Backfocus

Because I was seeing some shots I thought were a little fuzzy at my
focus point, using the focus charts available online I just tested the
focus of my Nikon 18-200 VR AF-S DX zoom lens on a D200. Turns out it
has about 10mm backfocus. Using the same setup I tested my Sigma
70-300 DG APO and it was spot on perfect. Since that's not an AF-S
lens (it has the screw drive) I'm fairly certain that the camera is
fine and the fault is with the 18-200 lens... I think..., right?

Anyone know of a way to adjust that? I see Canon has an AF micro
adjustment on some of their cameras. Is there any kind of hidden menu
option on the D200 to do something similar? It's bothering me enough
that I'm going to have to send it in for service if I can't adjust it.
It's under warrantee, and something like this better be covered. But
then what are the odds they'll say it's the camera and not the lens
even though the camera is fine with a different lens?

Steve

 

[A complimentary Cc of this posting was sent to
Steve

Hmm, where did you find them? I needed to make my own to get
some reliable result...

This makes no sense. Backfocus is, essentially, due to misplacement
of the focussing element. It cannot be misplaced by 10mm. ;-)

What you mean is a misplacement of focus plane IN THE OBJECT SPACE -
but it would depend on the focussing distance and focal length. E.g.,
with 100mm lens and an object at 1m distance, 10mm in object space
corresponds to 0.1mm in the sensor space (1/10x magnification
corresponds to 1/100 magnification in perpendicular direction).

The moral is that you better state these numbers together with your
10mm figure.

This always has been puzzling me: how to explain why different lenses
may lead to different misfocus...

[Are you sure that you tested with the same focal length and focusing
distance?]

Hope this helps,
Ilya

 

It most definitely can depend on the lens. I have 7 lenses for my
d200, 6 of which are autofocus. 3 focus perfectly using AF (one is a
sigma 10-20 and one a tamron 90mm f/2.8). One is manual focus and is
focused when the green focus confirmation thing is on (but it is on
for a range of distances; best focus is when you are towards the near
end of this range). One is a 50mm f/1.8; AF is in front of the subject
at f/1.8 or so, seems to be ok when you stop down to f/2.8 or
thereabouts-this may have something to do with basewidth of the af
system, I don't know.

The last two lenses I got are interesting. I decided to get two f/2.8
zooms for when I am travelling and want mobility without much emphasis
on quality. So I got a tamron 17-50 f/2.8 and a tokina 50-135mm f/2.8.
The tokina consistently focuses behind where it should; I can get it
exactly focused manually (I have a split prism screen). The Tamron
also consistently misses focus, but always focuses in front of where
it should.

I have no clue what is going on. By the way, once you're used to a
tamron 90mm f/2.8 and a nikkor 28mm f/2.8 ais, it's quite a shock to
use any of these zooms (I have tried the nikkor 17-55 f/2.8, better
than the tamron 17-50 but still quite shocking coming from those two).
Maybe the new ultra-expensive ones are better.

By the way (stop reading if you're not interested in rants), I sent
the Tamron for service (it was also misaligned) last september. After
lots of calls to the representative here in Belgium from my dealer,
which produced promises of "next week we'l return it", 10 days ago he
was finally told (apparently by tamron, not the representative) that
the problem is that they can't get parts! Furthermore, normal service
time for them is 14 weeks (!), but they'll put me in front of the
queue because it is already more than 4 months... 14 weeks, if
everything goes well. I thought it was a joke or a mistake at the
beginning.

Presumably it didn't occur to them to offer to replace the lens
(seeing as they sold it to me misaligned and unabled to focus in the
first place, even before they kept it for 4 months without explaining
what is going on).

The representative here is De Beukelaer, by the way. Sometimes I
wonder if some companies are actively trying to lose business.

