"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
boggis the cat Veteran Member • Posts: 6,329
Your examples do not demonstrate 'equivalence'

walkaround wrote:

There has been a lot of talk by a few people here about "total light", and how an f/1.4 lens on a m4/3 camera supposedly acts like an f/2.8 in regards to "light on the sensor", etc.

This is based on the ratio of the sensor sizes.

Because 135 is four times the area of FT, for the same exposure the 135 sensor collects four times as much light.  It is as simple as that -- try not to let other "explanations" muddy this.

Now, the 'equivalence' argument (which is theoretically sound, though would not likely occur precisely in reality for various reasons) builds on this with an assumption that the same total light (not exposure -- exposure is total light divided by sensor area, or light density) applied to any sensor will yield the same noise.

So, if this assumption is correct (and it should be -- sort of -- in theory: it gets complicated and further assumptions must be brought in) then the argument then proceeds that you should be able to raise the ISO on a larger sensor and obtain the same noise.

So, for your two shots we should be able to obtain 'equivalent photographs' with the following settings:

  • GX1 50 mm @ f/1.4 1/250 ISO 160
  • 5D 100 mm @ f/2.8 1/250 ISO 640 (-2 stops aperture, +2 'stop' ISO)

(Note that the ISO on the 5D is raised 'two stops' to counter the relative aperture being lowered two stops -- ISO isn't really measured in 'stops', thus my quote marks to appease the anal types.)

What you should get, in theory, is two 'identical'(ish) shots with the same characteristics -- framing (actually no, due to 3:2 v 4:3, but you should get the same angular coverage across the diagonal), DOF, and also apparent brightness and noise.

You will most likely find that this is not quite the case, due to the 5D being an older design and having a comparatively worse sensor.

However, if you ignore the fact that this doesn't really work in reality, it is a reasonably solid idea.  This is why you get people claiming that a given lens on FT is 'equivalent' to a lens with 2 stops less aperture on 135.

Refer to DPR using this concept when explaining the utility of the fast Sigma zoom on an APS-C body, and how that f/1.8 zoom 'equalises' APS-C with a 135 system using an f/2.8 zoom:


Sigma's choice of F1.8 as maximum aperture isn't a coincidence; it means that the lens will offer the same control over depth of field as an F2.8 zoom does on full frame. What's more, it will also offer effectively the same light-gathering capability as an F2.8 lens on full frame. By this we mean that it will be able to project an image that's just over twice as bright onto a sensor that's slightly less than half the area, meaning the same total amount of light is used to capture the image. This is important as it's a major determinant of image quality. Essentially it means that APS-C shooters will be able to use lower ISOs when shooting wide open in low light and get similar levels of image noise, substantially negating one of the key advantages of switching to full frame.

(My emphasis.)

Now, you have to bear in mind that unless noise is an important issue then this is something of an irrelevance (IMO).  My preference is to simply understand that a larger sensor will yield real benefits when the ISO must be raised quite high (and this changes: so my E-5 is pretty good through to ISO 1600, then falls off severely; where the E-M5 is good through to ISO 3200, then falls off less severely), or when you want a shallower DOF for a given focal length.

Oh, and also bear in mind that my caveats above with respect to 'equivalence' can also be ignored if you use the phrase "all else being equal".  This is what Joe (Great Bustard) prefers to use, and is perfectly fine provided you realise that this is not likely to be the case in reality.

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