Why are some of my photos tack sharp while others in the same series are out of focus?

[AO]

My camera is a Canon Rebel XS (dSLR) and I primarily use the 50mm f/1.8 lens. 

I shoot in Manual Mode with Autofocus ON.

I am having difficulty with my photos coming out incositant - some are out of focus, while others in the same series (taken a moment apart) are tack sharp.

I've attached two examples - one of the out-of-focus image, the other sharp images (both have been blown up).

It is not camera shake and my settings are identical for both photos. (ISO 200 | f/4.5 | 1/100sec in this instance.)

This has become such a problem that I find myself needing to take 10x more photos of the same subject/pose in order to hopefully get one sharp image from the series.

I have asked multiple people about this problem (though I have yet to take my camera and lens to a shop due to lack of funds) and no one has any idea of either what I'm talking about or what the problem could be.

I would appreciate your thoughts and help!

Thank you! 

Fuzzy/Out of Focus:

Sharp:

[AO]

Thank you so much!!

I plan to perform the focus test in the next couple days.

[bmac]

I would check your focus with live view to determine if it's lens or camera body. Live view and viewfinder use different systems for autofocus, Canon has had a huge problem with viewfinder autofocus consistency with the 7D and now the 70D.

Good luck, I'm wrestling with the save viewfinder autofocus incosistency on my 70D.

BMac

[ebiggs1]

I also think it is camera motion or shake.  As nothing seems in focus. With a 50mm lens at f4.5 one would think something would be in focus.  I am not a fan of ths Canon lens.  I have never been but lots of folks are and they like it.

I agree the lens may be faulty and you must confirm it is OK

[bmac]

If it's tack sharp in live view and not in viewfinder thens it's not the lens.

BMac