On Ways To Check Lenses

[The following can also be used for checking lenses on
 digital cameras...]


For the numerically inclined, the obvious way to 
establish the optical performance level of a lens may 
be 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 well 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 imaging performance: 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.


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 (using 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 opposite 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]).


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 linear 
distortion and from flare and ghosting are characteristics 
understood to be present for good lens performance):
- Super-wides (6-15mm) - must perform well at all distances 
   by f11.
- Very-wides (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 (especially tele-only zooms) as similar  
   focal-length non-zoom lenses at the same apertures (the 
   far corner sharpness and distortion characteristics may 
   be slightly worse and still be acceptable).


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 or one of the better 200-speed newer 
color negative films, which can allow simplification of the 
checking process by eliminating the need for a tripod if the 
shutter speeds are high enough. (What, no tripod in lens checking?!?! ;-) A tripod does aid in accurate focusing of 
longer lenses, though, and I do recommend using one for 
lenses over about 200mm. Shooting the tests in bright 
sunlight also helps. 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 very-wides and super-wides) 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).


A good 10X magnifier is sufficient (less magnification makes 
everything look good; more magnification makes film reading 
too difficult, can result in "empty magnification", and can 
make it more difficult to distinguish between image detail 
and the film grain or sensor noise; 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 biological magnifier (less than about $6). If 
you use a magnifier with a skirt, the magnifier 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 (not necessarily a 
rigid or sturdy one, but one that quickly stops
after being shocked)
. A well-aligned camera body is 
essential for lens checking (see below for more on this),
although problems with this are rare.


Before attempting to check lenses, it is necessary to check the camera body for good alignment. There is more on this 
here. 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 on the body 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 a short-tele is less likely to be misaligned than 
zooms, wide angles, or even normal lenses) 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. Lenses can form images 
that are not parallel to the film plane but be otherwise 
sharp - but that is not acceptable to me.

-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 maybe 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 (especially if 
an infinity-focus target also shows correct marked focus 
on the lens barrel scale and is sharp on film). 

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.

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 diaphragm 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 lens, 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 
few hairline scratches in the glass have so little practical 
effect on the lens image (especially if they are near the 
edge of the glass) that they can be generally forgotten 
(unless you are like me, and are 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 necessarily all. Also, visible 
lens defects mostly affect the contrast and brilliance of 
a lens, not its resolution.


Well, enough of all this blather - and on to some ways you 
can quickly and easily determine if a lens is good, and 
well-aligned. If you are lucky enough to have a camera body 
with an excellent viewfinder (100% coverage, no linear 
distortion, with high contrast and excellent sharpness 
everywhere in the frame), you can make good preliminary 
checks for lens alignment (lack of image "tilt" relative 
to the film) using the viewfinder. A viewfinder this good 
appears only on the Nikon F, F2, F3, F4, F5, and F6, alas, 
and all others that I have seen have shortcomings that make 
visual checks of lens alignment with the VF useless - but 
if you happen to have one of these cameras, you can 
carefully use it to focus on a very distant, contrasty 
subject, placing the subject at different locations on the 
screen (the four edges, the four corners, and the center) 
and repeatedly manually focusing the subject and checking 
the focus points on the lens focus scale to see if you get 
the same results for all of the points. You can also check 
for lens linear distortion on these bodies.

It is practical to check out lens alignment and sharpness 
on film using a minimum of film. I check it by first locking 
all camera controls and features in manual mode. I take a 
vertical photo (with the shutter release end of the camera 
up) of a detailed, distant scene (cityscapes and distant 
lines of bare trees work well, and a mix of these is 
better yet), with the detail at the top of the frame. Then, 
carefully not touching the focus ring, I 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 
the same way at f5.6 and maybe also at f11. Then shoot 
diagonals with the detailed subject material running from 
one corner to the opposite corner, and then reversed in 
camera tilt, at the same stops as before (compensating with 
appropriate changes to the shutter speed, of course!)

Take notes as you shoot! You will not remember what you 
shot afterwards!

Develop the film (leave it uncut and/or mounted), and before 
checking it, write on the film frames in unneeded areas how 
you shot them (FL, aperture, etc.) so that you can use the 
film as a reference later. Examine the film 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 (in digital, you can flip one 
of the frames, making it even easier to compare the image 
detail). 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). In the diagonal frames, you can see if 
there are any anomalies, and how the image center sharpness 
compares with the edges and corners. If the lens passes this 
check, it is probably well-aligned and not affected by any major optical manufacturing flaws. 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 for its type, model, and best samples - but the wide 
open results may tell you if you are satisfied with a 
particular lens used wide-open). Few lenses approach perfection (darn!), but I do expect good performance for 
the purposes I want to use them for! For MUCH more on this, 
see my "
SUBJECTIVE Lens Evaluations (Mostly Nikkors)". 

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!

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