A beginner's guide to "is equal to" vs "is equivalent to".

[bobn2]

Excuse me, but it starting to be ridiculous.

I had that feeling too, but I suspect that we differ on the source of the ridiculousness.

I don't deny equivalence theory, I use the underlying principles for a long time.

Good, so we are agreed there.

I don't insist on any my definition of Equivalence. I just provided evidence, that the current use of term equivalence when applied to lenses is not fully correct.

I disagree with you there. You said 'Sorry but there is just one definition of Equivalence relation:' then proceeded to define what you believed was that one definition. It's a different definition from the one I'd use, and also the one I pointed you at, so what you were saying was that there was one definition, and it was yours.

The Equivalence relation is indeed reflective, symetric and transitive, witch is expressed by the expression I provided.

Not 'the', 'an'. There are many relations that are equivalence relations, of which one is the Equivalence relation, with respect to photographic images, as defined by GB.

The problem is that *equivalence* relation as described by OP is NOT symetric when applied to lenses.

But it doesn't apply to lenses. As I explained, it applies to photographs. Lenses and photographs are different things. You can ask the question as to whether two lens/camera systems can take equivalent photographs, and they might be able to over some range of the available settings.

Domains of two *equivalent* lenses are not the same. There are some shots you can produce with one lens but not equivalently with the other. And usually vice versa.

I'm not sure what the 'domain' of a lens is. I know what the domain of a relation is, but a lens isn't a relation. Maybe you could clarify this.

So once more, I can live with equivalence theory as it is. I just wanted to note one aspect which is often missing. To help OP and DPR to make it more correct.

Unfortunately, rather than making it more 'correct' I think you've muddied the waters a little.

 

[Lee Jay]

I said what was debatable about what so many of the equivalence evangelists keep touting about total noise in the image being so important.

Because some of us shoot in light-starved situations where gathering as much light as possible is important.

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Lee Jay

 

[FingerPainter]

Exactly, but if you understand my needs, you can see that my situation is just the opposite. I need deep DOF and not shallow DOF and therefore a FF sensor does not represent much of an advantage to my photography.

Moti

So you never take photos where you only need to get one player in the field of acceptable sharpness?

 

[FingerPainter]

Because of physics yer 300mm lens is going to be the equivalent of a 480mm lens when you switch it from a Canon camera with a full frame sensor to a Canon camera with a smaller APS-C (crop) sensor.

Formula works on any lens.1.6 is the magic multiplication number (300mm X 1.6 = 480mm).

So ... wide angles lenses get less wide angle going from full frame to crop. Telephotos get more telephoto.

That's pretty much all you need to know.

You left out Depth of Field, noise, and diffraction blur. Are you suggesting all photographers should ignore such visible attributes of their photos?

 

[FingerPainter]

The debate about equivalence for a beginner photographer is catastrophic.

The discussion about equivalence for a beginner forum user is enlightening (about the forum).

How is it catastrophic? I once was unaware of equivalence. Learning it has helped my photography.

 

[bobn2]

The debate about equivalence for a beginner photographer is catastrophic.

Your usage of catastrophic is hyperbolic.

 

[Zed Man]

Because of physics yer 300mm lens is going to be the equivalent of a 480mm lens when you switch it from a Canon camera with a full frame sensor to a Canon camera with a smaller APS-C (crop) sensor.

Formula works on any lens.1.6 is the magic multiplication number (300mm X 1.6 = 480mm).

So ... wide angles lenses get less wide angle going from full frame to crop. Telephotos get more telephoto.

That's pretty much all you need to know.

You left out Depth of Field, noise, and diffraction blur. Are you suggesting all photographers should ignore such visible attributes of their photos?

This is where it gets fun.

Take the example above.

With a 300mm lens @F8 and 100 feet, on a 7D the DoF is 10.2 feet.

With a 300mm lens @F8 and 100 feet, on a 5D the DoF is 16 .2 feet.

(These are equal parameters not "equivalent")

When using longer lenses we often want as much reach as possible. The 7D user is not going to back up to get the same FoV. We don't want equivalent.

And interestingly according to DoF calculators, the smaller sensor gives you a shallower DoF.

 

[Airolo]

If you believe, that there is any two equivalence relation which is not symetric, than well...

With domain of the lense I mean set of perspectives (one of the 6 GB,s parameters) for which is a lens is able to produce a shot with something in focus. I supposed it's clear. Sorry.

And you are mistaken, the DPR's article does apply equivalence to lenses. Without any note, that the sets of meaningful perspectives for related lenses are not the same. So strictly speaking...

Regards

P.

 

[Iliah Borg]

99.9% of readers know what the poster is discussing.

"70% of all statistics are made up on the spot by 64% of people that produce false statistics 54% of the time they produce them."

I'm with you on this.

More seriously, https://www.thebereancall.org/node/8045

Hmmm, so when a tree falls and no one hears it, does it make a sound?

Not in the meaning of sound as something perceived, but sound waves, yes (same thing with colour). I take here the position proposed by Scientific American, April 5, 1884, p. 218 - “sound is vibration, transmitted to our senses through the mechanism of the ear, and recognized as sound only at our nerve centers. The falling of the tree or any other disturbance will produce vibration of the air. If there be no ears to hear, there will be no sound.”

I knew some serious scientists who refused to consider similar questions at all.

More to the point, if a species ceases to exist but was never known to exist, did it ever exist? This is going far afield here, but what I think is being overlooked by the author of that commentary is that species do go extinct at some rate and knowing if that rate is being accelerated or not by humans may be hard to quantify, but two things are certain:

1.) Diversity is decreasing. Put another way, currently the rate of extinction of existing species is faster than the rate of emergence of new species.

2.) Habitat is decreasing. Related to the first point, and regardless of how many species other than humans survive over the coming millennia that's an issue in and of itself.

Regarding the loss of 2% per year, as the number decreases each year and putting forward one guess after another (8.7 million existing species, some minimal level of new species emerging, an unceasing trendline, along with whatever the actual rate is) we might guess that it could take 1200 years before we were the only species that hadn't gone extinct. I don't think we are at all likely to be the last species to go extinct, nor do I think we will only last another 1200 years, which leads me to believe that we will come into balance with the earth's ecosystem whether by design or by some more brutal happenstance.

I have no knowledge of modern politics, but for ancient Greece the quote of Henry Adams saying “Practical politics consists of ignoring the truth” is a stretch.

The irony of the article is that the author is using basically the same methods he is criticizing, "inaccuracies come from claims based on non-existent or inadequate historical data, extremely crude estimates, computer model projections, or are simply incorrect".

I prefer this (not that it explains all of it, or is applicable to every case):

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[J A C S]

Exactly, but if you understand my needs, you can see that my situation is just the opposite. I need deep DOF and not shallow DOF and therefore a FF sensor does not represent much of an advantage to my photography.

Moti

So you never take photos where you only need to get one player in the field of acceptable sharpness?

In situations like those, getting all in acceptable focus is impossible with any format; and I'd rather blur some more than less.

 

[Dennis]

The debate about equivalence for a beginner photographer is catastrophic.

The discussion about equivalence for a beginner forum user is enlightening (about the forum).

How is it catastrophic? I once was unaware of equivalence. Learning it has helped my photography.

Likewise. I remember arguing "f/2.8 is f/2.8" with Great Bustard. He was, as he always is, very patient and I quickly "saw the light". I'm not sure it's helped my photography, but the concept of "total light" certainly has (I now use Auto ISO in M mode, setting exposure to maximize light on the sensor).

• Dennis
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