My dilemma is that I know nothing of this art. I had bought a Sony ds75 to just capture some pics and got hooked. I had it a year and it left me empty. I just purchased a canon 10D and a Sigma 24-135 lens f:28-5:4. I just don't know how to use the equipment. I could have never guessed that photography could be so frustrating trying to get the pics that you want. This lens, when it is working, gets super pics. I am not sure what itís capable of. Of the 200 or so pics I have taken only a small fraction are worth keeping. With the 10Dís focus issue, I am not sure if itís me or the lens. It is very frustrating and any help would be appreciated.
Some of your problems may be with the equipment but you won't know that until you learn a lot. I'm going to assume that in the past all you did was point and click to get the picture that you wanted. This won't work as well now because the equipment that you have is not as forgiving as a point and shoot but that is also why you can get more pleasing pictures with it. You have to learn what aperture is and it's affects on the picture. To go along with that you also have to learn about shutter speed and the different ISO settings. These 2 settings combined with the aperture all work together to give the resulting picture. If you let the camera make all of the settings for you then it is not always going to choose correctly. By correctly I mean that it's not going to choose the exposure and DOF (depth of field) that you wanted. That's why some of your pictures look good and some don't.
It's my opinion that you need to put the camera in manual mode and shoot like that. You will learn very fast and you'll probably find yourself shooting like that most of the time. Now I almost always shoot in manual since I put it in that mode to learn. After you begin to grasp exposure and DOF then you will also need to learn about how to process your pictures for the best results. It may all seem overwhelming to you right now but as you start to learn you will realize that it's not as complicated as it seems to learn the basics but it takes a long time to perfect your own style of photography.
Thank you for your comments but there are much better sites out there. I only put this together to try and help people like yourself get started. Go to this site and learn, not just read, lesson 3. It's talking mostly about P&S cameras but it does a good job of explaining shutter speed, aperture, ISO settings and how they all work together. If you learn and practice what is written there then you will have a good start to build on.
Once you get started in learning how to control your equipment then you will be able to figure out if it was the equipment or you that didn't get the shot. The 10D is a good camera with some limitations. However, I have not heard good things about that lens. I would try to stick with Canon lenses or the EX line of Sigma lenses. Yes they are a little more expensive but most of the time they are worth it. After you learn how to control your camera then you will be able to make better choices as to what lenses you want. For now use what you have. I think you'll find that if you stop down that lens a little then you will get better (sharper) results. If you don't understand what I just said then "learn" that lesson above and you will.
Don't get frustrated. Keep at it and you will be happy that you did.