I have been out taking some evening photos (just after sunset) and I have noticed that in a number of instances the higher f numbers ( 14+ ) are appearing more blurred than the lower f numbers ( < 14). I use a tripod and take a wide range of photos so I have been comparing the same photo with different f stops.
I am using a Nikon D3000 and begin by framing the photo and then setting the F Stop and adjusting the apature based upon the meter on the camera display. I use a tripod so I was expecting the majority of images to come out sharp. I was wondering if I may have set the F stop too high and not let enough light in.
For example, a photo taken at 62mm focal length at f/14 is appearing more sharp than an image taken at the same focal length and at f/22.
Answer
You've hit the diffraction limit. That link has some amazing answers with a lot of detail, so I won't be redundant, but in short, once the aperture gets to be below certain physical size, diffraction causes inevitable blur. For your camera (and any other camera with an APS-C-sized sensor), the limit is a little beyond f/11.
The amount of light let in doesn't really matter. If that's the case, your image will be underexposed, but this effect will happen in either case.
The bad news is: you can't really do anything about this. But, you might still get an overall benefit in across-the-frame sharpness — the overall sharpness may be a bit lower, but near and far details may be closer in sharpness giving a more unified appearance. See Do smaller apertures provide more depth of field past the diffraction limit, even if peak sharpness suffers? for more.
The good news is: you now know your equipment better, and can decide if sharpness of the point of focus is more or less important than depth of field for a given scene.
Also, if your goal in stopping down is actually to take a longer exposure in bright light, take a look at What are neutral density filters and how do I use them to create long exposures in daylight?
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