Backfocus - I don't understand

[rasputin]

I've had a few lenses with backfocus issues, and it's starting to look too coincidental:

Two different (brand new) Sony 24-105mm lenses - both had backfocus when focusing more than a couple of metres away, but fine close-up.

Tamron 70-300mm lens - perfect up to 20ish metres, then gets a bit dodgy like it's focusing past infinity.

And now I've just bought a Minolta 100-300mm APO lens that has awful backfocus from close-up to infinity.

BUT... I've tried these on two bodies and at all apertures and didn't notice any difference, and I have/had loads of lenses that work perfectly from 11mm to 300mm and at all distances. So I really don't think it's the bodies that have a problem.

How can I check if it's the lenses or my camera bodies? Both bodies are still under warranty so I'd really like to know...

[Simpo Two]

I'm stumped. With fast lenses sometimes the focusing precision needed at max aperture can exceed the accuracy of the AF system, but your lenses aren't that fast.

Throw everything away and start again

[rasputin]

The lenses that have had back-focus issues were never very fast, and I tested them from f/3.5 to f/30ish with no improvement.

I can AF my 50mm f/1.8 and 75mm f/2.8 wide open at closest distance with a DOF of just a couple of MM... and they're perfect.

It just seems too coincidental that so many lenses have the same problem...

[Simpo Two]

Well, I haven't had to do it, but I understand you can send them off to somewhere and have body and lenses calibrated together. Hopefully somebody will be able to recommend a company that does it.

[Cabinet Enforcer]

rasputin said:

I've had a few lenses with backfocus issues, and it's starting to look too coincidental:

Two different (brand new) Sony 24-105mm lenses - both had backfocus when focusing more than a couple of metres away, but fine close-up.

Tamron 70-300mm lens - perfect up to 20ish metres, then gets a bit dodgy like it's focusing past infinity.

And now I've just bought a Minolta 100-300mm APO lens that has awful backfocus from close-up to infinity.

BUT... I've tried these on two bodies and at all apertures and didn't notice any difference, and I have/had loads of lenses that work perfectly from 11mm to 300mm and at all distances. So I really don't think it's the bodies that have a problem.

How can I check if it's the lenses or my camera bodies? Both bodies are still under warranty so I'd really like to know...

If the bodies are the problem then you would likely see inconsistency between auto and manual focus.

If the lenses are in warranty then you should be able to have them adjusted as simpo says above. If lenses aren't in warranty then you'll have to pay, I don't know who would be able to adjust the minolta lens, try asking on http://www.dyxum.com/index.asp

[rasputin]

Been hearing about "micro focus adjust" on some higher-end bodies.

Is this just an ability to fine-tune the back/front focus for particular lenses, so the body adjusts itself depending on which lens you have attached?

So maybe I'm just being more picky than someone with a budget body should be

[Simpo Two]

rasputin said:

Been hearing about "micro focus adjust" on some higher-end bodies.

Is this just an ability to fine-tune the back/front focus for particular lenses, so the body adjusts itself depending on which lens you have attached?

Yes - but on the two such cameras I've tried (Nikon D300 and D700) it's only an adjustment for each lens as a whole, not front and back. The camera knows which lens you've fitted and applies the correction. They may have to be Nikon lenses, not sure.

rasputin said:

So maybe I'm just being more picky than someone with a budget body should be

Well, when you demand more from you kit than it can deliver, it's a valid reason to trade up. See 'throw it away and start again'

Edited by Simpo Two on Monday 1st March 15:19

[neilr]

rasputin said:

So maybe I'm just being more picky than someone with a budget body should be

No. Personally, I don't think expecting lenses to focus correctly is expecting too much, whatever the cost. Speed, image quality build quality etc etc maybe but IMHO expecting the lens to focus where you want to focus isn't expecting too much.

It's pretty depressing that camera manufactures have put themselves in the position of supplying what is now in essence consumer electronic goods rather than precision optical goods (when they should be both). IMO there has been a marked change, I've had a lot of issues with top of the line Nikon cameras and with Canon also that I simply shouldn't have had, maybe i've been unlucky, but I NEVER had as many issues with film gear. The race to the bottom seems in full swing if you ask me.

[gary71]

I've become a little more fussy of late regarding focus accuracy and have dialled in my D300s to better match the Sigma 24-70 lens I used most of the time. Subtle, but worthwhile tuning, although as mentioned above the lens should be correct in the first place!

[Simpo Two]

neilr said:

It's pretty depressing that camera manufactures have put themselves in the position of supplying what is now in essence consumer electronic goods rather than precision optical goods (when they should be both).

I agree, but the majority of people want lots of buttons and want them cheap.

Still, there's always Leica!