[ripit]

I know this has been brought up before and I have discussed some of the issues. One of the working theory was that it is just not focusing on what you want (its grabbing the highest contrast item even if its way off center). I just got a new lens though that seems to imply its something else (perhaps others have posted this and I missed it).

So I just got a sigma dl zoom 75-300mm 4-5.6. I haven't been able to finds much of anything about this lens but its a push pull auto focus lens. As best as I can tell it was made in the 90's and had the dreaded zen coating (which I have already cleaned). I found it was missing focus bad on the long end. It worked fine with my ds though. I have a newer sigma 70-300mm ldo macro (actually its quantaray brand but its a sigma). I had not notices any problems with it. So I compared the two and the newer lens nailed focus but the older one din't (but looked fine when manual focusing and worked find on my ds). This is under identical shooting conditions and settings. The old one was mainly having problems on the long end.

One of the shots was the face of a clock, at 300mm and f5.6. The face of the clock filled the frame so there was nothing else to focus on. The entire frame was on the same focal plain. The newer lens did fine with the k-01 but the older one missed focus a lot. When manual focusing it was fine. With the same parameters and the older lens (that didn't work with the k-01), it worded fine with my ds.

Does anyone have any other theories or has this been discussed further and I missed it? Fyi, I could care less about getting that lens to work as I have the newer one. Its just an observation.

[Na Horuk]

I really don't know. Could be that the older lens' contrast is so different that the AF is mistaken. Could be that at the long end the aperture is so narrow the camera can't focus properly (zooms have a slower speed at the long end than at the wide end). What is the widest aperture at the long end for both lenses?

[ripit]

I really don't know. Could be that the older lens' contrast is so different that the AF is mistaken. Could be that at the long end the aperture is so narrow the camera can't focus properly (zooms have a slower speed at the long end than at the wide end). What is the widest aperture at the long end for both lenses?

They are both 4-5.6 so 5.6 at 300mm. The older one is a sigma 75-300mm 4-5.6 and the newer one is a quantaray (built by sigma and nearly identical if not identical to the sigma branded version) 70-300mm 4-5.6. Specs wise they are nearly identical though build wise, they are drastically different. It would therefor seem to not be the aperture. Different contrast abilities of the lenses certainly seems like a working theory. I kind of wonder if the lens coatings play any part. I think both lenses are multi coated and both are full frame lenses designed for film, though there is probably a decade or 2 difference in age so it could be different coating technology. Then again, some have reported problems with current lenses that have coatings designed for digital and I have plenty of lenses designed for film that work fine.

[Rob22315]

I too got poor focus results from my Pentax 80-320 zoom at the long end (~250mm+). My theory is that it's a combination of the smaller aperture and the long focal length that creates the problem.

Less than about 250mm, the lens is very sharp, the focus reasonably accurate.

[ripit]

I too got poor focus results from my Pentax 80-320 zoom at the long end (~250mm+). My theory is that it's a combination of the smaller aperture and the long focal length that creates the problem.

Less than about 250mm, the lens is very sharp, the focus reasonably accurate.

Perhaps longer focal lengths and or smaller apertures are factors but in this case at least there has to be something else. 2 lenses, an older sigma, and a newer sigma, both at 300mm and both at f5.6. One auto focuses fine, the other doesn't. That is using the same target and a flat focal plane to avoid the possibility that it is grabbing something unintended to focus on.

Since the problem does seem to be at the long end perhaps the long focal length and smaller aperture both weaken its ability to focus accurately but doesn't quite completely eliminate it. Perhaps there is another factor (lower contrast glass seems like a feasible possibility) that pushes it over the edge where it can not focus right? That might explain why one lens can focus at 300mm f5.6 but another can not. I strongly suspect that longer focal lengths and or smaller apertures play a part but I think there may be another factor at play.

[THoog]

Try manually focusing both lenses on the same subject with the same lighting setup & aperture, shutter speed, and ISO manually set. Compare shots, and see what's different.

[ripit]

Try manually focusing both lenses on the same subject with the same lighting setup & aperture, shutter speed, and ISO manually set. Compare shots, and see what's different.

I didn't try that yet with the lens that was autofocusing fine, bud did already with the lens that did not auto focus fine. It was set to 300mm f5.6 and I do not recall the iso but I never set it to auto so it was at a fixed iso and the same iso where it would not auto focus. I was able to manual focus and get a good shot under identical conditions where the lens could not af and get a good shot. The Af was consistently off.