Originally Posted by Apodidae49
Tammy just arrived, amazed at the compact size! Just done a few photos for comparison alongside 70-300 and for the same focal length, 300mm, there’s no difference in size of an object in the frame. Thought the Nikon lens, being DX would be 450 or have I got it wrong and is it the camera sensor that gives the extra length effect?
The focal length of a lens is a physical property of the lens. It is a measure of the distance from the lens to the focal point (where the lines converge). A 300mm lens is always a 300mm lens (the actual distance can differ a little depending on the lens formula). The focal length does not vary based on DX or FX. The _field_of_view_ depends on the sensor, not the DX or FX size of the lens.
A full-frame lens needs to have an image circle that projects on the sensor that fits around the 36mm x 24mm rectangle sensor. So, they need to be a certain diameter to project that circle for a given minimum f-stop (largest opening).
A crop-sensor, like DX, is smaller (24mm x 16mm), so they need a smaller image circle (about 1/2.25 = 1/(1.5^2) the area). This means that for the same focal length and the same minimum f-stop, a DX lens can be lighter and smaller than an FX lens.
Because an FX lens has a larger image circle than a DX sensor, it can be used without vignetting on smaller sensors. A DX lens cannot cover a FX sensor, so a FX camera will crop down to DX when using a DX lens. But in both cases, 300mm is 300mm.
Using a DX sensor can be thought of like always cropping a photo. A Nikon d850 has more-or-less the same sensor density (pixels per mm) as a Nikon d500. If you shoot a d850 with a 300mm lens in FX mode you see the full field of view. If you crop it down to DX size (1/2.25 the area) on the same camera, you will get about the same image as if you shot that exact lens on a DX camera. You just see less of the field of view.
This is why DX camera and other smaller formats have greater depth of field (the amount that looks in focus forwards and backwards) for the same field of view. That is because a 200mm lens on a DX has the same field of view as a 300mm on FX and the 200mm will have greater field of view at comparable f-stop for the same amount of light on the subject.