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:

[hsbn]

50 1.8 lens auto focus is not that consistence. So miss-focus is expected. No lens is perfect but good lens perform better than cheap lens. That kind of performance is expected on a $100 lens. If it happens a lot, there maybe something wrong with your lens. You can send it to Canon but I don't think it worths it.

[AO]

Thank you so much! I'll have to test out how my camera does with a higher quality lens.

Thank you again for taking the time to reply - you're the first person who's given me a helpful answer!

[cuthere4]

No Image stabilisation on that lens either

[Skirball]

It's possible you got a lemon.  But I never had that kind of inconsistency with my 50mm 1.8, unless I was really pushing it with the shutter speed (I'd shoot as slow as 1/20 with it).  But at 1/100 you shouldn't have problems unless your hand is really unsteady.  Do you have other lenses?  Do you experience the same?

[ScottyP]

Good exposure for shooting manual. Do you always shoot manual, or only when using flash?

This shot could be a bit tricky for a relatively basic focus system. You have two subjects, and the center of the photo is the background. Are you using all the AF points, or have you selected a single point? Are you trying to do the "focus lock and recompose" method?

This one is stopped down to f/4.5' but in other cases, You will also definitely have a harder time shooting anything when wide open at f/1.8 because the in-focus depth of field will be razor-thin. Whenever possible, it is best to use one single AF point and place it over the key element in the photo (usually the eye closest to you for a person) and leave it there- don't try to recompose the shot.

[AO]

@Skirball- Thank you for commenting!

I do have my kit lens - 18 - 55mm - but I rarely use it so I am not sure if it gives me the same results. I will have to test it out. 

@ScottyP - Thank you! I always shoot manual - for the last year I've been using the ExpoDisc which I love because it gets my exposure and white balance at least 95% correct (often more).

This particular shot is actually of 5 subjects - but it is zoomed in at 100% in Lightroom to show more detail.

I always select a single AF point - usually the center crossfocus point. I do use the "focus lock and recompose" method at times - though whenever possible I will move around my focus point in order to avoid needing to recompose. 

For this particular shot, I center-focused on the man in the right of the image you see - he was dead center in the group of 5.

I am always in 'ONE SHOT' AF mode and in 'Single Shooting' Drive mode, if that helps.

Yes, I have noticed it is near impossible for me to acheive a sharp image when in f/1.8 - I typically stay at f2.2 or higher, depending on the number of subjects and their distance from me. 

@TCampbell- Thank you also for taking the time to consider my problem and examine the images I provided!

While I will not eliminate the possiblity that it camera shake may be contributing to the problem (or at least to this particular image), I do tend to doubt it as my hand is usually steady; I take precautions to steady my whole body;  and I have this problem even with images taken at faster shutter speeds.

This particular image was shot at ISO 200 | f/4.5 | 1/100sec.

Since this is not an isolated problem, but rather one that happens to me literally every shoot I do, I will definately have to do the test you recommended with my kit lens and 50mm. 

Thank you all so much for taking the time to reply and offer your suggestions and help. I appreciate it very much!

[AO]

If it helps, below I've included another example from the same session.

This image was taken of one subject and I focused on one of his eyes (I don't remember which one). This is also zoomed to 100% in Lightroom.

The settings are ISO 200 | f/3.5 | 1/100

Fuzzy-

Sharp-

[TCampbell]

This new pair of images does not look like camera motion -- this one looks like it missed focus.

If you use the included Canon Digital Photo Professional that came with your camera, you can right-click an image and tell it show you the focus point which were used (the camera wont know if you did a focus-and-recompose technique... so if you recompose after locking focus then the focus points indicated by DPP wouldn't be meaningful). But I bring this up because it would allow you to determine which focus point the camera claims it used.

I think you mentinoned that you used "One Shot" focus mode -- not AI Servo. One Shot should block the camera from taking the shot until it thinks it confirmed focus-lock on the selected AF point (or on at least one AF point it you allow the camera to pick from any of its available focus points.) In AI Servo the camera WILL take the shot when you fully press the button -- whether it successfully locked focus or not.

That brings us back to the subject of the focus test.

If you do not own a commercial chart, then you may want to download one from the Internet and print it. Here's a good site: http://regex.info/blog/photo-tech/focus-chart

Print the chart and follow the instructions. I deliberately run the camera out of focus to nearest focus distance and force the camera to re-focus and shoot... and I repeat this at least a half-dozen times. I then repeat... but this time deliberately run the camera out of focus to infinity before forcing the camera to focus and take the shot.

Track each shot as you do the focus test. You may see a pattern emerge (e.g. perhaps it nails focus when going in one direction to find focus... but misses in another, etc.) Such a pattern might indicate the lens is getting sloppy and needs service (it's usually not worth servicing the 50mm f/1.8 due to it's low cost.) The USM and STM lenses allow full-time manual focus override (even if the lens is auto-focusing). The basic auto-focus motor lenses (like the 50mm f/1.8) do not support this and forcing manual focus without disengaging the focus system can be harmful.

[AO]

Thank you so much!!

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