Reddit reviews Canon EF 300mm f/4L IS USM Telephoto Fixed Lens for Canon SLR Cameras - 2530A004
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Focal Length & Maximum Aperture:300mm 1:4.0Super multi-layer lens coating reduces flare and ghostingCompact L-series telephoto lens with an Image Stabilizer which compensates for camera shake with the equivalent effect of a shutter speed two stops fasterTwo Image Stabilizer modes are provided: Stabilizer Mode 1 and the new Stabilizer Mode 2 which steadies the image during horizontal or vertical panningMode 2 detects the panning direction automatically. The closest focusing distance is 4.9 feet 1.5 m
Heck, even the EF 300 f/4L is $1350 from Amazon.
It isn't that a smaller sensor magnifies the image more. It's that the smaller sensor is capturing and resolving a smaller section of the entire image projected by the lens. This is a good illustration of it. Say the entire circle is the image projected by the lens. The black square would be the 5D's full frame sensor, while the red square is the 7D's crop sensor. It isn't magnifying the image, it's just capturing a smaller portion of the projected image as a whole. It creates the illusion of more magnification, when in reality, it's just resolving a smaller portion of the projected image as a whole.
The concept of a divergent lens between the lens and the camera's sensor is illustrated by teleconverters. I shoot Canon, so I'm going to stick with their equipment for my examples. Canon makes a 1.4x teleconverter, and a 2x teleconverter for their cameras. These apply a multiplicative factor to compatible lenses (usually Canon's L series lenses). So, with a 2x teleconverter, a 300mm lens on a full frame sensor would become a 600mm lens. Unfortunately, these teleconverters reduce a lenses' widest possible aperture. The 1.4x reduces the maximum aperture by 1.5 stops, and the 2x reduces maximum aperture by 2 stops. This becomes really limiting in anything but very bright light, because realistically, your minimum shutter speed should be the reciprocal of your focal length in order to eliminate camera shake. So, if you're using a 600mm lens, you should be shooting at at least 1/600. So, let's expand on this. This is the longest lens I own. It's a 300mm f/4 L, compatible with my 2x teleconverter. With the teleconverter, it becomes a 600mm f/8 lens, on a full frame camera. So, without taking image stabilization into consideration, your minimum settings will be 1/600 at f/8. ISO, or sensor sensitivity, can certainly make up for this. However, it's also important to understand that adding this teleconverter has added distortion, and is degrading overall image quality. Additionally, most modern cameras can only autofocus with a lens that has a widest aperture of f/5.6 or lower. Off the top of my head, the only exception to this that I know of is Canon's 1D-X, their flagship model. So, it isn't that it's impossible to generate fairly long focal lengths on a moderate budget, but image quality and overall capability are going to suffer from doing so. Canon's longest lens that is under $10,000 is the 400mm f/5.6 L. There are versions of the 300mm and 400mm lenses with wider maximum apertures, but those are also extremely expensive. So yes, overall, this is very possible, but to do it effectively, it would be extremely expensive.
As for the idea of adding a close up filter, those don't really magnify an image too much. They really just magnify it in such a way that a plane of critical focus can be achieved at less distance than you would usually be able to achieve it with a lens. Additionally, they add a considerable amount of distortion and degradation of image quality. This is something to consider any time you add an element between the photographic subject and the sensor itself. Every layer of glass will degrade image quality to a certain extent. Some lenses are made and treated in such a way that it reduces image quality significantly less than others. Again though, glass that nice is usually pretty expensive.