How Far Is 50mm, 200mm, 400mm, Etc??

[SixTwoImages]

If I know how far away I will be from a given subject and know what framing I am looking for... How do I convert that to lens length?

I usually shoot with a Nikon 18-200 which has great range for most of my shots, but recently I was at the local aquarium and was on the opposite side of the sea ray display from my wife and baby... This was maybe 60 feet... and the 200mm was just not enough for a tight shot of my wife and baby's reactions

I would like to plan ahead and bring the correct lens for what I will be shooting.


This would also be helpful when I know how far away I will be from the subject at concerts, sporting events, and the like.

Any help on this would be very appreciated.

Thanks

SixTwoImages
Atlanta GA

EDIT

I'm thinking I asked the question incorrectly... or maybe I made it to complicated.

IF I have a subject framed with a 50mm lens 10' from the camera, and I was to move to 100' away from the subject... What lens focal length would I need to accomplish the same framing?

[firebox40dash5]

The only easy way that comes to my mind would be to do the math with your distance from a subject (the radius of a circle with the camera as the center and the subject as the perimeter) and the angular field of view. For example, 60' from your subject, a 360 degree FOV would have a circumference of ~377'. Nikon states the FOV of your lens @ 200mm as 8 degrees. So your FOV would be 377*(8/360), or roughly 8.5' You'd have to make sure to use the DX field of view, as FX/film is wider- the 200mm f/2 tele has a stated FOV of 12 degrees, 20 minutes.

Edit: Now I'm doubting myself whether it's the arc of that circle, or a flat plane. If it's a flat plane, you can either calculate the chord of that arc, or take half the angle and use tan(angle of view)=((1/2 the width)/(distance)). That works out to about 8.4'. It won't be precise (due to your subject being calculated on the perimeter, not the chord), but gives you a rough idea.

[RichardTaylor]

This will help. Just scroll down to the Dimensional Field of View Calculator

http://www.tawbaware.com/maxlyons/calc.htm

[SixTwoImages]

I just blew a blood vessel on that one...

[RichardTaylor]

Glad to help out!

[OsmosisStudios]

Originally Posted by SixTwoImages

I just blew a blood vessel on that one...

As long as it wasn't anything more... personal.

[sk66]

This is why they make zoom lenses!

[firebox40dash5]

I like Richard's way a lot better than mine! A quick google of "focal length conversion" didn't turn up anything useful for me, though.

Originally Posted by sk66

This is why they make zoom lenses!

My first thought was "Why do you need to calculate field of view when you're using the lens with the 3rd widest zoom range available?"

[SixTwoImages]

I'm not really interested in calculating field of view, or maybe I am and I don't know it...

I am simply looking for a rule of thumb to decide lens choice when I know distance to subject.

Thanks everyone..

RJ