 

Autofocus maximizes contrast so it really has nothing to do with the
mechanics of the lens.
If the lens will not focus manually and hold focus then something could be
wrong with the optical or mechanical elements.
When the Nikon 55-200 first came out I went through two which could not find
or hold focus: hence my opinion that is one of the worst Nikon lenses ever
made. Reviewers did not have my experience or were given better lenses to
test.
Realize that despite its lofty price the 18-200, and I have one, is kind of
a doggy lens: it ain't all that sharp and at the wide end it has a ton of
linear distortion.

 

Try for yourself:
http://focustestchart.com/focus21.pdf

 

Actually that's true only for compact digital cameras and is one of the main
reasons why they have such a large shutter lag.

dSLRs use phase detection which is much faster and more accurate.

Actually it does because phase detection let the camera dial the correct
focus distance directly without incremental try-and-error like in contrast
measurement. If the lens reacts differently than the camera computed, then
you got a problem.

jue

 

Focusing is done by the camera, not the lens.

 

The better compact digitals do not have large shutter lag -- shutter
response on my Panasonic DMC-FZ8 is near instantaneous.

Faster yes, but not necessarily more accurate -- it all depends on what
the camera is trying to focus on.

 

[A complimentary Cc of this posting was NOT [per weedlist] sent to
acl

I do not doubt this. Myself, I tested thoroughly only one lens; but I
saw many reports on the differences between lenses.

What I asked for was an explanation HOW it is possible (some
"semi-scientific" explanation ;-)!

So it is a mild backfocus... This is what I see on what I tested, in
luminiscent light (backfocus varies 0..7mm). But in sunlight, it is a
major backfocus (10..30mm, varies a lot).

[The EV of the subject was varying in the same range for both types of
lighting.]

Puzzled,
Ilya

 

<http://www.fredmiranda.com/forum/topic2/241524>

According to the major material in Canon's "Lens Work III," the
description in their US patent application, and remarks by Chuck
Westfall, to put it briefly:

The AF system sensors are located in the floor of the mirror box.
They receive the image through the semi-silvered mirror, which is
then reflected downward by a secondary mirror hinged to the back of
the main mirror. This forms a virtual focusing plane that is supposed
to be at the exact same plane as the sensor (a point of possible
miscalibration).

Each AF sensor consists of a pair of short lines of pixels forming an
array. One array comprises the outer sensors. Two crossed arrays (one
vertical, one horizontal) comprise the center sensor. With lenses or
f2.8 or faster, the camera activates a second vertical array in the
center.

The arrays are sensitive to linear details that run perpendicular to
the orientation of the array. Therefore, the horizontal arrays
(identified by the horizontal rectangle marks on the viewscreen) are
sensitive to vetical linear details; the vertical arrays (identified
by the vertical rectangle marks on the viewscreen) are sensitive to
horizontal linear details.

They are blind to linear details that run parallel to the array
direction. The center array, being a crossed combination of a
vertical and a horizontal array, is sensitive to linear details
running both vertically and horizontally. When the second vertical
array is activated, it's combined input increases the accuracy by a
factor of three.

The pixel arrays are actually three times longer than indicated by
the viewfinder markings. This is to cover the fact that the
viewscreen has a significant amount of "slop" in its horizontal-plane
positioning (what you see as left/right/up/down in the viewfinder).
Therefore, the sensors actually see details that are somewhat outside
the viewfinder markings, and may focus on them instead of details
within the sensor markings, if those outside details are more
perpendicular to the array than the details inside the markings.

When you mount a lens (whether the camera is on or off), the camera
interrogates the lens for its characteristics, including maximum
aperture, which one of the focusing parameters.

When you half-press the shutter release (or the * button, if you've
used the custom function to move focusing control there), the
activated AF sensor "looks" at the image projected by the lens from
two different directions (each line of pixels in the array looks from
the opposite direction of the other) and identifies the phase
difference of the light from each direction. In one "look," it
calculates the distance and direction the lens must be moved to
cancel the phase differences. It then commands the lens to move the
appropriate distance and direction and stops. It does not "hunt" for
a best focus, nor does it take a second look after the lens has moved
(it is an "open loop" system).

