I have an important shoot coming up and I really need your help. Can you guys tell me the formula/method (in english) to calculate the distance of the subject that you are photographing and the camera, by using the focal length to do so. By the way am shooting with the 100-400 and the 75-300 from canon on a 40D Canon so that,s the x1.6 factor.

do any one is this forum know how to do this?

I guess I don't quite understand what you're asking?

I think maybe you need to ask this question in a different way. It does not make sense as worded.

I am not sure if there is a formula to do this. Old lenses had a chart on the focusing ring, so depeding on where you had your focus set for the focal distance, it would tell you how far the distance was to the focus point. To do this you would look down at the focus ring when it was set. This was how people would "Zone focus". They would manually set the focus ring to say 10 feet, and they would know that as long as they were 10 feet fromt he subject, they would be in focus. This method was used on hard to focus things. Maybe there is a mathmatical calculation for it, but I can't image there would be, since there is not that third element in the equasion... the focus. You don't have any number or any idea on the focus on the newer lenses. (Does this make sense to anyone? I feel like I am talking in jibberish... I know what I am trying to say, but don't really know how to say it!)

What I think you're trying to ask is "what focal length should I use for a pleasing perspective on a group of 'x' number of people."

Well, it depends. I think a pleasing perspective is from about 8-15 feet away for most portraits. Then you'll need to use the appropriate focal length to frame it the way you want it. If you want more compression, use a long lens from 20 feet away. If you want a more intimate portrait, use a shorter lens from 5 feet away.

This is why zoom lenses are good for portraits. You can maintain the same perspective for an entire session no matter the size of the group while still framing the image in different ways. Think "formals at a wedding" where your camera is on a tripod and doesn't move, but the size of the groups could be anything from one or two to 30 or more.

But really, most folks don't care that much about maintaining the exact perspective. I've done sessions with one prime lens with lots of different sizes of groups. No one has ever complained about the different perspectives between individual or group portraits. I doubt anyone would notice without looking for it, unless it was really obvious. As long as you use a long-ish lens (like an 85mm) so you're never too close, the change in perspective will be minimal and hard to detect -- even for someone looking for it.

There really isn't much anyone can tell you with how you've worded your question.

Let me break it down. Lets say am using a 400mm zoom lens on a full frame DSLR ,right. I am photographing a man 6 feet tall, and from where I stand to photograph him, when I take picture of him I get his face alone, ok.
Can you guys tell me in feet or meters, the DISTANCE the man is away from me, or please give it to me in an equation....................

You see I want to get into some nature photography and I want to be able to calculate the distance from the subject to me photographically speaking without having to miscalculate what lens or tele-converters to use? Because i may end up missing the shot.
And also I want to be able to calculate the DISTANCE (in feet or meters) when I take the shot from the subject to me for informational reasons.

Now there's is a National Geographic calculation for you guys................................

I'm sure it could be figured out mathematically, but I doubt anyone would. It would take too long. And your example with a zoom lens -- I'm going to assume it would be set at 400mm. The height of the man wouldn't be part of the equation unless his entire body were in the frame, and not just his face.

What you're asking for really comes with practice. Do it enough and you know what you're going to get before you set up the camera. Most nature photographers (landscape, at least) work slowly to make sure they have exactly what they want. If you're working quickly, a zoom lens is your friend -- it would be easier to just zoom to what you want.

http://www.dofmaster.com/charts.html

Any help?

Now thats what am talking about.............................................
thanks guys for all the help I owe you guys an icy cold coconut from Trinidad and maybe a steel pan to go with it