feature,
>it's actually not the biggest benefit of the Sony DSC-R1's
>design. It turns out that eliminating the large mirror box
>required in digital SLRs conveys huge optical benefits to
>the lens system. What's involved is reducing the
>"back-focus distance" of the lens, which makes it much
>easier to reduce chromatic aberration and other optical
>defects. The illustration at right shows a cutaway view of
>a typical digital SLR. The green arrow highlights the
>distance between the rear lens element and the sensor
>surface. (Image courtesy Sony corporation.)
>
>As noted, back-focus is basically the distance between the
>rear element of the lens and the surface of the imager, and
>has much to do with how difficult it is to engineer the
>lens system for low chromatic aberration and other
>distortions. One way to understand this is to consider that
>the lens has to "project" the image across the gap between
>the back of the lens and the sensor itself. It makes sense
>that the smaller this distance, the more accurate the
>"projection" would be.
>
>In a conventional SLR, the minimum back-focus distance is
>set by the space required for the rotating mirror assembly.
>SLRs built on conventional 35mm bodies typically have
>back-focus distances of 30mm or more, while special
>digital-specific lenses (like Canon's EF-S series) have
>back-focuses of about 20mm. By contrast, the R1's optical
>system has a back-focus distance of only 2.1mm. This should
>translate into noticeably lower chromatic aberration at
>wide angle focal lengths, and in fact, we saw very little
>chromatic aberration with the R1 our test shots. (There was
>a small amount of chromatic aberration present, but it was
>quite a bit less than we'd normally expect on a high-end
>"prosumer" digital camera.) The DSC-R1's lens also showed
>considerably better corner sharpness than we're accustomed
>to seeing from cameras in this price range."

Reduction of the 'back-focus distance' is not per se a way to reduce 
aberrations of any kind. The argument that the rear 'projection' 
distance is a problem is silly.

The benefit of allowing lens components to encroach into the space 
that the mirror assemble takes up in other cameras is that it allows 
the lens designer greater freedom, witness the higher general 
performance of Leica M wideangles compared with any SLR lenses. SLR 
lenses can make up the deficits               to a certain degree by 
using more complex designs, but this leads to larger sizes and 
greater costs.

The best digital lenses, the so-called telecentric designs, use a 
very complex design which is helped by not having to leave a space in 
front of the sensor for things like mirrors. If they had to do so, 
they would be even larger than the monsters they already are, and 
they are not even true telecentric designs if they are for general 
photography!

The R1 has many interesting features, and if Zeiss did design the 
lens I'm sure it has in it much of what Zeiss has learned in 
designing high-end digital lenses for industrial and movie 
applications. If only Sony had incorporated decent electronics so it 
could shoot faster and had a larger buffer....

-- 
    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
  |[ ]|     http://www.archiphoto.com