"nospam" wrote in message
...

On 22:20 24 Dec 2018, "Sam Brown" wrote in
news
"Ken Hart" wrote in message
news
On 12/24/18 6:10 AM, nospam wrote:
In article , Sam Brown
wrote:

As I understand it, on a 35mm camera if 2 different lenses have
the same focal length then the angle of view would be the same in
each case.

only if the sensor size is the same.

a '35mm camera' can have at least five different sensor sizes.

A '35mm' _style_ _digital_ camera can have different sensor sizes.

An actual 35mm film camera has one sensor size, 24x36mm. (Unless, of
course, it is a half-frame 35mm camera, or one of the early 35mm
cameras made before the 24x36 standard, but that's a different
animal.)

Is it the same on a smartphone (only focal length determines angle
of view) or does sensor size also affect the angle of view?

it does, the same as with any camera.

In theory, if the film/sensor size is increased, the angle of view
from a given focal length lens will increase, and the magnification
of the image will be less.

An 80mm lens on a medium format camera (60mm film size) is a
'normal' lens. An 80mm lens on a large format camera (4"X5" film) is
a wide angle lens. An 80mm lens on a 35mm film camera (24mm X 36mm)
is a long or telephoto lens.

(In the real world, a lens is designed to cover a specific
film/sensor size. Putting a 35mm camera lens on a 4"x5" camera will
most likely give you a small image in the middle of the film.)

Very useful.

I used an online calculator for field of view but it calculated a
narrower horizontal field of view using my smartphone camera's specs
(69 degrees) than the smartphone could see (just over 80 degrees).

To check this, I printed a protractor and marked on the printed
protractor the two limits of the horizontal angle of view I could see
on the screen. I drew lines back from these points to the center of
the protractor. However when looking through the smartphone with the
protractor center was held against the lens, the lines did NOT
converge to a point. I could only get the point of convergence (the
protractor centre) in the viewfinder if I held the protrector center
about 6mm in front of the lens.

Presumably this is what is called the "front principal plane" in the
page David Taylor linked (figure 2).

https://www.edmundoptics.eu/resource...notes/imaging/
understanding-focal-length-and-field-of-view/

Is it usual to have the front principal plane of a smartphone quite
so far in front of the outer lens element?

In my case this made a significant difference.

By pulling the camera back 6mm from the point at which the lines seems to
converge, it increased the angle of view (horizontally) by 10 degrees.

It's almost pointless having a 35mm equivalent is it is so wrong. I was
asking if anyone knew the the explanation.