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09-18-2008, 11:16 AM   #1
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AF Performance vs Viewfinder size, brightness.

How related are those to.
I understand focusing screen (ground glass vs micro lens etc), pentamirror/prism assembly and eyepiece optics(coatings, aperture) has a lot to do with the VF brightness, but how about the mirror itself.

As i know its half reflective, the rest (looks like the least part) of the light goes trough it and is directed to AF sensors by smaller mirror.

If you change the reflectance/transmittance of the mirror you "steer" light from sensors to VF.. is that true?

Could it be, that the AF performance is the price for better viewfinders, since APS-C camera designs are quite similar... or they use some sort of dielectric mirror to direct visible light...wait..IR (even near) has different "focusing distance".. unless you correct.. (so you could actually have bright VF without reducing light available for AF)

..anyway, as you can see, i dont know for sure, so maybe there is someone who can enlighten me about this topic.

I apologize for my cluttered text.


Last edited by ytterbium; 09-18-2008 at 11:16 AM. Reason: title correction
09-18-2008, 11:25 AM   #2
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I'm pretty sure the mirror is far more than half reflective (towards the viewfinder). Probably there is some tradeoff between brightness and extreme low light AF performance, but probably the af sensor doesn't really need much light to work.

I think making a dielectric mirror with a wide and flat enough reflection region (in wavelength space) would be difficult.
09-18-2008, 07:34 PM   #3
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I believe the mirror is about 90% reflective, and a lot of this is due to the angle the light hits the mirror.

Much of the super bright viewfinder is related to the focusing screen itself, and the K10D is a good example of going too far.

The reflection off axis of the metering sensors acutally cause metering errors that are a function of maximum apature.

this results in unusual metering errors with manual apature lenses. To prove this poiint, I replaced the K10D screen with the *istD screen and it metered identical to the *istD when using the same lens on both cameras.
09-19-2008, 10:36 AM   #4
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So the worse (than competitors) AF performance in low light (the speed, i guess is tradeoff for accuracy), for example, is all about dumb AF algorithms and lower grade/resolution/sensitivity AF sensors.

09-19-2008, 11:40 AM   #5
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QuoteOriginally posted by ytterbium Quote
So the worse (than competitors) AF performance in low light (the speed, i guess is tradeoff for accuracy), for example, is all about dumb AF algorithms and lower grade/resolution/sensitivity AF sensors.
Before you go too far down this road, I believe somewhere I saw a post that analyzed the autofocus, and concluded that there was a trade off between speed and accuracy, where some canikons were reportidly faster to get close, but tended to have residual errors and poorer photos, where pentax reportidly checked the focus again, making the achievement of focus slower but more accurate.

Again, this is what I recall reading, and am not sure how much truth is in it, but to be honest, all cameras hunt in low light, low contrast situations.
09-19-2008, 11:55 AM   #6
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QuoteOriginally posted by ytterbium Quote
So the worse (than competitors) AF performance in low light (the speed, i guess is tradeoff for accuracy), for example, is all about dumb AF algorithms and lower grade/resolution/sensitivity AF sensors.
I have heard a lot about the "worse" AF performance relative to the competition, but generally do not make any comments since I am very happy with the AF (when I use it). I will take this opportunity to relate an experience I had a couple of days ago.

I was taking some photos from Chanticleer Point in the Columbia River Gorge and this fellow carrying a C*non 40D with the kit lens walks up next to me and my tripod, raises his camera, and attempts a shot of the same scene I was doing. His auto focus went wild from lock-to-lock, but could not attain focus. After two or three tries, he took the camera from his eye and slid the AF switch on the lens to M and proceeded to take the picture using manual focus.

Now the light was not bad and the subject was at infinity. The only complicating factor would have been a fair amount of haze, though not so much to affect the focus confirm on my K10D. (I was shooting with a manual focus lens.)

Here is the subject so you can see what I mean:



Steve
09-19-2008, 01:44 PM   #7
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Sweet shot.. i like its atmosphere.

Well.. im quite pleased with my AF too..
Talking in general (not just AF and cameras)...
It just sometimes seems fun for me, to think about all the technical details that make up the difference and maybe some ideas that could make an improvement.

What usually confuses me is those solutions and ideas which are already out there and have been perfected during many years but not implemented.. simple things that could make life easier virtually at no cost at all, or could be much better than currently used solutions
Hyperfocal AF, APS-C compact's - same camera, different sensor to name few.

I guess its marketing... because all those things are being introduced..but in a way, that makes you upgrade, buy stuff.
Offtopic...

Last edited by ytterbium; 09-19-2008 at 01:55 PM. Reason: typo
09-19-2008, 02:00 PM   #8
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QuoteOriginally posted by ytterbium Quote
Sweet shot.. i its atmosphere.
Thank you. It was a beautiful evening at a beautiful place. Sort of took its own picture...

QuoteQuote:
What usually confuses me is those solutions and ideas which are already out there and have been perfected during many years but not implemented...
I come from pretty much the other direction regarding auto-focus. I am always totally in awe that it works at all! I remember the early lens-based AF solutions based on infrared sensors and their limitations. The technology has improved immensely over the years. Consider what the system must do:
  • Acquire the target (no small task in itself)
  • Run an iterative process to determine whether optimum focus has been attained
  • Do so consistently under circumstances where the depth of field may be razor thin.
  • Do all of the above in a sufficiently short time window to allow capture of a fleeting event.

It is F***ing magic I tell you!

Steve

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