zoom lens issues!!???!? not holding focus during zoom...

jenningsp

jenningsp

Well-known member
i just bought a mint second hand sigma 24-70 2.8 nikon mount zoom....

when i set focus at 70mm it's sharp. but when i zoom out to 35 or 24 it goes soft... the focus needs readjusting slightly to be sharp again... so it's not holding focus perfectly during the zoom.... is this normal?????

should i take it back? can it be repaired easily?


thanks
 
Or you could have your SGpro backfocus properly adjusted. That's probably the issue. Unless the 24-70 doesn't track.
 
i have the exactly same issue with my sigma 20-40 f2.8. I was just testing a few minutes ago my lenses and adjusting collimation on my gg. all my nikon and mamiya primes are good and spot on the focus marks except this sigma zoom.
 
Actually, some 35m lenses do and some don't hold focus through a zoom. If the lens is designed to hold focus then if it doesn't that is a sign of incorrect back focus.
My Nikor 80 - 200 and my previous Tokina 80- 200 hold when the back focus is correct, but my Tokina 28 - 70 won't.

How do you whether it is supposed to though? No easy answer, but Often when a lens doesn't hold focus and isn't designed to you may see 2 or more focus marks on the barrel. On my Tokina one says 70 and the other 28 for opposite sides of the zoom.

Lenny Levy
 
damn it.... the only reason i bought this thing was to be able to zoom during a shot... :(

the only tell on the lens is it's got an infinity mark and a line leading slightly past it... which i now know is the 24mm infinity mark.

i wanted some cool crash zooms and whips with zooms and the always awesome slowly zooming in when someone is having a dramatic moment :)

80-200 is just in the wrong range for me. ideally i'd have a 40-135

so is there any reason to keep this thing when i've got a decent prime set???
 
Good question. i'd like to hear about that. You're right, it is a drag for video shooting, that's why all video and cine zooms do hold focus.

How about those nice Nikkor zooms anyone out there able to give us feedback?

- lenny
 
hmmmm..... i chucked the lens on my nikon FG20... and it seems like it does hold focus after all...

it goes off very slightly off at the 50-60 point but comes back at 70..

though the focus shifts sometimes. meaning when i set focus at 70mm and zoom out to 24 and then zoom back to 70mm it's off... i'm not touching the focus ring at all..

i need to do more testing
 
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