[Ash]

Mike and Ash, thank you for the encouragement.

I'm not saying you turned a deaf ear - it's just the way you see "equivalence".


See the problem? There should be no doubt that "equivalence" is harmful, given the evidence - once again - present even in this thread.
But you're seeing it as useful, and it's only natural to defend (try not to "lose") something deemed useful to you.

Certainly, human nature dictates our biases towards what we tend to hold dear, and old habits do die hard. But you’re right. A concept that is too confused and conflated with unequivocal terminology can be detrimental once we’ve passed the stage of rudimentary understanding of “focal length equivalence”. So what you’re trying to say is we really should kick off the training wheels now, eh?

[BigMackCam]

"When used on Fujifilm’s APS-C based cameras, the 56mm f1.2 will have the equivalent focal length of an 85mm lens on a Full Frame camera."
Our 4 Favorite 85mm Portrait Lenses Under $1,000 for Thrifty Shooters

Happens all the time. Very confusing to a first timer i.e. the conflation between FOV and Focal length. This is an article on 85mm lenses and they are taking about a 56mm physical lens.

Look in the comments and see the reaction to mine. "Stop it" is my favorite with "This is not a physics site". Good Grief.

OK, I just skimmed the article, and before I even got to the comments, I picked up on this statement:

"When used on Fujifilm’s APS-C based cameras, the 56mm f1.2 will have the equivalent focal length of an 85mm lens on a Full Frame camera."

As such, your comment to that article - which I read afterwards, and which clearly picks up on the same point - is one I agree with entirely. He's confusing focal length and field of view.

On that basis, I have to agree that equivalence is sometimes, even often-times, used to promote nonsense... when it's mis-understood or explained poorly. But again, that's the fault of the author - not the concept. Either the author doesn't understand, or - as I think is true in this specific case - he understands but has explained it very poorly, with misinformation that can be damaging to the understanding of others (a nod to Alex there on the damage aspect). Either way, I see that as the author's problem, not an issue with the basic concept. Had he phrased it correctly, that 56mm lens on APS-C does in fact produce a very similar field of view to an 85mm on full frame.

Incidentally, this article perfectly demonstrates why some folks can benefit from the concept of equivalence. They want the equivalent of an 85mm portrait lens on full frame, to use on their cropped sensor camera... That's a pretty common situation, I'd say.

I'm risking Alex shooting me down here, but I know he understands my motives are respectable enough. I just need educating on the alternative approach and/or knowledge base to solve the same problem(s)

[PDL]

-snip
The idea of shallow depth of field being a desirable (even essential) thing is a digital-era invention designed to make so-called "full frame" cameras seem all-superior. Back in the film era, most of us just saw shallow depth of field as the price we had to pay for shooting with fast lenses in low light at relatively low ISO.

-snip

With 55+ years of photographic experience I have to disagree here. I always used the DOF scales on my lenses to control "what appeared to be in focus" as a matter of intent. When switching to digital, the most frustrating thing was/is the removal of DOF scales from lenses. Remember, back in the film days we did not have the ability to change ISO on the fly, we were stuck with the ISO, or for us really old guys ASA/Din, cooked into the emulsion. I used the DOF scales to define those parts of the image that were "in focus". In fact its very easy to use hyperfocal focusing with them, where it is a real pain in the brain/backside to do it on digital lenses. Even the distance scale is pretty much dummied down on digital lenses, which is just plain wrong.

[Kunzite]

Certainly, human nature dictates our biases towards what we tend to hold dear, and old habits do die hard. But you’re right. A concept that is too confused and conflated with unequivocal terminology can be detrimental once we’ve passed the stage of rudimentary understanding of “focal length equivalence”. So what you’re trying to say is we really should kick off the training wheels now, eh?

The other way around: learn the basic concepts the proper, "classic" way. Present "FoV equivalence" as a shorthand, at some point.

[swanlefitte]

The alternative i put forward is distance needed from the sensor to the lens axis to change the angle. Then the lens focal length makes sense like a 8mm socket makes sense. If I have a bolt that is 1.5x bigger and I need a socket to fit tight like the one on an 8mm it needs to be 1.5x bigger.

[k1man]

Hi Mike, you have started a good read here, I personally understand none of it, haha if I like a pic taken with a certain format of camera and lens it is all good for me I dont try and analyze it, as said before I am just a happy snapper, regards Ian

[BigMackCam]

Mike and Ash, thank you for the encouragement.

You're most welcome We might not always agree, Alex, but I don't think we've ever fallen out over it

I'm not saying you turned a deaf ear - it's just the way you see "equivalence".

See the problem? There should be no doubt that "equivalence" is harmful, given the evidence - once again - present even in this thread.
But you're seeing it as useful, and it's only natural to defend (try not to "lose") something deemed useful to you.

