Myths


swifty

Senior Member
Oct 12, 2004
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Not sure if it's even worth clarifying but for those who care:

Equivalence:
It's not a dirty word and it doesn't favour any format and is brand agnostic. It's just a way to compare the RESULTANT IMAGE taken by equipment of differing formats with respect to: DOF, FOV, motion, noise characteristics.
It assumes the underlying sensor tech are the same and it doesn't take into account other factors such as lens quality, colour reproduction, pixel count etc.

For those who don't know, for historical reasons we sometimes express gear from other formats in FF (135mm) terms so it is easier to compare equipment from differing formats. It's a simple equation of multiplying the crop factor. Eg. m43 has a 2X crop factor. So a 25mm f/1.4 lens is equivalent to (not the same as) a 50mm f/2.8 lens in FF.

f/2.8 is f/2.8 is f/2.8:
Yes, the f-number is always the same. It doesn't change. Just like the focal length doesn't change. Nothing actually changes.
We are just trying to express how a particular lens on one format would perform if we are on a different format. Because the format has changed, we need equipment of different specifications to produce the same RESULTANT IMAGE.
The f-number is a ratio and describes the per square unit light gathering ability of the lens, not its absolute light gathering ability.
The aperture opening of a 25mm f/1.4 lens is 17.8mm.
The aperture opening of a 50mm f/2.8 lens is also 17.8mm
The light from the 25mm lens on m43 has higher intensity spread over a smaller light sensitive area (the smaller sensor size) whilst the light from the 50mm lens on FF has lower intensity spread over a larger surface area.
They collect the SAME amount of light if you keep the shutter speed the same and sensor tech the same.
That's why a 25mm f/1.4 lens on m43 is equivalent to (but not same as) a 50mm f/2.8 lens on FF because they will collect the same amount of light and that is the definition of EXPOSURE.
The exposure determines the noise characteristics when the underlying sensor tech is the same (ie. the electronic noise added by the circuitry is the same).

Smaller sensors are NOT noiser
Smaller sensors collect less light by virtue of a smaller photosensitive area compared to a larger sensor.
The light that is collected gets turned into a signal to form an image but along the way, because of the properties of light as well as the electronics required to convert the photons to a signal there will be noise in the image. So it is the Signal to Noise ratio (SNR) that matters for image quality.
In modern sensors, the electronics add very little noise so by and large, a noisy image from a modern sensor is due to lack of signal (lack of light).
It's an important distinction because it means smaller sensors are NOT by default, noisy. You just need to make up for the smaller sensor real estate with more light, such as longer shutter speed or higher intensity (higher f-number) but you do run into practical limits of a system hence why FF systems (compared to m43) are still superior when light levels drop.
FF images can often have more noise but it's the abundance of signal that drowns out the noise and that's the important part. If I could invent a word, it would be more accurate to say larger sensors have the potential to be 'signal-ier'.

At equivalent settings, you get the SAME image
Images from a smaller formats eg. m43 do not have less DR, more noise, etc. by default. It will have the SAME noise characteristics as well as all the other parameters I listed above for equivalency when you use an equivalent setting in m43, compared to FF. This is because the exposure will be the SAME in both instances.
You may get better results when you use settings in FF beyond what is practically available in m43, but this is scene dependent.

ISO is NOT an exposure parameter
Think of ISO as what the camera does with the exposure. It cannot change the amount of signal but may affect the noise characteristics depending on how it is implemented.
ISO will affect the brightness of the resultant image.
The exposure triangle concept, whilst easy to understand and has some practical applications is a fallacy and contributes to misunderstanding about digital photography.

Resources to consider:

I'm writing these in good faith but no one is infallible and may be prone to mistakes especially as our understanding improve. If you don't agree with what I've written, feel free to discuss and point out where I have made mistakes.
 

grantyale

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Oct 4, 2004
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Overall, I agree that current FF and M43 are comparable when shot using equivalent settings, and each format's limitations are mostly due to its unavailable operating points.

On the point about ISO -
It is messy because it's defined with regard to image output brightness or sensor saturation, neither of which is suitable for direct use in cross-format comparison.
It can be normalized to the total imaging area to form an "Equivalent ISO", so roughly speaking, M43's ISO 200 is equivalent to ISO 800 on 35 mm. This also retains the exposure triangle.

Regarding noise in images -
It's not just the signal versus electronic noise, but light intensity itself determines the signal-to-noise ratio except at extremely low levels (less than a dozen photons, for example). This is known as photon shot noise, and it is the square root of the expected (mean) photon count. This is in fact the justification for Equivalent ISO - to linearly correlate with the reciprocal of the number of incident photons per frame area.
As for M43 vs FF - Current M43 sensors have very similar sensitivity per unit area compared with current FF sensors, and M43 reached this point a few years earlier than FF.
 

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swifty

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Regarding noise in images -
It's not just the signal versus electronic noise, but light intensity itself determines the signal-to-noise ratio except at extremely low levels (less than a dozen photons, for example). This is known as photon shot noise, and it is the square root of the expected (mean) photon count. This is in fact the justification for Equivalent ISO - to linearly correlate with the reciprocal of the number of incident photons per frame area.
As for M43 vs FF - Current M43 sensors have very similar sensitivity per unit area compared with current FF sensors, and M43 reached this point a few years earlier than FF.
Yes, I didn't mean to imply SNR only referred to electronic read noise. I thought I referred to photon shot noise in "because of the properties of light as well as the electronics required to convert the photons to a signal there will be noise in the image" though not explicitly so thanks for the clarification and chiming in.
Cheers
 

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