"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
Great Bustard Forum Pro • Posts: 40,694
Re: 'Equivalent' can mean many different things

boggis the cat wrote:

Great Bustard wrote:

They're all quantative.  We can quantify perspective (subject-camera distance).  We can quantify framing (width and height of the scene on the focal plane).  We can quantify DOF (total distance front and back of the focal plane that is within critical focus).  In other words, they are all quantative just like shutter speed and display size.

Here, let me ask you a question:  would anyone disagree that 25mm f/2.8 1/100 ISO 400 with the 14=54 / 2.8 on an Olympus E1 is equivalent to the same settings with the 12-35 / 2.8 on an Olympus EM5?

I would disagree.  The E-1 produces a very different photograph to the E-M5, even when using the same lens on both.

So I would say that such photos were "of the same subject" or even "the same photo, using two different cameras".  The point would actually that they are not equivalent.  If they were, I could have saved a lot of money upgrading bodies to obtain a technically better photograph.

For the record, you are saying that a photo of a scene taken at 25mm f/2.8 1/100 ISO 400 on an Olympus E1 is *not* equivalent to a photo of the same scene also taken at 25mm f/2.8 1/100 ISO 400 on an EM5.


I'm thinking no one would say they are not "equivalent", despite the fact that the noise, detail, dynamic range, etc., would all be quite different.

You have a very narrow idea of what 'equivalent' should mean -- or rather 'equivalent photographs', where you use 'equivalent' as a short-hand for that concept.

Because of this narrow idea, you then have trouble with the concept that a 50 mm f/2 lens on FT is equivalent to 100 mm f/2 on 135.  In this case, we'd be considering equivalent focal length in 135 terms: 100 mm EFL (where "EFL" is an 'industry standard' terminology using the 135 film system as a basis for comparison).

Wait, wait, wait -- you just said above that 25mm f/2.8 on an E1 is not equivalent to 25mm f/2.8 on an EM5.  So how is it that 50mm f/2 on 4/3 "equivalent to" 100mm f/2 on FF?

It is not in fact necessary to 'correct' such a statement to "50 mm f/2 lens on FT is equivalent to 100 mm f/4 on 135" if you are discussing EFL.

It's not necessary to mention the f-ratio at all if you are only discussing EFL.

The advantage that a larger sensor has (or may have) is always 'present', and may be pointed out if relevant.

If 50mm on 4/3 is "equivalent to" 100mm on FF because it has the same EFL, then f/2 on 4/3 is "equivalent to" f/4 on FF because it has the same aperture (entrance pupil) diameter for the same EFL (50mm / 2 = 100mm / 4 = 25mm) which will result in not only the same DOF, but the same total amount of light projected on the sensor for a given shutter speed, which, in turn, will result in the same noise for equally efficient sensors.

In a word -- "Equivalent".

Thus, I see no reason not to use the word "equivalent" to describe photos that have the same perspective, framing, DOF, shutter speed, and display size even when other elements of IQ are not necessarily the same.

Once again, I suggest that you use 'equivalent photographs' as a clearer indication of what you are referring to in preference to your short-form usage of 'equivalent' or 'equivalence'.

Equivalent settings produce equivalent photos.

This prevents confusion when, for example, you jump into a discussion comparing lenses where other 'equivalents' may already being discussed and essentially try to narrowly redefine the word 'equivalent'.  It is irrelevant to the point that you are making (about total light capture and sensor noise level) and may lead to unnecessary confusion and argument.

'Equivalent' does not have the narrow meaning that you may prefer it to have.

Well, first you say that 25mm f/2.8 1/100 ISO 400 on an E1 is not "equivalent to" 25mm f/2.8 1/100 ISO 400 on an EM5, then you say that 50mm f/2 on 4/3 is "equivalent to" 100mm f/2 on FF, so I'm thinking that if anyone is causing "unnecessary confusion"...

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