Portrait perspective issues investigated

[cowanw]

Quite right. Poorly worded.
What I was trying to indicate was that to fill the picture at a larger format size with the same size lens requires the lens to be positioned closer to the subject. Despite this, it is commonly said that ULF images do not need longer (calculated as normal) lenses, that the virtual extension makes it a longer lens.

This sentence doesn't make sense.

The angle of view for specific lens remains constant, as the lens moves farther from the film the projection at the film gets larger, more coverage. This is the reason that coverage is also measured normally at infinity Focus.

If a 420 and 210 lens both had the same angle of view, then both would cover the same area when focused at the same bellows draw, what would not be the same is the focus.

What Sheldon was getting at though is the limitation of the camera if all I can hold is 4 x 5 film moving from 210 to 420 change is the angle of view that the camera can see, it does not change the optics of the lens but it does change where the plain of sharp focus falls.

[cowanw]

Yes to this (I think). I apologize if I am too dense to fully understand.
but again what I was addressing was the equivalent of the question of taking your 50mm lens and using it as a portrait lens for 8x10. Perspective must change (as the lens moves) yet upsizing this question to 20x24 changes things, according to LHF lore (apparently because the putative length of the lens changes.
It would be much easier for me to see and comparison example.
What I would really like to see is two same full face subject photos done with say a 360mm lens on 8x10 and 20x24. Then both printed to 20x24.
I am trying to think how I could test this with max 8x10 gear.
The 8x10 full face is no problem.
If I measure a 1 inch object and then I move in to make it 2.4 inches, for a second photograph, I will have a 8x10 section of a 20x24 print.
Then I enlarge the 8x10 full face to make the 1 inch, become 2.4 inches and compare the two.

Why so much talking? This is really easy to demonstrate:
A 12" lens is a bit shorter than 8x10 diagonal, which is about 12.5". 35mm diagonal is about 42mm, so a 42mm lens would be a hair longer equivalent on 35mm film to the 12" on 8x10, but it's close enough. If the two function equally at portrait distances, they should cover the same field when the lenses are at the same distance from the subject, right?

I just set up my 8x10 with a 12" lens pointed at my book case at portrait distance so that the vertical field of view was two shelves---30 inches. The total bellows extention is 15.5 inches, which is conveniently close to equivalent to 50mm on 35- approximately diagonal plus 20%, in both cases. The extension of the 50mm at this distance is, of course, negligible. If the case under discussion is true--that it's not the lens but the extension--then the 50mm (which is a long lens on 35mm by the normal rules) at the same distance (same lens to subject distance) should be cover the same as the 12".

And it does. A 35mm lens, which isn't too far from being a 12" equivalent, is wildly wider--another shelf and then some more. That makes a 12" on 8x10 about equal to 50mm on 35mm film. I suspect that with longer lenses on 8x10 the situation becomes even more in favor of these lenses acting like their extension, not their focal length.

The important point is that perspective = field of view compared with eye to subject distance. All the rest is extraneous.

You can do this at home, if you have a camera. :-) In fact, someone who's more fussy than I am should do it and report back.

[jb7]

Yes to this (I think). I apologize if I am too dense to fully understand.
but again what I was addressing was the equivalent of the question of taking your 50mm lens and using it as a portrait lens for 8x10. Perspective must change (as the lens moves ...

I think you have to imagine a camera that focuses by moving the rear standard.

Putting the lens for each format in the same position relative to the subject is what controls perspective; moving the ground glass until it is co-incident with the in-focus image is a good way to focus without altering perspective in that close up zone.

[jb7]

ok, maybe I didn't read far enough...

yes, perspective will change as the lens moves- the purpose of this comparison is not to compare the same lens on two different formats, but to compare lenses that will give the same perspective on different formats. Since the convention for measuring the length of lenses is at infinity, and not at close up distances, it's impossible to directly correlate the equivalence of two optical systems except at infinity, or close to it, if there is significant magnification involved.

Those who say they would like to use the equivalent of an 85mm lens on 35mm for close up portraiture on an 8x10 camera might find themselves wondering why they don't have enough bellows, if they get a lens that produces an equivalent angle of view at infinity...

[cowanw]

Quite right, regarding your comments, jb7.
It is the other that I can't get my wee brain around.

[Mark Barendt]

I think there are a few important side notes that should be made here.

First, the perspective of the taking lens is for the most part defined by where it is and where it's pointed. It does not matter what film size is behind the lens. A 4 x 5 is just a crop of an 8 x 10 which is just a crop of a 16 x 20. The prospective relationships stay the same, the angle of view changes (the crop).

Second, is that the instant that you turn a three dimensional scene into a two dimensional representation, the prospective relationships in the two-dimensional representation is fixed. The variable that controls whether or not when you see distortion is our relationship to the two-dimensional print or negative. What we call print viewing distance, although I'm not sure that properly defines the concept.

[cowanw]

Yes and yes.
Sheldon N brought up and I am discussing the close up portrait, wherein the position of the lens is changed to occupy the film size with the same image.
The flip side of perspective coin as it were.

[Sheldon N]

Yes, another term for it is "focus breathing". Filmmakers complain about it, since the field of view of their shot changes as they pull focus. Some complicated lenses with internal focusing can avoid it, but since we're using a simple lens design and bellows focusing in large format we get to experience the full effect. It's also why macro photographers tend to use focusing rails instead of just focusing the lens. Each time you refocus you change your framing and angle of view, it's simpler to just move the whole camera on a focusing rail than it is to focus/reposition/focus/reposition/focus.

To get into some of the lens comparisons mention above (50mm on 8x10, 360mm on 8x10 vs 20x24, crops, etc) , I'd suggest thinking about it this way....

1) How big is your negative?
2) How big is your subject?
3) How much magnification is this going to result in?

Once you know the magnification (and the focal length of your chosen lens), you can figure out how much bellows extension the shot will need. Then this will give you a rough idea of your effective focal length. You can then compare that to your format size to see what your field of view is.

An example....

My negative is 8x10 inches
My subject is 8x10 inches (tight headshot)
My resulting magnification is 1:1
I'm using a 50mm lens, so I need 100mm of extension to get to 1:1.

100mm field of view on 8x10 (if the lens could cover 8x10) is still a super wide angle. Your picture will be very distorted because you'll have to be super close to the subject to fill the frame. Replace the 50mm lens with a 360mm lens, and you end up with 720mm of extension and an effective field of view a lot like a 100mm lens on 35mm film. You'll have a pleasing perspective because you had to pull the whole camera back to frame the shot.

As you go smaller in format, less magnification is needed for a given picture so there's less bellows extension and less of an effect on the field of view.

[jb7]

... It's also why macro photographers tend to use focusing rails instead of just focusing the lens. Each time you refocus you change your framing and angle of view, it's simpler to just move the whole camera on a focusing rail than it is to focus/reposition/focus/reposition/focus.

...

This is not a universal truth, it relates only to cameras in which the film plane is fixed, and the lens is moved in order to achieve focus. Using a large format camera which can focus by moving the film plane, these reiteration a are not necessary, though they might form part of the picture taking process anyway. Sometimes it's not possible to achieve focus at all by moving the lens position only, hence the need to move the whole camera as a unit.

[Sheldon N]

Yes, lenses with internal focusing that are free from focus breathing solve that problem. It's the bellow style or true extension style macro lenses that are more difficult.

Focusing with the rear standard solves the problem that focusing with the front standard has, where you are pushing the lens toward your subject and working against yourself when you actually need to back the lens up. It doesn't totally get rid of the iterative process of focus/reposition, because there's only one correct distance of lens to subject for a given magnification/framing and you need to find that somehow.