Discussion of 'Equivalence'

[JohnMM]

....the DX foto will have been created using less dense light.

Wrong way around ? Lower density of light (=lower exposure) on the larger (FX) sensor.

[Bjørn Rørslett]

"Maybe the further points will be cleared up when we agree on what 'framing' means."

I rephrased following Andrea's suggestion to be same angle of view (of the captured scene that is).

However, as you apparently are aware of, if perspective is to be held and formats differ then primary magnification has to differ because different focal lengths are used, and with that follows a cascade of other issues to violate one or more of the listed 'Equivalence' criteria. In my humble opinion, this makes the whole approach fall like a house of cards. Others might disagree which is their prerogative. I'm not on a mission to convince anyone.

Whatever the attitude, we end up in a round-robin manner at the conclusion that a larger format in most cases needs a smaller degree of secondary magnification. A fact that is centuries old by now. A side effect of this not mentioned by any of the model articles (at least, I couldn't find it mentioned) is of course the danger of empty magnification also is reduced. An example: for true grand landscape photography one needs a longer lens to get details more magnified and avoid empty magnification; and unless stitching is applied the wider angle of view usually is provided by a [much] larger format than optimal elsewhere.

[simsurace]

"Maybe the further points will be cleared up when we agree on what 'framing' means."

I rephrased following Andrea's suggestion to be same angle of view (of the captured scene that is).

However, as you apparently are aware of, if perspective is to be held and formats differ then primary magnification has to differ because different focal lengths are used, and with that follows a cascade of other issues to violate one or more of the listed 'Equivalence' criteria.

Great. Yes, primary magnification is different between formats under the stated conditions (1,2). One can think about everything in terms of primary magnification. Using the concept of equivalence does not diminish the value of primary magnification in any way. But can you measure primary magnification from a finished print?

So, which of the following criteria (3-6) is violated without possible remedy?

[Bjørn Rørslett]

It's more the impossibility of getting all in line at the same time. Trust me, I wasted a lot of time trying to accomplish this.

[simsurace]

A side effect of this not mentioned by any of the model articles (at least, I couldn't find it mentioned) is of course the danger of empty magnification also is reduced.

If you are thinking of empty magnification because the primary projection of the scene onto the sensor has a finite resolution due to aberrations, indeed this is not part of the model. Within the theory, there is a very coarse simplification that treats all lenses as diffraction-limited, or to have the same relative resolution in terms of line widths per picture height (which is again a very coarse description, because this is just for one value of the MTF curve, e.g. MTF50). There are far too many lenses to distinguish a clear trend, and far too many ways to measure 'sharpness'.
The compensation of diffraction-related loss in resolution in small formats by a suitable aperture is likewise not without limits; there are obvious theoretical and practical (i.e. availability) limits on the range of apertures that can be used.

[Bjørn Rørslett]

Empty magnification  even rears its head for FX systems - just imagine a ultrawide lens used for a mountain landscape. That capture will very soon run into empty magnification even for pretty modest print sizes. The ubiquitous recommendations for landscape lenses fail to mention this simple fact.

[simsurace]

It's more the impossibility of getting all in line at the same time. Trust me, I wasted a lot of time trying to accomplish this.

What about the example I posted to Jakov? Is this not feasible?

[Jack Dahlgren]

Using your analogy, you already have the 'equivalent' measure to compare cars: acceleration. This is what matters, let's say, on a test track. A more massive body has to be compensated by a stronger engine.
In engineering and physics, the reformulation of relationships in terms of dimensionless quantities usually represents a step towards a deeper understanding, since stuff that is irrelevant for the qualitative behavior of the system gets moved out of the way.
Of course 'qualitative behavior' might not be so straightforward to define in photography. Still, I think that the methods of Joseph James are not too far off the mark.

What is missing from this discussion ARE the units or dimensions.

Without units you end up with strange ideas like the number of pixels increases the dynamic range.

The desire for high dynamic range is present during capture and processing. I don't think there are any printers out there that have 14 stops of range. So defining at output is the wrong target   

So much fuzzy math here that I'm going to slap the helicon on and take photos.

[simsurace]

Empty magnification  even rears its head for FX systems - just imagine a ultrawide lens used for a mountain landscape. That capture will very soon run into empty magnification even for pretty modest print sizes. The ubiquitous recommendations for landscape lenses fail to mention this simple fact.

Yes, I know. I stitch a lot to get around this problem. But stitching is in a way like using a massive sensor (that quickly exceeds the size of any commercially available sensor), and the same equivalence principles can also be applied to this scenario in order to choose a suitable focal length, aperture etc.

[simsurace]

What is missing from this discussion ARE the units or dimensions.

Without units you end up with strange ideas like the number of pixels increases the dynamic range.

Who said that?

The desire for high dynamic range is present during capture and processing. I don't think there are any printers out there that have 14 stops of range. So defining at output is the wrong target   

I'm not following. The output target is merely there in order to get a fair comparison. It can be anything: a 1000px web image, a 10 meter print, etc. and depends on the application. The important point is that it is the same across the formats that are being compared.

Why do we need high DR during capture and processing? I think it is to be able to massage the file in order to get a good end result afterwards. If we don't have the DR to start with, our options are limited to some extent. However, how we make use of the available DR is entirely subjective and an artistic choice.

The point is that the DR of the capture device defines an upper limit of what can be achieved. Especially if we pull up shadows we are going to run into limits. We do not need an output medium with 14 stops of contrast range to get to see the effects.

[Bjørn Rørslett]

You think from end to front, I think from the starting point and know where the end might be sought. Thus not necessary to even think about aperture or lens. Possibly a side effect of working with a wide range of formats for many years, going back even to large format view camera (I used up to 8x10"). Ingrained habits fade slowly.

[Jack Dahlgren]

Who said that?

I'm not following. The output target is merely there in order to get a fair comparison.

John Maud said DRFX=1.5*sqrt(N)*DRPIX where N equals number of pixels.

If output is only to get a comparison, then why compare at all?

Equivalence is a tarbaby.

[simsurace]

John Maud said DRFX=1.5*sqrt(N)*DRPIX where N equals number of pixels.

From that formula your statement follows only if you keep DRPIX fixed. What is the problem? I think John made his notation quite clear. Otherwise you can always ask for clarification.

If you disagree with the formula for fixed DRPIX, please explain why!

You did not say why it is a strange idea, and where the units are missing. Incidentally, DR is dimensionless.

If output is only to get a comparison, then why compare at all?

This is not what I said. Please read again. I was talking about why the equivalence theory assumes a consistent output target.

[David H. Hartman]

Will, for example, increasing the sheer number of pixels in the first step mitigate some of the problems caused later by large magnification?

I don't think it's just more pixels that needs to be considered. The larger format collects more light, more photons. So the question I have is does this greater sampling of photons, more light translate to improved dynamic range. At this point I'm just watching to see if something gets hammered out.

Another point I'd like to make is the graphs I posted without comment were FX v. DX simply because they were available and hopefully eliminated some variables between various cameras.

I invite consideration for DX v. FX v. mid medium v. traditional 6x4.5cm medium format though I have no idea how one might find a DX, FX and Medium Format camera with similar technical qualities. So what if any are the advantages of mid medium and medium format camera?

Dave who is reading and hoping to learn.

[simsurace]

The larger format collects more light, more photons.

This holds only for the same exposure.
Without the condition, the statement is too easy to misconstrue.