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F number "equivalence" and M43

eddystone

Member
There seem to be a number of posts on other forums (I know, get a life!) insisting that, for example, an M43 75/1.8 is equivalent to an FF 150mm 3.6 (or thereabouts) if such a thing existed, in all respects including exposure. Surely this is not right. As far I'm aware if you take cameras of different formats the same F number will give the same exposure if the other parameters, i.e ISO and shutter speed, are the same, provided you don't stick something like a teleconverter or extension tube between the lens and mount. I haven't actually tested this with digital but in the film days I had 35mm and 6x7 cameras and if I transferred settings from an incident reading with a separate exposure meter I would assume F4 on a 35mm lens was the same as F4 on 6x7 for the purpose of correct exposure.
 

MGrayson

Subscriber and Workshop Member
There seem to be a number of posts on other forums (I know, get a life!) insisting that, for example, an M43 75/1.8 is equivalent to an FF 150mm 3.6 (or thereabouts) if such a thing existed, in all respects including exposure. Surely this is not right. As far I'm aware if you take cameras of different formats the same F number will give the same exposure if the other parameters, i.e ISO and shutter speed, are the same, provided you don't stick something like a teleconverter or extension tube between the lens and mount. I haven't actually tested this with digital but in the film days I had 35mm and 6x7 cameras and if I transferred settings from an incident reading with a separate exposure meter I would assume F4 on a 35mm lens was the same as F4 on 6x7 for the purpose of correct exposure.
:lecture:
The rules of equivalence (this is math, not reality) require that for two cameras of different sensor sizes (ratio k) to take the same picture, the shutter speeds have to be the same (motion blur). The location and FoV must be the same, so the focal length of bigger camera is k times larger. For equal DoF, the f number of the bigger camera must be k times as large (not immediately obvious - it's a calculation), and to get the same exposure with that setup, the ISO of the bigger camera must be k times as large. This assumes that they use the same sensor technology, and a ton of other assumptions not met in real life. But these are a starting point.

The only thing missing in the claim above is that the ISO of the FF camera would have to be twice the ISO of the µ43.

Oh, and the 75/1.8 is such a fantastic lens that who cares? If you want ultra-shallow DoF, get a Canon 85/1.2.

Matt
 

pegelli

Well-known member
Matt, that's the most concise and clear story I have read on the equivalence "theory" comparing sensor sizes.

I'll bookmark it for later reference :thumbs:
 

Godfrey

Well-known member
Ultimately, I've found it best to dispense with all this equivalence thinking and just learn whatever camera and lens I have in hand so I can get the best out of it. As you intimate, the f/number give equal exposure while depth of field varies because of differences in format, focal length, and the resulting physical size of the lens opening.

I tend to think of it this way:
  • What is normal? For m43, that's about 25mm. For 35mm, about 50mm.
  • What is wide? For m43, about 18mm or shorter. For 35mm, about 35mm or shorter.
  • What is portrait? For m43, about 35mm and longer. For 35mm, about 75mm and longer.
  • For m43, I want larger lens openings by about two stops to get similar DoF than 35mm.

Obviously limitations set in regarding how shallow/how fast a lens I can get to work with m43, and how wide I can get by how short a lens. The limits in 35mm (and larger) format are determined by how big and heavy a lens I'm willing to carry and, again, what's available. So I use smaller formats like m43 and APS-C for when I want longer reach, and I use larger formats like 35mm and 6x6 for when I want wider FoV.

One system, one format, can never do all things best. :D

G
 

eddystone

Member
Ultimately, I've found it best to dispense with all this equivalence thinking and just learn whatever camera and lens I have in hand so I can get the best out of it. As you intimate, the f/number give equal exposure while depth of field varies because of differences in format, focal length, and the resulting physical size of the lens opening.

I tend to think of it this way:
  • What is normal? For m43, that's about 25mm. For 35mm, about 50mm.
  • What is wide? For m43, about 18mm or shorter. For 35mm, about 35mm or shorter.
  • What is portrait? For m43, about 35mm and longer. For 35mm, about 75mm and longer.
  • For m43, I want larger lens openings by about two stops to get similar DoF than 35mm.

Obviously limitations set in regarding how shallow/how fast a lens I can get to work with m43, and how wide I can get by how short a lens. The limits in 35mm (and larger) format are determined by how big and heavy a lens I'm willing to carry and, again, what's available. So I use smaller formats like m43 and APS-C for when I want longer reach, and I use larger formats like 35mm and 6x6 for when I want wider FoV.

One system, one format, can never do all things best. :D

G
Well yes I get all that concerning FOV and DoF which is what I've always understood but I've always believed in 1:1 equivalence when it comes to exposure for a given F stop, i.e. F4 is the same exposure on all formats assuming ISO and shutter speed is the same. Otherwise, you would have to apply an adjustment when using an external meter. Frankly I haven't used one for a very very long time, but I don't remember that when using T90 and P67 side by side.
 

pegelli

Well-known member
Well yes I get all that concerning FOV and DoF which is what I've always understood but I've always believed in 1:1 equivalence when it comes to exposure for a given F stop, i.e. F4 is the same exposure on all formats assuming ISO and shutter speed is the same. Otherwise, you would have to apply an adjustment when using an external meter. Frankly I haven't used one for a very very long time, but I don't remember that when using T90 and P67 side by side.
I think the way to look at it is that a smaller sensor with the same lens/aperture gets the same amount of light per unit area, so for a given scene you need the same f-stop/time/iso irrespective of sensor size.

