In a message dated 2/14/2001 11:51:15 AM Eastern Standard Time,
[EMAIL PROTECTED] writes:
> Does anyone know if the actual focal length of the following
> lenses is different than their nominal focal length, and if so,
> what the actual focal lengths may be?
> 77mm/1.8 Ltd
> FA*85/1.4
> A*85/1.4
> M85/2.0
> K85/1.8
> SMCT 85/1.8
I have no experience with the 77/1.8 (maybe someday), but I do have some
visual evidence on the relative (but not absolute) focal lengths of those
85's. Having previously shot each of seven different 85mm lenses at
identical targets from identical distances, I just now made some side-by-side
comparisons of some of the images (taken from my "85mm Lens Shoot-out" at
http://www.cetussoft.com/pentax/85compar/index.html).
If you were to take a look at the shots (taken at the maximum aperture for
each lens) of a specific portion of the Boston skyline (from the "Prudential
Tower" to the "Dorchester Gas Tank")
(http://www.cetussoft.com/pentax/85compar/85boston.jpg, or click on "Boston
Skyline from Same Location @ Maximum Apertures" link at the above URL), you
would see that all seven 85's I tested are within a percentage point or so of
each other in focal length.
If you were to take a look at the shots of a portion of a brick wall (taken
at f/5.6 from a fairly short distance)
(http://www.cetussoft.com/pentax/85compar/85bricks.jpg, or click on
"Identical Bricks Shot at Identical Distances @ f/5.6" link at the "85mm Lens
Shoot-out" page), you would see that all but one of the seven 85's that I
tested are within a percentage point or two of each other in focal length,
and that the FA* 85/1.4 has a focal length significantly (although not
drastically) shorter than the focal lengths of the others.
I have no data to determine the absolute focal length of each lens, but I can
determine the relative focal lengths of each from the size of each image in
pixels. Furthermore, I can then "go out on a limb" and make the assumption
that most 85mm lenses are indeed nominally 85mm, that an average focal length
for a number of 85mm lenses might in fact be 85.0 mm, and then come up with
the following estimates:
[WARNING: SERIOUSLY UNSCIENTIFIC REASONING WAS APPLIED TO OBTAIN THE
FOLLOWING NUMBERS]
For infinity shots of Boston:
84.8 mm - FA* 85/1.4
84.6 mm - A* 85/1.4
85.4 mm - M 85/2
85.2 mm - K 85/1.8
85.4 mm - SMCT 85/1.8
84.8 mm - ST 85/1.9
85.2 mm - K 85/2.2 Soft
For close shots of bricks:
81.4 mm - FA* 85/1.4
85.5 mm - A* 85/1.4
84.5 mm - M 85/2
84.8 mm - K 85/1.8
84.4 mm - SMCT 85/1.8
85.3 mm - ST 85/1.9
85.6 mm - K 85/2.2 Soft
(Of course, the scientist in me cringes at the implied precision of the above
falsely long significant figures, but I have to admit that the "armchair
naturalist" in me just loves the pseudo-scientific appearance of such focal
lengths "measured" - <heh-heh> - to the tenth of a millimeter.)
As you can see, only the FA* 85/1.4 demonstrated a noticeably different
(shorter) focal length from the others, and only at a close focus distance.
The FA* focal length difference was admittedly not huge, but the resulting
difference in image magnification was in fact visually noticeable when
setting up to take brick wall photos with the FA* lens before and after doing
the same with other lenses. I assume that this is because the FA* 85/1.4 is
an IF lens, so that it is supposed to "lose" some focal length when it is
focused at close range. (Maybe?)
Anyway, I hope this helped answer your question, Shel.
Fred
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