A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions
Great Bustard
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A few words on Equivalence and comparing systems
Apr 2, 2013

I was reading in another thread where the 35-100 / 2.8 on mFT was being compared to a 70-200 / 2.8 on FF, and, well, you know.

So, let's see if this post can't serve as a reference for such future "discussions".

First of all, I don't think that anyone disagrees that 35-100mm on mFT has the same [diagonal] AOV (angle of view) as 70-200mm on FF.  This will result in the same framing (plus-or-minus due to the difference in the 4:3 and 3:2 aspect ratios) for the same perspective (subject-camera distance).

Furthermore, I hope everyone understands that f/2.8 on mFT results in the same DOF (depth-of-field) as f/5.6 on FF for a given perspective and framing, or, conversely, that f/2.8 on FF results in the same DOF as f/1.4 on mFT.

In addition, I believe everyone understands that f/2.8 results in the same exposure on all systems for a given scene luminance and shutter speed (give or take for the difference between the t-stop and f-ratio of the lens).

But there are two points that do cause some amount of confusion:

  • All systems suffer the same from diffraction softening at the same DOF
  • The same total amount of light falls on the sensors for the same scene luminance, DOF, and shutter speed for all systems, which will result in the same total noise for equally efficient sensors.

Failure to understand these last two points by some people are where the vast majority of the problems come from, in my opinion (and experience), so let me flesh these points out just a bit.

Saying that all systems suffer the same from diffraction softening at the same DOF does not mean that all systems resolve the same amount of detail at the same DOF.  It simply means that the blur introduced by diffraction is the same for all systems at the same DOF.  Clearly, an otherwise sharper photo will still be sharper if the same amount of blur is applied to it as a less sharp photo (well, I hope that's clear).

Next, the primary sources of noise in a photo come from the light itself (photon noise) and the sensor and supporting hardware (read noise).  The exposure is simply the density of light falling on the sensor, whereas the total light is, well, the total amount of light falling on the sensor.  We can relate exposure and total light in the following manner:

Total Light = Exposure x Effective Sensor Area

The sensor efficiency comes in two parts.  First of all, there's the QE (Quantum Efficiency) which is simply the proportion of light falling on the sensor that is recorded.  For example, if a sensor has a QE of 50% (which is pretty much where most modern sensors are at), half the light falling on the sensor is recorded.  The second component is how much additional noise is added by the sensor and supporting hardware (read noise), which is often a function of the ISO setting (ISOless sensors, on the other hand, have the same read noise regardless of the ISO setting).

Now, no one would say that a 35-100 / 2.8 on an EP1 is not equivalent to the same 35-100 / 2.8 on an EM5, even though the EM5 will record more detail and have less noise.  So, simply because 35-100mm f/2.8 doesn't necessarily record the same detail or have the same noise as 70-200mm f/5.6 on FF does not mean that they are not equivalent:  "Equivalent" does not mean "equal".

This is an important point that cannot be understated.  When we say that 35-100mm / 2.8 on mFT is "equivalent to" 70-200mm f/5.6 on FF, what is meant is:

  • The photos all have the same AOV (diagonal angle of view) and aperture (entrance pupil) diameter: 35mm / 2.8 = 70mm / 5.6 = 12.5mm, 100mm / 2.8 = 200mm / 5.6 = 35.7mm.

  • The photos all have the same DOF (as well as diffraction softening) when they have sameperspective (subject-camera distance), AOV, aperture diameter, and display size.

  • The photos all have the same motion blur and the same total amount of light falls on the sensor when the aperture diameter and shutter speed are the same (e.g. ISO 100 on mFT and ISO 400 on FF).

  • The photos all have the same same noise when the same total amount of light falls on the sensor if the sensors are equally efficient (less noise if the sensor is more efficient, more noise if the sensor is less efficient).

  • Other elements of IQ, such as resolution, bokeh, flare resistance, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".

Lastly, one doesn't choose a particular system to get images which are equivalent to another system.  Instead, one chooses a particular system for the best balance of the factors that matter to the them, such as price, size, weight, IQ, DOF range, available lenses, and/or operation.  By understanding which settings on which system create equivalent images, the difference in their capabilities is more easily understood.

In other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF, it doesn't make one "better than" the other.  But it may well have a great deal with why one chooses one system over the other, or, in fact, why so many seem to enjoy both.

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Sergey Borachev
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Re: A few words on Equivalence and comparing systems
In reply to Great Bustard, Apr 2, 2013

Thank you very much for the clear explanations.

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PC Wheeler
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Re: A few words on Equivalence and comparing systems
In reply to Sergey Borachev, Apr 2, 2013

Sergey Borachev wrote:

Thank you very much for the clear explanations.

