Archived posting to the Leica Users Group, 2007/04/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Or an opportunity to redesign the Noct. Aspherics have become a lot cheaper
to machine and the focus shift is almost all due to spherical aberration.
Now that Noct's are not being made in Canada anymore I suspect that a new
design will be forthcoming. Say a 35 F1 or maybe a 40.
On 4/24/07, Tom Schofield <tomschofield@comcast.net> wrote:
>
> Rather than jumping to the conclusion that the Noctilux is unusable
> on the M8, perhaps adjusting one's technique in anticipation of the
> problem is in order. You said as much yourself - instead of
> focusing on the eyeball, focus on the tip of the nose when shooting
> at f 2.0, for example. Like Ted practicing to "lead" the peak of
> action when releasing the shutter in order to get the basketball in
> the frame instead of p-shopping one in!
>
> I myself have not visited Erwin's site in a while, and am enjoying
> his many M8 articles. Here's another excerpt discussing this issue,
> of which Leica was evidently aware, as they increased the tolerance
> of the RF mechanism by a factor of ten to help compensate:
>
>
> "Sensor based cameras
> The main characteristic of the sensor surface is its absolutely flat
> surface: there is no depth like we have in film emulsions. Here we
> cannot count on emulsion thickness to compensate for mechanical
> errors in accuracy or focus shift.
>
> Leica does know this of course. And the tolerance level of the whole
> rangefinder adjustment chain has been narrowed accordingly. The
> tolerance level in film based cameras is a few hundreds of a
> millimetre, in the M8 that level has been reduced to a few thousands
> of a millimetre. That does not imply that the factual tolerance level
> is a factor ten narrower!
>
> In reality the factor is about three to four times. This makes the M8
> the most accurately machined and assembled M camera in history. This
> is done to compensate for the lack of image capture thickness.
>
> When focussed accurately the focus plane is spot on on the sensor
> surface.
>
> The other side of the medal is the fact that focus shift might be
> more noticeable when stopping down: another argument not to stop down
> too much when you need critical sharpness at the focus point.
>
> We should also realise that any mechanical part and any manual
> adjustment has a small level of tolerance that may cancel out (good)
> or add up (bad). So some misalignment of the rangefinder mechanism
> should be kept in mind. You cannot work at zero tolerance.
>
> Here we run against the limits of mechanical precision equipment."
>
>
>
> Tom
>
>
>
>
> On Apr 24, 2007, at 4:30 PM, Eric Korenman wrote:
>
> > Much thanks for the info. I did know that some focus shift existed.
> >
> > As others have speculated, the virtual perfect flatness of the digital
> > sensor prob makes matters even more extreme. Film's 'thickness' was
> > probably
> > the fudge factor.
> >
> > That being said, the noctilux, as such can not be focussed with the M8
> > without mentally compensating.
> > IE, focus on the tip of the nose if you want the eyes in focus.
> >
> > Eric
> >
> > On 4/24/07, Tom Schofield <tomschofield@comcast.net> wrote:
> >>
> >> Hi Eric,
> >>
> >> We should introduce you to the work of a former LUG member-turned-
> >> author of the Leica Lens Compendium, Erwin Puts. You can spend many
> >> hours on his website: www.imx.nl
> >>
> >> Here is an excerpt discussing the focus shift with the Noctilux
> >> (tables did not paste well) In a nutshell, as you stop down, while
> >> the focus shift continues, depth of field compensates for it, so that
> >> it is most problematic at f 2.0, where the focus shift is greater
> >> than, the depth of field:
> >>
> >> "MTF measurements of the Noctilux and the Summicron can be compared
> >> to give this discussion a more quantifiable dimension.
> >>
> >> The Noctilux however has some very special problems in this area.
> >> Consider the results in the tables below.
> >>
> >> % Contrast in center Noctilux-M
> >> lp/mm 1.0 2.0 5.6
> >> 10 84 79 90
> >> 20 59 41 72
> >> 40 27 1 33
> >> The 10lp/mm value at f/1,0 is very good, the 20lp/mm is still quite
> >> good and the 40lp/mm result is acceptable: the Summicron has the same
> >> value for the 40lp/mm in the extreme corners. (at f/2,0 mind you!).
> >> At f/2,0 we notice an overall drop in performance , especially at
> >> 40lp/mm, while at f/5,6 the performance is slightly better than at
> >> full aperture. Modern Photography tested the Noctilux a number of
> >> years ago and noticed the same behaviour. They just accepted the
> >> figures. We know that the Noctilux cannot be fully corrected for
> >> spherical aberration. One important result of this defect is a slight
> >> focusshift. ('Blendendifferenz' the Germans call it). When refocused
> >> the Noctilux showed these results:
> >>
> >>
> >> % Contrast in center Noctilux-M (refocused)
> >> lp/mm 2.0 5.6
> >> 10 94 96
> >> 20 80 91
> >> 40 51 79
> >>
> >> The focusshift of the Noctilux is 74 micron when stopping down from
> >> 1,0 to 2,0 and 120 micron when going to 5.6. The reason that the f/
> >> 5,6 values do not drop as much as the f/2,0 values (in the first
> >> series of measurements) is the depth of field. At f/5,6 this depth of
> >> field is greater than the focusshift, so the results stay within
> >> tolerances. By the way: 120 microns is the total thickness of a
> >> typical fine grain 35mm film! These results show the pitfalls when
> >> testing an extreme aperture lens. As most objects in the real world
> >> are three dimensional and have depth, the effects of the focusshift
> >> are hardly noticable. What you could notice is a slight softening of
> >> the image at f/5,6, because the focusshift will produce a somewhat
> >> larger diameter of the blur circle."
> >>
> >>
> >>
> >>
> >>
> >> Enjoy Erwin's site, but keep in mind some of the technical issues he
> >> discusses are so minute that most people will never realize them in
> >> ordinary shooting.
> >>
> >> Tom
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> On Apr 24, 2007, at 1:15 PM, Eric Korenman wrote:
> >>
> >> > So I spent an evening with my M8, a sturdy tripod and a focus
> >> chart.
> >> > I know this is obsessive naval gazing and not art but here is what
> >> > I found:
> >> >
> >> > The 35mm asph summicron and 4th generation 50mm summicron are dead
> >> > on at
> >> > near focus and wide open.
> >> > Stepping down aperture shows the focus stays near center.
> >> >
> >> > However- the Noctilux, while dead on at f/1.0 shows severe back
> >> > focus the
> >> > higher the f stop.
> >> > At f/5.6, the plane of focus is nearly 6 cm behind the target, at
> >> > target is
> >> > blurred.
> >> >
> >> > I plan to rerun the test with a borrowed M6 and film.
> >> >
> >> > Eric
> >> >
> >> > left calls with Leica NJ - everyone techincal seems to be out of
> >> > the office!
> >> >
> >> > _______________________________________________
> >> > Leica Users Group.
> >> > See http://leica-users.org/mailman/listinfo/lug for more
> >> information
> >>
> >>
> >>
> >> _______________________________________________
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> >> See http://leica-users.org/mailman/listinfo/lug for more information
> >>
> >
> > _______________________________________________
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>
>
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--
Don
don.dory@gmail.com