I was talking with someone today, and they mentioned that some digital SLR's have some sort of teleconverter built in. They mnetined that for example an 18-70MM lens on a DSLR may actually equal to a 36-140MM lens.
My first question is why most DLSR's have a teleconverter built in (if this is the case)? If this isn't the case what was this person talking about? Secondly does the D70 have this issue? If so what is it's conversion factor?
well no, there is no teleconverter. but there is a 'crop factor'. the digital image sensor in most digital slr's is smaller than 35mm film. when adapting digital technology to existing 35mm lens technology, they just put the digital sensor where the film would be, basically. the fact that the digital sensor is smaller than 35mm film just means that it will capture the center of what a 35mm frame would normally catch. the result is an image that seems 'zoomed'. some people describe it as increasing the focal length of your lens, when actually, it's just a crop. in the case of the D70, the crop factor is 1.5x. eg. a 18-70mm lens will have a field of view equivalent to a 27-105mm lens. there are some other subtle differences in depth of field, but they can be a bit confusing. do a search here on nikonians and i'm sure you can find controversy.
Although, there is a 1.5x crop factor going on with D70, the camera will capture exactly what you see through the viewfinder of the camera right? In another word, the ViewFinder is also cropped at 1.5x factor it is WYSIWYG, not what you get is just a factor of what you see, is that right?
I just hope that it is not like you see a full 35mm during composition but then you will only get 1.5x cropped of the frame when you pull the image out on your computer screen.
Before you get mired in the details, just keep in mind that the smaller sensor crop will make your lenses behave as if you had a 1.5x converter mounted with your lens on a film body. The angle of view is the same in either case.
The 18-70 will deliver the same angle of view as a 27-105 on a film body (or on a full frame digital body with a sensor the same size as a 35mm film frame).
The focal length does not change, the 1.5x factor just lets you compare the angle of view with lenses on 35mm that you're familiar with.