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artmustel Sep 10, 2008 3:30 PM

Hello. I am a bit confused about focal lenghts in DX format...

By instance, a 18-55mm kit DX lens for the D40 is equal to those focal lenghts x 1.5 on a FX camera? If so, let's say ifI use a 18-55mm lens designed for a FX camera...will that factor be the same (1.5) ? :?


goomer Sep 10, 2008 4:12 PM

sorry, I totally misunderstood your question...yes, if you put a full frame lens (FX or other full frame lens) on a Dx camera you get the 1.5 crop factor which gives you the 35 mm equivalent focal length.

artmustel Sep 10, 2008 5:35 PM

Goomer, I still have doubts about all this.

If a18-55mm DX lens on the D40 gives the equivalent of 27-82.5mm in a 35mm camera, then a 18-55mm lens designed for a 35mm camera if installed on the D40 will give the same factor of 1.5x, 27-82mm?


ReneB3 Sep 10, 2008 6:09 PM

Yes, it is the same. They don't use different focal lengths for the FX or DX lenses. All are designated for the FX or 35mm based focal lengths and we have to use the 1.5 conversion factor for our DX cameras.

tjsnaps Sep 10, 2008 8:00 PM

Maybe this will help

The mm designation on a lens in simply a measurement of the distance between the main element of the lens and the film or in the case of digital, the censor. It is the size of the film or censor that determines the angle of view the lens will produce. Therefore a lens of any given focal length , say 80mm , will give the same angle of view on a DX camera no mater what type of camera the lens was designed for.

The 1.5 crop factor is designed to make it easier for people who are used to shooting in 35mm to judge the effect of their lenses when used on a DX camera. But if you never used 35mm cameras it might only serve to confuse you.

This is nothing new to photography people who switched between 35mm and 220 film went through the same process.

artmustel Sep 10, 2008 8:45 PM

Thanks to you all!


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