Any math whizs out there?

[sldi]

Even though geometry was my favorite math class in school, its been awhile and I can't remember a formula.

I want to be able to figure out the distance I need to be away from a subject of a give height, given the angle of view of the lens. I know there is a bit of difference in landscape and portrait, as well as my aspect ration. I now its going to be somewhat based on Pythagorean logic

So If I am shooting a 6' subject with a lens that has a 29degree FOV, placing the central axis of the lens midway at 3ft above the ground, what subject to focal plane distance do I need.

[cooltouch]

It's actually not a geometry problem, but a trig problem. Assuming you have a calculator with trig functions, make sure it's set to degrees first. This is your formula, where a = half the height of your subject, or 3', b = distance from focal plane to subject, and T (for 'theta') = half the field of view of the lens, or 29/2, or 14.5:

tan T = a/b

-or-

tan 14.5 = 3/b

solve for b:

b = 3/(tan 14.5)

b = 11.6 feet

[ronocnikral]

what he said...or you can just go out and shoot it.

[ramiro]

What is this used for? Is this to determine at what distance from focal plane the subject needs to be for perfect focus?

[sldi]

thanks for the formula, I could see it in my head but hated trig and couldn't get that part figured. I realized I could go out and shoot and get figured out, but I wanted to have a base so I could I have a general starting point when thinking of lens/distance combos for a shoot

[ramiro]

Originally Posted by ronocnikral

what he said...or you can just go out and shoot it.

I was going to say, call me dense but I just press the AF-ON button until the little green dot lights up in the viewfinder then I press the shutter.

[dryicerx]

Originally Posted by ramiro

I was going to say, call me dense but I just press the AF-ON button until the little green dot lights up in the viewfinder then I press the shutter.

It's not the focus, I think he was trying to get full coverage of the body how far back to stand.

Also you can google the equation (replace HEIGHT and FOV)

( HEIGHT /2)/tan(( FOV /2) degrees)

[cooltouch]

Back in ancient times, when photographers were doing, say, macro photography with rangefinder cameras -- using extension tubes and such -- photographers routinely had to calculate focus, determine depth of field, etc.

The value of this formula in these modern times is to be able to quickly determine the distance from focal plane to subject in which the subject occupies the maximum field of view of the lens.

-M

[Coelus]

Just to muddy the Mississippi; does a crop sensor change the FOV?

[The Philos]

Yes, you have to look up the AOV or calculate it yourself-in fact that's all a crop sensor changes. It doesn't change the focal length of the lens it changes the AOV. To have a 29 degree AOV on 35mm you need an 85mm lens, on Nikon and Canon crop it's ~55mm lens. The focal length you need to have a consistent AOV changes due to the crop. You have to decide what you're going to hold constant when looking at AOV and the focal length of the lens.

There's a chart on B&H:

[kenw]

or you can use this:
http://www.dofmaster.com/dofjs.html

kenw added 0 Minutes and 33 Seconds later...Double Post Merged Below

Originally Posted by Coelus

Just to muddy the Mississippi; does a crop sensor change the FOV?

yes

[Howard Barlow]

You lost me at 1+1.

[Coelus]

Ken that's a great link. Bookmarked on my phone now.

[kenw]

Originally Posted by Coelus

Ken that's a great link. Bookmarked on my phone now.

me too, and I downloaded the app as well. best $2 you'll spend.

[Coelus]

Originally Posted by kenw

me too, and I downloaded the app as well. best $2 you'll spend.

No iPhone Getting a myTouch on Saturday.