To UncleFrank: I also have this issue, and can't resolve it. Especially when it comes to old lenses.
Resolution - imagine we have 2 lines. How far apart should they be to appear as 2 separate lines, not as one object? That is why normally we define resolution as ability to record certain number of line pairs per millimeter of film (abbreviations used lppm and lpm - second one is confusing, as it is also used in the meaning of lines per millimeter, that is twice as many as line pairs per millimeter).
Things are not so easy if the dimensions of the recording media are different, like in 35mm film vs. medium format film, or when it comes to comparison of different digital cameras with different sensor sizes.
In these cases we need to account not only for resolution in line pairs per millimeter of media, but also for how many line pairs we are able to record on the whole width of media.
In 35mm and medium format films we can have same film, with th same resolution in line pairs per millimeter. Yet, because of more millimeters in medium format, there is more information recorded on it. Let's assume film is 110 lppm. On 35mm film we can record 3850 line pairs totally, while on medium format film it will be 6050 line pairs.
Now, let's imagine we have 35mm film with 170 lppm resolution. In this case we will record same number of line pairs as with medium format film with 110 lppm film.
Digital analog of this situation is D2x vs. 1DsMkII.
D2x resolves more line pairs per millimeter, but 1DsMkII has larger sensor and more pixel count. To make a fair comparison, we first need to measure how many line pairs each of the cameras can record across sensor width.
Now, if we are able to fill the frame with the scene (say, by using 200mm lens on D2x and 300mm lens on 1DsMkII), we can predict (less taking into account the weight of lenses/shake and the fact that often 300mm lenses have less resolving power then 200mm lenses) we can compare resolution in this scene.
Imagine we can not fill the scene - like in both cases same focal length is used (I'm shooting on the arena next to Canon guy, and we both have 300/2.8 lenses on - very typical). In this case, knowing the dimensions of recording of the subject of interest, we compare the resolution on the subject.
The above two cases are called scene resolution and subject resolution.