If the starting point is so far out of focus that the sensor can't
identify a phase difference, the camera racks the lens once forward
and once backward to find a detectable difference. If it can't find a
detectable difference during that motion, it stops.

Although the camera does not take a "second look" to see if the
intended focus has been achieved, the lens does take a "second look"
to ensure it has moved the direction and distance commanded by the
camera (it is a "closed loop" system). This second look corrects for
any slippage or backlash in the lens mechanism, and can often be
detected as a small "correction" movement at the end of the longer
initial movements.

When the camera determines how far and in what direction the lens
must move to cancel the phase difference, it does so within a
tolerance of "within the depth of focus" of lenses slower than f2.8
(down to f5.6) or "within 1/3 of the depth of focus" of lenses f2.8
and faster. The depth of focus is the range at the sensor plane
within which the image of a point will be reproduced as a blur
smaller than the manufacturer's designated "circle of confusion"
(CoC). Canon's designated circle of confusion is 0.035mm for the
24x36mm format and 0.02mm for the APS-C format. The CoC is based on
maintaining the appearance of sharpness in a 6x9 inch print at about
an 10 inch viewing distance (as revealed by the Euro-Canon web site).
There is no guarantee that images enlarged any greater than this will
appear sharp.

The depth of focus increases when the aperture of the lens decreases
(like depth of field at the subject plane), but it does not change
with the focused distance or the focal length of the lens (according
to Canon, unlike depth of field). That is why the camera interrogates
the lens for that information; it calculates the depth of focus
tolerance from the maximum aperture, not the set working aperture.

As a result of this tolerance (within the depth of focus or within
1/3 of the depth of focus), the camera can place the actual plane of
focus at random anywhere within the tolerance range, and not
necessarily at the same place each time.

A non-exhaustive list of information about focusing:

1. The center focus square in the viewfinder represents has both
horizontal and vertical sensors, so it can focus just as well on
vertical and horizontal lines of detail. The outer focusing
rectangles are represent sensors that are oriented either vertically
or horizontally (according to the shape of the marks), and focus best
on lines of detail that are perpendicular to them. You can test this
easily: Line up a vertical focusing rectangle on a vertical detail
(like the corner of a wall or the edge of a door) and try to focus.
The camera will not be able to focus on it. But put a horizontal
rectangle against that vertical line, and it will snap instantly into
focus (you can turn the camera, and the same will be true). This is a
valuable tool. If you are struggling with a background that competes
with the foreground, look at whether either has linear detail (say, a
squirrel on a tree branch). You can activate one of the rectangles
and turn the camera so that the rectangle is either parallel with the
linear detail that you want to ignore or perpendicular to the detail
you want to focus on.

2. The actual focus sensor arrays are three times larger than the
viewfinder marks. A user could put an intended subject in the mark,
but if there is a strong detail just outside the mark (but within the
sensor area), the camera would focus on that strong detail. This is a
source of much of the complaints of the back- or front-focusing --
especially with the "ruler tests." Also, as far as the camera is
concerned, a focus lock on anything within the sensor area is good,
which sometimes covers more area than the photographer intended.

3. Auto focusing with the 20D only works with lenses with maximum
apertures of f5.6 or greater (as determined by the information passed
to the camera by the lens). This means the total maximum aperture of
the lens, not the aperture you're shooting with at the momement. With
a lens slower than f5.6, you have to focus manually (unless you fool
the lens somehow into reporting an incorrect aperture to the camera).

4. On the 20D, the center marks have additional sensors to increase
accuracy three times greater than the 10D, but these only come into
play with lenses that have maximum apertures of f2.8 or greater (not
the aperture set for shooting, but the maximum aperture). On a
variable aperture zoom lens, if it drops below f2.8 while zooming,
that information is passed to the camera, which cuts out the
additional focusing sensors. The outside focus sensors of the 20D are
normal accuracy.

