Re: Nikon Lens suggestions

Kevin, Ryan,

So we don't leave a bunch of folks scratching their heads, lets make sure we are using the correct terminology in this discussion.

First about lens focal length. When you see numbers published such as 50mm, 200mm, 70-300mm, etc, those are the lens focal lengths based on use with a 35mm film camera. The camera and lens makers continued to use that standard for digital cameras because it was already well known and a convenient way to compare one lens to another. The actual focal length of the lens (zoom lenses the zoom range) always stays the same no matter what camera it is mounted on, film or digital. If it's a 50mm lens, it will always have a focal length of 50mm. The crop factor comes in when the differing sizes of digital sensors is considered. I borrowed the illustration below to try and show how that really works. This discussion applies to all manufacturers cameras the same way, just with differing crop factors. Most Canons are 1.6 for instance.

In the illustration, the maximum image circle for this camera is shown. If it is an Nikon FX series camera (D3-4, D800,etc) it will have a sensor the size of the red frame. If it's a DX camera (d300, d7100, etc.) it will have a sensor the size of the green frame. It's pretty apparent that the green frame literally crops a section of the red frame. What you get with the DX camera is a narrower field of view with the smaller sensor. If you were to mount a 50mm lens on an FX body, since the digital sensor is about the same size as a frame of 35mm film, you will get a field of view just like you would in a 35mm film camera. If you mount the same 50mm lens on a DX body the smaller sensor effectively crops the image. In Nikon's case since all their crop sensor cameras have a crop factor of 1.5, you get a field of view of 50mm X1.5= 75mm. IOW an image taken with the DX body and the 50mm lens looks like it would if the lens had been a 75mm. This conversion works for all focal lengths, no matter the lens.

This can be a big advantage if you want longer telephoto lenses, not so much if you want wide angle lenses. In my case, I have a D3 (12mp FX camera). If I mount my 70-300mm on it, I get the 70-300mm field of view I also have a D-300s (12mp DX camera) if I mount the same 70-300mm lens, I get the field of view of a 105-450mm lens. (the lens is still a 70-300mm) The entire DX sensor is being used, just taking only the center part of the image projected by the lens. If I wanted the field of view of a 70-300mm lens with the DX camera I would need a lens with a focal length of 47mm-200mm.

What further confuses things is Nikon (and other manufacturers) make "DX" lenses. A DX lens such as the 18-300mm actually has a focal length of 18-300mm. What it doesn't have is a large enough image circle to fully cover the entire FX size sensor. The corners will vignette on a FX camera body. It will fully cover the smaller DX sensor. If you use the 18-300 on a DX camera, your field of view will be 27-450mm.

The megapixel density of the sensor doesn't change anything in this discussion, I only included it to show how it could be used to advantage by those looking for longer focal length lenses.

Clear as mud?





Edited 5 time(s). Last edit at 11/21/2015 01:21PM by jgunning.