M E T H O D S O F L E N S C H E C K I N G (Version 1)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
David Ruether

SOME BACKGROUND ON LENS CHECKING--
For the numerically inclined, the obvious way to establish the
optical performance level of a lens is to photograph lens test charts
and then to read the finest resolved lines at different apertures off
the film, converting the results to a solid-sounding set of numbers.
For the casual lens user, shooting a roll of film and looking at the
4x6 prints may be sufficient to establish that a lens is good enough.
For those not convinced that either method is very meaningful for
showing the detailed information about how lenses perform for taking
pictures, for separating the barely adequate optics from the truly
excellent, and for quickly and easily spotting poor lens designs and
poor samples of good lens designs, I offer some methods of lens
checking that have served me well for over a quarter century (Yikes!).

Before getting into my methods, let me give some of the reasons for
my misgivings about using some other methods (while noting that ANY
method that satisfies a user is fine for that user - there are few
absolutes here).

Using lens test charts has many pitfalls for establishing lens
quality: the commonly used distance of 25X the focal length may place
many otherwise fine lenses at a distance that is unlike what would be
used in normal photography, and at which the lens performance is not
representative (and may not be very good); it is difficult to test
lenses using test charts at all the relevant distances (especially
near infinity) that would show how a lens performs (lens performance
varies at least some with distance, and may vary a large amount,
depending on the type and design of the lens); it is VERY difficult
to focus with sufficient accuracy on a flat target to make the test
meaningful without using sequence focusing techniques at each aperture
(and a lot of film and eyestrain); it is difficult to align the film
and target planes sufficiently well to make the off center target
information reliable without special alignment tools or great care -
and an otherwise sharp lens may have slight field curvature which
could make the edge test results look poorer than the lens actually
is in normal use; resolution tests may not tell much about contrast,
which in some images may be more important than resolution (both
contribute to the look of sharpness in a lens) - also, in lens design,
it is hard to improve both contrast and resolution together, and much
easier to trade one for the other (it is quite possible for a lens to
be very high in resolving ability, but look poor in picture taking -
and the other way around); it is difficult to duplicate resolution
test results, even when using the same setup, equipment, and
materials - which leads to questions about the reliability of
solid-seeming test result numbers; a meaningfully thorough chart
testing of even one fixed focal length lens (let alone a zoom, or
two lenses tested together for comparison) is an exhausting ordeal
(not a suitable process for anyone but the most dedicated lens tester)
and the resulting sea of numbers may be overwhelming, and may be less
than easy to translate into a good understanding of how one lens
performs compared with another.

On the other side, taking a few photographs to judge the optical
quality of a lens also has pitfalls: what looks sharp enough on a
print (even an 8x10) may not look sharp on a negative examined with
a good magnifier (or on a larger print or projected slide);
cross-lighting on the subject photographed can mask faults that
would be evident in a larger collection of photographs taken under
a variety of lighting conditions; optical faults can easily escape
detection (until after the return period [or warranty period]
has expired - when they have a habit of suddenly making themselves
very obvious); many people seem completely unaware of image quality
anywhere but in the center of the image, but a photograph is
EVERYTHING inside the frame edge, and when the photo viewing (and
taking) sophistication level improves, the "sharp" lens (with the
soft edges and corners) may not look so sharp anymore; one person's
"looks sharp to me" comment may not serve another's desire to know
how good a lens is, since there is no reference standard of quality.

REASONABLE ASSUMPTIONS ABOUT LENS CHECKING--
Of the two methods of lens checking in common use (one attempting
to quantify performance using test charts and numbers, the other
qualifying performance using normal subjects and descriptions), I
prefer a variation on the latter, but with the addition of some
references to better quantify the quality descriptions (subjective
lens evaluation numbers). Here are some reasons for this choice
(and some assumptions that I think are reasonably made): familiar
normal subjects at various subject distances can be used repeatedly
for different lens checks; most normal subjects are at least somewhat
three dimensional, which probably insures that some parts of the
subject will be in focus even with a slight focus error (which makes
focus errors more detectable than they would be with flat subjects,
and helps reduce the focusing accuracy standards to practical levels);
a subject near infinity presents no subject-film plane alignment
difficulties (unless the lens shows field curvature - and, even then,
opposite sides of the frame should match in sharpness [and the four
corners should also match] if the lens is well-aligned); distant
subjects may be moved around to various parts of the film frame
without introducing focus errors (which near subjects would do if
they were moved in the frame); and the same subject moved around in
the frame presents a familiar (and comparable) rendition on the film.

