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View Full Version : Deffinitive Guide to A200/A2 Lowlight (AF) Autofocusing Problem and Solnt Comments??



eqqmc2
03-19-2005, 06:09 AM
I posted this message at dpreview and at networks54.Figure it could help here too:

After seen so many threads on this subject on a variety of prosumer digital cameras forums 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 (anyone knows of any technical reason??). There are a few patents from Minolta detailling their designs. Contrast Detection method for the continous AF mode is documented in patent No 6636263

http://www.freepatentsonline.com/image-6636263-1.html or
http://www.freepatentsonline.com/patents/us/663/6636263/6636263.pdf

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)
enddo
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. Any comments here ???

As you can see, this type of AF WILL NOT FOCUS 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.

Solution for LOW LIGHTING: I have tried a laser pointer which has different patterns (red light vertical contrast) and gotten excellent results in focusing the A200 in TOTAL DARKNESS (using my 5600HD flash or built in flash). I will post the details if I see a few requests).



As for the comments on IF AF Assist: To make this systems work you must have an IR sensor that measures the time that it takes for the IR beam to travel to the subject and back to the camera. Knowing the time and speed of the IR light the object distance is computed. This mechanism is also documented in several Minolta patents. The A200 does already have an IR sensor, however, this doesnt mean thats is located in the proper area for the computations of the time. So even if minolta would put out a firmware upgrade to use the AF assist light of the 3600 or 5600 there could be some other problems such as deciding how to optimize the IR focused image with the vertical contrast algorithm already used in the camera and the additional logic required to make it work with the flash. Here is note to those "bright" Minolta business decision makers: if you had built the a200 or a2 so that it could have used the IR assist mechanism in the 3600 or 5600 (but only thru these flashes), you would have force us to purchase those flash units, creating a great product synergy and increasing the revenue needed at such a bad economic position as KM is at right now.

Last note, if there are any minolta engineers reading this forum, as they should be, please bring out some coments and some more details. I am not too impressed with your engineering judgement (of the AF system only, everything else is AWSOME) of this camera design and if i am mistaken please point it out. Also why not implement horizontal contrast design?? Why remove the wireless flash capability that existed in the A2.

It is a shame, cause this camera and the A2 could have been the best prosumer cameras in the market.

Hope this helps!!

gregtelma
04-12-2005, 06:08 PM
It was interesting reading, I know that Minolta made a big mistake in their choice for auto focusing mechanisms for the A-2. Probably the best prosumer camera made that can't focus consistantly. I hear the A-200 is better. As for the A-2, I recomended shooting in the manual focusing mode as often as possible! Greg :eek:

Geoff Chandler
04-13-2005, 11:46 PM
I haven't quite read all the above literature - but will do when I get home!! I reckon the A-200 must be better focusing than the A2. Mine rarely gets caught out - and the manual focus is easy peasy so it's not a problem anyway. Night shots recently taken were no problem at all - the only time it does have a problem is in extreem lack of contrast - ie sky shots - I took the moon during the day in the sky and it struggled a bit - also if you are right on the edge between needing macro and not needing macro - then it may struggle and you have to decide to move out a little or switch to macro or whatever..
Anyway - where it was stated AF will not focus in low light - well how low? I have taken some night shots so it would have to be virtually no light I reckon as mine had no problems

eqqmc2
04-18-2005, 06:01 PM
Thanks for the feedback. Here is some more info I have found plus some tips

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

http://www.minoltaphotoworld.com/cms/456+M52087573ab0.html


"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."

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 camerabright.com 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 laserpointers.co.uk 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

