What is Infinity -- Photographically, not Philosophically?

Experimenting with manual focus on G6. In some situations, it is difficult to see a focus change between a person and his background (as on a stage). The focus "guide" on my LCD reaches infinity fairly quickly. The manual says this scale is only a guide, but it raises questions about exactly what means of focusing will be more reliable. How far away is infinity?
Some options are "landscape mode", "manual trying to see the focus change". "manual using infinity as guide", or if it is clear of foreground material, "auto." Or none of the above.
Any tips on the most reliable method. Thanks.

Infinity is the point where the rays of light coming from your subject are practically parallel through your lens. Infinity is the distance where you lose the parallax movement that you get when you move your camera. Basically if you see a mountain or a house in the far off distance, then you moving your camera a foot to the right or a foot to the left will not change the look (focus sharpness, relative placement of object in viewfinder) you get through your camera (assuming your at wide angle).

With a digicam which has such great depth of field due to its small sensor size, you hit the "infinity" point pretty quickly. On a camera like a G3, infinity can be used for objects maybe 10 meters and beyond in the wide angle mode. The human eye has an optical infinity of about 6 meters.

I am an engineer and should be able to give you a better definition using optical terms, but this is how I understand it, and I've forgotten the optics I studied 12 years ago.

Hope this helps. A couple of the other regulars will chime in with better answers I am sure...

Thanks x3 !

Sorry about that. I don't know what happened?

pradeep1 wrote:
With a digicam which has such great depth of field due to its small sensor size, you hit the "infinity" point pretty quickly. On a camera like a G3, infinity can be used for objects maybe 10 meters and beyond in the wide angle mode. The human eye has an optical infinity of about 6 meters.

I am an engineer and should be able to give you a better definition using optical terms, but this is how I understand it, and I've forgotten the optics I studied 12 years ago.

Let's deal with the part I "understand." I'll just accept the small sensor size because you sound like you're sure of it. And I'll go along with the wide angle mode, but can you explain or tell me more about this? I'm dealing with the exact opposite situation. I'm using a teleconverter fully zoomed in (optically). At what distance approximately would I hit infinity in this configuration?

Thanks for your help.

Don't know. Run a test. Put the camera on a tripod. Put camera in manual focus. Aim at objects that are approximately known distances, manually focus until they are in focus (on the LCD at least). Snap the picture. Go from objects ranging from 10 ft. away to 30 ft. away to 100 ft. away with your lens fully zoomed in. Once you've taken the pictures, come back to your computer look at them. You should be able to make out the point where "infinity" starts.

Of course, this will also be effected by what aperture size you use.

Just ran a quick experiment. I seem to hit infinity at about 20 ft. at f/4 when fully zoomed in.

Do you know what the sensor size is on the G3?
Actually, not the actual sensor but the magnification factor to make it
equivalent to 35mm.

The focal length of the lens is 7.2 mm to 28.8 mm. It has an equivalent of 35-140mm when converted over to 35mm standard frame.

So the magnifaction factor is 4.86 (7.2 X 4.8 = 35, 28.8 X 4.8 = 140).

Are you still using the G3? I have not spent much time in this area lately, been
mostly in the SHARE PHOTOS section.

Yes, still using it. Too poor to afford a 20D at this time.

pradeep1 wrote:
Yes, still using it. Too poor to afford a 20D at this time.

OK, I know how that feels.

4walls...do you have an answer to the original question?

Optically, infinity is any distance where the focusing error (when focused at inifinity) is small enough to not be noticable. Not noticable could be due to: your vision, lens resolution limit, CCD resolution limit, diffraction limit (an unavoidable fact of physics, which is dependant on imaging device size and the wavelength you are capturing.).

Regardless of the quality of the optical system and the viewer's fussiness, the diffraction limit is always present. So perfect focus is always achievable because diffraction will overtake the focusing error at some point.

Edited: Typo and:
I disagree about the parallax error disappearing because the explanation is not general enough. For example, a small aperture (same size sensor) can give you sharp focus mere meters away when focused at infinity. However, moving the camera from side to side will most deffinitely change the picture at such a small distance.

There is another point to consider. Some lenses are manufactured to be able to focus past infinity. While there is theoretically nothing past infinity, such lenses used to be made to accomodate expansion and contraction of the lens materials due to extreme cold and heat, which cause a shift in the infinity focusing point.

But I don't think the G6 is advanced enough to do that.