K10D & Sigma 70 lens, focusing problem

I purchased my Pentax k10D because of the ability to use
my old non-digital lenses. One of those lenses was a Sigma 70-300mm 1:4-5.6 APO lens. Although not the most expensive of lenses it served my well on the old 35mm SLR, however I cannot get this to produce properly focused images. Even set to manual focus no part of the image is in sharp focus. Is this lens a dead loss or am I doing something wrong?:

To be honest sharpness is probably the Sigma's weakest point (as has been discussed here many times). You probably wouldn't notice so much on film as you rarely look at prints with the same level of scrutiny as a digital picture blown up to 100% on the computer screen.

To get the best out of it try and avoid the extremes (use 260mm instead of 300mm etc and always stop it down at least 1EV from the maximum aperture).

Welcome to the forum by the way

Thanks for the welcome I appreciate your quick and helpful responses. I have tried taking images at 70, 100, 135, 200 & 300MM respectively, with no apparent improvement. I am debating replacing this lens with the new Tamron 70 - 200 f2.8 with macro. Although this will not give me the same focal distance I am hoping it will provide sharper images with less frustration. Do you have any opinions on this? I notice that you have used the Tamron 70-300 is it any good.

I have the APO DG version of this lens and have had no problems with image sharpness though I do tend to use it stopped down at least to f8. My only other comment would be that the lens is quite long when extended and becomes somewhat unbalanced which could lead to camera shake when hand held. Did you use it on a tripod? Certainly to use its macro function, a tripod is essential. The only other possibility is that it is back or front focusing!

John

The Tamron 70-300 has its own problems; it's sharper than the Sigma but it's also a lot more prone to really ugly purple fringing (which the Sigma seems to control much better). When deciding between the two it's really a case of deciding which is the lesser of the two evils...

From what I've seen (and from what others on here have said) the Pentax DA55-300 is a far better lens than either of them; it's as sharp as the Tamron but has none of the issues with abberations. It also costs around £200 whereas the other two cost nearer £100 (less in the Tamron's case) so it should offer quite a step up to be worth its money.

The Tamron 70-200/2.8 is, of course, a different class of lens entirely and one that I would expect to be very competent. We have users on here that have bought this lens and I can't recall hearing any complaints about it; again it's a much more expensive optic and with nearly all lenses you really do get what you pay for!

I have had the same issues with the Sigma and would agree with Hefty that the Pentax 55-300 is a good step up. I also have a Tamron 70-200 and it is a cracking lens - I use it for wildlife photography so the 2.8 is very useful. However, not only is it 3x the price of the Pentax 55-300, it is also 2x the weight which can count for a lot when you are trying to get a shot in very difficult conditions with no support, even with SR.

Thanks for the f8 tip John, I will give that a go. In all my tests I have used a tripod and tend to get the effect of the image being unfocused rather than camera shake. However my lens is not 'D' rated so I wonder if trying to get a lens that has been optimised for 35mm to focus on a CCD is the cause?

Thanks also for the comments regarding the Pentax 55-300 I must confess that I had not considered this one as the ability of the Tamron's f2.8 was quite attractive. Does anybody know a camera shop that actually keeps Pentax lenses in stock so I can pop in and have a look at one. Preferably one that has an enthusiastic and patient staff behind the counter in the North East.

The only "Pentax Pro Centre" listed on the Pentax UK website for the North East is:

CHARLES EAGLES & SON
5 MARITIME STREET
SUNDERLAND
0191 567 9308

Might be worth giving them a call to see if they have a DA55-300 in stock or if they're prepared to get one for you to try.

Thanks I'll give them a call.

Having taken the advice onboard I whent for a wander around our local nature reserve. I set my camera at 400 ASA, f8 & 250mm, this did give a marginal improvement to the quality. So thanks for all your advice,it will at least give me something to compare alternative lenses against.

I've had similar problems with the same Sigma lens (APO 70-300, no DG), there was back or front focusing I believe. But when I do focus manually (which is what I was used to do most of the time on my MZ-5n) it is okay. Check the viewfinder if you haven't shifted the dioptric adjust, it happened to me a few times. Unable to do manual focus then. I'm glad to hear that the 55-300 is a good lens, I might buy it since the Sigma got suddenly oiled inside on the lens! But the dream is still 100-300/4 or eventually 70-200/2.8 with converter.

[/quote]Thanks for the f8 tip John, I will give that a go. In all my tests I have used a tripod and tend to get the effect of the image being unfocused rather than camera shake. However my lens is not 'D' rated so I wonder if trying to get a lens that has been optimised for 35mm to focus on a CCD is the cause?[/quote]I don't think that the lens being pre-digital would have that effect. I have a Vivitar 105 mm Series 1 macro which I purchased before digital cameras were perhaps though of and that gives superb results. As I understand it, one of the benefits of a lens being optimised for digital is the reduce fringing.

John