It is reasonable to assume that the center of the lens image is no
worse than any other part of the image, so comparisons can be made
between the center and any other part; it is reasonable to assume that
the center performance of virtually all good lenses is excellent by
f5.6, so that performance at other apertures, and in other parts of
the frame may be compared with the reference center performance at
f5.6 (when this is not correct, frames exposed at smaller apertures
or experience gained checking other lenses should indicate that);
it is reasonable to assume that a well-aligned lens will have equal
performance at oposite frame edges, and also at all four corners
(assuming that experience checking a few lenses [or using the method
for body checking given below] has established that the camera body
used for lens checking is well-aligned [the lens mount is parallel
with the film plane and the viewed image, and the film and viewing
screen have the same focus]).

STANDARDS OF LENS QUALITY--
If the requirement for a lens to be considered sharp were for the
lens to show excellent sharpness at all points in the frame at all
apertures and focusing distances, there would be VERY few lenses that
would be acceptable. It is necessary to determine your own standards
for the lenses you use. For me, if a lens is not reasonably sharp
in the corners and sharp at the edges at a particular aperture and
in the focus range appropriate for that aperture (relating lens
performance to how the lens will generally be used), it is not sharp
at that aperture, no matter how astoundingly high the resolution is
in the center. Whether a lens can resolve 56 or 100 lines in the
center at f5.6 at 10' is irrelevant information for me - focus is
rarely accurate enough to make the difference noticeable (unless the
subject has enough depth to have SOMETHING correctly focused, a large
enough photograph is made [and examined closely enough] to see the
difference, and the camera is held steady enough [and the subject
does not move] to realize the higher resolution on film). What does
matter to me is good wide-aperture performance (which can aid
focusing, and allow lower-light photography with good quality),
and good off-axis performance (which is so poor in so many lenses
that it is a very relevant issue concerning lens image quality,
and it is of primary interest to me). My standards follow (good
resolution and contrast center-to-corner, uniform performance at
opposite sides and all four corners of the frame [good optical
alignment], even illumination, and reasonable freedom from distortion
and flare are characteristics understood to be present for good lens
performance):
- Superwides (6-15mm) - must perform well at all distances by f11.
- Verywides (18mm-24mm) - must perform well at all distances by f5.6.
- Wide-angles (28-45mm) - must perform well at all distances by f5.6,
and reasonably well at most distances wide-open.
- Normals (50-60mm) - must perform well at all distances by f2.8,
and reasonably well at most distances wide-open.
- Short teles (85-135mm) - must perform well wide-open, though not
necessarily near the closest focus distance.
- Medium to long teles (180mm, and up) - must perform well at all
distances wide-open.
- Zooms - must perform nearly as well at particular focal-lengths as
similar focal-length prime lenses at the same apertures (the far
corner sharpness and distortion characteristics may be slightly
worse).

MATERIALS NEEDED FOR LENS CHECKING--
Almost any film is suitable for lens checking. Resolution is a
function of both film and lens resolution - increasing or decreasing
either lens resolution or film resolution does not result in an equal
change in on-film resolution (which is why using extremely slow,
very sharp film does not result in the often expected vast increase
in image sharpness). Useful information in lens checking can be had
by using a film like Tri-X, which can allow simplification of the
checking process by eliminating the need for a tripod (What, no
tripod in lens checking?!) if the shutter speeds are high enough.
(A tripod does aid in accurate focusing of longer lenses, though,
and I do recommend using one for lenses over about 200mm.) Using very
slow films will slightly increase the differences in the performance
characteristics that you are looking for, but the problems introduced
(having to use a really reliable tripod [rare], with mirror
pre-release, etc.) discourages me from this choice, especially
when an easier one is available. Note that a few lenses (mostly wide
angles) do show differences in edge and corner performance on color
and B & W film (most look better on color film when there is a
difference).

