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Re: [Rollei] DOF Indices, on R-TLR and other lenses



At 01:17 PM 11/12/2001 +0100, you wrote:
>  From Vincent L. Gookin, about DOF scales:
>
>> ...I engaged a knowledgeable Swede at the H********* counter in a
>> discussion of DOF and the index markings on their lenses.
>
>> He declared that the markings are theoretical, not very precise 
>
>Definitely, I would not say that. The engravings are as precise as any
>good analog scale. And something theoretical can be as precise as you
>want if the model is correct and input values are correct !!! But
>don't forget that, whatever model you are using, "garbage in, garbage
>out." ;-)
>
>For all serious lens manufacturers, DOF scales engravings are
>*precisely* designed according to the most simple law : H=f*f/(N*c),
>and this model is not questionable except for macro work.
>
>**BUT**:
>
>> ...and overly generous. You're not going to get what the indices
>> indicate in your photograph.
>
>... you have to decide on a realistic value for "c" !!! On the H******
>planar CF 100, the engravings are coherent with an hyperfocal value H
>= 6 metres    if you extract the value of the corresponding
>circle of confusion "c", you eventually find
>
>*** c = 75 microns *** !!!! 
>
>i.e. an "acceptable" sharpness limit of about 6-7 line pairs/mm !!! for
>a lens capable of "passing" 100 lp/mm in good conditions, "overly
>generous" is an understatement...
>
>> Use the indices for the NEXT WIDER APERTURE than you are setting for
>> your exposure. OR do as he always does: use the indices for that
>> f-stop which is 2 stops wider than you are using for your exposure
>> in order to get all you expect/need in focus. 
>
>My understanding is that c=75 microns is simply not a realistic value
>in practice!!!. Changing the reading by 2 f-stops means dividing the
>value of "c" by 2, i.e taking c=37.5 microns. This sounds more
>coherent with actual performance of Z**-H** lenses, capable on the
>whole MF frame of performances in sharpness quite similar to a good
>35mm lens on 35mm format. I've also checked on a 50mm Z**-H** distagon
>C (with the Synchro-Compur, moving red DOF indices) and found a circle
>"c" of about 50 microns (H=2,2m @f/22) identical to the one Rollei
>uses on R-TLRs (see below).
>
>
>Other values for those interested 
>
>lens         focal length     H(metres)@f/22   c=f*f/(H*N) (in microns)
>
>H********* (read from W***i's book) 
>planar            80            4.9               60 microns
>	
>on a 35mm Voigtländer Bessamatic	    
>skoparex          35            1.8   		  31 microns
>septon            50            3.6   		  31 microns
>super-dynarex    135           18.    		  46 microns
>	
>on a Rollei 35 SE	 
>sonnar            40            3.    		   24 microns
>
>> I would infer that the same applies to Rollei lens markings as well.
>Rolleiflex T 
>tessar            75            5.                 51 microns
>
>---------------------------------
>
>A first remark is that a great fantasy seems to be behind the choice
>of the "c" value.
>
>Another remark is that the DOF scale on a R-TLR focus knob is
>reasonable, being based on a circle c=50 microns. However this means
>10lp/mm, and for a lens capable of passing a minimum of 60-80 lp/mm (I
>hope I do not open a passionat RUG thread here ;-);-), again a
>sharpness criterion of 10 lp/mm is, at least, "generous".
>
>Good'ol Voigtländer lenses are based on a reasonable c=30 microns for
>35mm format ; except for the telephoto : probably they cheated a
>little, did not want to frighten the users with an "impossible to use"
>lens "afflicted" by excessively narrow DOF limits... ;-);-)
>
>Now the strange 75 microns for the H**** planar CF 100 is for me a
>real puzzle in contradiction with a legend I used to believe in (until
>today ;-) "Z*** used to design its lens DOF scales with tighter
>sharpness tolerances tha other manufacturers". A last remark to be
>confirmed is that manufacturers seem to increase the value of "c" for
>longer focal lengths even if used for the same image format.
>
>Well eventually most (not all) modern manufacturers of 35mm auto-focus
>lenses have definitely solved the question : they ignore the formula
>H=f*f/(N*c) and do not engrave anything at all... not even a distance
>scale. So the customer gets what he deserves and cannot complain being
>"cheated" on non-existent DOF or distance engravings... ;-);-)
>
>-- 
>Emmanuel BIGLER         
><bigler  
>
  FWIW, Kodak states that the depth of field charts published for their
large format lenses is based on a circle of confusion equal to 2 min of arc
or approximately 1/1750 of the focal length. 
  I don't know what criteria was used for the depth of field scales on
their focusing lens. I rather think it depended on the application and
assumed magnification of the final image. Kingslake may say something about
this in his old book on photgraphic lenses.
  In any case depth of field is an optical illusion and can be calulated in
a couple of ways depending on whether one chooses a fixed circle of
confusion (as makes sense for a camera with interchangible lenses, or a
fraction of the focal length. DOF seems to generate more confusion than
almost any other subject in photography probably because it can be defined
in more than one way. 
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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