Doug Kerr
Well-known member
I have recently conducted a multi-faceted "contemplation" of the autofocus system used in Canon EOS dSLR cameras. A recurring thread was to answer certain questions that recur in connection with this system (I will highlight questions as I go).
The project included:
• Extensive testing with various bodies and lenses here.
• Contemplation of hints found in a Canon patent (not actually dealing with the matter of autofocus, but describing an AF process as part of a context for discussing the actual topic of the patent, an improved interface between the lens and the body).
• Parsing of information found in the Canon repair manual for some early EF-series lenses.
• Some comments made by Chuck Westfall during dialog on part of this topic on the ProPhoto Home forum.
I was assisted in my investigation by the stimuli and insights afforded by my colleague, "WilbaAtProPhoto", on the ProPhoto Home forum.
Not every aspect of the topic will be covered in this series. I will in particular not speak here of the matter of selection of AF points or issues of AF point sensitivity.
I will also not speak for now of the matter of "focus search", which refers to the fact that, when AF is called for, if the system is not able to make a satisfying "defocus" determination (the camera is so far out-of-focus that there is no workable image on the AF detector), the lens focusing mechanism is moved until a satisfying determination can be made.
Note that in any case, very little of what I describe here has been unequivocally confirmed by Canon. It is much the result of reverse engineering and forensic engineering.
The reader who knows otherwise is urged to come forth.
************
The big picture
I describe the overall basic AF scheme as "closed-loop overall, with open-loop movements".
Closed loop overall means that, overall, the AF system does what it needs to until proper focus (within a certain tolerance) is indicated by AF detector measurement.
Open loop movements refers to the fact that when the system puts the lens focusing mechanism into motion, it has already determined how much movement from the current position should lead to the attainment of proper focus.
Let's see that in action.
We will of course assume AF mode, in particular one-shot AF.
In this scenario, I will leave out an important aspect, to allow best clarity of the basic scheme. I will introduce it presently.
1. We full press and hold the shutter release.
2. The body makes a numerical defocus determination by observing the deviation from perfect alignment of the image on the two autofocus "subdetectors" of the AF detector of interest (visualize a split-prism focusing aid on the focusing screen). An adjustment is made based on information from the lens (we will discuss that in a later part of this series - remember, this is the "big picture").
3. If the result is "it's perfect" within the established tolerance (which is, incidentally, fed from the lens), then focus confirmation is declared, and the shutter is allowed to fire. This is the consummation of the "closed loop overall" nature of the process.
4. If not, then, via a calculation involving the measured misalignment, certain factors of the AF system geometry, and certain information provided by the lens (more details on this latter) the camera determines how much movement (in "ticks" - a unit specific to the lens), in which direction, should bring the camera into perfect focus for the object of interest.
5. The body tells the lens to move its focusing mechanism that many ticks in that direction. (This is one "open loop movement".)
6. The lens does that.
7. The lens reports when it has done that.
8. We loop to step 2.
Thus we may make zero, one, or several open loop movements, until we have "success" in step 3. Why several? Because at the completion of the first one, the check of defocus shows it not yet within tolerance. Why might that happen? More on that later.
What does it typically look like if two or more open loop movements are needed? If we watch the focus indicator scale, we will see it move to nearly the final position, pause a very tiny time, then move some more, and perhaps pause a very tiny time, and move again, eventually "closing in" on the final point. Then (assuming we are still in full press) the shutter trips.
"WilbaAtProPhoto " calls this "the twitch".
An added wrinkle
This aspect was not mentioned above for simplicity's sake. Its is wholly a conjecture on my part.
When the lens is on the way under its first open-loop movement, part way along (I have no idea how the body decides when) the body evidently takes another defocus measurement and updates its view of where the focus mechanism should end up in terms of ticks from its current position (essentially redoing steps 2-5, "on-the-fly").
Suppose that when the body tries this, it finds that there is not enough luminance of the object to make such an "on the fly" defocus determination (even though there was plenty of luminance for it to do it "leisurely" in the original step 1). Then it has the lens stop its motion, makes a "leisurely" defocus measurement, and then updates steps 3-5, sending the lens on its way to its (possibly changed) destination. It is essentially a "mid-course correction" feature.
Note that if the object is moving, this will give us an update for that (even though we are not in AI servo mode).
What if no defocus determination can be made, even on the "leisurely basis" (perhaps the lights have just gone out)? The process stalls - we cannot take the shot.
************
Well, this may all be quite startling to some. I'll let you digest this.
