> Haven't read here for a while. Wondered when all this would surface
> here. Well, it has and it won't go away, so maybe some clarification is
> in order.
> This whole subject is what happens when the tech geeks and the arty
> types fail to communicate, things get lost in the cracks, important
> things. We have focal lengths, equivalent focal lengths, angles of
> view, image area, depth of field, frame composition, perspective...
> real hodge podge!
> Two things that weren't considered, and are important, are:
> 1) The interface between photographer and camera, the view finder.
> The one thing that does not ever change is the natural field of vision
> of an individual, the inherent angle of view. That's the bench mark.
> Now, an individual looks throught the viewfinder of a camera. If it's
> an SLR, the viewfinder presents a perceived angle of view that does not
> change. It does not allow the viewer to perceive the change in the
> lens' angle of view.
> Look through a rangefinder with interchangeable lenses. Different
> marked off fields of view, so the viewer can perceive the changed angle
> of view. Since the viewfinder is not part of the lens' optical train,
> the perceived angle of view *of the viewfinder* does not change, and the
> sense of the differences between the "perspectives" of different focal
> length lenses is not apparent.
> What happens with the SLR is that the eye is fooled into thinking that
> what he/she sees has changed, when it has in fact not changed at all.
> How that happens is obvious: A "standard" lens is one that will present
> to the viewfinder what the eye would see if the viewfinder were a simple
> window, unaltered by an optical train. One can often shoot with both
> eyes open using a standard lens, without suffering the effects of
> difference perceived by each eye.
> Can't do that with a telephoto or wide angle. Well, at least I can't...
> So what is going on here is that the camera user comes to identify the
> perceived effect of different focal lengths, and comes to associate
> particular differences with particular focal length lenses. And this is
> why the persistent popularity of the "equivalent focal length".
> Are the tech geeks right? Yep, they are. Is that important? Yep, it
> is. Why? Because some things don't change, and one of them is DOF.
> Another is relative distortion of the scene image. Both of these are
> very important, of course.
> Which leads to the second thing.
> 2) Perspective.
> It turns out that there are two different definitions in common usage.
> Now never mind the dictionaries, we know this without consulting them.
> The first and literal definition is "what you see from where you are".
> Here, we're talking about what you can and cannot see from a point of
> view, which changes when the point of view is changed. You change your
> viewing position, and some things that were not visible become so, and
> some things that were visible, now are not.
> That's not the same as the second definition, which has to do with the
> relative size of viewed objects. The problem here is that this second
> definition can easily be assumed to be a subset of the first: Things
> get larger as one gets closer; given the natural angle of view, moving
> closer makes an object take up more room in the field of vision.
> However, the relative size of objects in the scene can be made
> independent of the point of view created perspective. That is easily
> accomplished with optics (and recorded in camera) by distorting the
> scene optically. Artists do this all the time to achieve the effects
> they want. They call it a change in perspective, and their use of the
> term is what has established the second definition. Artists use
> cameras, and so photography uses both definitions.
> What's the problem? Well, there's a "gotcha!!" lurking here. And that
> is that a photographer expects the camera to record reality as it is,
> because that's what cameras are supposed to do. Right?
> Well, that's as may be, but the reality is that no, they do not. What
> they do is record an optically produced image, whatever that may be, and
> the unwary can really get confused when optical distortion is not
> Here's the reality check: If things change in relative size, expect
> different things to be visible. If different things are not visible,
> you're looking at an optically produced distortion. And that's how you
> tell which definition is being used.
> The arty types don't care about all this "semantic detail". Perspective
> change via distortion and perspective change via change in position are
> deemed equivalent for their purposes, or so it seems. Not being an arty
> type I wouldn't know for sure.
> The tech geeks insist that they are entirely different things and should
> never be confused. Perspective change is never produced by a change in
> focal length, so they say, and according to the first definition, they're
> right. And for them, apparently, that's all that matters. Not being a
> tech geek, I wouldn't know for sure.
> The truth is that both are missing an important aspect of this business,
> and that is the effect these changes have on the viewer. Trouble is,
> both types are also viewers, and because of this lack of complete
> understanding, cannot come to an agreement about what's what.
> There are some truths in some of the statements made, of course.
> You cannot change perspective of the first definition by changing focal
> lengths. But it's equally true that you certainly can change
> perspective by the second definition by changing focal lengths. What
> makes both true is that one makes the change by distortion and the other
> does not.
> As a camera user of some experience, you can equate the angle of view
> provided by a focal length in one format with the perceived angle of
> view in another. But the lenses themselves do not change focal length,
> nor do they change angle of view. Enter the crop factor.
> Incidentally, an optical train is an optical train whether displaying a
> recorded image, or recording a displayed image. Optical crop is optical
> crop. No, the two trains are not designed alike, but that's irrelevant.
> For what its worth, that is...
> Maybe if you guys would think all this through, you would realize that
> reality does not change, and that differences in opinion are the result
> of different perceptions. You want to be completely right? Better
> think these things through a bit better. Otherwise, all you guys have
> stuff to learn, and are better off listening to each other and trying to
> figure out what you've missed.
> Myself included, of course. I walk the walk I talk: tell me where I
> ed up here, or what I missed and I'll willingly stand instructed.
> Will D.