L.E. My experience with "equivalence" being obviously very different; first, it didn't teach me anything. So I have no reason to cut it any slack, seeing so many people confused by it - and it being misused in silly Internet fights between smaller and larger formats.

OK... So I ask, as a favour, please point me to any resource - a website, or a book (I'll buy the latter, and be happy to do so as an investment in knowledge) - that will help me to understand things better, so I can apply the correct fundamental principles to achieve the same end goals. Please just bear in mind, my maths knowledge is probably at high school level (applied and statistical) and I have zero knowledge of optical physics and related maths... So I need material that is accessible for someone like me

[Rondec]

I'm about done with the discussion, but I will say that in general I think that "equivalence" is something that folks who shoot shallow depth of field images tend to talk more about than folks who prefer to be stopped down.

If you want one eye in focus portraits, you may be more interested in full frame than in APS-C or micro four thirds. If you shoot stopped down to f8 or f11 all of the time, then full frame will require you to stop down a bit more to maintain that depth of field. Personally, I shoot a lot of landscapes and I do like having deep depth of field and I do find that I need to stop down more to get the same results on full frame. I guess I could say that I am trying for equivalency, but since that seems to be a dirty word to some folks, I will just say that I was satisfied with the depth of field I was getting with my K3, I just wanted more dynamic range and I found the K-1 offered that, particularly when I used pixel shift.

Anyway, there are a couple of folks on this thread (I am one of them) who have said that equivalence helped them. There are a bunch of others who have said that our lives and many others have actually been destroyed by it, at least photographically speaking. I still enjoy making images and I suppose I will keep making them and posting them around here, even if my techniques are a bit tainted.

[BigMackCam]

Hi Mike, you have started a good read here, I personally understand none of it, haha if I like a pic taken with a certain format of camera and lens it is all good for me I dont try and analyze it, as said before I am just a happy snapper, regards Ian

Thanks, Ian

Actually, you're probably one of our members (and there are many in the same position) who could be positively or adversely affected by the concept being debated. So I'm glad you're reading the thread.

If it's not abundantly clear already, my use of this thing I've called equivalence is a learned concept that I've personally found useful, but I have insufficient in-depth knowledge to appreciate the reasons why some of our other respected members object to the concept. I'm trying to improve my knowledge in that area, and once I've been able to (hopefully with some help!), I'll re-visit this. I'd love to be able to shoot down my views in this thread!!

[BigMackCam]

I'm about done with the discussion

I'm sorry to hear that Vincent, though I completely understand. In case you do leave things there, thanks for your invaluable input. This was never going to be a thread of consensus, yet the disagreement has been largely respectful and certainly productive (in my opinion, at least), so I'm grateful for that and to all who've participated, yourself very much included.

I will say that in general I think that "equivalence" is something that folks who shoot shallow depth of field images tend to talk more about than folks who prefer to be stopped down.

I suspect you're right. Yet I still see the concept of equivalence, or whatever the more fundamental and correct alternative, as having value beyond extremely shallow and extremely deep DoF. Despite all the perfectly reasonable statements around individual lens rendering and how DoF and "background blur" don't match precisely, there are - I maintain - use cases where, even if just for the purposes of emulation and / or learning, equivalence (or a better understanding therein) can be of benefit.

Anyway, there are a couple of folks on this thread (I am one of them) who have said that equivalence helped them. There are a bunch of others who have said that our lives and many others have actually been destroyed by it, at least photographically speaking. I still enjoy making images and I suppose I will keep making them and posting them around here, even if my techniques are a bit tainted.

As you obviously know, I believe the concept of equivalence has helped me - in spite, it would seem, of its inadequacies and faults. But I am genuinely willing and eager to learn, and I'll re-visit this topic once I've increased my knowledge. At the very least, I hope you'll stop by and offer your opinions on my revised views

[photoptimist]

Sadly, ten years into my photography journey, and (perhaps unjustifiably) considering myself to be somewhat beyond the beginner stage, I'd still be one of the subjects of your pity

I have a couple of well-respected books on lens design in my small library, but both (one more so than the other) presume a level of both mathematical and optical knowledge I simply don't possess, so both were - despite my best efforts - relegated to the book-shelf shortly after reading the first few chapters.

I would love to understand this stuff at a grass-roots level. But I'd need literature or resources that can appeal to my level of mathematical and optical knowledge... or else - like many, I suspect - I may just have to accept I will never understand it, and simply take what I'm told at face value

I don't pity you because you are aware of what you don't know and seek to fix it. That puts you in exalted company in my view. And I'd wager there are many good lens designers both past and present who have not mastered the contents of those books you have on your shelf. For many purposes, it's sufficient to understand basic physical relationships at the approximate phenomenological level or know how to use the right software (e.g., ray-tracing algorithms) to get good results.

