Perspective and 2X flowers

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ChrisR
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Post by ChrisR »

ChrisR wrote:
Pictures taken with long lenses tend to look flatter because we're usually further away.
But only for the same total final image i.e. the magnification, with the frame covering the same portion of the subject.
I think what you're saying is that if you use a wider lens, from the same spot, there has to be other stuff in the picture which contributes to depth. Is that right?
If so then consider that the view of first tree is through a hole in a huge white wall, parallel to the sensor. The expanse of white on the print wouldn't help with depth, because it has no depth of its own. If you could see the grass in front of the wall, then it's got the depth back.


The viewing distance of the print, is often, in the words of one our great leaders, misunderestimated. If you look at the print through the taking lens, you get the same perspective.

This pic by Ken Rockwell, with a 13mm lens on 35mm, is so perspectivationally affected that it looks distorted. But fill your computer screen with it, and get your eye close enough, and it all looks right.
If you look at it from too much distation, it's more depthified.

http://www.kenrockwell.com/tech/images/ ... -dubai.jpg

---------

Surely, the extension is a factor in the viewing position? (I have to admit uncertainty here).
Not entirely sure what you mean.
If you take a pic at 1:1 with a 200mm lens, you need to move the sensor 200mm further from the subject, to focus, than you would for infinity.
Yes that DOES have the effect/angle of view you'd normally expect from a 400mm.
Is that what you mean?

Harold Gough
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Post by Harold Gough »

ChrisR wrote:The viewing distance of the print, is often, in the words of one our great leaders, misunderestimated. If you look at the print through the taking lens, you get the same perspective.
That is a way of agreeing with my first point.
ChrisR wrote:
Surely, the extension is a factor in the viewing position? (I have to admit uncertainty here).
Not entirely sure what you mean.
If you take a pic at 1:1 with a 200mm lens, you need to move the sensor 200mm further from the subject, to focus, than you would for infinity.
Yes that DOES have the effect/angle of view you'd normally expect from a 400mm.
Is that what you mean?
No. I was refering to bellows/tube extension behind the lens (but not excluding teleconverters).

Harold
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ChrisR
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Post by ChrisR »

I should have writtten
you need to move the sensor 200mm further from the lens, not
you need to move the sensor 200mm further from the subject.

I think that's what you mean by "Behind" the lens?!

In case anyone's wondering why you can't just go right up to something with a long lens, the distance between the lens and the subject is given mathematically by
Lens Focal length (like 200mm) + (LFL divided by Magnification)

So you can never get closer than 200mm in this case.

This strictly is for "siimple" lenses. In reality working distance may be very different.

Harold Gough
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Post by Harold Gough »

ChrisR wrote:I should have writtten
you need to move the sensor 200mm further from the lens, not
you need to move the sensor 200mm further from the subject.

I think that's what you mean by "Behind" the lens?!
Empirically, yes.
ChrisR wrote:In case anyone's wondering why you can't just go right up to something with a long lens, the distance between the lens and the subject is given mathematically by
Lens Focal length (like 200mm) + (LFL divided by Magnification)

So you can never get closer than 200mm in this case.

This strictly is for "simple" lenses. In reality working distance may be very different.
See here for non-simple lenses:

http://en.wikipedia.org/wiki/Telephoto_lens

Harold
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rjlittlefield
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Post by rjlittlefield »

There's a big problem that "FOV" means different things to different people.

To some people FOV is a linear dimension, like "15 mm horizontal".

To other people FOV is an angular dimension, like "5 degrees".

Regarding perspective, as Chris says, FOV the linear dimension makes no difference. You can shoot 15 mm across the field with perspective ranging from extreme in-your-face (using an endoscope) to none at all (using a telecentric lens). Whether you can tell the difference between those two depends on having actual depth in the subject. For any given actual depth, you get more apparent depth from a close-up perspective.

But as Harold says, FOV the angular dimension makes all the difference in the world. If the linear dimension is fixed, then large angular FOV goes hand-in-hand with close-up perspective, while small angular FOV goes with far-away perspective.

It came as a great surprise to me -- a Christmas present in 2005 -- to discover that you can get different perspectives using exactly the same lens and sensor, in the same positions, simply by changing the aperture position. It is completely possible, quite simple actually, to shoot telephoto perspective from a close-up position. Simply place an aperture so that the selected ray cones are all parallel to the optical axis. But shooting a close-up perspective from a long distance is not possible, since the light rays that represent that perspective go shooting off at angles nowhere the lens.

How and why this works is explained in Theory of the “No-Parallax” Point in Panorama Photography. Obviously I learned it in the context of stitched panoramas, but the underlying theory is quite general. These days I use it mostly to think about perspective in macro setups, the better to set up telecentric systems, understand inverted perspective in lens combos, and so on.

In that paper, I use the less ambiguous term Angle of View (AOV). But FOV has been used for so long in both ways that there is no hope of straightening out the terminology at this point.

