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-   -   A200, Ax Auto Focus Mechanism Description and Guide (Newbies (

eqqmc2 Apr 19, 2005 2:04 PM

I am posting this thread at the request of some folks in dpreview where I posted yesterday.

I have decided to post this thread on the KM focus mechanism due to the constant level of threads dealing with the issue.I did some research on the patented autofocus mechanisms that Minolta has put out. As many of you know for passive autofocusing (ie no AF assist IF light) the two most common mechanism/algorithms are the phase detect method patented by Honeywell, in 1980 (Minolta was sued and had to pay a lot of money for illegally using this patent) and contrast detection method. The phase detection method uses at least two AF detectors and assumes that a focused image is when the light that passes through the two separate paths are in phase. This is a more expensive and involved mechanism and Minolta use it primarily on their SLR cameras.

According to the Technical Section of the konica minolta photo world website: (you may have to register to get in)

"Phase detection autofocus is mainly used in SLR cameras. A part of the picture is mirrored out. The picture is projected by two lenses onto a special CCD consisting of many small elements.

The autofocus-software now detects contrasts in the picture and adjusts the sharpness using the lens's focusing until the picture has the maximum sharpness. The focusing is more accurate and faster, the higher the contrast in the subject is – the speed of the autofocus is dependent on the maximum aperture of the lens.

A disadvantage of phase detection autofocus is, that it has to be integrated in the optical system of the camera. Therefore it is for example not used in the DiMAGE A1, since it was designed to include an optical system as compact as possible.

A standard sensor of a phase detection autofocus is only sensitive to vertical contrasts.

Under unfavourable lighting conditions this system needs an AF illuminator.

When using a camera with phase detection autofocus, it is important that the subject has contrast in a vertical direction. If this is not the case, the photographer should try to focus on an object in the same distance from the camera as the main subject."

For the A200 and Ax cameras KM uses what is called Video AF. According to the same website:

"Video autofocus is, like phase detection autofocus, a passive system and is exclusively used in digital cameras. This system works by evaluating the signal of the CCD, where the autofocus-software tries to detect contrasts. The focus is adjusted, until the contrast has reached its maximum value. The time it takes to determine the focus, depends for example on the frequency, with which the CCD is read from. In the DiMAGE 7, it is read 15 times per second, in the modern DiMAGE A1 50 times a second.

The most important factor for the speed of the autofocus is the capacity of the autofocus processor, which is one reason why the speed of the cameras in focusing has, like resolution, improved significantly over the years.

The flexible focus point of the DiMAGE 7 and A series makes it possible for example, to focus on a single spot that can be freely moved around the image. One disadvantage of the video AF is, that this system is not capable of detecting if the sharpness is located in front or behind the subject. In older cameras, it is possible that the focus is "pumping", that means that the camera is continuously focusing back and forth across the point of maximum sharpness, without stopping the focusing-process.

For video AF to work properly, it is necessary that the CCD generate image data during focusing. To improve focusing in a camera with video AF, it is possible to activate only one autofocus sensor (flexible focus point). The camera isn't then wasting time analysing different parts of the image, but only the desired spot."

Video AF is nothing more than "Vertical Contrast detection method". How do I know this?

Now on for the technical description :

There are a few patents from Minolta detailling their designs. Contrast Detection method for the continous AF mode is documented in patent No 6636263 or

This patent and others are from the middle 1980's to 2001.

The algorithm is pretty simple: for the AF area (which is a subset of the total CCD area, if anyone knows whats is this AF area is (No of pixels) for the A200 or A2 that would be very helpful)calculate:

Every pixel (i,j) has intensity X(i,j) where i=column number and j=row number of the AF area

For every pixel location i,j in AF area do
Build the matrix:
Delta(i,j)= X(i+1,j)-X(i,j)
Sum over all pixels in AF area:
Xn = Sum((Delta(i,j),i=1,N,j=1,M)

Xn = what Minolta refers to as the AF Evaluation Value which is nothing more than a Focus Measure

Now, the AF mechanism moves the lens at small steps increments (computing Xn at each step) and determines the position that has the MAXIMUM Xn.Thus, n is the position of the lens at every step. This is the position where it is assumed that the image is focused. The lens movement happens when you depress the shutter halfway. The location of maximum Xn happens when the locked AF signal shows on the EV or LCD.

If you carefully notice, the matrix Delta(i,j) is nothing more than the vertical contrast at each pixel location with respect to the right neighbour pixel. The patent doesnt specify which component (R,G,B) of the light intensity is used. Someone has already offered that it could be the green component but I am not sure.

As you can see, this type of AF WILLhave a hard time FOCUSING IN LOW LIGHT unless you are focusing in an area that has some small light source with enough high relative intensity and will focus correctly in LIGHTED SCENES only if you have vertical contrast in you AF area, ie vertical lines of relative bright and dark fringes. I have tested this algortithm and it is true all the time.

Now here is what i have done to mitigate the problem:

1) For Indoors when I need flash I found out that using the KM wideangle lense really improves the focusing and sharpness of the image. Granted, I use the 5600 HD flash cause the built in flash can overcome the size of the converter lense. I still need to go backto the optics equations to figure out the reason

2) For low light scenes I bouth the camerabright extended range device from that attaches to the tripod hole of the a200. It works reasonable good as long as I focus on objects with vertical contrast

3) For total darkness I use the laserpointer from with vertical patterns. It works remarkably well

4) When in P mode I first lock on shutter speed and exposure by half pressing the shutter, then release and adjust the dial of shutter speed to the fastest setting plus one or two levels more than the camera algorithm predicts. Then i refocus and take the pic with that shutter and exposure settings. The sharpness of the shot improves quite bit.I use mostly S mode given the limitations of the camera algorithm and use the HSS mode for Xfin and RAW modes cause is better at least I get 1 of 3 or 5 shots with all the right attributes

Sorry for the lengthy article but I like to help


stk Apr 19, 2005 5:17 PM

Another approach for shooting in total darkness is to use manual distance readout with a reasonably small aperture (f5-6 or so) and the hyperfocal approach. I tried this in a totally dark room (all lights completely off) with flash and it worked fine.
I'm not sure exactly how it got the exposure but the Flash exposedperfectly too!

When used correctly this gives good focus from about half focus distance to infinity. Considering that the flash is only really effective for about 4m, then this isn't too difficult to judge. Could be a simple method for snaps at parties!


eqqmc2 Apr 20, 2005 2:21 PM

For the hyperfocal distance: how you recompute it everytime, or eyeball it?

Just explain it in a littlemore detail



stk Apr 20, 2005 4:13 PM

1 Attachment(s)
For the tests I did, I just eyeballed. As the flash only covers about 4m anyway the focus is set fairly close.

From memory I used something like F6 and 50mm effective (which I felt was about right for party type shots). I think this gives a hyperfocal distance around 3m, so everything from around 1.5m to infinity should be in focus. The only awkward thing to judge is the readout. At wide angle settings (even 50mm) the increment is fairly coarse.

Strictly Idon't think it's necessary togo quite to the hyperfocal setting as the flash can't cover very far anyway, so I tended to set focus a bit closer.

I think I got the source info from

If it helps, I took this quick picin mygarden F5.6 manual 3m, cropped to about 25%. Front flowers are about 1.5m away, red tuplips around 3m, greenhouse about 8m.


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