Archived posting to the Leica Users Group, 2007/04/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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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>
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