p.1 #1 · How does a camera see? mk III related (maybe)
This may be a dumb question, but when it comes to tech, I honestly have no idea. So maybe someone can possibly answer my question.
Im not sure how a lens/camera finds its focus, but as I was writing a message in another thread, the idea popped into my head. (Please dont make fun - lol).
On a warm sunny day, when you look down a road for example, you see heat. You know, that wavy aura that distorts what you are looking at. Sometimes, if its hot enough, the black asphalt will look like water. Sometimes you can even see this effect coming from the top of an athletes head or back when he/she has been playing hard.
I imagine that when hot enough this distortion occurs all around us. Coming mostly from the ground. Now, our eyes may not be that sensitive to it, or maybe they adjust to it, and we dont even "see" it. But does the mk III?
Since the mkIII seems to have worse AF issues on Sunny days, or still worse on HOT sunny days, is it possible that the camera is so sensitive that it is picking up this effect and causing the AF to miss-focus?
It is also said that its worse on Longer reach lenses, meaning that you are shooting at something with more distance than normal, which kind of sets up the same scenario of looking quite a bit aways down the road and seeing this effect with the naked eye.
Its just a guess, especially since I honestly do not know how a camera achieves focus. I thought Id throw it out there.
I hope I was able to explain well enough what Im trying to say...
p.1 #2 · How does a camera see? mk III related (maybe)
Intresting idea, I haven't heard this one brought up before. It seems all the Canon techs would have thought of this, but maybe they don't get this kind of weather where they live and work.
p.1 #4 · How does a camera see? mk III related (maybe)
Autofocus cells in essence are just a row of photosites as you will find in a sensor. As an image is increasingly in focus, these photosites record more contrast. When contrast is highest, the best focus is achieved.
Moving air (which is what you refer to) can screw up contrast or make the contrast shift slightly. I'm not sure how much effect this would have.Perhaps the temperature of the sensors itself maybe more of an issue. Hot sensors are noisy sensors. The light through the lens warms up the sensors, as the light is continually hitting it. On sunny days this effect is stronger, on hot day it will be even worse. Just a thought.
The problem seems to be worse in AI Servo than in Oneshot, though, so it may be a software or speed thing.
p.1 #5 · How does a camera see? mk III related (maybe)
Autofocus is not detemined by contrast. Autofocus uses a phase difference detection system similar in principle of operation to a split prism viewfinder.
Gene
Rijsberman wrote:
Autofocus cells in essence are just a row of photosites as you will find in a sensor. As an image is increasingly in focus, these photosites record more contrast. When contrast is highest, the best focus is achieved.
Moving air (which is what you refer to) can screw up contrast or make the contrast shift slightly. I'm not sure how much effect this would have.Perhaps the temperature of the sensors itself maybe more of an issue. Hot sensors are noisy sensors. The light through the lens warms up the sensors, as the light is continually hitting it. On sunny days this effect is stronger, on hot day it will be even worse. Just a thought.
The problem seems to be worse in AI Servo than in Oneshot, though, so it may be a software or speed thing....Show more →
edit: I partly stand corrected: http://en.wikipedia.org/wiki/Autofocus
The 1D series seems to use the phase difference system. Still, this is a system that could suffer from higher temperatures, as described.
p.1 #8 · How does a camera see? mk III related (maybe)
My (unsubstantiated) bet is on phase difference detection, at least in the case of a Canon SLR. Thus the need for a circular, rather than linear, polarizer to maintain AF operation. On a p&s, I'd say it is contrast-based, which is why a linear polarizer works OK.
edit: I partly stand corrected: http://en.wikipedia.org/wiki/Autofocus
The 1D series seems to use the phase difference system. Still, this is a system that could suffer from higher temperatures, as described.
I don't think any digital SLRs use contrast AF anymore, it is inferior to phase difference.
Why would this suffer from temperature issues? It is used in all other EOS digital bodies with no such issues.
p.1 #11 · How does a camera see? mk III related (maybe)
It depends on tolerances. Off course I don't know the exact tolerances Canon ses.
The sensors are less accurate if temperature increases, while components will expand. Minute differences, sure, but so is the difference between an image being in focus and slightly out of focus. And the tolerances will be smaller still as pixelpitch of a sensor decreases. The faster and more accurate you want the AF to be, the smaller the allowable tolerances will be. A change in temperature COULD be an issue at some point.
p.1 #12 · How does a camera see? mk III related (maybe)
Actually, apparently the current crop of Nikon DSLRs use both phase (normally) and contrast (in live view mode) AF. Of course, this doesn't make any difference in the Mk III case, but I just thought I should point out that contrast detection is not "dead" in DSLRs
