 Deb James |
D70 autofocus issue I'm having issues with autofocus not finding a focus point and locking on it. I primarily use two zoom lenses - AF-S Nikkor 18-70mm and an AF Nikkor 70-300mm. Most of the time the problem occurs when I'm zoomed in with the 70-300. The autofocus can't seem to find anything to focus on. I've specifically tried changing to the single focus mode and centering the image on an object in the distance with distinct contrast to the background. This doesn't help. I generally have to zoom back out, focus, zoom back in and try again. Sometimes it takes several iterations of this to get it to lock focus. I keep my camera and lenses very clean, but I've never had the low-pass filter cleaned. Could this have anything to do with it? Does anyone have any other ideas?
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robert G. Fately |
Debra, you don't mention the lighting conditions in which you're shooting, but it may just be a matter of not enough light to let the autofocus mechanism do its job. Your camera's autofocus is based on maximizing the sharpness between contrasting areas - meaning it has to perceive a line or edge to work. In low ambient light conditions, an f5.6 lens may not be able to pass enough light to the sensors to give them that ability - it's just a limitation of autofocus systems. I don't think anything on your low-pass filter would hve an effect - certainly not sporadic.
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Deb James |
Thanks for the response! This happens in bright daylight. I was just in Seattle this past weekend and trying to take pics of the mountains in the distance. There was low contrast so that could be the issue. I was also trying to take pics of the Orca whales again in bright daylight. Plenty of contrast in this case. I also typically use a circular polarizer. I thought that might be the problem, but it happens without it too. Also, I was reading somewhere else about the low contrast possibly being the issue. However, the D70 uses TTL phase detection for auto focus. I thought that eliminated the issues with low contrast?
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robert G. Fately |
Debra, I believe TTL (through the lens) Phase Detection is the fancy name for contrast-based auto focus. (there's a "phase change" when going from light to dark, which we percieve as contrast). This is as opposed to, say, the sonar based auto-focus which was used in Polaroid cameras (SX-70?) some years ago - where the camera sent out a sound and timed the delay to the echo - much like a bat.
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Deb James |
Thanks! I'll do some experimenting to see if that's the issue. I'm betting it is though.
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