5. The camera's AF sensors require some details in the image to
determine the phase difference. It's harder for the camera to find
focus when the light is dim or there is little subject detail.
Contrary to recent remarks on another topic, the camera CAN
distinguish contrast between equally bright hues of red and green
just as the eye can--the sensors are color corrected. Although the
sensors can distinguish some quite subtle detail differences, they
don't see quite a sharply as the eye. If the lens starts from a very
out of focus condition, it can miss very fine detail that the eye
sees clearly, such as the mesh of a speaker grill from across the
room. In this case, it can be helped if the photographer manually
moves close to "focus" and allows the camera to find the actual
focus.

6. AF controls: Shutter release. By default, when you half-depress
the shutter release, the camera will focus with the active sensors on
the strongest contrasts within those sensor areas. Whether or not it
will hold that focused distance depends on what AF mode you're
shooting in.

AE/AF Lock Button. The asterisk button on the back near your right
thumb. You can set this button to be the focus button in the Custom
Function menu (CF4--choose option 1). When this is set, you focus by
putting the active AF mark in the viewfinder on your subject and
press the asterisk button. The camera focuses on that spot and does
not change focus until you press the button again. In AI Servo mode,
the camera continuously evaluates focus only as long as you have the
button pressed.

Multicontroller (joy button) and AF Selection button. These controls,
plus the control wheels, allow you to select which focus marks are
active--they provide multiple ways to do the same thing, so take your
choice. You can either select one point or you can set the camera to
choose its own points as you focus. If the camera chooses the points,
it will usually focus on any number of points that are closest to the
camera. About the only time this is better is when you're focusing on
fast-moving activity that you can't keep under a single mark (say, a
soccer player). Otherwise, it's usually better to select your own
point. The diagonal points on the 20D are very close to the "Rule of
Thirds" intersections, so sometimes it's convenient (if you use that
composition rule to place your subject in the frame) to select one of
those points.

7. AF Modes: One Shot: When you set the camera to "One Shot," you set
the condition "The subject is definitely not moving." The camera is
in a "focus priority" mode. The shutter release is locked until the
camera achieves what it thinks is the proper focus. This is best if
your subject and the camera will be motionless, because it allows you
to focus and change the framing without the camera refocusing
automatically.

AI Servo: When you put the camera into AI Servo mode, you have set
the condition "The subject is definitely moving." The camera is in a
"shutter priority" mode. Therefore, the camera goes into a routine
that continually collects data to predict the subject movement and
move the lens to intercept the subject at its new position. You can
shoot even if out of focus (however, the camera cannot release the
shutter if the lens is actually in motion). If you know your subject
will be in constant motion, this is the best mode. If the subject is
actually not moving, the chance of a misfocused shot increases as the
camera goes through its data-collection routine. However, often a
handheld camera does move (as the photographer sways naturally) for
AI Focus to detect and correct for the sway. AI Servo will use
whichever focus point you have activated. However, if you activate
all the focus points, you must put the center point on the subject
and half-press the shutter release for about half a second for the
camera to "acquire" the right subject. After that, while you hold the
shutter release, the camera can intelligently "hand off" the subject
focus from point to point as the subject "wanders" over the
viewscreen.

AI Focus: The camera is normally in One Shot mode and the shutter
will lock until it achieves focus. However, if it detects the subject
moving (that is, the subject goes out of focus), it will
automatically switch into AI Servo mode and try to maintain focus. If
you are focusing on something that frequently stays still but could
move suddenly (like a toddler) this mode comes in handy. The
important point wiht AI Focus is that it does not lock the shutter.
However, the camera will usually interpret "focus and recompose" as
movement of the subject, and will refocus.

