Re: [Leica] How to read film data sheets

[nbwatson@juno.com (N. B. Watson)]

I stand corrected: resolution figures are out, MTF is in...and they prove
positively that PKR is sharper than RVP.   But pedantic dissertations
aside,  Velvia *looks* sharper to my eye, to the eyes of droves of
photographers who switched from PKR almost overnight and have never
looked back, and to clients and editors who pay for the images.  I've
never sold an image of a test chart.  As soon as such an assignment comes
my way, I will surely shoot it with PKR.  

Regards,
Nigel

On Mon, 16 Nov 1998 23:28:32 +0100 Erwin Puts <imxputs@knoware.nl>
writes:
>A comment has been made that Velvia is better than K'chrome because 
>the
>resolution figure of V is higher than K (±160 versus ± 125). No doubt 
>that
>the cited figure is put in the data sheets. Has it any relevance?
>No. I will as usual give a solid explanation why not.
>
>Film manufacturers produce data sheets with info about resolution. 
>They
>give resolution figures for low (1:80) and high (1:1000) contrast 
>targets.
>And they give an MTF graph. For optical analysis the resolution data 
>are
>completely obsolete and so they should also be buried for film 
>emulsions.
>Because of the same reasons. I do not have to recall these, as they 
>are
>amply documented.
>Now look at the high contrast figure. What does it mean. The test 
>pattern
>is the well known and much abused barline target: black and white 
>lines of
>diminishing width per mm produce a test pattern of ever increasing 
>spatial
>frequency. (more lines per mm). This target is illuminated in such a 
>way
>that the luminance dofference between adjacnet black and white lines 
>is
>1:1000 or 10 stops contrast difference. This type of contrast you 
>might
>encounter when taking a silhouet aganst blue sky. But than we have a 
>low
>resolution target (only the silhouet line has the contrast figure). It 
>is
>nay impossible to find in high res targets. Look at any picture you 
>took
>the last decade and see if you can find a detail with very fine 
>structures
>in it and ask yourself: do I see two adjacent very small object 
>details
>that differ in luminance by more than 10 stops? You will not find any
>detail! So the high contrast figure is meaningless. If you are in need 
>of a
>figure go for the low contrast value and now we see that V and K are
>identical. NO advantage for either one.
>Take a look at the MTF graph and now you see a big difference, The K 
>graph
>tells you that from 1 to 20 lp.mm the MTF value is far above 100%! The 
>same
>values for V are lower. So in the critical areas where  sharpness  is 
>all
>there is the K wins. Why the threshold of 20 lp/mm. That is exact the 
>value
>Leica lenses are calibrated for!!
>Why then is K for many purposes the better film: it is grain based 
>where
>the V is dye cloud based. Recall that a dye cloud image is being 
>generated
>by arificially restraining the growth of clumps of grain and replacing 
>them
>by dye clouds of about the same dimension at about the same location. 
>Note
>the vagueness here? A grain image is an exact replica of the optical 
>image
>falling onto the emulsion. The dye cloud image is a chemical 
>interpretation
>of this image.
>The capture of fine detail is better preserved with the grain based 
>image
>and its 'hard' edges against the finer (smaller) dye cloud based image 
>with
>the soft edges.
>
>This is same the reason why fine grain developers in fact kill fine 
>detail
>and acutance developers enhance fine detail up to the limit of grain 
>noise.
>Recall the Rodinal discussion?
>
>As in optical evaluation we must become accustomed to the fact that
>resolution figures are out and MTF graphs are in. Thats reality.
>
>
>Erwin
>
>
>

___________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
or call Juno at (800) 654-JUNO [654-5866]