Can anyone explain to me the relationship between these two numbers?  I understand what each does but the knowledge that eludes me is how to perform a rough calculation on what the fStop should be at any given focal_Distance.

To explain further, I can set up a scene with a focal_Distance of say 10 units, which may only require an fStop of about 4 to ensure that the entire subject is remains in focus.  However, I change the focal_Distance to something closer, say 3 units, and now the fStop has to be much higher, like 7 or 8, to accomplish the same task.

It's taking a lot of trial and error to find an fStop value that works each time I move the camera so if anyone can provide some insight into how to come up with a rough calculation to put me in the same ballpark at least, I would be most grateful.

To simplify much more than what is stated in the Poser manual, here it is.  The Focal Distance determines at what range from the lens you will have the sharpest focus.  For completely accurate calculations you would need to take into account the focal perspective of the camera, as well as it's maximum distance (yon).  I suggest not worrying too much about doing the heavy math.  The F-Stop, when altered with Focal Distance will increase or decrease the range of focal clarity (that is how much further away from the point of focus things will remain in focus).  Larger F-Stop numbers give a wider range of focus, while smaller ones give a narrow range.  Your focal field increases with F-Stop toward infinity the higher it goes in relation to the Focal Distance of the lens.  The further your Focal Distance and higher your F-Stop combined the greater your focal field will become.  The easiest way to get a feel for it is to run some render tests at different settings with simple objects spread throughout the scene.  Otherwise you will need to pick up a good photography reference book.

Both of these settings only have an effect if you have the render settings with depth of field enabled.  This is because it is a virtual camera, and without actually rendering depth of field, there is no focal plane, F-Stop becomes irrelevant, and there is no focal distance as everything is in focus.

Thanks for the breakdown Kage.  I feel that I'll start getting a better feel for fStop values that match the focal distance the more I play with it.  Generally, my subject is no more than 2 units in depth so it should be fairly easy to draft a quick and dirty table of focal distances and corresponding fStops.  I just kinda hoped someone had already done it.  :)

Thanks again!

I kinda wing it. I get a rough idea, and then do some test renders to fine tune it.  You could check some photography reference sources online, or search google for Docal Distance and F-Stop.  Also, I set my Poser to use Meters, that way I have a real world basis of measurment to go on. I find it easier (got used to this in Lightwave).

Try here, there's a chart, but it's in meters. http://www.conent.com/ConAdv/Encyclopaedia/Photography/CNQ_CAPhotography001.asp

Also, keep in mind that with real world cameras, the F-stops affect how much light reaches the camera: as the numbers increase the image gets darker unless you either use faster film, or a slower shutter.  This is something Poser does not really address.

Keep in mind, in pursuing DoF in Poser, you must meet specific criteria to achieve realisitc results.  You will need to target your subject precisely using the Poser builtin guidelines.  Dr. Geep's has an excellent tutorial how to perform this.  Pixel sampling in your render settings must be pushed to 10 or greater to maximize the effect.  However, this will slow your render time tremendously; be prepared to allow plenty of time based on your scenes requirements. 

I may be completely wrong, but just thinking about it seems to me that depth of filed would be equal to the distance to the focus point squared. So that if you moved the camera twice as far away, the depth of field around the focus point would be quadrupled. This is only a guess, I may be wrong.

I can imagine any pro photographers grinding their teeth at a lot of the above, because focal point / focus distance has nothing to do with the f-stop.

The focal point is where a desired point in the subject is in sharpest focus at the object, the subject being the scene, the object being the image plane (e.g. the camera film or sensor).

It's what you focused on.

Strictly speaking, it doesn't produce a point at the object but a circle, which is referred to as the 'blur circle' or 'circle of confusion'.

The size of this blur circle is determined by the aperture (f-stop) through which light passes; as the aperture becomes smaller, so does the blur circle. This means that reducing the size of the aperture increases the range of the subject's depth (Z-plane in 3D-speak) that appears to be in sharp focus. It increases the depth of field.

I have to stress this: what you focus on and the size of the hole in your lens are not related. 

Poser - and other 3D apps - attempt to simulate the depth-of-field effect but I've yet to find one that does it really well, and in Poser you'll need to (a) crank up your render settings and (b) endure very long render times to get anything acceptable.

A far quicker, more controllable method is to render out a depth-map (Z-plane map), and apply depth-of-field effects in post processing.

One of my favorite Renderosity threads:

http://market.renderosity.com/mod/forumpro/showthread.php?thread_id=2014883

Bushi did some cool work here and provides a nice graphic to use on the Poser floor plain.

Best Wishes,
Rªnce

Quote - I can imagine any pro photographers grinding their teeth at a lot of the above, because focal point / focus distance has nothing to do with the f-stop.

I never said they did, hawever it has everything to do with your focal field and depth in relation to each other.  You use your focal point in combination with F-stop to determine your depth of field. F-stop affects the depth of field of your camera directly. When trying to achieve a wider range of Depth of Field in photography, you want to use a greater F-stop at your hyperfocal distance.  This is commonly misunderstood by amature photographers and hobbyists. 

Quote - The size of this blur circle is determined by the aperture (f-stop) through which light passes; as the aperture becomes smaller, so does the blur circle. This means that reducing the size of the aperture increases the range of the subject's depth (Z-plane in 3D-speak) that appears to be in sharp focus. It increases the depth of field.

See you basicly just argued against yourself here.  Unless you are just cut-and-pasting from a photography website, I don't know why you would say they are unrelated, as from this statement it seems you have an understanding of the relationship.

Quote - I can imagine any pro photographers grinding their teeth at a lot of the above, because focal point / focus distance has nothing to do with the f-stop.

Quite correct, but as far as I can see no one said that it did. What has been said above is that the depth of field is dependant on the apature, and the focus point (camera-to-subject distance). Which it is.

Quote - I may be completely wrong, but just thinking about it seems to me that depth of filed would be equal to the distance to the focus point squared. So that if you moved the camera twice as far away, the depth of field around the focus point would be quadrupled. This is only a guess, I may be wrong.

This is actually not a bad stab, it is close. If you move your focal distance twice as far away, you need to increase your focal plane by a factor of four to keep everything in focus (though greatly effected by both the closest object and the farthest - F-stops provide a focal range from X to Y at given distances).  F-stops are exponentially incremental, each one doubling or halving the aperture size.  So in this example you would need to raise the value by 2 stops (thus decreasing aperature by a factor of four and increasing focal depth likewise). So if you were using an F-Stop 2.8 and moved the camera twice as far away you would go to F-stop 5.6 (remember F-stop 4 falls in between on a camera, it is not as simple as doubling the F-stop value).  The real trick is, different distances will use a different starting F-stop based on your apparent lens size.  The chart page I linked to is really useful for seeing the ranges, in meters, different F-stops give you on different lens for focal field.

That may not have come out right - trying to oversimplify I think.