With the same FL and f-number, a DX lens has more light covering the APS-C area?

Started 2 months ago | Discussions thread
MartyMarathon New Member • Posts: 22
Re: With the same FL and f-number, a DX lens has more light covering the APS-C area?

This is the way I understand it, maybe really simplified.

The lens creates a circular focused image inside the camera. The difference between an FX and DX sensor is how much of that image is captured. The FX lenses create a large enough circle to effectively cover an FX sensor, DX lenses will only create an image large enough to cover a DX sensor. The image circle created by the lens inside the camera does not change in a DX or FX camera.

If I use an FX lens on a DX camera some of the image will fall outside the sensor area. If I use a DX lens on an FX camera the image will not be large enough to cover the entire sensor without some vignetting. The reason that a DX lens is smaller and less costly is that it does not need to create as large of an image circle.

The amount of light reaching the sensor does not change, a DX lens just captures less of the image than an FX lens does.

When using an FX camera in DX mode the image is only being recorded from part of the sensor. On my Z6 the DX images is about 12 megapixels, I believe on the Z7 the DX image is about 20 megapixels.

I don't believe that the angle of view of a lens changes on an FX of DX camera but capturing less of the image creates an image using less of the angle of view.

Using my 70-200 f2.8 zoom on my Z6 provides a maximum aperture of f2.8 and a 70-200 view. If I put the zoom on my D500 it sill provides and aperture of f2.8 but since the DX sensor does not capture the entire image I get the equivalent of a 105-300 zoom.

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