active or passive autofocus? what's better?
Bernhard Baeumle
Hi,
what is the difference between *active* and *passive* autofocus ????
I think of buying a compact camera (NON-SLR, with 35mm film!) and for that
I'd like to know the difference between *active* and *passive* AF?
- Am I correct that *active* AF uses some kind of infrared light?
- Does it work in complete dark?
- What is better from the technical point of view? (accuracy?)
- What is better for the ease of use??????????
- What about the speed of the AF? ...
The SPEED of AF is another question:
I heard that for some compact cameras it takes about 1/4 up to 1/2 seconds
from the time when you press the button until it really takes the picture!
It seems that the camera needs some time to measure the distance and then
the focusing also takes a while...
==> IS THAT TRUE???
My SLR-camera does the focusing every time I push the button half-way in.
Why do some compact cameras wait until the button is pressed completely????
PS:
I have a 9 year old Canon EOS SLR-camera with (passive!?) autofocus.
In dark settings I have problems with focusing : sometimes it takes me
several seconds to find a spot with at least some contrast (eye, nose,
lips, ear) so that the camera can focus... Now I want a small, lightweight
pocket zoom camera that has no problems in the dark and (for snapshots)
I cannot use a camera which 'waits' 1/2 seconds before taking the picture!
THANKS IN ADVANCE for ANY infos!
Beni.
+-----------------------------------------+--------------------------------+
| Bernhard Baeumle / |
| Swiss Federal Institute of Technology / Phone: (+41) 1 632 51 53 |
| Electronics Laboratory, ETZ H60.1 / FAX: (+41) 1 632 12 10 |
| Gloriastrasse 35 / Email: bae...@ife.ee.ethz.ch |
| CH - 8092 Zuerich (SWITZERLAND) / WWW: http://www.ife.ee.ethz.ch/~bb |
+-----------------------------------+--------------------------------------+
Steve
I believe active focus is the kind like SLRs have, where the camera reads a
portion of the image from the lens and adjusts the focus until the vertical
or horizontal edges are sharp. Passive AF is like Point and shoots. When
you take the picture, it reads the distance with a seperate sensor, adjusts
the focus, and takes the picture. Point of shoot cameras have much wider
angle lenses that are not very fast (usually around f11 or so), so depth of
field is very deep.
I think that your SLR used active focus. I do not know of any which
focuses in the other way. All AF is done through the lens...
No camera is going to do very well in the dark. Some cameras have an AF
assist light, which illumitates the subject with a pattern for AF to focus
on. I have used my Elan IIe in complete darkeness with a 380EX with pretty
good results. Of fourse, focus is not instantaneous, and it takes some
getting used to. Also, the AF light will let your subject notice you (bad
for candids)
You cannot have everything. Photography is a tradeoff. If you don't want
to wait forever for AF, then you have to get an SLR. With P&S cameras, you
don't even know if the picture really is in focus until you get it back.
With an SLR, you can see all the time.
A great deal of AF speed is due to the lenses. With Canon, you can get
ultrasonic lenses which focus very quietly and quiclky. Ultrasonic does
not refer to how the camera figures distance, but rather the kind of focus
motor which the camera uses.
Canon are the only cameras with affordable consumer grade lenses with
ultrasonic motors BTW.
P.S., which EOS did you have, and which lens?
--
:- ) Steve
Bernhard Baeumle <b...@ife.ee.ethz.ch> wrote in article
<5d7c8b$k...@elna.ethz.ch>...
Godfrey DiGiorgi
"Steve" <swe...@ix.netcom.com> responded to this question
with rather a lot of incorrect information. There are several
types of passive and active autofocus, Norman Goldberg's excellent
book "Camera Technology - The Dark Side of the Lens" details several
of them with their plus and minus attributes. I wont try to
reconstruct them all from memory, but anyone desperate can email me
and I'll try to synopsize the material.
Most small point and shoots use a type of active autofocus, which uses
an IR transmitter and a pair of lenses to measure distance by comparing
the luminance of the return across a baseline like a rangefinder. Most
SLRs use a passive autofocus system which has some lenses and looks for
contrast differentials.
