Is it me, or my 450D? Focussing and image quality

This one is not bad, the one on the left is slight OOF but still consider it good enough for a small print or for web view. A greater DOF by stopping down the aperture would've helped. The 450D has no cross type AF points around the perifery, only the center AF point is a cross type, and IIRC also with a more sensitive AF point (f/2.8 or slower instead of f/5.6 and slower). This will help focus better using faster primes.
So use center AF point, but where to AF when two toddlers are in the frame and the center AF point typically is on the background? I'd use BBF (Back Button Focusing). That means that you dive into your custom fn menu and set AF on the * button. That way you can start and stop AF when you want and be able to shoot pictures separate from AF.

Steps for good focus:
- Make sure you find a position so that both are on the same focal plane, ie they are are both equally far away from the camera)
- AF using BBF on the face of one toddler, recompose and immediately take a picture or two
- Rinse and repeat. You can shoot more pictures without refocusing but it's probably best to refocus when they've moved to make sure you've got a couple of good shots.

Ideally you would use an off center AF point but as none of them on the 450D are cross type accuracy is going down, esp. if they are the normal f/5.6 sensitivity type. The most recent 650D and 60D cameras do have all cross-type AF. With those you could stop down to f/4 or f/5.6 and get a good keeper rate.

Moving toddlers are a challenge to focus on, and the closer they get the harder the AF has to work to lock focus, and the harder you have to work to keep them in the frame and keep the AF point on the eye or head (which is where you want the DOF ideally). Taking a bit more distance and crop later in post will increase DOF and AF accuracy and will allow you to easier track your toddler. Getting a longer lens like the 85mm f/1.8 will also help. It can AF much faster and much more accurately than the 50mm f/1.8. Because it's a longer lens you can take a little bit more distance. Frame a bit wider and use center AF, crop later to the proper composition.

f/18 will most likely slow the shutter speed down too slow for a sharp shot. You've used flash for fill but the blurred ambient light makes for two ghost apparitions Bump up iso to 800 or 1600, make sure the shutter speed is around 1/200 and aperture is between f/4 and f/8. Line the kids up so they are at equal distance to the camera, use center AF and focus & recompose:

Same goes for this shot:

The focus is pretty good, around the viewers left eye. DOF is shallow and with this pose it's difficult to get both in focus unless you stop down, but that will make the background sharper as well. I don't think it's objectionable to have one eye OOF, but YMMV.
You could also try focusing on the bridge of the nose and hope AF will place the DOF so that both eyes are within range. Stopping down to f/4 should work. I usually use f/2.8 with the 85mm f/1.8 at this framing and most shots come out ok.

This one is good too in my book. It may seem a bit soft due to the flat overcast lighting. You can sharpen it up a little in post, and downsizing for web view and sharpening after that will also improve things.

Here I've made a quick edit of the image above, please view at original size.

Hope this helps!

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Kind regards
Imqqmi

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