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This question is not about the main imaging sensor, but about the autofocus sensors and circuits on a DSLR.

I read that sunlight may damage the shutter or the main sensor. But when using the DSLR, since the shutter and the main sensor will only see sunlight when the picture is taken, the time that they are exposed to direct sunlight is weakened. Meanwhile, using ND filters further reduce impact on the shutter and the main sensor.

However, the auto-focus sensors and elements are always getting the light from the mirror, and sunlight is always reachable, even if it is weakened via ND filters. Therefore, when doing a long time (for instance, hours long) time lapse, will the extensive exposure to sunlight hurt the auto-focus sensor? If so, is there any solutions or workarounds? Thanks!

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It's difficult to say that time lapse can't possibly have any effect on an autofocus sensor, but if there is any, I think it's little enough that it's basically irrelevant--a camera is almost certain to be quite obsolete before it has any noticeable effect.

Back when it was still quite new, I bought a Minolta Maxxum 9000 with the matching Program Back Super 90. The program back features a built-in intervalometer, and I used it a fair number of times, including some shots of the movement of the sun through the day. That includes one series taken in Wood Buffalo National Park in Canada during June, so the sun was up for around 18 hours, if memory serves.

That body is now over 30 years old. Its autofocus works pretty poorly--it's horribly slow, and it tends to hunt a lot, even when there's pretty obvious contrast that it seems like it should be able to detect. But that's exactly how it worked when it was brand new. Time has simply moved on, and the Maxxum 9000 is pretty thoroughly obsolete, especially in terms of autofocus performance.

Summary

My experience tends to suggest that deterioration of AF sensors from excessive light exposure simply isn't a significant concern.

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