Contrast Detection Autofocus
Flynn Marr 3 August 2024
The AL-1 was Canon’s first foray into a 35mm interchangeable single lens reflex camera. The FD lens mount was not capable of fully automatic focus .
I have been gathering information to update my post on Canon’s AL-1 camera. It is an interesting camera in that it has what Canon call’s its Focus Assist System or Quick Focus feature. Although not a fully automatic focus camera it used Contrast Detection to tell the user which way to turn the lens barrel for correct focus and to indicate when the lens was focused. And that got me wondering how Contrast Detection in the AL-1 works.
This post is not a technical analysis and I am certainly not equipped to discuss the math or the electrical wizardry involved but in simple terms what follows is what I have learned.
This method of focusing is based on the fact that the sharper the focus on a scene the greater the overall contrast. That means that the measure of light intensity between the highlights and the shadows is maximised with sharp focus and reduced more and more as the lens moves away from focus. How can we turn this into a focusing system?
The Dealer Notes Canon supplied to their dealers explains how the system in the AL-1 works but it can be a little hard to understand.
The AL-1 main mirror has a series of lines on the mirror that are partially silvered allowing about 50% of incident light to pass through the mirror to be deflected downward by a secondary mirror into the bottom of the mirrorbox. Why the network of lines looks as it does I have not figured out yet, however, a portion of the light from the center of the viewfinder is deflected downwards. In the bottom of the mirrorbox is what Canon calls a Triple Beam Splitter where the deflected light is divided into three equal
The Dealer Notes for the AL-1 talks about the Quick Focus system and how it works.
A CCD, initials for a Charge Coupled Device, is a solid state device that accumulates electrical charge in proportion to the light falling upon it. The AL-1 has three rows of 112 cells each to sample across a narrow strip of the center of the image.
Each CCD array is seeing 1/3rd of the same light sample. The three samples are obtained from what Canon calls its Three Beam Splitter. This is a glass plate that has been sliced horizontally into four pieces at a 45 degree angle, the sloped surfaces silvered, partially or totally, and then the pieces glued back together.
The light reflected down on the top of the Beam Splitter hits the first mirror where about 33% passes through it to the first CCD Linear Array. 66% of the light is reflected horizontally to the second mirror which reflects 50% of the remaining 66% downwards to the second array, and the mirror passes 50% of the incident light, which is now 33% of the original light. That final amount of light hits the last mirror which is totally reflecting and the last 33% of the original light is reflected down onto the third Linear Array.
So that is the hardware. We have a slice of light from the center of the image, it is divided in three and it illuminates three identical Linear CCD Arrays. So how can we focus a lens with this hardware?
For the moment, lets consider a single Linear CCD Array and two points of light. In the diagram on the right we can see these points in focus at the bottom and increasingly out of focus as we go up. Underneath the points we see the charge level on the CCD Array if it was across these points. Looking at this diagram we have to remember that the energy in a point of light is the same whether it is in focus or not. If it is in focus all of that energy is concentrated in one spot and the electric charge on the CCD’s will be high but if it is out of focus the size of the illuminated circle is larger which means less charge spread over more CCD elements. That is the basis of Contrast Detection Autofocus.
You can see in this illustration that if we plot the electrical charge built up on the CCD’s across the two points that the closer to accurate focus we are the higher the peaks and the lower the valleys on the plot. At we travel further from actual focus the shallower the peaks and valleys become.
Because the collected electrical charge also varies with the brightness of the scene it will be more accurate if we measure the difference between the maximum signal and the minimum signal. This is represented by the letter “A” in our diagram and you can follow the value of “A” from good focus to very out of focus and you can see it decreasing. Accurate focus is obtained when “A” is at its maximum, in other words, at maximum contrast.
So why the three CCD Arrays? Well, a single Array can be used and it will indicate when accurate focus is obtained. But, if you are out of focus there is no information to tell you which way you have to move to move towards accurate focus. That is where the three come in.
CCD Charge Variation with Focus
Ideally the CCD Array should be at the same distance from the lens as is the film plane. In this way if the image is in focus on the ground glass it will be in focus on the film and it will be in focus on the CCD Array. This is where the center array is placed. However, the other two are placed closer to and further from the lens which means they will be out of focus when the center one is in focus.
So, think about this: when the lens is in focus the center Array is showing the maximum “A” and the other two, which are out of focus, are showing smaller but equal values for “A”. Remember that each Array is looking at exactly the same sample from the image. Now if we move the lens out of focus think about what happens. The two outside CCD Arrays will have different values for “A” because one of them will be closer to actual focus than the other. And knowing which Array has the largest value for “A” will tell you which way you have to move the lens to move towards improved focus. When the lens is correctly focused, the outside Arrays will both have the same value for “A”.
Now, we could get so far from accurate focus that the outside Arrays appear to have the same value which could give a false indication. But the center Array will also show the same value as the other two and if we are in focus that cannot be. So the center CCD Array is a control making sure that a false indication of “in focus” is not given.
Auto Focus and the AL-1
The AL-1, being an “A” Series Canon Camera is controlled by some advanced electronic circuitry which reads the values on the 3 CCD Arrays and performs calculations on these readings. However, because the camera has an FD lens mount, there is no way to actually control focus automatically by turning the lens barrel. So Canon came up with a system in which the camera tells the user which way to rotate the lens barrel for correct focus. This is done by two red arrows in the viewfinder which illuminate depending on which way the lens should be turned. When correct focus is obtained a green signal light comes on.
Canon called this their “Quick Focus System”. One advantage it had was that it would work with any lens that the FD lens mount would accept. Because the system is based on measuring differences in contrast it does not work well with low contrast subjects. It also struggles in low light situations because the amounts of light diverted into the system is small and even smaller when it gets to the CCD Arrays. Low light is just too big a challenge for this method.
And that is how the Contrast Detection Autofocus System works in the Canon AL-1.
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