help! disappointed with Sigma 18-50mm f2.8 EX DC

You can also do the battery test and probably find out its front focusing.

I agree that the Sigma lens shouldn't be that blurry, but my copy was. Under normal circumstances I would probably get it fixed, but since I am traveling in a few weeks for Africa, then Asia and September and October, then Europe in November, I really want a lens that works out of the box. I don't have time to send it back to Sigma for recalibration and then hope that what I get back works better. I may still get a new copy of the Sigma lens, but I will try it out in the store before taking it home and not order it online. That's the lesson I learned through al of this!

If you can replace it with a good one by testing the new one in-store that would be a good quick solution.
Go on a bright sunny day and simply shoot something out in the street
Just make absolutally sure that the center focus point is catching reliably with both lenses.A strong vertical or horizontal line with no detail around it would be ideal.

I have one and find it to be sharp even wide open but you do have to be careful with the focusing.

Don't forget the kit lens is 3.5 wide open vs 2.8 with the sigma = much smaller DOF

Just like you can't use a 50mm 1.8 wide open and expect every shot to be in focus.

Sometimes I find that if I take a pic wide open it seems to slightly back focus but it isn't consistant, sometimes it does it and sometimes it doesn't.

I have also found that if you pump the shutter release a couple of times the lens will adjust a tiny bit and by doing this the amount of sharply focused shots increases a great deal.

It seems to be in back focus and then you press the shutter again and it's ok and then another press makes it slightly back focused and so on, it seems to go between two slightly different settings.

You could try this just sitting on the couch, take a pic of something wide open then inspect it on the lcd, then try pressing the shutter a few times for the next shot and inspect it again.

When I do this the first pic is usually soft but the second one very sharp and if you really concentrate you can just make out the difference in the viewfinder so you can tell if it's focused properly and not have to take several shots.

I don't know if it's a fault with the lens or just down to the shallow DOF but I find in most cases I can now get a decent shot every time even wide open.

I also find that by the time you stop the lens down to the 3.5 of the canon it is tack sharp.

I'm pretty sure mine back focuses ever so slightly when it first locks on but like I said, after a couple of presses of the shutter release it seems to get a much better lock.

I dont have the problem at all once I'm stopped down to f4 or so.

I'll post an example of what I mean later if I get the time.

AF with shallow DOF is definitely a challenge. it really helps to select the best sensor (preferably the center, but only if its really appropriate) and to know what the sensors are most sensitive to (vertical, horizontal) and their range accross the image space. even a tiny change in your position or the subject's position (or sometimes even lighting) can cause a refocus which has noticeable results.

Ok here we go:

Centre AF point, one half press to focus then fully pressed

And this time exactly the same settings but with TWO half presses before fully pressing the shutter and taking the pic.

fatdeeman wrote:
And this time exactly the same settings but with TWO half presses before fully pressing the shutter and taking the pic.

Try giving it a go, if you get a similar experience then it's probably just down to the small DOF and also lowlight doesn't help.

But like I said, you can usually see the minute difference through the viewfinder and avoid having to take several shots to get the sharp one.

I thought there was something wrong when I first got mine too but since then I've just learned that it's more to do with the shallow DOF and a little time spent getting the focus right makes all the difference.

Wow, what a difference!

Thanks for the pics. You're right. That does make a huge difference. I'll definitely give it a try. Are you finding this true with only the sigma lens or is this a trick you use on other lenses as well?

Only my sigma and the canon 50mm 1.8

Alough some lenses have naturally poor focusing like cheap 300mm zooms and always benefit from a bit of trial and error but that's a bit different realy.

I think a lot of it is down to the shallow DOF.

It might be different for you and I'm not saying all sigmas are perfect but it's just something to try out before taking more extreme measures because I thought exactly the same thing, how can my kit lens be sharper and focus more reliably than the sigma? It was frustrating because I got the sigma with the sole intention of getting rid of the kit lens.

But after a while I worked out how to get the focus working right and the results were much better, I'm quite impressed by the results even wide open.

Also the kit lens has a greater DOF wide open because it's 3.5 vs 2.8 which makes a difference too.

Basically if I shoot under f4 I will make sure the focus is good but anything over that I've never had a problem because the DOF by then is sufficiant to cover up a slight misfocus.

fatdeeman wrote:
And this time exactly the same settings but with TWO half presses before fully pressing the shutter and taking the pic.

Try giving it a go, if you get a similar experience then it's probably just down to the small DOF and also lowlight doesn't help.

But like I said, you can usually see the minute difference through the viewfinder and avoid having to take several shots to get the sharp one.

I thought there was something wrong when I first got mine too but since then I've just learned that it's more to do with the shallow DOF and a little time spent getting the focus right makes all the difference.

Used this trick with my 18-125 Sigma and it works most of the time. I did wonder if it was just me , thanks for sharing.

I use CFn 4-1 or 4-3 to get around this type of problem.
I use Servo AF for all type of shots and it seems to help with all types of lenses.
If you give the button a half second push it'll be re-focusing all that time and usually settles down.

It was the only way I could get in focus shots with my Sigma 18-125 also. Tested many times thanks to fstopjojo's findings _ finally got annoyed enough and sold it.