why the fascination with full frame?

Started May 19, 2009 | Discussions thread
Joseph S Wisniewski Forum Pro • Posts: 34,130
OK, normals and fast lenses explained...

kwik wrote:

Joseph S Wisniewski wrote:

A normal is a normal is a normal, it has a focal length about equal
to the sensor diagonal (about 15% larger for fast normals on SLRs,
and I won't get into the physics of that) and covers about 53

...I wouldn't mind a quick education on the "physics of that". I've
always wondered why that is.

OK, here goes...

An SLR has a mirror that sits at a 45 degree angle, and has to be able to "light" the focusing screen from top to bottom. That means its height is the image height (sensor or negative height) * sqrt(2). For 35mm FF, the image is 24x36mm, so the mirror is 24*sqrt(2) = 33.9mm tall. When the mirror swings up for shooting, that 33.9mm arcs forward like a swinging blade. There's a shutter mechanism some 4mm thick in front of the image plane, the mirror swings fron the front of that, so it swings out to at least 37.9,, from the image plane. That means the rear element of the lens can come no closer than 38039mm from the image plane, or it's in danger of hitting the swinging mirror. That 39mm is called the "back focus" of a lens. (this has nothing to do with the focus error commonly called "back focusing").

Now, the simplest lens that works well at speeds like f2.8 or 2.0 is a symmetrical pair of achromats. An achromat is just two lens elements, a positive ("bulging" or "convex" curves) made of one of one kind of glass, and a negative one ("hollow" or "concave" curves) of another kind of glass. The use of two different glasses causes the two elements to cancel some of each other's chromatic aberration (tendency to produce color fringes).

We use a pair of achromats, facing "back to back" with the positive elements facing outward, and the aperture between the two achromats. A matched pair of lenses cancels several other "aberrations" such as distortion or astigmatism, producing a very sharp image with a very simple four lens element. Because it's symmetrical, its very nearly "balanced" around an optical center. So a 44mm lens (the "perfect" normal for 35mm FF) has one achromat forward of 44mm, towards the subject, and one achromat behind 44mm, towards the sensor. That leaves us 5mm (44mm-39mm) from the optical center to the "safe" rear element location. And, because the lens is symmetrical, another 5mm from the center to the front element. So the two achromats have to fit within a 10mm long tube. The diameter, for an f2.8 is 15mm. This is just barely possible to make, but it's difficult to make a four element lens that wide and thin.

If we tried to go to f2.0, it would have to be 22mm in diameter, and still 10mm thick. And f1.4 would be 30mm in diameter, and still 10mm thick. Neither of those is technically possible with known optical glasses.

The reason is that lenses are sections of spheres. Imagine a large sphere, say 12 inches in diameter, Now, if we carved off a little lens, a circle 1 inch in diameter, it the sphere doesn't curve very much in just 1 inch, so our lens would be very thin, a fraction of an inch.

But if you tried to carve a 6 inch circle, you'd be taking 1/4 of the sphere, and your lens would be huge, several inches thick.

So, the larger a diameter we give each lens element, the thicker it gets.

By the time we make an f1.4 lens, it's grown to about 35mm thick. That means that the half of the lens behind the center is about 18mm thick. But there's only 5mm clear between the center and the mirror for a 44mm lens, so we're stuck.

The solution is simply to make the normal longer. The shortest normal that won't whack the mirror is 39mm + 18mm = 57mm. And back in the late 50s to early 60s, Nikon, Canon, Pentax, Topcon, etc. all launched 58mm f1.4 normals. The photographers hated them and complained bitterly.

So the camera companies made the normal assymetrical. They made the group in front of the center a shorter focal length than the group behind the center. This pushed the center back, and let the lens move forward. 44mm would have been a huge push, 14mm. Making the lens assymetrical meant the rear group did less to correct the front group's aberrations. So distortion increased, sharpmess decreased, life uas ugly...

So the camera companies compromised, they met the photographers half way, around 50mm. Not as wide as the photographers wanted, but not as much distortion and loss of sharpness as if they had gone the full 14mm.

And this is how it's been, ever since, the normal being longer than photographers want, to make the lens more "buildable" with acceptable distortion and sharpness.

(MF somehow or other managed to miss the boat, and end up with a
28mm diameter normal, 80mm f2.8. That's a long story. If MF were viable,
and pursued by companies as aggressive as Nikon or Canon, we'd be
seeing a lot faster and better lenses).

I'd love to hear the story!

That one will have to wait till tomorrow.

(hint, 6x6 was the most popular MF format, then 645 overtook it)

(second hint, the aspect of 35mm is 1.5, while the aspect of 645 is 1.33 and the aspect of a square format is 1)

-- hide signature --

Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph


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