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MLSTB-I SOA
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12 May 19s9
mum cleti opening cl the surface which is
actually u;ed in forming an image
in any
part of th( field:The mount aperture at each
surface sk d] be at least as large as the clear
aperture :
n order that vignetting will not
exceed the computed value. The clear aper-
ture is ust,ally circular and specified by its
diameter. ;t is sometimes referred to as the
free aperw re.
&2JiRe, u~ctiaperture.s The relative aper-
ture shall .}edefinedas the ratio of the EFL
to thediameter of the effective aperture. The
SVITIUOJ for relative aperture Sha]i be A“j
Ioiiowed
by a numerical value. it IS~vritten
as a iraction, for exampie, f/2 signities that
Lhediameter [~t’theeffective aperture is one-
half the focal length. For an object at an
infinitedistan.e, thedenominator of therela-
tive aperture ,md the second member, N, of
the aperture r.kio are identical, provided-the
image is formed in air and the imageryobeys
the sine condition.
32.5.1 ~-?wnJbw,6The f.number ~ha]] ~
definedas the denominator in the expression
for the rel:itive aperture. Thus, if the rela-
‘; ve aperture is f/2, the f-number is 2.
3.2.6 T-a‘op and T-nwnber.7 The T-stop is
referred to as the aperture of a lens cali-
brated photometrically and assigned a T-
number, ,rhich is the f-number of a circular
opening in a fictitious lens having 100 per-
cent tram: uittan- andwhich gives the -e
central image illuminance as the actual lens
at the specified stop opening. Hence, for a
lens with a circular aperture, the
f-number
T-niiixber = ---
(8)
Vt
.-. —.
Wher?t is th~transmittance. For a lens with
.tr eflect+.ve::nerture ,>”any shape and area
A, the corresponding formula is:
f
T-number = —
r
w
— ,1)
2
At
The transmittance of thelens shall be defined
as the ratio of the transmitted light flux to
the incident light flux. The symbol for the
T-stop shall be T followed by a space and a
numerical value —
for example, T 2. The
numeral 2 r+:presentsthe T-numher. (F r
test procedure, see 5.L2.1O.)
3.2.6.1 Area weightdd average T-wtt .... ,
The T-number ~s defined in 3.2.6 is a c, i, -
parative measure of illuminanceon the ax:.
of a lens.Since theilluminanceusuallyvarie=
over the field,a need may exist for determin.
ing T-numbers for
OfT axia] im4ge ~in~ ~~
computing an average T-number. In accord-
ance with the basic photometric relation-
ships involved, the general detlnition of T-
number is given as
1
I
(5)
Since, in accordance with this definition,
l—
–d
=B = TO
d
Eti
2
d
.——
T,
%
= T,
(6)
E, “
In these expressions, T, is the T-number for
an image point in a zone i, To is the axial
T-number, B is the object luminance, E. is
the illuminance on the axis, and E, is the
average illuminancefor the zone. Compatible
units should be used for quantities B, EO,
and El. when the illuminance is averaged
over the field, weighting the average by the
area cifthe circular zone in which the iMni-
nance is determined,and this average is sub-
stituted ior El in equation (6), the resulting
T-numkr i~ called the area weighted aver-
age T-number, or AWAT. For cmcular zones
which extend beyond the ~undaries of the
SUptwaks Page s d 12 May1959
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