Some thoughts....
- On a crop body, especially with a high pixel density, diffraction will make your images look soft with apertures narrower than f/16. In fact, even beyond f/8 you may see MTF figures dropping. I don't know what body ot lens you are using but your success will be influenced by equipment capability as well as your own skills and the environment as a whole.
- Assuming you are shooting with wideish to ultra wide focal lengths, say 35mm or less on a crop body, then even at f/8 and 35mm the hyperfocal distance is 26.6', giving a close focus distance of just 13.3'. So focus at a distance of roughly the length of a through lounge and your close focus will be around the length of a dining room all the way through to infinity. These distances should not be so hard to judge. At shorter focal lengths it should be even easier to keep more of the scene in focus. However, be aware that lenses often do not deliver their best performance at either extreme of the focal length range. Some can be especially weak at their widest setting. Check out MTF figures at varying focal lengths and apertures for different lenses here -
http://www.photozone.de/Reviews/Canon EOS Lens Tests.
- Things that are in the far distance can easily be affected by atmospheric effects like heat shimmer, mist/haze (especially over water), pollution etc.. Check with your own eyes that the atmosphere is clean when you take the shot, before assuming the equipment is at fault.
- When you shoot distant objects with a wide angle lens the details are simply too small to register as individual details on the sensor pixels. When the leaves on a distant tree cover less than one pixel each it will simply not be possible to record those details. Pixel peeping at 100% or 200% cannot magic out details that are physically impossible to capture. In general terms, you need to concentrate your focusing efforts nearer to you, to render sharply the objects where details can be seen easily and captured well. You may expect a sharp horizon 3 miles away, but you cannot expect to see fine details in your typical wide angle shot at that distance.
Other factors such as high ISO and consequential noise processing should be considered, as well as shutter speeds or equipment setup that may allow camera shake to show through.
Perhaps if you posted a sample image complete with EXIF data people could offer some more ideas. Ultimately, though, the hyperfocal distance and online calculators for these things should serve you perfectly well. Just don't be fooled into using a smaller aperture than you need - f/8 will likely be fine for many landscape situations.