What's interesting is that focal length itself is more of a human-created mathematical model than it is some inviolate physical property of glass, air, and light. I know of no lenses that fit the "focal length" model exactly. And the simplest lenses have the worst fit to the focal length model. If you attempt to measure the focal length of a real lens, you'll find it varies with wavelength (chromatic aberrations), location across the field (distortion and field curvature), and other variables (other aberrations). The fact that making the Pentax' new DFA* 50/1.4 required designing 15 elements, using exotic glasses, and putting them in a barrel twice as long as the focal length show how unnatural focal length is. At some level Pentax' new DFA* 50/1.4 is only "equivalent" to "50 mm focal length lens"!

And if you delve into science, you find it's chock full of models and approximations like this. Your quest to understand things at the grass-roots level is the quest of science. Alas, we humans may still be beginners in this quest. We can see the grass, have begun tracing the blades and stems, but the roots still allude us. And that's what make life interesting and internet forums so lively!

[Ash]

The other way around: learn the basic concepts the proper, "classic" way. Present "FoV equivalence" as a shorthand, at some point.

And I’m with you on that point also, personally. I teach my kids the fundamentals of music before they get to learning pieces, but I wouldn’t bog them down with deep music theory. That’s what I meant when referring to learning photography: yes, learn the exposure triad and focal length vs. subject distance first before shooting for improvement, but enjoy the first 10,000 exposures without the stress of trying to grasp mathematical concepts that not everyone is gifted in understanding.

[swanlefitte]

I think the top down view is a better model as focal length and angle of view are the main elements. The crop (edit 0pps image) circle view makes no reference to focal length. Since equivalence is a direct computation between focal lengths it needs to be part of the model. how about the scissors? a rough diagram I made.

[swanlefitte]

I find myself thinking equivalence from this model. What is equivalent is the triangle. and changing the length of the base of the triangle decreases the scope of what is on the other side of the fulcrum. magnification doesn't seem to make sense because that focuses on a single element in the frame. This way focuses on what else is also on the frame. Imagine a dot in the upper open triangle. You don't think its larger in the yellow field. You think there is less of the rest.
.

what we really say about magnification is If I take a small piece of a bigger thing and magnify it by x times so its the size of the bigger thing then it will look x times bigger provided I don't also magnify the bigger thing too. Obvious until your screen automatically does this resize.

[Fogel70]

OK, I just skimmed the article, and before I even got to the comments, I picked up on this statement:

"When used on Fujifilm’s APS-C based cameras, the 56mm f1.2 will have the equivalent focal length of an 85mm lens on a Full Frame camera."

As such, your comment to that article - which I read afterwards, and which clearly picks up on the same point - is one I agree with entirely. He's confusing focal length and field of view.

On that basis, I have to agree that equivalence is sometimes, even often-times, used to promote nonsense... when it's mis-understood or explained poorly. But again, that's the fault of the author - not the concept. Either the author doesn't understand, or - as I think is true in this specific case - he understands but has explained it very poorly, with misinformation that can be damaging to the understanding of others (a nod to Alex there on the damage aspect). Either way, I see that as the author's problem, not an issue with the basic concept. Had he phrased it correctly, that 56mm lens on APS-C does in fact produce a very similar field of view to an 85mm on full frame.

Incidentally, this article perfectly demonstrates why some folks can benefit from the concept of equivalence. They want the equivalent of an 85mm portrait lens on full frame, to use on their cropped sensor camera... That's a pretty common situation, I'd say.

I'm risking Alex shooting me down here, but I know he understands my motives are respectable enough. I just need educating on the alternative approach and/or knowledge base to solve the same problem(s)

To be fair, equivalent focal length is something that is widely spread in the industry, and something every camera manufacturers use. FI Ricoh describes GR II as having 28mm equivalent focal length, and they specify equivalent focal length on K-mount lenses. And in the EXIF of every image I capture with my APS-C cameras there is a field with equivalent focal length.

I'm pretty sure Fuji promotes this lens as being 85mm equivalent. So it easy to get this wrong if not being careful or not knowing what equivalent focal length really means (equivalent FOV).

Edit: this is how Fuji describe this lens.

A fast F1.2 lens with a focal length of 85mm* that delivers beautiful background bokeh so it's ideal for portraiture. The lens' incredible resolving power is thanks to a new optical design featuring 11 elements in 8 groups. Two ED (extra-low dispersion) elements and one double-sided aspherical element are used to control spherical and chromatic aberrations, while a further four of the elements feature convex surfaces to ensure excellent light coverage, even when shooting wide open.
Like other XF lenses, the XF56mmF1.2 R is designed to be comfortable in use and has a premium quality look and feel.

* 35mm format equivalent

---------- Post added 27-01-19 at 07:13 ----------

Equivalent focal length is standardized by CIPA, with below short desciption.

This means the focal length of a digital camera lens converted into that of a 35 mm {film} camera lens that has the same input field angle as the digital camera lens.