--Rik

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Post by PaulFurman »

Chris, could you explain that diagram? All I can get is that's DOF graphed somehow...

elf
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Post by elf »

Here's another (not quite macro) example of perspective: http://www.flickr.com/photos/43894176@N ... 745355484/

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Post by augusthouse »

elf wrote:
Here's another (not quite macro) example of perspective: http://www.flickr.com/photos/43894176@N ... 745355484/
The images at that link are rather special (^^)

Craig
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Harold Gough
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Post by Harold Gough »

rjlittlefield wrote:In that paper, I use the less ambiguous term Angle of View (AOV). But FOV has been used for so long in both ways that there is no hope of straightening out the terminology at this point.
Historically, it has been a struggle to get "depth of field" accepted as more appropriate than "depth of focus" in general photography. Perhaps the preference for FOV shows some consistency.

Harold
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rjlittlefield
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Post by rjlittlefield »

Harold Gough wrote:Historically, it has been a struggle to get "depth of field" accepted as more appropriate that "depth of focus" in general photography.
That problem is not restricted to general photography.

Quoting from Nikon's MicroscopyU: The term depth of focus, which refers to image space, is often used interchangeably with depth of field, which refers to object space.

Two things I have learned over the years are that A) you can never trust that two people are using the same word to mean exactly the same thing, and B) trying to fix A is an exercise in futility.

Whenever I read technical material, I spend a little extra effort specifically trying to make sure that I understand how the terms are being used by the person who wrote the material. Even in mathematics, widely considered to be a bastion of precision and consistency, it is not unusual to find that the same word has different meanings even in apparently similar constructions. My personal favorite is to note that f(x)=3x+5 either is or is not a "linear function", depending on which of two common definitions is being used.

--Rik

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Post by Harold Gough »

rjlittlefield wrote:There's a big problem that "FOV" means different things to different people.
Further thoughts on this:

The angle of view of a prime lens is something you would expect to find in its specification. However, once you put a teleconverter or extension behind the lens you change the effective angle of view. Consider also the factor of variation of sensor size, with or without the foregoing, and the angle of view of the lens specification is rather disconnected from the reality of image capture, except in relation to an alternative lens in the same setup.

While the angle of view of a prime lens, used directly on the camera body, is of some practical interest, that of e.g. a bellows lens is so only at the magnification around that to which it was designed (perhaps optimised) to be used.

Thus, ideally, for all photography we might better think in terms of magnification at the film/sensor, linear for our understanding, in area where adjustments of exposure are required.

Harold
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Post by ChrisR »

I was coming back to this , regarding the different meanings applied to FOV, and whether Angle of view was safer. We're wrangling with things we've been used to for a long time so each have a good understanding, but when expressed either differently, or loosely, they look "wrong" to one or more of us and alarm bells start ringing.
It can be quite awkward to find out what the difference in usage or understanding really is, and then somehow deal with it. Overdoing it, in going over too much, is not always helpful, neither is digression.

I daresay if we each tried to write a definition of "perspective" we'd differ, as the dictionaries do.
Angle of View is another one. Not many would be familiar with say what a 12º angle of view is, but we do know when to pick up say a 200mm lens. But as Harold points out, as soon as you focus the thing the angle changes.

Depth of field/focus/sharp region - here we go again. We can all work it out from a theoretical viewpoint (perspective...) perhaps accepting a CofC of 1000th the sensor size, but for what shows in a small print, that would be the wrong measure.

Magnification onto sensor is safe at least some of the time, for exposure purposes, but I'm not at all sure what happens e.g. if you poke your camera down 2 metres of endoscope. The Fuchsia might be life size on sensor, so you add 2 stops, right? Maybe.. I 'd bet there are other factors in there somewhere!

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Post by Harold Gough »

ChrisR wrote: I 'd bet there are other factors in there somewhere!
Not that I can think of. Magnification is what matters with tubes. If you start stacking (several) teleconverters (unlikely in this application) there may be light loss due to imperfect transmisson. This is covered in a string somewhere.

Harold
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Post by rjlittlefield »

ChrisR wrote:Magnification onto sensor is safe at least some of the time, for exposure purposes, but I'm not at all sure what happens e.g. if you poke your camera down 2 metres of endoscope. The Fuchsia might be life size on sensor, so you add 2 stops, right? Maybe.. I 'd bet there are other factors in there somewhere!
I'm with Chris on this one. I certainly don't know enough to predict the effective f-number of his endoscope. I'm pretty sure it has one, and I'm pretty sure that the usual relationship would hold between effective f-number, magnification, DOF, and diffraction. But what the f-number is, and where the limiting aperture physically resides, now those are mysteries.

--Rik

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Post by rjlittlefield »

(PaulFurman, if you're looking for replies to your questions, I split those posts off into their own thread, HERE.)

--Rik

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