Illumination only. This should be intuitively obvious to the most casual
observer. Most other characteristics are independent of whether the lens is
"for digital" or "for film", are independent of flatness of field, etc., and
are simply a function of tradeoffs in lens design or care in manufacture.
This "light falloff" due to sensor designs has been the bone of contention
in many circles.
-------------------------------------------------------
Procedure is:
Snap with the digital lens.
Snap with the film lens.
For each lens:
For each of red, blue, green:
Separate into layers.
Pick a small center spot in each and average all pixels.
Using green as "standard", Calculate the multiplying factors for red
& blue
in the averaged spot.
Multiply the red & blue over entire image by each's multiplying
factor.
We now have the complete field for each lens calibrated to a center
"gray".
Multiply the film lens's entire field (each RGB layer)
by the digital center gray / film center gray.
We now have each field, D lens & F lens, calibrated for equal center
grays.
Pick four small edge spots and four small corner spots in each lens
and average all pixels.
For each of the eight spots for each lens:
calculate the difference, F lens / D Lens in EV.
-------------------------------------------------------
For me, if the difference is less than 1/6 stop, I don't give a damn, at
least at that aperture.
Regards,
Bob...
----------------------------------------------------------------------------------------------------
By all means, marry. If you get a good wife, you'll become happy;
if you get a bad one, you'll become a philosopher.
- Socrates
From: "Godfrey DiGiorgi" <[EMAIL PROTECTED]>
But what are you trying to compare, Bob? Evenness of illumination,
resolution, contrast ... what? A gray card will only show you evenness of
illumination. At f/2.8, it's a toss up between the two 50mm lenses to
predict which will do better, although at copystand distances I'd put my
bets on the dedicated macro lens.
A D-FA50 macro lens is designed to be best at flat-field imaging, stopped
down to f/8-f/32, in the near-focusing range. An A50/1.4 is designed for
general pictorial use at wide apertures, and will not perform at its best
at copystand distances.
Comparing a DA40 and a M40 pancake makes more sense as they are both
designed to do the same job of general pictorial work.
Unfortunately, the only lens I've got of the above four is an A50/1.4. I
can shoot a gray card for you, but I doubt that it's going to tell you
very much.
Godfrey
On Sep 5, 2005, at 1:52 PM, Bob Blakely wrote:
What I would be interested in, then, is D-FA50 Macro wide open and the
A50/1.4 at the lowest common stop, or the DA40 and the SMC 40 at the
lowest common stop.
Regards,
Bob...
----------------------------------------------------------------------
------------------------------
By all means, marry. If you get a good wife, you'll become happy;
if you get a bad one, you'll become a philosopher.
- Socrates
From: "Godfrey DiGiorgi" <[EMAIL PROTECTED]>
There are no "digital" prime lenses between 20 and 35mm. Pentax lenses
designed for digital SLRs, to date, include:
DA14
DA16-45
DA18-55
DA40
DA50-200
D-FA50 Macro
D-FA100 Macro
If I get a chance to set up my copystand and lighting, I'll make a
gray card exposure with the A24/2.8 or FA35/2, but I doubt very much
that you'll see anything different compared to making the same
exposure on a 35mm film negative and then cropping the negative to the
16x24 sensor format dimensions.
Godfrey
On Sep 5, 2005, at 12:56 PM, Bob Blakely wrote:
Yes. Also, if one of you digital dudes would shoot a gray card frame
edge to frame edge with one of your digital primes (say between 20
and 35 mm or so) and one of your film primes of *equal* focal length,
I would be interested in comparing light fall off at the corners.
Tripod mount camera on centerline wit perpendicular from the center
of the gray card. Light gray card evenly with two side lights placed
on each side of the camera and such that there is no equal angle
reflection from the light to the card to the lens. No flash. Just
like you were photocopying art.