EQUIPMENT NEEDED FOR LENS CHECKING--
A good 10X magnifier is sufficient (less magnification makes
everything look good; more magnification makes film reading too
difficult; 10X is a good choice for easily seeing the differences
in sharpness that you are looking for) - we are not looking for 100
lines-per-mm, just equality and sufficiency of detail in the parts
of the frame of interest in the lens check. Cheap is O.K. - I like a
folding field magnifier (less than $5). If you use a magnifier with
a base, it often works better turned around backward (the focus
distance in the normal position is rarely correct, unless it can be
focused). You will also need a broad, even light source, such as a
window on an overcast day, a frosted globe lamp, or a slide-sorter.
A sharp-tipped china marker is useful for writing on the processed
film what each frame represents. A well-damped tripod may be useful.
A well-aligned camera body is essential.

CHECKING THE CAMERA BODY FOR GOOD ALIGNMENT--
Before attempting to check lenses, it is necessary to check the
camera body for good alignment. Here are two ways to check camera
focus alignment. The first will tell you about parallel alignment
of the viewing screen and mirror with the film and bayonet (if there
is a problem, it is usually a mirror alignment problem, rather than
a bayonet mount-film plane alignment problem, though a slightly tipped
viewfinder screen is not unknown). The second will tell you more about
focus accuracy. A camera that fails the first test will probably fail
the second, but a camera that passes the first may fail the second.

-1) Put a fixed focal length lens (preferably a good short tele,
which is the easiest lens to use for checking viewfinder alignment
if the viewfinder is not very good, and which is less likely to be
misaligned than zooms, wide angles, or even normal lenses) on the
camera and aim it at distant detail (like a tree-edged horizon, or
distant buildings), focus the same distant subject carefully at the
four edges of the frame several times (observe the focus marks on the
barrel each time you focus). If the focus is almost always the same
in the center of all four edges of the frame, the lens is probably
forming an image of the subject that is parallel to the viewing
screen. It is likely that the lens mounting bayonet is parallel to
the film plane (it is unusual for it not to be), but a film check
is a good idea to confirm it. Shoot the same distant subject (at
about f2.8) in the center of each of the four frame edges without
changing focus, then check the four frame edges on the film (using
a good 10X magnifier) to see if they are equally sharp. If there is
a difference in focus around the edges of the frame, try another lens
(BTW, lenses can form images that are not parallel to the film plane,
and be otherwise sharp).

-2) Put a good 50mm (f1.4-2) lens on the camera and shoot a newspaper
at about 45 degrees at about 2' at f2. Focus carefully on one letter
in a paragraph that you can identify easily on the processed film.
Shoot the target 6 or 8 times, refocusing each time. Look at the film
with a good 10X magnifier. If you hit the focus right-on more than
1/2 the time, and the other focus points are randomly in front of and
behind the correct point, the viewing screen focus almost certainly
agrees with the film focus.

-3) If you want to know more about your camera viewfinder, you can
check framing accuracy (film area shown, centering, and rotation)
and linear distortion by photographing a distant lake shore, road
center line, window edge, etc. at the very top of the frame, turning
the camera 90 degrees with each photo until all four sides of the
frame have been used. (If this is done hand-held, I suggest repeating
each photo two or three times to average your framing errors.) On the
film (preferably unmounted, if slide film) you will see how far the
photographed lines are from the edge of the film frame, whether the
lines are equally spaced from the edges of the film frame, and whether
there is a consistent parallelism between the lines and the frame
edges (if there is a consistent angle instead, the viewfinder mask is
rotated slightly - a not uncommon fault in viewfinders, which makes
leveling the camera difficult). When photographing the straight lines,
you may notice that they bend inward in the viewfinder - this is more
likely due to pincushion distortion in the viewfinder (very common),
than to linear distortion in the lens (unless it is a zoom, in which
case the two distortions will add in the VF), but you can check the
straightness of the lines on the film to see which it is.

