“Full Frame Equivalence” and Why It Doesn’t Matter

Started Feb 4, 2013 | Discussions thread
sean000 Veteran Member • Posts: 7,297
Re: “Full Frame Equivalence” and Why It Doesn’t Matter

Chez Wimpy wrote:

sean000 wrote:

Actually I don't mind comparing formats since I shoot two formats, but the thing that chaps my hide is when someone says, "That 45mm f/1.8 is really a 45mm f/3.5." No, it isn't. It's a 45mm f/1.8.

But isn't this a strawman? Where did you see this statement? The "chaps my hide" for most is the "The 45/1.8 is equivalent to 90/1.8 on FF" which is most definitely purported... and leads to The Troubles.

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I've seen it stated both ways numerous times. Most of the time when someone says the 45mm f/1.8 is equivalent to a 90mm f/1.8 on FF they are talking about the two things that matter most: Field of view equivalency and exposure equivalency. Exposure isn't really equivalency. Under the same conditions using the same ISO, a 45mm f/1.8 will let you use the same shutter speed as a 90mm f/1.8. Does that mean it's capturing the same total light or providing equivalent depth of field? Nope, but this is where the confusion comes in since people don't always say clearly what they mean by "equivalent."

I think the problem is that when we used to discuss equivalency on these message boards, we only meant the field of view. For years and years on the Nikon APS-C messages boards we talked about equivalency with FF and 135 film, but everyone knew that we were just talking about the field of view. We knew that it didn't mean you'd get exactly the same results as you'd get with a longer lens and a larger sensor, but the important question most people were trying to figure out was, "I came from 35mm film, so what focal lengths do I need for APS-C to get the same FOV as my favorite lenses for 135?" And a couple of years ago on this m4/3 forum it was, "I want focal lengths that match the FOV of my favorite lenses for APS-C or FF DSLR." It wasn't until the last six months or so that I started to see so many threads arguing over total light and depth of field. Maybe it's because of the newer fast lenses like the 75mm f/1.8, 12-35mm f/2.8, and 35-100mm f/2.8. I've also seen it in the comments for lens announcements like the MKII version of the Oly 75-300mm. People say things like, "Why would you want a 600mm f/12.5?" I suppose the problem is that Olympus bills the lens as equivalent to 150-600mm, but I know they are they are talking FOV. I thought that was obvious, but I guess it isn't.

This is why, to avoid there being any confusion on the subject, whenever I use equivalency I am careful to say that the 45mm f/1.8 has a FOV equivalent to 90mm on 135." What matters to me though, is that it's a nice portrait focal length on m4/3 that is fast enough for low light (thanks to the f/1.8) and it delivers enough subject isolation for my tastes.


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