The first sample image in the question is focused well in front of the central pillar. The second is focused well behind the flowers.
You have told us that you are using single point AF but you haven't told us which AF mode you are using: One Shot, AI Servo, or AI Focus? If you're trying to focus and recompose using AI Servo the camera will refocus when you move the camera to point at a different spot. If you use AI Focus the camera will initially hold focus as it would in One Shot mode, but if you recompose and hold the camera too long in the new position the camera will sense that your selected AF point is no longer in focus and will switch over to AI Servo.
There appear to be other issues at work that might also be contributing to your results:
Diffraction The 70D is a Canon APS-C camera with 4.1µm pixel pitch. The Diffraction Limited Aperture (DLA) of the 70D is f/6.6. This is the point at which the effects of diffraction begin when viewed at the pixel level. As apertures are narrowed beyond the DLA the results get more and more noticeable at normal viewing sizes. The best way to avoid this is to shoot at around f/8 or wider and at f/6.3 or wider if possible.
Camera movement Not everyone can hold a camera steady enough to use the 1/focal length rule-of-thumb, even when viewing at the standard 8x10 sizes for which it applies. You may get useable results for viewing at smaller sizes, but nowhere near the equivalent viewing size of looking at part of an image at 100% on your monitor. If you have an HD (1920x1080 pixels) monitor that measures 23" diagonally you are viewing images at 96 ppi. That means an 18MP image viewed at 100% is being magnified at the equivalent of 54x36 inches! That's 5X the magnification of the standard 8x10 print.
The optical limits of your lens I'd like to know where you read excellent reviews of this lens. I've never seen any critical reviews from reputable reviewers written about it that impressed me very much. Before you can blame AF you need to be sure that something else isn't causing your images to be blurred. To do that you need to eliminate as many of the other possible causes as you can.
- Mount your camera on a stable tripod, turn off optical image stabilisation, and use a cable release or the self timer to release the shutter. This will help eliminate camera movement as the source of your problem.
- Shoot under bright enough constant lighting that your shutter speed at ISO 100 can be 1/100 second or faster. Use the fullest spectrum lights available to you. This will further help to eliminate camera movement including vibrations caused by the movement of the camera's mirror. Properly exposing using low ISO will also help eliminate poor image quality caused by a low signal-to-noise ratio and the resulting noise reduction.
- Use a flat target that is lined up parallel with your camera's image sensor and perpendicular to the optical axis of the lens. An easy way to do this is to aim your camera at a flat, stable mirror. Center the viewfinder on the center of reflection of the lens in the mirror. Then tape your focus target onto the mirror being careful not to move the mirror.
- Use careful manual focus with magnified Live View. Take several samples while refocusing manually between each sample.
- Repeat the test shots using One Shot AF mode with the single center focus point selected. Move the lens to infinity or minimum focus between each test shot. Use a half shutter press with your cable release to allow the AF to confirm focus before taking the photo.
- Compare the best of the manually focused shots to the best of the AF shots.
If there is a significant difference then you have an AF issue. If there is not a significant difference then your problem lies elsewhere.
Comment from the OP:
I still doubt that there is a focus issue. I have been using Single shot AF in almost all pictures. And shooting at 4-5 times of 1/focal length at ISO less than 800 I doubt the hardware. Of course its difficult to doubt on ones own abilities! :)
Look at the examples you posted carefully at 100% magnification. You can tell by the cobblestones in the first image that focus was missed (based on what you said you attempted to focus). The second image is focused well behind the flowers in the foreground. The clock tower in the background is the most in focus area ofthe image. There are areas in both images that are in focus, they're just not where you wanted to focus.