VISIBLE LENS FAULTS--
Check the lens to see if the lens is sufficiently free of mechanical
focusing wobble (or unsmoothness) for you; is free of oil on the
diaphram leaves (operate the aperture with the lens off the camera,
looking in both the front and rear at the blades to see if they are
dry, and open and close properly); focuses to infinity on the
viewfinder screen (beyond is OK, especially if the lens is AF, a zoom,
a mirror, or has Ed glass); and is free of obvious flaws in the glass
(by placing a bare light bulb behind and in front of the lens with the
aperture open, and looking into the lens from the side opposite the
bulb, but at an angle so that you can see the illuminated glass but
not the blast of light - this will show some dust inside, and maybe a
barely visible hairline scratch or two, but should not show major
amounts of haze, fungus tendrils, etched fingerprints, obvious
scratches, element delaminations, or other horrors - unless the lens
was very cheap). Minor faults like dust, a bubble or two, even a light
layer of haze, a small amount of fungus, mars in the coating, or a
hairline scratch in the glass have so little practical effect on a
lens image (especially if they are near the edge of the glass) that
they can be forgotten (unless you are like me, a "mint-nut"). The
reason this is true is easy to figure out: the defect occupies such
a small percentage of the lens area that it cannot affect more than
a tiny percentage of the light passing through the lens. Even if the
defect were great enough to cause light scatter at a level of, say,
5 stops below the average exposure, it would not register on most
films. A deep and extended scratch, a fingerprint, considerable haze
or fungus, or a lot of hairline scratches all over a lens surface are
defects that would affect image quality under some circumstances, but
not all. Also, visible lens defects mostly affect the contrast and
brilliance of a lens, not its sharpness.

// >checking image in VF PROCEDURE FOR LENS CHECKING--
Well, enough of all this blather - on to some ways you can quickly
and easily determine
I'm still thinking about that article I've been working on on
practical and easy lens checking, but it hasn't been easy to
write it, so it isn't here yet. But, maybe this should be it:
Since your lens is not a zoom, and has no floating elements,
it is practical to check it out using a minimum of film. I
would check it by taking a vertical photo (with the shutter
release end of the camera up) of a detailed, distant scene,
with the detail at the top of the frame. Then, carefully not
touching the focus ring, turn the camera over, and repeat the
photo (both should be shot wide open). If you feel unsure of
your focusing accuracy, refocus and repeat what you did a few
times. Then shoot the same target at f5.6 and f11 (one frame
each is fine). Develop the film (leave it uncut and/or mounted),
and examine it with a good 10X magnifier. In the pairs you first
shot, you have the same subject in two frames, but just across
the frame lines, where it is easy to compare. Check first the
centers of the edges of the frame pairs (where exactly the same
subject appears). It should be equally sharp and detailed in both
photos. Next, check the opposite corners along the adjacent frame
lines (which also have the same subject material), which should
also be equal in sharpness). If the lens passes this check, it is
probably well-aligned and not affected by any major optical
manufacturing flaw. Use the f5.6 and f11 frames as references to
see how well the lens can perform, and how well it performs wide-open
compared with optimum apertures (this is not for checking if the lens
is up to par - the wide-open checks tell you that - only if you are
satisfied with this particular lens design wide-open). Many good
teles perform badly at minimum focus (not a flaw, but a compromise..),
but checking this is difficult (and is probably best done with photos
of standard subjects taken with the lens wide-open at minimum focus -
then checking the in-focus parts with a magnifier to see if they are
sufficiently sharp for you. If your camera has very fast shutter
speeds, using a fast film with the camera hand-held works well for
this kind of lens checking. Uh, and don't get too caught up in
lens checking - it is a DISEASE, I tell you, a DISEASE!!!!!
Once acquired, it is very hard to get rid of it (just like reading
and writing posts on the 'net....!:<).

ADDENDA--
Consider all of the above to be copyrighted material (1995 -
David Ruether) which may be used freely for non-commercial purposes.
If this material is reproduced, reproduce it as is and as a whole
(unexerpted), including this paragraph - and please let me know
where it appears. Thanks.
Hope this helps.
David Ruether ( d_ruether@hotmail.com )