I think most people will find that focusing manually, through a viewfinder, is a skill which takes some practice to master. I've been focusing manually since the early 80's and it is by far my preferred method of operation, particularly where the
depth of field is very shallow and accurate focus is most critical.
MF will be affected by many factors and in the best circumstances can be relatively easy but in poor circumstances can be extremely difficult at best.
Some cameras, in fact almost all amateur cameras, are not designed to be very good at MF as most amateur cameras are used with AF so manual focus is not a big issue. They typically have a small viewfinder, low image magnification, and a focusing screen designed to give a bright image rather than one which allows accurate focusing (they are not the same thing). Modern 'pro' cameras such a 1ds3 (and similar) are designed to focus manually. Intermediate cameras like the 5d2 can be quite poor. I regularly use a 5d2 and have difficulty with it if focusing through the viewfinder (Live View is another matter). Some times you can replace the focusing screen in the camera to improve focusing accuracy, at the cost of brightness, and I certainly recommend doing that if you intend to use MF regularly.
The
lens you are using will also be a major factor. Consider that the image you see when focusing is the image that the
lens creates when it is wide open. Most
lenses are not at their best when wide open and some can even be a bit fuzzy/poor so if you are already focusing with an image which is not very
sharp then the task becomes even harder. A high quality
lens might have a better image when wide open so focusing can become much easier. The maximum
aperture of the
lens will also be a factor. An F1.4
lens will obviously give a very bright image but the image might not be as
sharp as that from a similar but slower
lens, say a macro. It’s not exactly a ‘trick’ but you can use the
depth of field preview function on a camera to
stop it down one or 2
stops and focus that way, giving you a sharper image with which to focus. The other extreme is the kit
lens which is typically not very
sharp wide open and then usually very slow as well. Frankly you are pushing shit up hill with these. There are other factors too, such as field curvature and focus shift, walk first, run later.
Even in the best conditions you need to practice to recognise what a
sharp image looks like in your camera. There's always a certain amount of 'wiggle' room where the image may not really seem to change very much but in fact the image will have changed significantly. Recognising the exact point where the image is perfectly focused with your camera may take some practice and comparing it to what you see in Live View is a good way to check how you are performing.
If you wear glasses then remember that the image you are viewing corrected to appear approx 1m away so you need your distance, not reading, correction.
JJ