"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
boggis the cat
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Re: Your examples do not demonstrate 'equivalence'
In reply to Great Bustard, Apr 29, 2013

Great Bustard wrote:

boggis the cat wrote:

(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 is ISO "really measured" in, then?

It's a proxy for sensitivity (or was, in film times).  It relates the exposure to the resulting image density for a given film (rated in ISO or an equivalent scale).

Unitless, I guess.

The point being that you need to factor exposure parameters (aperture and shutter speed) and sensitivity (ISO) to determine a "correctly exposed" photograph.  Use too high an ISO and it will be over-saturated; too low and it will be too dark (and possibly have excessive apparent noise).

Now, since you can start from a correctly exposed photograph of, for example, f/2 1/125" at ISO 100 then change the aperture to f/2.8 (-1 stop) and compensate by either changing the shutter speed to 1/64" or the ISO to 200 it can be useful to consider ISO changes in terms of a 'stop', even though that isn't technically accurate.

Indeed, I tend to consider 'exposure' to mean both exposure proper and ISO, even though that is completely incorrect from a technical point of view -- ISO is not an exposure parameter at all.  This is when I am thinking in terms of a correctly exposed photograph -- so, really, 'exposure' would be my short-form thinking for 'how I want this photograph to look' (generally, balanced or 'correctly exposed').

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.

In reality, it's pretty freakin' close for sensors of the same generation.

Well, you admit that your 'equivalent photographs' concept ignores many image quality parameters.  The 5D and GX1 are not very close so I would expect the results to be skewed against the 5D.

Now, you have to bear in mind that unless noise is an important issue then this is something of an irrelevance (IMO).

There's still DOF.  So I guess you're saying that if noise and DOF (which necessarily means the corners will be rendered significantly different for the vast majority of scenes) don't matter, then, sure.  May as well just use a compact then, right?

DOF is a fairly obvious difference.

Also, I am still wanting to pick up a Panasonic FZ200 to -- shockingly enough -- take photographs with.  It must amaze Panasonic that there are still people stupid enough to want a different set of compromises in some cases and not all want to use "full frame" (aka 135 format) for every purpose.

Have you considered writing a spy novel?  You could improve it by making the spy use a 135 system instead of one of those entirely useless tiny spy-camera things to copy the secret documents.

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.

Each stop decrease in total light reaching the sensor results in a 41% increase in photon noise.  However, as the light gets dimmer and dimmer, the read noise (the additional noise added by the sensor and supporting hardware) begins to become dominant, so, at a certain light level, the noise in the photo is dominated by the read noise.

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.

All else is never equal.  But "all else" can often be accounted for.  It's like gravity -- for the most part, we only need to worry about the earth.  But if we want to explain the tides, we have to include the moon.  If we want to explain spring tides, we need to include the sun.  Etc., etc., etc.

Sure, but you can't explain tidal behaviour at a given location purely with reference to the tidal pull of the Moon and Sun (and any other orbiting bodies you may care to include if you like meaningless precision).

This is not to say that the general theory is useless, of course.  Someone could have saved Bill O'Reilly some embarrassment if they'd bothered to explain the general gist of it to him when he was a child, for example.

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