However a smaller sensor has less surface area so it receives a lower amount of total light (again assuming the same lens/aperture). So the people who claim this are theoretically right but for taking photo's in the field it's not relevant to determine the settings you need to expose properly.
 

Godfrey

Well-known member
Well yes I get all that concerning FOV and DoF which is what I've always understood but I've always believed in 1:1 equivalence when it comes to exposure for a given F stop, i.e. F4 is the same exposure on all formats assuming ISO and shutter speed is the same. Otherwise, you would have to apply an adjustment when using an external meter. Frankly I haven't used one for a very very long time, but I don't remember that when using T90 and P67 side by side.
Did I say anything different?

With respect to exposure, the f/number system is normalized specifically to permit any lens, on any format, to pass the same amount of light energy per unit area to the receiving medium in order to permit commonality of concept and exposure consistency regardless of format size, lens focal length, and film vs digital.

I'm getting confused as to what your question is.

G
 

eddystone

Member
I think the way to look at it is that a smaller sensor with the same lens/aperture gets the same amount of light per unit area, so for a given scene you need the same f-stop/time/iso irrespective of sensor size.

However a smaller sensor has less surface area so it receives a lower amount of total light (again assuming the same lens/aperture). So the people who claim this are theoretically right but for taking photo's in the field it's not relevant to determine the settings you need to expose properly.
Thank you - that response answers my question perfectly
 

Shashin

Well-known member
There seem to be a number of posts on other forums (I know, get a life!) insisting that, for example, an M43 75/1.8 is equivalent to an FF 150mm 3.6 (or thereabouts) if such a thing existed, in all respects including exposure. Surely this is not right.
You are right to question this.

As far as depth of field, it is a good approximation, but there are conditions where this falls apart--usually with macro photography.

The problem with equivalency, as Matt points out, this is really just a collection of geometric relationships that require a lot of assumptions and do not take how photography actually works into account. F-number is lens speed, not depth of field. Yes, you can model the human visual system in its perception of a print (and depth of field is the area that is perceived to be sharp in a print and nothing inherent in the camera image). But f-number does not represent depth of field per se.

The other red herring is the idea of "light gathering." That argument is great if you want to pick winners and losers as the larger sensor always has the potential of intersecting more light. But here is the problem--pixels are discrete--what one pixel "gathers" has no baring on what another one does. Now, larger sensors can have larger pixels assuming the same pixel resolution (another assumption)), but the advantage is the pixel has a greater area, not that they cover a larger format. Pixel area is also known as detector size. But play a thought experiment: if pixel were perfect in that they would count every photon and have no noise, what would a larger sensor provide in and of itself?

The other problem with equivalency is that it assumes an absolute frame for comparison. But we don't use cameras the same way and there is a large variation in how a photograph can be taken. If you take random images and displayed them, could you actually identify the sensor size that produced the image. You would not probably succeed beyond random chance. This is why when equivalent images are compared, they need to be under very controlled conditions and then compared at extremes: 100% monitor view, high ISOs, extreme lighting, etc. None of those condition represent actual viewing conditions of a final image.

And lastly, equivalency is cherry picking. The comparison aligns the variables that supports an argument, but it is impossible to make every variable "equivalent." As a simple example, do you fix the number of pixels or the pixel pitch? The equivalence would argue to fix the number of pixels, but it is the pixel pitch (detector size) that is related to pixel efficiency. Their argument is the the image should "look like" each other. But here is the thing: if pixels are not resolved by the human visual systems, can a viewer distinguish between two images of different pixel resolutions? You don't have to have the same number of pixels to make the same visual response.

Yes, equivalency becomes an interesting learning tool to understand some of the relationships to sensor size and things like depth of field. But it is a model to make comparisons. It is not a scientific imaging theory.

Sensor size does not change focal length or f-number. Just like you cannot use focal length or f-number to change sensor size (I mean if they those are real relationships, then they work both ways).
 

Godfrey

Well-known member
... Thus my earlier response: I gave up on equivalency some time ago. I learn what my cameras' FoV and DoF relationships are with my various lenses, and learn what their response are in DR and rendering qualities. And then I start making photographs. :)

G
 

ptomsu

Workshop Member
Meanwhile shooting m43 (and 43) for the last 16 years I must say I no longer care about anything like "equivalent" between FF, APSC, MFD and m43.

I know how my lenses will draw on my m43 camera and what are their weaknesses and strength. I know the same for any of the other formats as I shot all of them and will shoot Leica M (FF) asap if I can get hold of a good second hand M240 or M10 body.

Why this answer? I think everyone shooting a certain system should become familiar with the properties of that specific system and sensor size and lenses. This takes some time and maybe some use of 2 or 3 systems in parallel but then this magically happens - it did for me and I am sure it will do for everyone really interested in this stuff.

So just try and do no fear too much failures - you will learn by shooting and then switches between systems (and sensor sizes) become second nature. And don't believe the BS that you cannot get shallow DOF with m43 - it is easier than you think, but maybe not as easy as getting super DOF with FF (or MFD).

Just forget about all these theoretical considerations and equivalences, take whatever system you have out and shoot :thumbup:
 

pegelli

Well-known member
I don't think the OP was really worried about equivalence, he was only asking about the story he read somewhere that you needed to adjust exposure for a smaller sensor (using the same lens/shutter speed/iso) since the sensor gets less light because it's smaller. This was debunked in several different ways and his question was answered.
 

eddystone

Member
I don't think the OP was really worried about equivalence, he was only asking about the story he read somewhere that you needed to adjust exposure for a smaller sensor (using the same lens/shutter speed/iso) since the sensor gets less light because it's smaller. This was debunked in several different ways and his question was answered.
Indeed it was.
 
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