right i may have found a better way to explain what i want to know now lol
say i wanted to put a big photo on the side of a double decker bus
presuming im on the right track i would take a picture of the object with a digital camera with a high resolution ...... yes?
then when i blow the picture up to the size of the bus it would still look good because its high resolution ie:- megapixels?
but what would i have done if digital cameras were not invented?
could you still do it with film?
i am not taking the mickey here folks im really intrigued
By trial and error we knew the limits of the different film types on different formats.
'Slow' film - 25-64iso film was used for high quality images.
General-purpose film was about 100-200iso. Most hobbyist photographers would use this, but also a lot of Press work when light wasn't an issue.
High-speed was anything from 200-400iso.
Most press work was done with this film (first exclusively B&W and later on Colour as newspapers introduced colour-printing) and here the real test of your abilities occurred as you eked every grain of quality out of that film as you pushed it, sometimes as high as
1600iso 
to make the most of what light you had available...lol
Real men were masters at finding different ways of manipulating the processing-chemistry to get better images at higher iso's...
Also, colour-transparency (slide) film was used for most professional applications, whereas colour-neg film was generally regarded as the preserve of hobbyists...
Whereas now we just use a camera with a higher Mpx-count such as a D3x to produce large images, in
'Ye Olden Days of Filme' we used different camera formats.
35mm was generally but not exclusively the preserve of Press-toggers.
Medium-format: Hasselblad, Pentax 6x7, Mamiya etc. were generally used for fashion, advertising and editorial as they were generally expected to be used for larger prints, glossy-magazines and bill-boards.
5x4 Plate Cameras such as the Sinar and Toyo were used for very high-quality work, some portrait photographers used them and some fashion and advertising work as well as a lot of product photography - that which called for very high-quality images. Otherwise that too would have been done on medium-format.
For ultra-ultra high-quality work, 10x8 plate cameras were used. massive group photos are still done on these cameras as digital still isn't up to that unless you perform a multi image 'stitch' (which we still do on occasion, such as a Battalion Photograph - 800+ people - in front of Edinburgh Castle or similar).
