Sigma 18-35mm f1.8 focusing issues!

af_thefragile

Today I finally bought the Sigma 18-35mm f1.8 lens and it's brilliant! But for one issue, the AF (or phase-detection manual focus for that matter) is useless below f2.8!!!

It seems to be a known problem with the lens and surely mine isn't immune to it. It's front focusing like an absolutely *****! At 35mm at about 2 feet away it's about 2-3inches off while at 18mm at around infinity it's upto a foot off! This is pretty appalling for a lens that's optically so good! To note it does focus fine when in live view (which on my camera is painfully slow!).

Now I bought this lens mainly cuz I'm getting into shooting video lately in which case AF isn't an issue as for video shooting it's gonna be all manual focus (not an issue with Magic Lantern's focus peaking dots). But for stills photography it's gonna be a big nuisance if I'll need to switch to manual focusing in live view when shooting below f2.8. Reading around this seems to be able to fixed upto an extent by microadjusting the focus with the use of the Sigma USB dock but the process appears to be lengthy, painful and it may not fix the focusing issue completely.

So I can't decide now whether to keep this lens or return it. On one hand it is amazing optically and when shooting video the AF isn't going to be an issue but on the other hand if I wanna quickly shoot some cool shallow DOF shots, I'll have to resort to manual focusing under live view to get the focus right which defeats the purpose of having AF in the first place. It's a great lens and there's no other lens like it in this price range but it has it's issues and I can't decide whether to keep it or return it. What do you guys recommend I should do?

magicbastarder

af_thefragile wrote: »

at 18mm at around infinity it's upto a foot off

a foot off infinity doesn't sound *that* bad...

DaireQuinlan

magicbastarder wrote: »

a foot off infinity doesn't sound *that* bad...

It's worse than that. 1 foot at infinity is actually 2 feet long.

af_thefragile

Here, I did a semi scientific test of measuring the focus accuracy of the lens at 18mm, 24mm and 35mm focal lengths and at 113cm and 64cm focal distances. I also used the Canon 50mm f1.8 as a control lens. Each image was focused at the '0 line' on the scale. Here are the results:

18mm at 113cm:

It doesn't even focus on the chart!

24mm at 113cm:

Still a little off.

35mm at 113cm:

It's still focusing more than 3cm in front of the focus point.

Canon 50mm at 113cm:

Surprisingly this is off by >3cm too!

Sigma 18mm at 64cm:

At least it's focusing on the chart.

24mm at 64cm:

Front focusing by about 1-2cm

35mm at 64cm:

Front focusing by about 1cm

Canon 50mm at 64cm:

Off by only 1-2mm which is quite acceptable!


So you guys reckon I should keep this lens or return it back??
I might post some more images of focusing at near infinity sometime later once I figure out a good way to measure that!

Zascar

Is this an issue with All of them?? I've read loads of reviews and they are all brilliant, how much of an issue is this really? I'm not really into charts etc but how much will this really affect a normal shooter? I'm still hugely tempted by this lens...

Can I ask where you bought it and how much you paid? The reviews say its €800 which is €600 - but i cannot find it for that price anywwhere. It's $1000+ on ebay and £649 on amazon uk

af_thefragile

Zascar wrote: »

Is this an issue with All of them?? I've read loads of reviews and they are all brilliant, how much of an issue is this really? I'm not really into charts etc but how much will this really affect a normal shooter? I'm still hugely tempted by this lens...

I've been using this lens for a while now and I've gotten used to it. In everyday shooting it works fine. The AF is just a little less accurate than Cannon lenses but it's not too bad and once you realise that you can learn to live with it. This is gonna be a problem with most lenses that aren't Canon because the lens manufacturers need to reverse engineer the lenses to work on the Canon mount which leads to such inaccuracies. Also a reason why Zeiss only makes manual focus lenses for Canon and Nikon mounts.

I found out testing lenses with horizontal lines isn't a good way to measure it's accuracy as the phase detection AF isn't very accurate at distinguishing between horizontal lines and it will snap onto any horizontal line that near the focus point without necessarily focusing on where you want it to focus. Which is what gives the lens its inaccuracy. At 18mm the field is pretty wide so even millimeters of inaccuracy will shift the focus by a good margin but that's only apparent if you start pixel peeping.

It will focus on people's faces fine. It will also focus on things at 35mm focal lengths at f1.8 fine too. The only issue that may bother is that it doesn't focus well at infinity. At 18mm focal length things are quite small at infinity distance for the lens to focus accurately using phase detection probably. So if you're focusing on something at infinity, it's best to either a) use the lens guide to manually focus the lens at infinity distance and take the shot or b) use the live view mode to get accurate focus at infinity or c) try the focus microadjustment calibration thing if your camera has it or you can use the Sigma USB dock to have a go at it.

The guys here at DP review talk about this issue: http://www.dpreview.com/lensreviews/sigma-18-35-1-8/5

I bought this lens mainly for shooting video so for that I won't be using AF as I'll be pulling manual focus using the focus peaking feature on magic lantern (which makes pulling focus a doddle!). For other purposes like low light night photography I'll be resorting to live view to get accurate focus if I need that.



cameracentre.ie seem to be the only ones who have it in stock in Dublin. I think I bought the last one the store on Grafton street had. I bought it for €800 as that's the price it's for sale at everywhere in Ireland-UK. $800 seems to be the price for US/Canada only. Amazon has it for slightly cheaper so if you wanna save around €50, I'ld say order it for amazon! They seem to have it in stock too.