DOF and f-number

[Infrared digital]

Just wondering if someone can help me understand this.

When using the VRay physical camera - I want to control the intensity of the Dof however, the only parameter that seems to control its intensity is the f-number. The problem here is that this plays havoc with the exposure of the scene.

What I don't get is that when using our dSLR - its the position of the camera to the subject and the zoom which control the intensity of the Dof and nothing to do with the f-stops.

I am assuming that I am exposing my lack of knowledge here but, if someone could put me straight on this I would appreciate it.

Thanks

N

[Neilg]

I'm pretty sure DOF does go off the target distance too.

You can work out how much you need to change your shutter speed for however much you've changed the f-stop easily enough, but i cant remember how they relate to each other atm. I used to do it all the time.
Now I use standard cams if dof/MB is needed...

[tricky]

DOF is absolutely connected with aperture (f-number) and subject magnification. Wide apertures (small f-numbers) will shrink the DOF. Larger subject magnification (using a more telephoto lens or moving closer to the subject) will also yield shallower DOF. The combination of the two is therefore critical.

I used to have a Canon slr. It had a clever feature where you would target the furthest point you need to be in focus, then the nearest point, and it will calculate an aperture/speed setting to best achieve this.

[^Lele^]

I use my own scripts to adjust focus and exposure: first i autofocus the camera, then i change the in-focus depth to suit my needs playing with the F number, and lastly i expose with ISOs through the exposimeter. Easy as one, two three

[Infrared digital]

Lele, I have used some of your other scripts and they are excellent so, will try this one soon.

Thanks

N