# Re: Field of view - need help with the math

Discussion in 'Digital Photography' started by David Dyer-Bennet, Nov 15, 2011.

1. ### David Dyer-BennetGuest

Signet <> writes:

> I respectfully beg to differ. Using the diagonal of the frame
> size for the normal focal length was conceived of as a rule-of-
> thumb, not really scientifically. My first camera was a Pony 135
> and it had an f/3.5 44mm lens. It was decidedly moderately wide-
> angle. A later precision camera, a Minolta SR-1 featured a
> correct 55mm f/2. Minolta wisely chose that focal length because
> it provided a 1:1 image in the viewfinder. In other words, if
> you viewfind the focused subject with your left eye and
> simultaneously sighted the same subject with your naked right
> eye, the two images are coincident. Thus, the field of view of
> the camera and eye are the same to the limits of the frame,
> which is the real definition of a "normal" focal length. Sadly,
> fixed 55s are next to impossible to find these days.

But that's not field of view you're comparing; that's magnification.

David Dyer-Bennet, Nov 15, 2011

2. ### PeabodyGuest

David Dyer-Bennet says...

> But that's not field of view you're comparing; that's
> magnification.

To be clear, I want to take "the same picture" with my T2i
as with the Medalist. So I'm looking for the same field of
view.

I think my 26.4mm number was right. It's kinda hard to tell
by looking through the Medialist viewfinder because I don't
know what percentage of the actual field shows up in the
viewfinder. But it does look pretty close.

Well, I bought a roll of film today, so I'll know soon
enough, assuming the respooling goes ok.

Peabody, Nov 16, 2011

3. ### Kennedy McEwenGuest

In article <>, Peabody
<> writes
>David Dyer-Bennet says...
>
> > But that's not field of view you're comparing; that's
> > magnification.

>
>To be clear, I want to take "the same picture" with my T2i
>as with the Medalist. So I'm looking for the same field of
>view.
>
>I think my 26.4mm number was right. It's kinda hard to tell
>by looking through the Medialist viewfinder because I don't
>know what percentage of the actual field shows up in the
>viewfinder. But it does look pretty close.
>

Fortunately both the sensors are 3:2 aspect ratio so it doesn't matter
what dimension you use, provided you use the same on both. The posts
about image diagonal are just irrelevant in this case.

Your Medalist has sensor width of W1, sensor height of H1, sensor
diagonal D1 and focal length F1. Your T2i has sensor width of W2,
sensor height of H2, sensor diagonal D2. You are trying to find focal
length F2.

F2 = F1 x W2 / W1 = F1 x H2 / H1 = F1 x D2 / D1 = approx. 26.4mm
as you first estimated.

If the aspect ratio of the sensors was different then you would need to
decide which FoV you wanted to be the same - horizontal, vertical or
diagonal - they would all be different.

However, you may well find that the focal length of the lens on your
Medalist is only nominally 100mm, and could have a tolerance of as much
as +/-10% which might even change with ambient temperature. Your
required focal length will have the same tolerance.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

Kennedy McEwen, Nov 16, 2011