33RIVERS Loc: Central MN

Jerryc41, Now it makes perfect sense! I spent 6 years as a propulsion test technician in the aerospace industry, worked on everything from satellites to the Space Shuttle and still have a hard time wrapping my brain around the subject! Your diagram brought it all into focus (slight pun intended). Now my only question is...Does your diagram come printed on a cocktail napkin?

Crazycameralady Loc: California

I think my head hurts :o. TMI.....

mcveed Loc: Kelowna, British Columbia (between trips)

Fill them in, or do them in?

dachs

Start over; the focal length of a glass is the distance to its point of focus, doesn't vary except zooms do by re-arranging all the glasses inside. It's apparent working angle of view stays the same too, but if you put smaller film or sensors behind it, the actual angle of taken picture becomes restricted and we get a somewhat telephoto effect.

100 years ago 6cm by 7 or 9cm film was common, and ordinary folk got contact prints off the negatives. So, family snaps were 6 x 7 or 6 x 9 (later, square ones 6 x 6) (35mm cameras were also all the rage but usually used by enthusiasts)

The point was that at normal reading distance, say 9 to 11 inches, a 6 x 7 contact print looked about right if the original negative was taken with an 80mm focal length lens. People's noses seemed the right length, the car and house looked normal.

A similar thing emerged with 35mm, and the standard for that became 40 to 50 something mm focal length for similar 'angle of view' and 'perceived perspective' reasons. Though, usually, the pictures had to be an enlarged print to make anything of them, which introduced a whole extra set of variables such as the enlarger lens, however, much less effect on the finished print (which was flat and from a flat 2-D negative) than the camera.

If we look to large format, we'll see 180 mm standard lenses on those old mahogany boxes on tripods. In that case the lens had to be computed to throw light inside the box all over a 5" by 4" negative - around 100 times more area than an APS C sensor!

Back to 35mm film or sensor; due to folk like Cartier Bresson, Capa et al, (though I never met Al) the 35mm lens became a standard; very little distortion, usually large aperture (for the day) and when computed and built by serious firms, very high quality - as good as the film was in resolution terms in the post War years. But if you do a big enlargement of a lady taken close to with a 35mm lens, and stand close to the print you'll not like the result - big nose, receding hair, odd facial proportions. To completely avoid that (though it is geometrically "true" it's not attractive unless you mean to have a clown effect), we have the concept of 'portrait' lenses.

For 35mm again, a portrait lens of choice would be 75mm focal length minimum, preferably 90mm, and they have the advantage of you standing back a little from the subject. Note that to change perspective you MUST move; the longer lens pushes you back to a point where you still fill the frame. Two different focal lengths from the same position are the SAME perspective, but the effects and picture crop vary of course.

Cropped sensors would give similar results with 50mm lenses for portraits.

The 3 to 6mm lenses in phones will distort whatever you do, which is why Facebook etc is full of unbelievably bad pictures of people.

CaptainC Loc: Colorado, south of Denver

Boy, talk about asking for the time and having someone build you a watch!
:-)

dachs

served him right I thought, and up to then , no-one had touched on the historic reasons for viewing pix and manufacturer's lens choices.

If you wish I'll stand down, but the OP doesn't have to read it?
Kindest regards, b0b0

Jay Pat Loc: Round Rock, Texas, USA

That's perfect!!

dachs

Dear JP - et tu, Brute? You meant the Captain's venerable bon mot was pertinent I think.

I was trying to work in the perception thing, which is mostly what it's about, with a brief history note.

If I am wrong in detail please tell me! I don't want to be doomed to wandering Flickr like the Flying Dutchman.....

johna800 Loc: Atlanta

Often, focal length refers to the distance between the front and back element of a given lens. It also provides a reference to the angle/area of coverage and magnification provided by the lens.
It gets more complicated but this is the basic understanding.

Keep Shooting,
John A

Pentony Loc: Earth Traveller

I love it when you guys fight oh I mean discuss things. :P :-P :lol:

Jen226 Loc: Cleburne, TX

Back in my 35mm film days, I remember reading that the 50mm focal length is fairly close to a person's optimal field of vision. A good analogy is if looking at the horizon, think of imaginary lines extending from each shoulder to the horizon, (you can place your hands on your shoulders and then extend them out), so what you see between your hands or the imaginary lines, is where the vision of the human eye is at its best. We have much more in the way of peripheral vision, but much detail, etc., is lost outside the "shoulder-width" area. Here's a link to a website that has an excellent article relating the camera's focal length (both crop sensor & full frame) to human vision:

http://www.cambridgeincolour.com/tutorials/cameras-vs-human-eye.htm

Hope this helps!

Jen

MT Shooter Loc: Montana

No doubt!

Weddingguy Loc: British Columbia - Canada

"Often, focal length refers to the distance between the front and back element of a given lens. It also provides a reference to the angle/area of coverage and magnification provided by the lens.
It gets more complicated but this is the basic understanding.
"



Absolutely not! . . . sorry!

johna800 Loc: Atlanta

"Rear focal distance" redirects here. For lens to film distance in a camera, see Flange focal distance.
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.

In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.

Is that a better general explaination?

dachs

and I am accused of watch making. Seems unfair