Depth of field and f-stop of scopes (1 Viewer)

[Fedster]

Hello, if I look at a camera lens, it has a f-stop value (which corresponds to the largest aperture, since the lens has a diaphragm that can make the aperture smaller). A fast aperture lets a lot of light in but has a shallow depth of field (thing are out of focus if they are even slightly off the focusing plane), a slow aperture is darker but has a deep depth of field (objects that are 'approximately' at the focusing distance are all in focus). This raises the question of depth of field -- ideally (for me!) a scope would not have a razor thin focus plane, but it would have some depth of field to avoid having to play with the fine focus all of the time.

Additionally, the wider the angle the greater the depth of field: a tele lens has a much shallower depth of field than a wide-angle.

Am I correct thinking that scopes' aperture is just a function of the size of the pupil (i.e. objective lens divided by whatever magnification)? would it be possible to make a scope that has both a high aperture and a deep depth of field? how does/would magnification play into this?

 

[opisska]

Since a scope does not have an iris for variable aperture it has "just one f-stop" - equivalent to the largest f-stop of a given lens, it is simply the ratio of its focal length to the diameter of the objective. You (when manufacturing a telescope) can make this arbitrarily low by making the focal length larger, but that's not very popular for birding, because it makes the telescope long. In astronomy, telescopes at f/15 and more were commonly used for planetary observations for example, because there are less aberrations when the optics is "slower". But then some of these are many meters long!

Magnification is given by the focal length of the eyepiece - you choose your magnification by choosing the eyepiece as it is given by the ratio of focal lengths of the objective and the eyepiece. So if you change the focal length of your telescope, you just change the focal length of your eyepiece and have the same magnification.

So yes, you can get a "slower" telescope with a large depth of field and larger travel through focus easily - it's even easier to make! - but it will quickly get unwieldy long. The only reason spotting scopes are made "fast" is this.

 

[henry link]

Depth of field in telescopes causes a lot of confusion, especially when concepts from camera lenses are inappropriately applied - which happens often.

The first thing to understand about telescopes is that, unlike camera lenses, they are afocal devices. The light that emerges from the eyepiece and falls on the eye has no DOF. It's no more focused than the natural light that fell on the the telescope's objective lens or the natural light that falls on the naked eye when there is no telescope involved. The eye's optical system is what brings that afocal light to focus at the retina. In a perfect telescope only two factors effect our perception of DOF: the telescope's magnification and the effective entrance pupil diameter of the eye.

I'm going to suggest using the concept of the "circle of confusion" to make the interaction between the telescope and the eye clearer. The circle of confusion is most readily observed by using a star point, either real of artificial. At best focus the star appears to be a point, but as it's gradually defocused through the scope it forms a disc of increasing size made up of an increasing number of diffraction rings, the so called "circle of confusion". What we see as DOF is the part of defocus so close to perfect focus that the circle of confusion is too small to distinguish from the point of best focus. For example, the circle of confusion at 3 diffraction rings of defocus might not look any different at 20x than the focused star, but might show as a small defocused disc instead of a point if the defocus is increased to 9 rings. Now. what if we increase the magnification to 60x. That 3 ring circle of confusion becomes the same size as the 9 ring circle of confusion at 20x. What we see as DOF has narrowed even through there is no change in true defocus. This is the way that increasing magnification inevitably decreases out perception of DOF, and nothing can be done about it by changing the scope's design.

However, there is one way to change the true DOF of the eye and scope combination, and that is to reduce the effective aperture of the eye at a given magnification. Unlike the scope's optics the eye's optics are analogous to a camera lens with a focused image formed at the retina just like a focused image forms at the sensor (or film) plane of a camera. When observing through a birding scope in daylight at 60x the eye's pupil might be open to around 3mm while the scope's exit pupil at the same magnification might range from as large as 1.92mm (Swaro 115mm ATX) to as small as 0.83mm in a 50mm scope. If we assign the eye at 3mm dilation a focal ratio of f/6, then its focal ratio effectively becomes f/9.38 when the entrance pupil is reduced to 1.92mm by the exit pupil of the 115mm scope and f/21.7 through the 50mm scope. So, DOF is wider through the 50mm scope, but the image is dimmer. There is simply no way to take advantage of the larger aperture of the 115mm scope without also decreasing the DOF.

 

[Tringa45]

So yes, you can get a "slower" telescope with a large depth of field and larger travel through focus easily - it's even easier to make! - but it will quickly get unwieldy long. The only reason spotting scopes are made "fast" is this.

The "slow" telescope will have exactly the same depth of field as the "fast" telescope for the same magnification and exit pupil size.
However, its depth of focus will be larger, i.e. the in and out focus travel of the eyepiece between circles of confusion of the same diameter.
Incidentally, I think that for terrestrial use, the light loss associated with small exit pupils will often counteract the benefits of higher magnification.
Tolerances will vary with the individual but floaters also become an issue for us old dogs at small exit pupils .

John