[DennisH]

I picked up an F*300 from KEH. Good news is it is just about spotless cosmetically and has the tripod ring.

Bad news is in viewfinder mode af works great, locks on quickly and stays locked. In live view, however, the af just very slowly runs all the way out and all the way in without locking on at all. Occasionally after racking all the way in and out it will quickly lock on, but even that has only happened a couple times.

Does this sound like something simple that KEH might be able to fix, or am I likely looking at a return for a refund? Or am I somehow missing something simple. As far as I can tell I’m locking the manual/AF ring correctly. Firmware is up to date.

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Last edited by DennisH; 03-28-2019 at 07:58 PM.

[vonBaloney]

Sounds like a camera problem, not the lens. If it works ok with the viewfinder, then the lens is fine. Focus in live view uses a totally different system (focus is done by contrast on the main image sensor rather than with the special PDAF sensor for AF). What you describe sounds like it is too dark, or you are using an older model body that wasn't very good at live view (like the K-7). (You do want to make sure you've got the lens set to AF mode properly though as it will create more drag if it isn't but it still works either way. Sounds like you do.)

[pschlute]

Have you cycled through the different AF settings in LV ?

[stevebrot]

Or am I somehow missing something simple.

CDAF, the type of AF used in live view, simply does not work well with some lenses and may do a full rack at least once before giving up with some subjects. Is the lens easy to focus manually using live view on the same subject and light? If not, it is unlikely that the CDAF system will have an easier time of it. If the problem is with the lens, it will show in the general image quality and given the reputation of the F* 300, the image should be sharp with good contrast. I suspect inadequate light and/or low contrast subject.


Steve

[richfam]

In live view, however, the af just very slowly runs all the way out and all the way in without locking on at all.

I hardly ever use my F* 300/4.5 in Live View, but this sounds normal to me, as it has happened to me as well. Live View uses contrast-detection AF which, while very accurate, is slow and can result in a lot of "hunting," especially with lenses that have a long focus throw like the 300.

If shorter lenses work OK in Live View, and the 300 is working properly when using the viewfinder (using phase-detect AF), then there's nothing to fix.

More info about the two types of AF:

Youtu.be

Youtu.be

[DennisH]

Thanks for the suggestions. I think bad lighting might be the issue, but I’ll have to continue testing tomorrow.

I did try all AF modes, and I don’t have any problems with my 20-40 Limited.

[vonBaloney]

I would think a newer body like the KP would do a pretty good job of it ( Are the improvements in EV sensitivity on the newer models mostly for PDAF?). Even on the K-5 & K-01 (that I'm still using) they usually don't hunt like that unless you've got an effective aperture that is f/5.6 or slower. But that's in the sun. If the scene is dark, it can struggle with an f/4.5 I'm sure. Come to think of it when I had the FA*300 (same optics as the F*), I do recall it having trouble locking in at times. Something about that lens? Or just the semi-slow aperture?

[UncleVanya]

I can try my fa* 300 on my k3 tomorrow. Honestly I can't recall ever trying in line view other than manual focus on the moon.

[monochrome]

Thanks for the suggestions. I think bad lighting might be the issue, but I’ll have to continue testing tomorrow.

I did try all AF modes, and I don’t have any problems with my 20-40 Limited.

Try to focus on a high contrast line in good light, not (for instance) something like a light gray shirt indoors under incandescent light.

[DennisH]

Little update: I tried with better light this morning (still not bright daylight) and got the same result, but I tried my other screw drive lens, D FA 100 macro WR, and it behaves somewhat similarly, so I’m leaning toward this not being a fault.

FYI, the behavior isn’t normal bad light hunting, but VERY slowly going to infinity and back to close-up repeatedly without locking on at all. I thought the KP had a pretty strong screw drive motor, but maybe it’s struggling.

Definitely will do better tests this weekend.

[vonBaloney]

Little update: I tried with better light this morning (still not bright daylight) and got the same result, but I tried my other screw drive lens, D FA 100 macro WR, and it behaves somewhat similarly, so I’m leaning toward this not being a fault.

FYI, the behavior isn’t normal bad light hunting, but VERY slowly going to infinity and back to close-up repeatedly without locking on at all. I thought the KP had a pretty strong screw drive motor, but maybe it’s struggling.

Definitely will do better tests this weekend.

The very slowly thing is "normal" as well, I'm not sure why it does that sometimes (lack of contrast probably) but still sounds like your lens is fine. (Make sure aperture ring is on 'A')