Telling it simply - just taking photos with a digital camera using an additional lens in front of the camera lens like any telextender - kinda. Just multiply the focal length of the lens and the add-on to get the new focal length when combined. Simple. Riiight.
Oh, there is so much more. What cameras, with what camera lenses and capabilities, what additional lenses to use, how to mount them and how to make them work together.
Cameras - those with small diameter front elements (lens opening) are best as they let you get closest to the lens you are putting in front of the camera lens. These include the Nikon 900, 950, 990, 995, & 4500 and some in the 800 series too (maybe others). These have a 28mm filter size. Some Sonys, like the Mavicas have a 37mm filter size and for my experience that is about the limits for this to work well. There may be others but these the ones I am familiar with.
Now that I've become a bit more familiar with the Minolta Dimage X-series, I believe this camera to be near ideal for this kind of photography. Very small lens openings, and the lens does not protrude from the camera body, make it easier to adapt to your choice of optics, but they have no threads on the camera or lens so a mount must be fabricated to hold the camera to the eyepiece.
You do not need high megapixel resolution cameras for this to work - they are a bonus - but the physical size of the lens, its' quality, the zoom ratio, are all of more consideration here.
Click here for a graphic showing you the size image your monitor will fully display.
(NOTE: make sure this graphic is displaying at full size - some Windows settings will shrink it to fit on screen and you must choose it to display full size. Check your settings). This graphic is much, much larger than any screen setting for any monitor.
Cameras with larger fronts - Sony 828s and similar for instance - must be moved so far from the eyepiece that the light falloff is too severe to get good photos. It can be done, but those cameras built in this fashion are just a pain to work with.
There are several cameras that have small lens openings, but have no filter threads at all. These can be used but are more difficult to use as there is no built-in way to connect the camera to the lens you are using. These must simply be held in place by hand, or with some sort of 'slip' mount - a temporary mount.
Next would be a zoom function that does not change the physical length of the lens. If not that then a mount must be used that is fitted to the length of the camera lens at full zoom.
The amount of zoom is critical. Most of these techniques will not work with short zooms or at wide angle settings. Zoom capability of 3x seems to be about the 'shortest' zoom setting that will allow you to take advantage of these techniques. Generally you will need to use the zoom at its' longest setting to avoid 'vignetting' (darkening of the corners of the shot). You could use the center portion - but the entire purpose of this is to get the maximum 'reach' of the combinations of camera and add-on lens.
Lenses - You can use nearly anything you'd like - as long as the eyepiece opening will 'project' an image circle that covers the camera lens opening.
Monoculars, binoculars, telescopes, spotting scopes - even other camera lenses. Each choice will require a different mount and support combination - and these are pretty few and far between. This is even more true if the camera you choose has no threads.
The Nikon 9xx series / 4500 are again a winner here - 28mm thread and that articulated body for ease of viewing. However, don't let that keep you from trying things. Most combinations can be ignored with just a little thought. Think of that large front element on some Canons and Sonys - well you know these are far larger that the eye opening on a set of binoculars (generally no more than about 6 mm or so). S m a l l is the key.
This can work with video cameras also - but ...
The biggest problem - after getting a suitable camera lens combo - is focusing. You must focus the 'extra' lens as if you were looking through it (so the image coming out the eyepiece is in focus for the camera). Then the camera will need to focus separately. If you are using an add-on that has some ring focus versus a separate focus knob, you will have to move the camera for each shot (not good for moving subjects).
Supports - These are a critical part of this. You must take steps to minimize movement and vibration. The angle-of-view for just about all of these long distance photos are way under one (1) degree! Any movement will reveal itself as blur - or even move the subject completely out of the photo. As the focal length increases this goes from just critical to obsessive - no point in doing this if you ain't gonna make it still.
Tripods need to be substantial (heavy or weighted). You can use a video head, but I prefer a geared head that turns a tiny amount instead of depending on my moving the arm just so. I find ball heads to be a real pain-in-the-butt - and joysticks are even more trouble.
Use a cable or electronic shutter release if you have the choice. The pressure from you pushing on the camera will move it slightly 99% of the time. Wind can be a problem too.
But the photos can be pretty amazing.
Zoom and images - These are a critical part of this. You must take steps to minimize movement and vibration. The angle-of-view for just about all of these long distance photos are way under one (1) degree! Any movement will reveal itself as blur - or even move the subject completely out of the photo. As the focal length increases this goes from just critical to obsessive - no point in doing this if you ain't gonna make it still.
You do not need a really big zoom for this to work for you - or a high resolution camera either. I will trade resolution for zoom though - any day.
This technique is not dependent on the size image you get - but the size of what you are shooting within the image. Often a 640 x 480 image is sufficient, especially if you are going to show it on screen. Remember - DVDs are only 640/720 x 480 - and on a 6 or 10 foot screen - impressive.
As far as using the zoom on your lens - most monoscopes and binoculars will be a fixed power such as 6x, 8x, 10x. Using a decent spotting scope seems to be the best choice for now - a 60mm diameter, 15x to 45x zoom, is a bit less expensive than the 77mm to 90mm and cover the most usable range of zoom. Next are the catadioptic scopes (mine begins at 50x, and can be pushed to 450x for viewing). These are lightweight and not at all like the long telescopes that come to mind. I'll see if it works out ok. Stay tuned.
It seems as though 20x to 30x on the spotting scope is the best all around for relative ease of focusing, stability, and maintains the light well. I want to see if we can push this out to the limits of a 45x scope with improved results.
I will also do a bit more with the Sony Mavica FD-92, 1.3mp & 8x zoom, camera for a range from 120x of up to 360x on that camera. (That is from 4,920mm to over 118,000mm equiv !!!) And not using any digital zoom.