Re: Realist vs. 4"x5"

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

Eric G. wrote:

>> John B wrote:
>> When you look at the MTF curves for lenses and for film and
>> consider their compounding, you find that in miniature format,
>> the film drives the system MTF, in large format, the lens
>> drives the system MTF, and in medium format they both
>> contribute pretty equally.  So a fine lens in a larger format
>> is working at a bit of an advantage, not being film limited.

> Ah, the very famous and oft-times referred to MTF curves.
> Might we have a bit more explanation, sir? Perhaps what the
> initials "MTF" stand for, and a look at the curves themselves?

Sir?  You must have me confused with someone else.  (Actually, my 
father was "Mr. Bercovitz" but my sister and I called him "Sir" 
for short.  8-)

Sorry to be so slow on the reply but ever oncet in a while they 
ketch me and make me work.  Today was like that.

MTF stands for "modulation transfer function".  Very simple 
concept.  You have a special resolution chart which has bars on it 
just like an ordinary resolution chart except instead of black 
bars on a white background (or is it vice versa?) the lightness of 
the bars changes in a sinusoidal fashion from black to white and 
back, ad infinitum, ad nauseum.  So the lightness of the bars is 
modulated.  As with an ordinary resolution chart, there are groups 
of bars with different spacings, say 1 line per 1 cm, 3 lines per 
1 cm, 10 lines per 1 cm and so on.  You point your camera lens at 
this special resolution chart and then you read the sinusoidal 
output pattern with a meter.  So let's say our special resolution 
chart is perfect and goes from perfectly black up to some peak 
light level in a perfect sinusoid.  Now the question is, how far 
does the output sinusoid of the lens swing?  Say it swings from
5% to 95% of the full input light swing.  So your transfer is 
95 - 5 = 90% of the input.  You have an MTF of 90% or 0.9.

The same thing applies to film.  If the lines on the chart are 
spaced closely enough, you can well imagine that you don't get 
the swing you should on the film; the lines just blur together 
and you lose information.

That's a non-technical explanation and I'm sure there's someone 
out there who is much more knowledgeable in the area (John R?) who 
can make corrections as required.

> Danke

Bitte

John B

PS: The plate resolution of a lens is limited by physical optics 
(that's wave interference and stuff like that there) to somewhere 
around 1400 to 1800/(f/no) for the number of lines per mm.  But 
there are other problems which I'm not completely clear on that 
make it more difficult to approach this limit as the lens is 
scaled up.  So it's not all gravy for the lens designer if he 
moves up to 4x5 or 8x10 format.

PPS: Oops!  You wanted a look at the curves.  Don't have a curve
on my so I'll do one for a hypothetical lens.  For film, you just
have a chart of MTF vs lines per mm.  For a lens, you have a curve
because MTF falls as you get farther off axis.  In other words, a
lens has its best res right in the middle of the film and it gets
worse as you go towards a corner of the field.  It's ASCII time!
(Ugh.)

   MTF

  100% |*
       |     *
       |          *
   90% |             *
       |                *
       |  *               *
   80% |      *              *  10 line pairs per mm
       |         *             *
       |           *             *
   70% |             *             *
       |               *              *
       |                 *              *
   60% |                  *                *
       |                    * 20 lppmm        *    
       |                      *                    *
   50% |  *                      *                        *
       |      *                     *                          
       |                                 *
   40% |         *                             *
       |                                               *
       |            *                   
   30% |                                  
       |                *  40 lppmm                 
       |                                     
   20% |                     *                  
       |                           *              
       |                                    *
   10% |                                              *
       |
       |
    0% |______________________________________________________

       0          5         10         20         30         40 

                 Distance from axis of lens, degrees


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