# Is there a way to calculate field width from the mm of the lens?

Discussion in '35mm Cameras' started by BobFlint, Apr 12, 2006.

1. ### BobFlintGuest

In other words, if I'm 12 feet from something and I want to have a 6 foot wide
field of view, what mm lens would I use?

Since I'm using a D70 I guess I'll have to figure with the cropping factor, no
big deal...

I want to get a wide fixed lens and there is a 24mm in the store, and I can
order a 20mm if I want... for \$100 more... I'm wondering how to calculate what
the different view would be...

I've been taking test pics with a zoom lens but that's tedious... some nice math
charts would be nice!

Thanks

BobFlint, Apr 12, 2006

2. ### Scott WGuest

The D70 sensor is 23.7mm wide, so if you are 12 feet from a 6 foot wide
traget you would need a lens that has a FL of 23.7mm * 12 / 6 or
47.4mm.

Scott

Scott W, Apr 12, 2006

3. ### Peter ChantGuest

It's fairly simple, you need to know the sensor width. You can then
calculate the field width using similar triangles.

Pete

Peter Chant, Apr 12, 2006
4. ### Matt ClaraGuest

Simpler yet, take a slide mount, use it to frame the shot you want, measure
from the slide mount to your eye, the distance in millimeters will
correspond to the lens you would use to frame the same shot. You can attach
a bit of measuring tape to the slide mount to aid in quick measurment.
Because the slide mount represents 35mm, you'll need to mask out part of the
opening to correspond to the D70's sensor size.

Matt Clara, Apr 12, 2006
5. ### JimGuest

Yes, it is just a matter of trigonometry.
Nikon shows the angle of view of their lenses. These charts are really
quite common.
Jim

Jim, Apr 12, 2006
6. ### bob.kirkpatrickGuest

Using a 35 slide mount as described in another post is the simplest but
if you have a calculator with you: f = focal length. M =
magnification. For a format width, height, or diagonal of distance X,
the angle of coverage along the width, height, or diagonal is
2*ARCTAN( X / (2*f*(M+1))). At infinity the magnification M is 0 so
the formula is 2*ARCTAN( X / 2*f ).

bob.kirkpatrick, Apr 12, 2006
7. ### BobFlintGuest

Thanks! That matches my test shot as well...

BobFlint, Apr 13, 2006
8. ### BobFlintGuest

As Spock would say... Fascinating!

There's nothing like empirical engineering!

thanks

BobFlint, Apr 13, 2006
9. ### BobFlintGuest

I figured that much...

Why not?

Maybe, but I haven't seen them... and they wouldn't apply to a D70...

BobFlint, Apr 13, 2006
10. ### BobFlintGuest

Thanks - I'll do a spreadsheet sometime...

BobFlint, Apr 13, 2006
11. ### JimGuest

The last few full catalogs have a table which lists every Nikon lens. This
list shows the angle of view for 35mm cameras and the APS-C digital cameras,
The D70 is such a camera.
You can also find such a list on the Nikon web site.
Jim

Jim, Apr 13, 2006