Originally posted by bmonki Your smarter than your own good my friend... If a lens says 50 on it and it becomes a 25 or 30 on medium format or a 75 on an APS-C it means 35mm is the standard. 35mm/FF has the lenses that a professional needs in the field I plan on going too. Like I said you can always do the math but most lenses are built with FF in mind therefore making FF/35mm the standard. Just because photographers adopted the format from film users doesn't mean it's not correct. You can throw equations and facts all day but when it comes down to it the fact is 35mm is the standard that most lenses are built around, deal with it... so that makes 35mm the standard format which all lenses are measured by... Just like I wish the US was on the metric system, it made things very complicated when I was working on air craft for the air force, you gotta have a standard format and that format is 35mm/FF...
The only time that I know that MF or LF lenses are compared to your "standard" 35mm lens is for people, usually new to the format, can determine which focal lengths they would be interested in, especially as the aspect ratio of almost any format is different, 4X5, 8X10 and 16X20 are the same but how many formats have a 4 to 5 aspect.
If you were to buy a Pentax 645 or 67 lens plus adapter and put that lens on your Pentax film or digital camera the focal length that is written on the lens for 645 or 67 will be the exact same focal length on your film or digital slr. Rent a couple of view cameras and one lens, say a 210mm lens and a 4X5 and 5X7. Mount the lens on the 4X5 and measure the distance from the lens' nodal point to the ground glass; it will be 210mm. Now mount the lens on a 5X7 and do the same and the measurement will be .......210mm. Now put a 35mm lens on a film camera and take two images on film. Have the film developed and cut out from the centre of one of them the size of a cropped sensor. The original images were both shot at the same distance and focus hence the lens in both cases is a 35mm lens. The coverage changes but the focal length will not. The older American made LF lenses are measured in inches and preceded 35mm lenses as well.
It is very common for people to compare the coverage of a lens from two different formats and in doing so it is usually the 35mm that is used as it had become the most common especially with non pros and I doubt any pros needed to compare their MF or LF lenses to 35mm in order to understand what was wide angle and what was a long lens. Car performance is some times given in quarter mile which was a standard length of some horse races.
Pentax DA and Nikon DX lenses were not built around 35mm nor were any of the MF or LF lenses. That they are still the most common for most people is true but it is also true that the focal length of a lens does not change no matter what format you put it on and going a little farther that if you put it on a MF lens it would not have the coverage that is needed to cover the sensor or the film. Not one of the MF digital cameras that I know of have a "full frame" sensor compared to their film predessors and yet I have not really read MF digital users always having to calculate the equivalence of their lens for the cropped sensor. A 35mm lens is normal on my digital and wide on my film Pentax and I do not have to figure it out.
You are arguing one thing and using some thing else to back it up and your back up to your point is incorrect. Focal length does not change. Field of view does