Most important when mixing mount types are the specs of both mounts, especially Diameter and Register (distance between lens and sensor / film). Obviously, a short Register of a lens cannot be extended and a small Diameter of a body also cannot be extended. So in general: a lens with smaller Diameter and longer Register than the original lens can be mounted - provided there is an adapter.
Example: Nikon lenses can be mounted on Canon bodies - but not vice versa. More about Canon and manual lenses: http://photonotes.org/articles/eos-manual-lenses/
Some more good reading here: http://www.markerink.org/WJM/HTML/mounts.htm
I recently had a friend with a similar problem (a good problem ). Too many legacy Nikon and RF lenses. So after going through the various options, I advised him to get a u4/3 system. It takes both the Nikon and RF lenses using an adapter. He got a G1 and has been very happy with it since.
I suggest you read up on the 4/3 system before you jump though. There are trade-offs, but in my friend's case, it made the most sense since he did not need to invest in new glass, had a limited budget, could live with the implications of using 4/3s and the RF lenses being small complemented very well with a smallish 4/3s camera.
My father is a old photography freak and a Leica lens fan. He own a few leica lens that are used to attached into his old manual camera and he wants to move on to a DSLR but his condition is that it has to be able to attached his old Leica lens and new ones.
I have taken a look at the Leica webpage but the R, M and S systems confuses me. and i have no idea what to look out for. Also there is no Leica section in clubsnap.
So therefore i think the best alternative is to get a DSLR which my dad's Leica lens can be fully utilize.
Have a look at Wikipedia: Leica for the different mount types. First you will need to determine which kind of lenses your father has. Are they M or R type lenses? AFAIK, the R type lenses can be mounted on DSLR.
What do you mean with 'full use'? The lens as an optical element will always do its job. On a crop sensor body you will have the same effects as with any other lens: crop factor will reduce field of view. It's the camera body you will need to pay more attention to since there is no electrical connection to the lens. All recent DSLR have communication with the lens for aperture, focus and also metering (Nikon). Read up about the various brands of cameras how they support other lenses via adapters. Of course it's never officially supported but many people try and bring their results online. Also, certain adapters come with integrated chips that simulate a lens in Manual Focus mode. As a result (e.g. for Canon) the AF confirmation works and helps to focus properly.
The Leica M lenses cannot be fitted onto ur typical SLRs because the flange to sensor/film distance is way too long for these lenses. even if u can mount them somehow, u need to wack out the SLR mirror assembly or lock it out of the way.. unless u dun care about focus to infinity
But micro four third systems can mount Leica M lenses with infinity focusing