# Can you tell the lens mm if you know the angle of the shot?

Discussion in '35mm Cameras' started by Pettygrugh, May 24, 2007.

1. ### PettygrughGuest

A friend asked me if I could tell him what lens was used to take a photo, where
the view angle was 30 degrees.

I'm assuming full frame 35mm film...

Is there a formula?

I just guessed and said it's probably somewhere between a normal 50mm, up to
maybe 75mm...

I forget most of my trig! (school was a long long time ago...)

Pettygrugh, May 24, 2007

2. ### Scott WGuest

If the 30 degrees is the horizontal field of view then the focal
length would be 18mm/tan(30/2)
or 67mm.

Scott

Scott W, May 24, 2007

3. ### Colin_DGuest

First, how is the 30° measured? Is it the short side, the long side, or
the diagonal? The answer to that is needed to calculate the focal
length. Second, what measurement did you use to ascertain that the
angle of view in the image is in fact 30°? I am not aware of any method
to do that just from the image.

However, assuming an AoV of 30° is right, and the angle refers to the
image diagonal of 43mm for a 35mm frame (usual), then 43/2 (half the
diag) divided by tan(15°) (half of 30°) is 21.5/0.2679, = 80mm
(rounded). So under those conditions the focal length is 80mm. (Half
the diagonal and half the angle is used to obtain a right-angled
triangle to do the trig with.)

Colin D.

Colin_D, May 25, 2007
4. ### PettygrughGuest

of geosync satellites... I take it you need a 30 degree angle to capture a
couple of satellites... Someone took a photo of 30 degrees and he wanted to
know what lens could be used. Perhaps it referred to the angle above the
horizon, from horizontal to 30 degrees up... which is a normal method used to
find space objects... so either way, it would be the angle of the cone of vision
at the camera. It's not too important, he was really wondering if it required a
special telephoto lens or just something anyone would have.
Thanks for the info.

Pettygrugh, May 25, 2007
5. ### Fred McKenzieGuest

Pettygrugh-

This doesn't add up! If you want to photograph an actual satellite from
the ground, consider the angle subtended by the tiny satellite from a
great distance. In your case, they are thousands of miles away. It
would be a Herculean task for even an extremely powerful telescope used
as a camera lens.

Normally you wait until after dark to see one of the larger satellites
in low orbit, while it is still in the sunlight. What you see is just a
small dot, like a moving star. With a long time exposure, it might show
up as a streak of light using a camera with a 30 degree view.
Geosynchronous satellites don't move much with respect to a point on the
earth, so you aren't likely to distinguish between them and the many
stars behind them.

For satellites in low orbits, it is possible to photograph them using a
telescope on a computer-controlled tracking mount. If you search the
this several years ago, and the technique even worked in daylight.

Fred

Fred McKenzie, May 26, 2007
6. ### PettygrughGuest

I think the idea is to get a few satellites at once, hoping the sun is lighting
them up at night! They would be the only dots in a sky full of streaks... I
guess you have to know where they are as well...
I didn't really look into it... I'm just quoting... I've taken a few moon shots
and once a trio of planets in close formation... but I wouldn't attempt
satellites!

Pettygrugh, May 27, 2007
7. ### Duncan ChesleyGuest

Geosynchronous satellites are all on the celestial equator and are all
22,000 miles (IIRC) up, You should look in the western third of the
sky after sunset and the eastern third of the sky before sunrise. They
will be very faint, probably only visible in a large telescope with a
long exposure time. For about 3 hours each night any given one will be
in the Earth's shadow and therefore invisible. There are a lot of
them, however.

Cheers,
DuncanC

Duncan Chesley, May 27, 2007