[Leica] Noctilux DOF Film vs M8

[kididdoc at cox.net (Steve Barbour)]

thanks Peter,

I believe I understand,

but you are coming close to the edge of my circle of confusion,

and I am not sure if you are just within it, or just outside of it.


Steve



On Apr 21, 2008, at 11:18 AM, Peter Klein wrote:

>
> Steve: It has to do with the range of the RF roller adjustment that  
> will result in acceptable focus with each lens.  The lenses are  
> calibrated to a standard, like the body.  But in the real world,  
> things may not be quite right, and the M8 is less forgiving than  
> with film--in other words, the tolerances before things go awry are  
> less.
>
> Note: The diagrams below will only look correct if you are using a  
> Monospace font like Courier to view your email.
>
> Think of a yardstick and a 1-foot ruler placed on a table, with the  
> ruler below the yardstick. Now imagine that the yardstick is the  
> adjustment range that work with the 50/2 Summicron, and the ruler is  
> the range that will work for the Noctilux at f/1. I've pictured it  
> below. The asterisk represents where the RF roller is adjusted. Here  
> is how your wide-open Noct and M8 may been set, before you adjusted  
> the camera:
>
>
> (Fig. 1)
> |SummicronSummicronSummicron|
>       |Noctilux|
>     *
>
>
> As adjusted above, the camera will focus properly with the  
> Summicron, but not with the wide-open Noct.  However, we could  
> adjust it so that the focus is OK with the Noct, and still be OK  
> with the Summicron. Below is what you probably did:
>
>
> (Fig. 2.)
> |SummicronSummicronSummicron|
>       |Noctilux|
>           *
>
>
> The problem with the Noct is that if you stop it down 2 or 3 stops,  
> you get something like this:
>
>
> (Fig. 3)
> |SummicronSummicronSummicron|
>                                      |NoctiluxNoct|
>           *
>
>
> Now there is no way to get the Noct exactly right without messing up  
> your other lenses like the Summicron.
>
> The Summicron also focus shifts a little bit when you stop down 2 or  
> 3 stops, but nowhere near as much as the Noct.  So you might get the  
> typical M8 "all depth of field in back of the point of focus," but  
> things are still usable with the Summicron. Not with the Noct.
>
>
> (Fig. 4)
>          |SummicronSummicronSummicronSummicron|
>                                      |NoctiluxNoct|
>           *
>
>
>
> On film, the shift is less than with either lens.  The problem still  
> exists, but it is within tolerance for the Summicron, and still  
> visible (though less) for the Noct:
>
>
> (Fig. 5)
>   |SummicronSummicronSummicronSummicron|
>                              |NoctiluxNoct|
>                *
>
>
> Does this make sense?  In real life, the initial position of the RF  
> roller adjustment may vary, (asterisk) as may the calibration or the  
> individual lenses (sideways position of the "ruler" or "yardstick."
>
> The point is that there is no absolute "perfect" point of focus.   
> There is only a range of acceptable tolerances.  With the M8, those  
> tolerances are noticably smaller.  It also appears that in order to  
> get some lenses to focus correctly wide-open, the RF and lens are  
> adjusted so that most of the DOF is behind the point of focus  
> (Summicron in Fig. 4), rather than the classic 1/3:2/3 distribution  
> most of us learned in the film era (Fig. 5).
>
> I'm sure the folks at Leica knew all this for a long time. It's just  
> that in the film era, most of us didn't notice.  The M8, with its  
> instant feedback and stricter requirements, opened our eyes.  The  
> confusion exists because the above is not intuitive--most of us  
> thing as focus as being either correct or not, and we don't think  
> about focus shift.  The situation has been made worse by quality  
> control problems, used lenses that were out of adjustment, and  
> perhaps by Leica initially adjusting lenses to a standard that was  
> viable for film but insufficient for the M8.
>
> I don't understand if or how focus shift also varies with respect to  
> subject distance. No one I've ever asked seems to understand it,  
> either. Sometimes DOF seems to cover focus shift at longer  
> distances. Some people have observed that focus shift is always the  
> same amount of offset "twist" from what the RF tells us, so it may  
> be in proportion to the subject distance, ie. linear with respect to  
> the focus cam.  This may break down at sufficiently large stops or  
> long focal lengths.
>
> --Peter
>
> Steve Barbour wrote:
>
>> a wonderful review and summary Peter...
>
>> what I really don't understand is the following two points and their
>> (apparant) incongruity...
>> re fixing "back focus"....
>
>> we either change the set screw in the body with an Allen wrench, OR  
>> we
>> send our lenses somewhere, WITHOUT the body, to be adjusted...
>
>> if you can fix the problem by changing something in the body, how can
>> fixing the lenses with NO regard to the settings/status of the body
>> possibly work?
>
>
>
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best, Steve


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