 

Adding to your puzzlement may be the fact that some wide aperture zoom
lenses (Nikkor 80-200, 37-70 & others) can tend to backfocus at close
distances at some zoom settings - something to do with spherical
aberration in the old (but otherwise very good) designs - perhaps with
few aspherical elements. Newer lenses such as 70-200 VR don't seem to
have the problem. I'm not sure, but suspect some classic designs (ie 85
f1.4) where the SA is significant enough to "add character" to the lens,
may be similarly affected.
It's noted in Nikon dslr user manuals - not that they backfocus, but
that the electronic rangefinder (focus confirmation light) should not be
relied on when manual focusing.
I have the 80-200. It's actually seldom a practical issue, as the
backfocus occurs at close range (<3m or so) and near or at 200mm. It's
at the range where the razor thin DOF at large aperture means that the
area enclosed by the focus brackets is too large to precisely gauge what
the real focus spot is on any subject other than something completely
parallel to the sensor plane.
I await getting a D300 to see if the lens focuses accurately under those
conditions when in liveview mode.

 

Ok, what I mean is that when I use the chart at
http://focustestchart.com/focus21.pdf with a camera on a tripod set at
a 45 degree down angle and a height about 5 feet with the target chart
on the floor about 5 feet in front of the tripod, and set the lens to
200mm, focus on the "focus here" bar in the middle of the page, the
text that should be in perfect focus is not as sharp as the 10mm line
above it. The 10mm line below the center (closer to the lens) is
blurry, about as blurry as the 40mm line above the center (further
from the lens)

When I replace the 18-200 VR with the Sigma 70-300 and keep everything
else the same (camera and target still in the same position, set the
lens to 200mm so that about the same view is in the viewfinder) and
focus on the "focus here" bar in the middle, the text that should be
in perfect focus is in focus. And the depth of field is centered.
I.e., the 10mm line above the center is about as blurry as the 10mm
line below the center.

I'll post some pictures tomorrow.

Steve

 

I agree. But obviously the lens has something to do with it since
different lenses focus differently.

Steve

 

Forget the 45 degree angle test chart method.
For all you know, you're testing positioning of focus sensor in the
focus brackets, which probably isn't center, rather than real back or
front focus.
Much better to use a series of stepped targets (like matchboxes), in
line with the sensor plane.

 

[A complimentary Cc of this posting was sent to
John Navas

Do not see how this could explain the RADICAL difference between
luminiscent and sun light (in a wide range of brightness)...

Yours,
Ilya

 

[A complimentary Cc of this posting was sent to
John Navas

Can somebody comment on this? As far as I can read, it is a total
confusion. Is there a "second look" or not? If yes, would it use the
same algorithm, or a different one?

Although not Canon, let me calculate how it would behave in my
situation (APS, lens in 100mm setting, f/4.5, distance 1m). The
allowed size of CoC is 0.02mm/3; with 1/4.5 lens this allows the error
of a focal plane position of 0.03mm. The magnification in
"perpendicular" direction is (100mm/1m)^2 = 1/100; so the allowed
misplacement of the focus position in the object space is 3mm.

Thanks,
Ilya

 

[A complimentary Cc of this posting was sent to
frederick

These conditions (2m distance, 200mm lens) match exactly what I said
in another posting of this thread: with APS sensor and dimmer than
1/2.8 lens, Canon's should have at most 3mm error in the position of
focusing plane.

To avoid this, I repeat the shots with the camera upside down. Do not
get any difference...

Hope this helps,
Ilya

 

John Navas wrote:
[]

Well, not quite. The camera measures the error in focus, and tells the
lens by how much it should change focus. The lens then attempts to
(closed loop) do what the camera has asked. Both compoments have a part
to play.

David

 

To some degree, but the lens is actually very dumb*, and is usually an
open loop (not a closed loop) system, which is how focus errors can
occur, much of which could be eliminated (or at least reported to the
user) if the camera checked the focus after the lens moved.

* The lens reports its parameters to the camera, and from these the
camera calculates what the lens should do. If the actual lens varies
from those parameters, then focus errors can result.

 

Yes. However, I believe that a DSLR camera does this checking in
continuous rather than single-shot focus mode, the system then becoming
closed-loop.

Cheers,
David