DOF discussions are never complete without circle of confusion specifications. (...)
So really, equivalence is very tough to define.
Aren't the CoC-sizes used by, for example, DOF-master OK for the discussion? I don't care about the exact values maybe but the relation between the different sizes of the CoC should be clear; 4/3 CoC needs to be half of the FF CoC to achieve the same resolution as I understand it.
But, I'm interested in learning more if needed for taking part in a discussion.
I have for example always assumed that people are talking about a common situation and goal when discussing equivalence, for example; the same motif, the same vantage point, the same print size as the end result and as a sidenote an agreement about not thinking about the differences in aspect of ratio. That seems complicated and simple enough for me. Is there anything here I have got wrong please let me know.
50/0.95 on NEX (details of areas in focus get mushed up- problem with the NEX sensor). The lens did not "become" a 75mm f/1.5 "equivalent" (and it never will).
What would the use of the same 50/0.95 on a G1 make it an equivalent of? 100mm f/2?!
A lens never becomes anything. How could it? We should be able to agree a lens is what it is no matter what adapter you put it on.
I have a 98mm f/1 lens that covers 6x6cm. Does it "become" slower when used on smaller format cameras?
Heck, no.
Really, why do ask/mention this? My guess is that you are very much aware about how the FOV changes when moving a lens between cameras with different sensor sizes. Right?
I'm also pretty sure you know, and very well so, how the DOF is changed by the fact that you use different aperture sizes
and take the images at different shooting distances. Right?
Then what are you arguing about?
If you want to discuss the differences in image quality and lens rendering styles between super fast lenses and medium slow lenses for a bigger format then we can do that. I started above when first posting about my impressions from the Nokton 25/0.95 samples.
I have owned FF cameras, 1.5, 1.6 and 2x crop cameras. I have done experiments and I have found the theories to work out surprisingly well in real life. Just saying.
:lecture:
The ONLY "Equivalent" would be what f/stop would be needed to achieve the same DOF, NOT light gathering power.
A 25mm f/.95 for a m4/3 shoot at f/.95 on a m4/3 camera will have the same DOF as a 50mm f/1.5 on a FF camera...BUT it still has the light gathering power of f/.95, even though the DOF is deeper BECAUSE OF the sensor size.
The senor size affects DOF, not light gathering power...
(...)
f/2 on a m4/3 lens, lets in the same light gathering power as a f/2 on a FF camera, or a 6x6 camera or a 4x5 camera... what changes is the f/stop needed to have the same DOF across the formats at any given distance.
That is the precise reason the f/stop method was developed. The formula used makes sure that regardless of the focal length, the iris opening is proportional to the focal length. fl/stop = opening size, 100mm/4 = 25mm, that is: f/4 = a 25mm opening. on a 300mm lens, f/4 = a 75mm opening. Yes it is 3x the size of the 100 f/4, BUT, it lets in THE SAME amount of light. With different formats, though, f/4 will have different DOF depths, controlled by the format (Film/sensor) size.
I would say DOF is what is interesting in these discussions. For the hard core equivalence enthusiasts "light gathering power" is important as they are interested in not only DOF but also noise. So they don't say the light gathering is the same between a µ4/3 25/1 and a FF 50/2 lens/camera combo. Using the same f-stops will make the lenses let in the same amount of light per area unit, not per image. And surely the f-stop method was developed for exposure reasons, not for DOF comparisons?
So, discussing DOF only here, and for the sake of convenience I'll round 0.95 to 1, OK?...
M5-Guy, how did you get f/1 equiv to f/1.5? In these discussions one usually multiply with the crop factor and finds a 25/1 lens is equiv to a 50/2 lens with regards to DOF. What's the reason for multiplying with a factor of 1.5?
Oh, and yes, I get bored by all this as well but I want it to be right now that people are here.
/Jonas