Not a flame war. A legitimate lens comparison

Jim Vincent wrote:

I could not find the panasonic. You would have to help me with info on this lens. I picked the 1.4 because it was the fastest available without throwing manual focus in. I was trying to get apples to apples. The 1.4 panasonic is equivalent 50 2.8?

I believe your comparison is incorrect.

In a way yes.

Yes, with the M43 sensor being half the area of a full frame camera, the 25 f/1.4 provides the same field of view as a 50mm FF, and, it's DOF would be equivalent to a 50mm FF at f/2.8.

correct

However, relative to exposure, at ISO 200 and the same shutter speed the FF 50 would have to be an f/1.4 to match the exposure of the M43 camera with the 25 f/1.4.

also correct i believe. Or you could adjust the ISO to 400. I see your point. I did not fully understand that when I started this thread

You are mis-representing the M43 lenses when you say their f numbers must be doubled to provide equivalence to the FF lenses.

I should have more accurately said "perspective equivalence"

Most people are primarily concerned with the "speed" of a lens (as well as image quality) when considering f stops - DOF is of secondary concern.

Image quality is obviously much more than just focal length and aperture. There are plenty of fast lenses that hve to be stopped way down to be sharp. Saying DOF is a secondary concern to "speed" is just as much an opinion as saying "DOF is more important than speed". Obviously speed is more flexible than DOF.

In summary you made a great point. I did not purposefully misrepresent things. I understood that aperture is always aperture it is relative to focal length. I understood that to achieve the same look you needed to multiply aperture and focal length by crop. I did not understand that when you use a larger aperture on smaller formats that you are introducing more light. So in an odd way the "perspective equivalent" lenses do have an advantage over larger format lenses. It seems to be that they are actually "faster". It is a little bit apples and oranges but when explained correctly is very simple