Auto Focus Problems

Hi All,

I'm new to Photo net, and am wondering if anyone has the same problem. I focu

on my subject, using my (mainly middle) selection point, and recompose, or

sometimes keep it right there, but my pix are soft. I know there's depth of

field issues, but when you're shooting a wedding, or people, and its happening

quickly, there's no time to ask the bride/minister/child to stand still, put

their hand out so you can focus a little in front, or if i'm doing a group,

and the shutter speed it too slow at F11, or F16, I land up with 'softness' -

how do I get around this problem (and I'm using flash!) Equipment - Canon 30D,

Sigma lenses, Flash 580EX - is it just a lens thing, should I sell them all,

I'm thinking of getting the 24-105mm IS L USM , but its not a F2.8! I have

the 70-200 IS L USM, but its damn heavy and the angle not wide enough for all

occasions!

Thanks

Robyn,

 

If you're having the same problem with all your lenses I'd have to look at my technique. You mention shooting at f/11 and f/16, apertures at which diffraction can come in to play and soften your images. Why are you using such small apertures for wedding/portrait photography ? The resulting low shutter speed will certainly contribute as it may be too slow to eliminate subject motion. Smaller aperture also equals greater DOF, something one usually doesn't want in this type of work. Unless I've misread your post in some way, I don't think the problem is the lenses so much as the aperture settings they're being used at. Good luck.

Robin, carry out this experiment, I would like to know your reply.

 

Buy a A4 sheet mainly dark green with golden stripes on it. local shops are selling those papers.

 

Then position your 70-200IS lens and camera, I used a 350D+70-200IS, on a tripod with remote release cable, cannot remember if I used a 12mm tube to bring it closer.

Take a rose, fix it to another something so it cannot move, the aim at it with centre focus, and zoom till it fills say about the frame, say 80% or so.

 

Now take that A4 green paper hold it still in one hand, look through viewfinder and shoot the rose say 10x times..Myne was sharp all 10.

 

Now move that A4 green paper up and down while taking the next 10 pics..Myne were all noticibly soft...

 

I am not sure what this prove but to me it proves that the movement on non-focussed area can have an influence on sharpnes..?

THanks guys for your contributions. As to F16, I've very harsh light here, as well as with a big group, I sometimes find on anything less (F4?) those on the edges are not always in focus, so am trying to find happy mediums Re the experiment - I must focus on the rose (no striped paper involved) and take pix, and then next set of pix, still focus on rose and moved the striped paper up and down?

I'm in South Africa, and we have hellish hot, bright sun here - so bear that in mind as well - which is why someitmes I'm going for F11/ F16 - reckon I should rather keep me shutter speeds up?<div></div>

There have been several postings lately in the various forums (fora?) recently discussing flatness of field. This should not be a problem at f11 or f16 but may become one at larger openings. Maybe you should try a few snaps of brick walls to test settings.

I can think of a few issues that might be contributing to your softness.

 

First, if you focus and hold, then turn you camera to re-frame your scene, your plane of focus turns with the camera, and the original subject is no longer laying in the plane of focus. This more commonly shows up with wide to normal lenses, as you generally turn more when you re-frame. Using a smaller apertures will reduce this of course, but the real solution is to use a side AF point so you don't have to move much after focusing.

 

If you are using a small aperture of f/16, you may be encountering softness due to defraction limits. f/8 or f/11 will usually be sharper on all but the longest lenses.

 

Sigma lenses are known for erratic AF issues on Canon cameras. Some focus close and others far, but most are just inconsistent, especially in low light. With my 18-50 f/2.8, it is very common that I can AF, release, and press the AF button again, and the focus will move a second time. Sometimes the focus point will visibly shift every time the AF button is pressed, and never seem to get it right. Most of those times the focus is not correct. This resulted in too many soft images, so I bought Canon's 17-55 f/2.8 IS. Now focus is sure every time, and slightly faster too. Besides, the addition of IS makes low light shooting far more reliable.

 

I think 17-55 is a better range for weddings than 24-105 when used on a 30D. Costs a bit less too. But of course it will not work with full frame cameras if you are planning to upgrade.

Here's a trick it took me a while to learn. When you auto-focus and then recompose the shot, generally you recompose by swiveling the camera to where you want it. No big deal, right? Well, that swivel loses the focus. I know, we're only moving the camera a little bit, but that's just enough to throw off the focus. The main reason for this, I believe, is that we're changing the angle at which the lens faces the subject, thereby changing the distance from the lens to the subject. So now, instead of swiveling, I slide the camera into position. Does this make sense? Think of a normal door; it swivels on its hinges. Now think of a sliding glass door. It moves from left to right. So next time you recompose slide, don't swivel, slide instead. With a little practice it will keep the distance uniform.

 

Another thing you may try is using prime lenses instead of zoom lenses; my primes focus way better than my zooms. If you must get zooms, get L glass (I can't afford it but I hear the 24-80 L is the bomb).

 

Another technique to try is shooting in "AI Servo" mode for your auto-focus. I use this for live concert photography and it really helps as it "tracks" moving subjects and allows you to capture moments you probably wouldn't be able to otherwise. Try out these techniques, I hope they help!