Originally Posted by Adam
Rob is correct. In an optical system, DOF is DOF. An image on a sensor or film is either focused or not.
But while the human eye is viewing an image through an optical system, the eye forms part of the optical system. There is another lens (within the eye) and "sensor" (retina) in the mix.
The human eye tries to make a blurry image clear, a process called accommodation
. This ability declines markedly with age.
So the apparent
DoF of a scope is influenced by the individual using it.
True. What accommodation does is allow the eye to shift the focus but it's really limited, it's going to be a hard task to make mega DOF disappear
The image plane is focused to one point, and I'm not sure this is really about DOF, but about the reticule being lost in favour of the image.
If the eye is accommodating, ie shifting focus to the image, then it would suggest that perhaps the cross hairs are not set correctly for the eye. A little bit of fight between reticule and image can be experienced where the two are close in focus terms to each other ie say your shooting at 30yds and parallaxed to 25... however when the focus of the image is way off (ie close or far away) the eye should jump clearly to focus on the reticule, not the image... the ret shouldn't be lost.
The image you see in a scope works like this...
You have the image... what you're looking at... that is focused to a point inside the scope which if the parallax is removed is at the same place as the cross hairs inside the tube. If the object you are looking at is further back or forth than what the parallax is set to, the focus point of the image is behind or in front of the cross hairs, and that's why you get parallax error because the two are apart. Its a tiny amount in the scope (millimeters or less), but magnification magnifies the gap and effect.
The eyebell adjustment focuses your vision to the cross hair. It should be repeatably in focus. Test it on high mag against a blank background so the eye isn't trying to accommodate another image and throwing your eye's focus behind or in front of the cross hairs.
So in order to see the image correctly, you need to be able to see the cross hairs. Then you know your eye isn't doing any unnecessary work and you wont lose the ret, and the DOF is true or at least common to someone else to a correctly setup scope for them.
What the eye bell adjustment can't cope with is eyesight beyond it's range of adjustment, or astigmatisms which means the eye doesn't settle in focus properly as it fights to focus vertical and horizontally due to it's oval shape of focus.
You can test scopes side by side... stick a camera behind them...it will show the actual DOF of your scope, and you can compare with others if you like.