Though a bit overlong and pedantic

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Bluephotons
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Re: A few words on Equivalence and comparing systems
In reply to Great Bustard, Apr 2, 2013

There is not a single spot that hasn't been kiked at least 20 times in this dead horse is time to buried the poor thing.      Or maybe you found one

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Portola
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Re: A few words on Equivalence and comparing systems
In reply to Great Bustard, Apr 2, 2013

Dear Mr. Bustard,

Your posts are always of interest and this one is interesting too, but isn't the 12-35 f/2.8 equivalent to a 24-70 f/5.6 rather than a 70-200?

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Great Bustard
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Yikes! : )
In reply to Portola, Apr 2, 2013

Portola wrote:

Dear Mr. Bustard,

Your posts are always of interest and this one is interesting too, but isn't the 12-35 f/2.8 equivalent to a 24-70 f/5.6 rather than a 70-200?

Oh dear -- how embarrassing is that?  Well, it could be worse, I suppose -- you could have pointed out my zipper was down when I wasn't wearing underwear. 

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Mike Ronesia
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Re: Yikes! : )
In reply to Great Bustard, Apr 2, 2013

Great Bustard wrote:

Portola wrote:

Dear Mr. Bustard,

Your posts are always of interest and this one is interesting too, but isn't the 12-35 f/2.8 equivalent to a 24-70 f/5.6 rather than a 70-200?

Oh dear -- how embarrassing is that?  Well, it could be worse, I suppose -- you could have pointed out my zipper was down when I wasn't wearing underwear. 

LOL, that was the first thing I saw and stopped reading. Maybe you can get a Mod to correct the original post for you.

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Great Bustard
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Re: Yikes! : )
In reply to Mike Ronesia, Apr 2, 2013

Mike Ronesia wrote:

Great Bustard wrote:

Portola wrote:

Dear Mr. Bustard,

Your posts are always of interest and this one is interesting too, but isn't the 12-35 f/2.8 equivalent to a 24-70 f/5.6 rather than a 70-200?

Oh dear -- how embarrassing is that?  Well, it could be worse, I suppose -- you could have pointed out my zipper was down when I wasn't wearing underwear. 

LOL, that was the first thing I saw and stopped reading. Maybe you can get a Mod to correct the original post for you.

I was thinking that, but then it seems such a shame to erase a blunder of that magnitude, don't you think? 

Mark James
A.K.A. Mike Ronesia

24-70 / 5.6

A.K.A. 70-200 / 5.6

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maljo@inreach.com
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Always great to read your posts!
In reply to Great Bustard, Apr 2, 2013

Maljo

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Great Bustard
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Kind of you to say!
In reply to maljo@inreach.com, Apr 2, 2013

maljo@inreach.com wrote:

Maljo

By the way, what's your email? 

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Anders W
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Re: Yikes! : )
In reply to Great Bustard, Apr 2, 2013

Great Bustard wrote:

Mike Ronesia wrote:

Great Bustard wrote:

Portola wrote:

Dear Mr. Bustard,

Your posts are always of interest and this one is interesting too, but isn't the 12-35 f/2.8 equivalent to a 24-70 f/5.6 rather than a 70-200?

Oh dear -- how embarrassing is that?  Well, it could be worse, I suppose -- you could have pointed out my zipper was down when I wasn't wearing underwear. 

LOL, that was the first thing I saw and stopped reading. Maybe you can get a Mod to correct the original post for you.

I was thinking that, but then it seems such a shame to erase a blunder of that magnitude, don't you think? 

Mark James
A.K.A. Mike Ronesia

24-70 / 5.6

A.K.A. 70-200 / 5.6

Are you sure about that Joe? What I thought you actually meant to say is that a 12-35/2.8 on MFT is equivalent to a 70-200/16 on MFE.

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Great Bustard
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Re: Yikes! : )
In reply to Anders W, Apr 2, 2013

Anders W wrote:

Great Bustard wrote:

Mike Ronesia wrote:

Great Bustard wrote:

Portola wrote:

Dear Mr. Bustard,

Your posts are always of interest and this one is interesting too, but isn't the 12-35 f/2.8 equivalent to a 24-70 f/5.6 rather than a 70-200?

Oh dear -- how embarrassing is that?  Well, it could be worse, I suppose -- you could have pointed out my zipper was down when I wasn't wearing underwear. 

LOL, that was the first thing I saw and stopped reading. Maybe you can get a Mod to correct the original post for you.

I was thinking that, but then it seems such a shame to erase a blunder of that magnitude, don't you think? 

Mark James
A.K.A. Mike Ronesia

24-70 / 5.6

A.K.A. 70-200 / 5.6

Are you sure about that Joe? What I thought you actually meant to say is that a 12-35/2.8 on MFT is equivalent to a 70-200/16 on MFE.

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Aleo Veuliah
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Re: A few words on Equivalence and comparing systems
In reply to Great Bustard, Apr 2, 2013

Good explanation.


