Originally posted by fplowman
Im stupid!
Can I quote you the next time we wallow into a political flame fest?
lol Can you tell me what all them numbers was?? Seriously though are you saying that this 1.6x multiplier somehow digitally converts this EF-S lense into a wide and telephoto?? Dual purpose? Am I correct here?? Also I assume 17 - 40 lense is for wide angle use?? Right?
Im a layman.. Without irritating anyone can someone give me a short course here??
Fred
OK. It's late, but I'll give it a go.... :cyclopsani:
Ignore the sensor and just imagine a lens focusing an image onto a flat surface. Depending on the diameter and design of the lens, the image will be fully illuminated and reasonably sharp over a limited area. At the edges, light loss will occur because some of the light entering the lens will be unable to make it to the focal plane. Also, the edges will get softer as various abberations become more apparent as you move away from the central axis of the lens.
So, this leaves you with a 'sweet spot' in which the formed image will be bright and sharp. With lenses designed for 'old' 35mm film, this 'circle of goodness' (TM by Greg) comfortably covered a 35x24mm piece of film. If you placed a somethat larger format film at the focal plane, you'd get a soda-straw effect, and the outside coners would be nearly dark, and very blurry.
With a smaller sensor (one of them digital thangies), the lens' image is identical, but the sensor, being smaller than 35x24mm, intercepts a smaller section of it. That section of the image that does fall on the sensor is all the camera can see. Since the section is smaller, it represents a 'cropped' version of the image 35mm film would see if used with the same lens. You can see that this 'cropped' image is a 'zoom' of the film image and represents a greater magnification of the oriinal scene. Thus, a sensor that is 1.6 times smaller than a 35x24mm chunk of film will produce 1.6 times the magnification when used with the same lens.
The (Canon) EF-S lenses are designed to be smaller (and, much more to the point, cheaper) than the 'standard' EF lenses that film based cameras established. These smaller lenses are disigned to produce a smaller 'circle of goodness' and cannot fully illuminate a 35x24 format sensor, be it film or silicon based. This may or may not be a problem in the future. Sensor sizes are creeping up for a number of technical reasons and many people think that they will wind up at 35x24, or maybe 32x24 - very close to the 35mm film format. Your $500 EF-S lens just won't work with that 2006 Canon model that features a full-frame sensor. (Even today, Canon, Nikon, Kodak, and others(?) produce pro-level DSLR cameras with 35x24mm sensors. 'Fortunately' you don't need to wory about them unless you want to take out a second mortgage on your house.)
To counteract the 1.6 magnification factor, and give digicam shooter a 'proper' wide angle option, camera manufacurers are producing EF-S (Nikon has a similar line, AFAIK) like lenses with very short focal lengths, like 10~40 zooms. These when used with a 1.6 scale digital sensor, this would provide the same image FOV as a 16~64mm lens on a film camera. Building such a lens (10~40mm) to cover the film format would require a large, very spendy, design. However, with the reduced COG requirement, an EF-S lens system can be build much smaller and at a semi-affordable price.
-Greg (Reveling in the price CRASH of manual focus Canon gear as the thundering herds 'go digital' and sell solidly built (ohhh, metal!), perfectly functional equipment for next to nothing!
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