Some of the better PnS cameras (notably the Contax T2 and TVS) use
a hybrid passive/active autofocus system with IR assist under low light
circumstances.
The way most of the PnS cameras work in terms of driving the focus is
to reset the lens to infinity focus and drive it back to the focus point
for each exposure. Obviously, there's both time and battery consumption
to pay for this, but the simplicity of the system reduces the cost. SLRs
generally leave the focus motors where they were sitting after the
exposure, so they have much less focus operation at the time of exposure.
One of the reasons for why the SLRs do this is that otherwise, your
viewfinder image would defocus after each exposure instantly, where the
PnS cameras with a seperate viewfinder always give you a clear, well focused
viewfinder image so they can use the less expensive design with impunity.
The Contax TVS allows the user to set a default option which will drive
the focus either at the half press of the shutter button, which reduces
the lag time considerably, or at the normal full press point, which saves
battery life.
A full discussion of camera mechanical lag, etc, is included in Goldberg's
book. It's quite an illuminating work and worth the purchase if your have
any interest in camera engineering.
If you want a very high quality compact 35mm camera with an excellent
lens and all the autofocus/exposure automation, I'd really recommend the
Contax T2 and TVS are worth a serious look. If you would prefer something
a little simpler without all the bells and whistles (and the high price),
look seriously at the Leica Minizoom or Rollei Prego series. The lenses on
all of these are top notch and produce professional quality images. I have
a TVS and a Minizoom; my negatives are invariably perfectly exposed and
are as sharp as my Nikon SLR and Leica M negatives. I've taken to carrying
the TVS instead of either Nikon SLR or Leica M as it's zoom lens
(28-56mm) is exactly what I'd otherwise be carrying in a series of
primes for either of the above, it weighs less, has everything I need in
manual adjustability for serious photography, and the only thing I give
up is some lens speed.
Godfrey
Aleksandr Noy
Steve:
It is actually the other way around. Your SLR uses passive autofocus,
i.e. it looks for the sharp edges in the image it gets from the lens. P/S
cameras use active infra-red autofocus which uses a bounced IR-light beam.
That's why P/S sometimes blink a barely noticeable red flash when you
press the shutter. In general passive focus is more reliable because it
wouldn't be fooled by the glass window in front of the camera of some
other transparent object (which is a classic case where active
autofocus fails). From the other hand, passive AF will perform very poorly
in low light situation and that's why your Elan has an AF-assist light.
Fortunately enough you have a luxury of looking through the lens and seeng
when you are not in focus (or hear the lens "hunting"). You can't do it in
P/S.
I believe that the new Canon APS (the Elph) has a dual AF system-
passive at normal illumination and a back-up active system that kicks in
in low light. It seems to be a very elegant solution and the way to go
for the future. Just my 0.02.
Alex
P.S. I am not Alex Iatskovski and I am not related to him whatsoever.
I also never used B&H, so I have no opinion on that subject. Finally I
own a Pentax AF camera, so I have no interest in Canon-Nikon war. So,
don't flame me...
Steve (swe...@ix.netcom.com) wrote:
: I believe active focus is the kind like SLRs have, where the camera reads a
: >
--
---------------------------------------------------------
Aleksandr Noy Office: Conant 015
Chemistry Department Phone: (617)-496-3918
Harvard University
Cambridge, MA 02138
Jim Williams
>what is the difference between *active* and *passive* autofocus ????
Sounds as if you've got a pretty good grip on the basic concepts. Active
autofocus actively emits a focusing beam (IR or, with Polaroid's instant
cameras, ultrasonic.) Passive AF examines details of the image to determine
focus.
>
>I think of buying a compact camera (NON-SLR, with 35mm film!) and for that
>I'd like to know the difference between *active* and *passive* AF?
>
>- Am I correct that *active* AF uses some kind of infrared light?
>- Does it work in complete dark?
Polariod's got a patent on the ultrasonic system, so all others use an IR
emitter and detector. It works like a rangefinder camera -- it measures the
included angle of the reflected infrared beam to calculate the focusing
distance.