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Alumna Gorp
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Re: A few words on Equivalence and comparing systems
In reply to Great Bustard, Apr 2, 2013

"in other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF"

Jeez man, not again, a 2.8`s, a 2.8 unless your being anal.

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Great Bustard
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Thanks!
In reply to Aleo Veuliah, Apr 2, 2013

Aleo Veuliah wrote:

Good explanation.

I also appreciate your fixing of my goof in the OP!

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Great Bustard
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On "f/2.8 = f/2.8" and "being anal"
In reply to Alumna Gorp, Apr 2, 2013

Alumna Gorp wrote:

"in other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF"

Jeez man, not again, a 2.8`s, a 2.8 unless your being anal.

Saying "f/2.8 = f/2.8" is no more or less true than saying "50mm = 50mm".  In other words, it makes no more sense to compare f/2.8 on mFT to f/2.8 on FF than it does to compare 50mm on mFT to 50mm on FF.

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exdeejjjaaaa
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Re: A few words on Equivalence and comparing systems
In reply to Alumna Gorp, Apr 2, 2013

Alumna Gorp wrote:

"in other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF"

Jeez man, not again, a 2.8`s, a 2.8 unless your being anal.

my cell ph camera lens w/ 2.6 aperture is better, right ?... makes me wonder why I am not getting a better image out of that and where is by OOF blur or rather where is my OOF at all ? ... it is a brighter aperture, is it not ?

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Alumna Gorp
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Re: On "f/2.8 = f/2.8" and "being anal"
In reply to Great Bustard, Apr 2, 2013

Great Bustard wrote:

Alumna Gorp wrote:

"in other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF"

Jeez man, not again, a 2.8`s, a 2.8 unless your being anal.

Saying "f/2.8 = f/2.8" is no more or less true than saying "50mm = 50mm".  In other words, it makes no more sense to compare f/2.8 on mFT to f/2.8 on FF than it does to compare 50mm on mFT to 50mm on FF.

Nope a 2.8 is a 2.8, and any exposer meter will tell you so

But your going on about DOF, a totaly different ball game.

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Great Bustard
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Re: On "f/2.8 = f/2.8" and "being anal"
In reply to Alumna Gorp, Apr 2, 2013

Alumna Gorp wrote:

Great Bustard wrote:

Alumna Gorp wrote:

"in other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF"

Jeez man, not again, a 2.8`s, a 2.8 unless your being anal.

Saying "f/2.8 = f/2.8" is no more or less true than saying "50mm = 50mm".  In other words, it makes no more sense to compare f/2.8 on mFT to f/2.8 on FF than it does to compare 50mm on mFT to 50mm on FF.

Nope a 2.8 is a 2.8, any light meter will tell you so

50mm = 50mm -- any lens will tell you so.

But your going on about DOF, a totaly different ball game.

Not just DOF, but the total amount of light projected on the sensor for a given shutter speed.  For example, 50mm f/2.8 1/100 projects four times as much light on the sensor as 25mm f/2.8 1/100 on mFT (resulting in half the noise for equally efficient sensors), or, alternatively, 50mm f/5.6 1/100 on FF projects the same total amount of light on the sensor as 25mm f/2.8 1/100 on mFT (which will result in the same noise for equally efficient sensors).

So, instead of quoting numbers without knowing what these numbers have to do with the visual properties of the final photo, you might wish to instead consider how these numbers relate to the visual properties of the final photo.

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Alumna Gorp
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Re: On "f/2.8 = f/2.8" and "being anal"
In reply to Great Bustard, Apr 2, 2013

Great Bustard wrote:

Alumna Gorp wrote:

Great Bustard wrote:

Alumna Gorp wrote:

"in other words, simply because 35-100mm f/2.8 on mFT is equivalent to 70-200mm f/5.6 on FF"

Jeez man, not again, a 2.8`s, a 2.8 unless your being anal.

Saying "f/2.8 = f/2.8" is no more or less true than saying "50mm = 50mm".  In other words, it makes no more sense to compare f/2.8 on mFT to f/2.8 on FF than it does to compare 50mm on mFT to 50mm on FF.

Nope a 2.8 is a 2.8, any light meter will tell you so

50mm = 50mm -- any lens will tell you so.

But your going on about DOF, a totaly different ball game.

Not just DOF, but the total amount of light projected on the sensor for a given shutter speed.  For example, 50mm f/2.8 1/100 projects four times as much light on the sensor as 25mm f/2.8 1/100 on mFT (resulting in half the noise for equally efficient sensors), or, alternatively, 50mm f/5.6 1/100 on FF projects the same total amount of light on the sensor as 25mm f/2.8 1/100 on mFT (which will result in the same noise for equally efficient sensors).

So, instead of quoting numbers without knowing what these numbers have to do with the visual properties of the final photo, you might wish to instead consider how these numbers relate to the visual properties of the final photo.

I`m a bloody artist not a ******* scientist

Get a life......

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