Geoff Chandler
04-19-2005, 05:10 AM
This is all quite interesting stuff - forgive me on the maths front I am a tiny bit dyslexic - so I will leave that side to you!
My non- technical findings (but I am a technically biased person!) have found it to work very well on focusing with restrictions as suggested in extreem lack of contrast situations. Only once did I seem to find it unexpectidly not getting the best focus - for no apparent reason. It was a bronze model at Dysney land - but I just selected manual focus and turned the ring until it was sharp then took the pic. This has happened on 2 other occassions - both of those had good excuses. Once when taking the moon in the middle of the day, (no contrast) so I used manual focus, and once when I was right on the borderline for macro, I had selected macro but was just outside the macro focus range - which I discovered as soon as I selected manual focus - so I reselected auto focus and moved in a touch and the problem was resolved. So in around 3-4000 photos the focus has only once genuinly failed me - and then it was only slightly off. ( Is there really a problem here??)
Whether the focusing system is better or worse generally than others is not an issue for me - the A-200 focuses quicker than my Oly C-740 99/100 times and I am happy with the Oly mostly so I don't have a problem with the A-200
Most people accuse it of being 'soft' this is because the contrast and sharpness and Saturation are not boosted up like others - my Oly has had to have them reduced to achieve natural looking photos! Additionally - with a little bit of time and experimentation almost any look can be achieved by adjusting the settings - I guess it would be fair to say that if you want a point and shoot on steroids then the A-200 is not for you But if you like high quality - f'ilm like' results that look vey natural and you are also willing to experiment then the A-200 is great.
I'll stop now - I'm begining to sound like a preacher!!!

eqqmc2
04-19-2005, 06:33 PM
Geoff,

It sounds from your comments that your a200 seem to works exceptionally well. Unfortunately it seems that such performance is highly dependent upon the experience of the photographer with these type of cameras. There is a learning curve specially for newbies when it comes to getting the bang for the buck for this camera. I had the expectation that the a200 would have worked just as you described for all kinds of users, but from the threads on dpreview, steves-digicam, network54 and others this was just a dumb founded dream. There is absolutely no question that when issues like this are mastered, the a200 produces outstanding results and with KM quality control issues with the A2 it is pausible that we could get some units that may not have been tweaked to get the best performance. I am glad that at least folks coming to this forum can get both a technical dissertation backed by good solid references as well as the experience of pros like yourself. The former is hard to get from manufacturers like KM. When all is said and done I am very happy with the camera but I thiink i will going the dslr route in the not so long future.

gary_hendricks
04-19-2005, 06:50 PM
Hi eqqmc2, thanks for the free patents link. I didn't know there's such a website with so much information!

eqqmc2
04-20-2005, 12:35 PM
You are welcomed!! I am just trying to shed more light on the subject!

Rob vdKam
08-24-2005, 05:11 AM
Just wanted to add my two cents...
I found this info very interesting and thought provoking. My own experience is more like Geoff's (except that I've only taken a few hundred pics in the last month). In almost all cases, the focus works find, and when it doesn't, I can always find a very clear reason. For example, yesterday I tried to take a picture of a nice ripe tomoato in my garden, but it couldn't find a focus with the spot (little box) method. The tomato was just too smooth and lacking in contrast. So I focussed on a nearby leaf, locked focus, snapped, and moved on. (In some cases, I would first lock exposure before finding a nearby object to lock focus on.) I have not tried to switch to wide focus area because it tends to select a small area to focus on that may not be the object I'm centred on. In general, I'm very happy with this camera although it has taken some learning. The Canon S2 and Sony H1 I tried had their own peccadilloes.

reyval2k
08-30-2005, 08:21 PM
Hi there, found this webpage on focusing with the A1/A2:

http://home.tiscali.be/johan.branders/A1/A1-focus.htm

My question to the A200 owners, does this apply to the A200 also? Thanks.

Rob vdKam
08-31-2005, 06:59 AM
That looks like a summary of the focus capabilities from the A1/A2 manual. The A200 manual info is almost identical (after a quick look). The A200 also has a mode called "spot AF areas" which is like a flex point but larger. But I don't see anything there on focus difficulties or workarounds.

nr1
10-11-2005, 11:06 AM
Hi,
I am a patent analyst working on project devoted to shutter lag problem in digital cameras. I would be very grateful if you can answer my questions.
Shutter lag is mainly caused by autofocus. I've figured out so far that there are two main autofocus approaches: contrast based and phase based. Is it correct that the phase based is the fastest? Why phase based autofocus is implemented in SLR cameras, why I can't find them in cameras with aside viewer? Is IR range finder absolutely necessary for phase-based autofocus approach?

Thanks
NR1