Passive AF cameras (mostly SLRs, as this system is more costly) work like
the split-image rangefinder screen you've probably seen in SLR cameras.
They use two rows of CCD arrays to compare the two halves of the image, and
a CPU chip to determine whether the waveforms from the two CCDs are "in
phase" or "out of phase." If out of phase, the CPU determined the direction
and amount of focus adjustment needed.
>- What is better from the technical point of view? (accuracy?)
Both can be equally accurate at short distances. Active AF starts to fall
down at longer distances because the IR emitter is limited in power -- it
can't illuminate subjects much beyond 3 or 4 meters. With a wide or slow
lens on the camera (as with most point-and-shoots) this isn't a problem
because at longer distances depth of field will be enough to give
acceptable results. SLRs need passive AF because they may be used with
longer lenses that require more focusing accuracy at long distances.
>- What is better for the ease of use??????????
Active AF is probably easier to use within its limits. Usually, it either
works or it doesn't. Passive AF sometimes operates more slowly, or "hunts,"
or needs to be aimed at a more "focusable" area of the subject to work.
It's more versatile, but takes more practice to use.
The closest thing to the ideal system is to use both active and passive,
but so far the only camera that does that is the Contax G2 (yes, I have
one, so I'm not perfectly unbiased : > )
>- What about the speed of the AF? ...
Again, active AF is pretty fast if it's going to work at all. Passive AF
can be even faster under good conditions; slows down under adverse
conditions, but works over a wider range than active AF.
>
>The SPEED of AF is another question:
>I heard that for some compact cameras it takes about 1/4 up to 1/2 seconds
>from the time when you press the button until it really takes the picture!
>It seems that the camera needs some time to measure the distance and then
>the focusing also takes a while...
>==> IS THAT TRUE???
>My SLR-camera does the focusing every time I push the button half-way in.
>Why do some compact cameras wait until the button is pressed
completely????
I believe that's mostly done to save battery power. A tyro could use up a
lot of power looking through the camera... motoring the lens into the
proper focus... then deciding not to take a picture after all. The way they
do it, the battery power doesn't get "spent" until the user has decided for
sure to take a picture.
A few newer compact AF cameras have a much shorter delay.
>
>PS:
>I have a 9 year old Canon EOS SLR-camera with (passive!?) autofocus.
>In dark settings I have problems with focusing : sometimes it takes me
>several seconds to find a spot with at least some contrast (eye, nose,
>lips, ear) so that the camera can focus... Now I want a small, lightweight
>pocket zoom camera that has no problems in the dark and (for snapshots)
>I cannot use a camera which 'waits' 1/2 seconds before taking the picture!
You'll have to pick out some compact cameras that have the features you
want, then try them (or read test reports) to find out which have the least
delay. It varies a lot from camera to camera.
Steve
Well thank you for correcting me. Of course, I said that it was what I
"believed" and should not be taken for fact.
BTW, Your english is too good for you to have been Alex! <g>
--
:- ) Steve
Aleksandr Noy <n...@login1.fas.harvard.edu> wrote in article
<5d7ui5$t25$1...@news.fas.harvard.edu>...
Captain Ron (Chris K.)
I believe some P&S cameras have an "in-focus" indicator.
Take Care,
Captain Ron (Chris)
Chris Schultz
Your comment applies to autofocus cameras in general, and
not to P&S in particular.
---- Chris
John Stewart
The first AF module (from the 70's) was invented by Honeywell in the USA.
This "Visitronic" module was used in EVERY AF camera at that time. It used a
contrast comparator (?) to determine best focus. Because it did not emit any
light, it was passive. Bad points: not good in low light. Good points,
could work through glass.
Today there are still more advanced active and passive systems. Be aware
that many PS AF'ers do not focus AF continuously like most SLRs. They fall
into "steps"...as few as TWO (near/far) and as many as 180+. A few PS have
continuous AF, but they are expensive.
John Stewart
http://www.acpress.com >>>free newsletter every Friday!
In article <5dgl0h$r...@boursy.news.erols.com>, cjsc...@erols.com says...