Originally posted by Wheatfield The exposures you are listing are around EV5 or thereabouts. The K5 AF is rated to EV-1, but that would be, I expect, a full spectrum daylight rating. I've had good results myself as low as EV3 in artificial light, with colour temp in the 3000K range, and no problems at all in higher light levels (EV 8-10) with tungsten.
I'm thinking what we are seeing is a combination of colour failure of the sensor at low light levels combined with a loss of the double check that Pentax dropped to pick up the AF speed.
Hopefully they can do a firmware fix that will mask the deficiency in the sensor, or at least program the assist light to come on at higher EV values, perhaps 5 or 6 rather than the present level of 3 or 4.
Time will tell, but in the meantime, I would venture to say that in very low light levels, manual focus might be the best approach.
Wheatfield,
thanks for the insight.
Could it be that Pentax chose a new autofocus sensor for the K5 to improve autofocus performance in general, but it came with this weakness at the tungsten temperature?
Even if they can turn on the green light for higher EV levels, it won't have significant range, is that correct? The good news with the K5, is that the camera's great iso performance can be used to mitigate the low lite autofocus weakness by using a smaller aperture/larger DOF. Even with the K20's lower iso performance, i use that same technique to cover up any AF weakness by a larger DOF that covers more of the theatre stage. works quite well.
This is a case where Pentax coming out with an official announcement about the situation would be more useful than a ton of explanation and rumours.
What some people don't realize is that most design decisions come with intrinsic advantages/disadvantages. thats the real world. Manufacturers don't want to talk about them because any announced weaknesses may be offputting to sales, but they are there in any product, e.g. better water resistance=higher cost, lens based vibration reduction=better viewfinder image=higher cost and more bulk, etc.