In the next part of this series, we will examine what data is stored in the lens, what happens if it is not "appropriate", and the matter of "lens calibration".
[To be continued.]
Best regards,
Doug
The project included:
• Extensive testing with various bodies and lenses here.
• Contemplation of hints found in a Canon patent (not actually dealing with the matter of autofocus, but describing an AF process as part of a context for discussing the actual topic of the patent, an improved interface between the lens and the body).
• Parsing of information found in the Canon repair manual for some early EF-series lenses.
• Some comments made by Chuck Westfall during dialog on part of this topic on the ProPhoto Home forum.
I was assisted in my investigation by the stimuli and insights afforded by my colleague, "WilbaAtProPhoto", on the ProPhoto Home forum.
Not every aspect of the topic will be covered in this series. I will in particular not speak here of the matter of selection of AF points or issues of AF point sensitivity.
I will also not speak for now of the matter of "focus search", which refers to the fact that, when AF is called for, if the system is not able to make a satisfying "defocus" determination (the camera is so far out-of-focus that there is no workable image on the AF detector), the lens focusing mechanism is moved until a satisfying determination can be made.
Note that in any case, very little of what I describe here has been unequivocally confirmed by Canon. It is much the result of reverse engineering and forensic engineering.
The reader who knows otherwise is urged to come forth.
************
The big picture
I describe the overall basic AF scheme as "closed-loop overall, with open-loop movements".
Closed loop overall means that, overall, the AF system does what it needs to until proper focus (within a certain tolerance) is indicated by AF detector measurement.
Open loop movements refers to the fact that when the system puts the lens focusing mechanism into motion, it has already determined how much movement from the current position should lead to the attainment of proper focus.
Let's see that in action.
We will of course assume AF mode, in particular one-shot AF.
In this scenario, I will leave out an important aspect, to allow best clarity of the basic scheme. I will introduce it presently.
1. We full press and hold the shutter release.
2. The body makes a numerical defocus determination by observing the deviation from perfect alignment of the image on the two autofocus "subdetectors" of the AF detector of interest (visualize a split-prism focusing aid on the focusing screen). An adjustment is made based on information from the lens (we will discuss that in a later part of this series - remember, this is the "big picture").
3. If the result is "it's perfect" within the established tolerance (which is, incidentally, fed from the lens), then focus confirmation is declared, and the shutter is allowed to fire. This is the consummation of the "closed loop overall" nature of the process.
4. If not, then, via a calculation involving the measured misalignment, certain factors of the AF system geometry, and certain information provided by the lens (more details on this latter) the camera determines how much movement (in "ticks" - a unit specific to the lens), in which direction, should bring the camera into perfect focus for the object of interest.
5. The body tells the lens to move its focusing mechanism that many ticks in that direction. (This is one "open loop movement".)
6. The lens does that.
7. The lens reports when it has done that.
8. We loop to step 2.
Thus we may make zero, one, or several open loop movements, until we have "success" in step 3. Why several? Because at the completion of the first one, the check of defocus shows it not yet within tolerance. Why might that happen? More on that later.
What does it typically look like if two or more open loop movements are needed? If we watch the focus indicator scale, we will see it move to nearly the final position, pause a very tiny time, then move some more, and perhaps pause a very tiny time, and move again, eventually "closing in" on the final point. Then (assuming we are still in full press) the shutter trips.
"WilbaAtProPhoto " calls this "the twitch".
An added wrinkle
This aspect was not mentioned above for simplicity's sake. Its is wholly a conjecture on my part.
When the lens is on the way under its first open-loop movement, part way along (I have no idea how the body decides when) the body evidently takes another defocus measurement and updates its view of where the focus mechanism should end up in terms of ticks from its current position (essentially redoing steps 2-5, "on-the-fly").
Suppose that when the body tries this, it finds that there is not enough luminance of the object to make such an "on the fly" defocus determination (even though there was plenty of luminance for it to do it "leisurely" in the original step 1). Then it has the lens stop its motion, makes a "leisurely" defocus measurement, and then updates steps 3-5, sending the lens on its way to its (possibly changed) destination. It is essentially a "mid-course correction" feature.
Note that if the object is moving, this will give us an update for that (even though we are not in AI servo mode).
What if no defocus determination can be made, even on the "leisurely basis" (perhaps the lights have just gone out)? The process stalls - we cannot take the shot.
************
Well, this may all be quite startling to some. I'll let you digest this.
In the next part of this series, we will examine what data is stored in the lens, what happens if it is not "appropriate", and the matter of "lens calibration".
[To be continued.]
Best regards,
Doug
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