I’m confused. Last summer after much research and on recommendation on how good it was I purchased the Nikon 70-300mm f4.5-5.6 G AF-S VR IF-ED Lens to use on my Nikon D5200 camera.
Now the camera is DX format (eg the smaller of the 2 sensors Nikon put in their cameras) and the lens is an FX format that is designed to work on FX cameras but that also works on DX cameras albeit with the image cropped. Everything I read says that the lens is the equivalent of using a 105-400mm lens on a DX camera. Sounds good and what I was keen on as I already had the 55-200mm covered. See quote below from WEX Photographic webpage:
The AF-S VR 70-300 f/4.5-5.6G IF-ED is a high-performance super telephoto zoom lens that has been designed to meet the demands of both digital and 35mm format SLR users. It offers a 4.3x zoom with a range of 70-300mm (105-450mm on a DX Nikon D-SLR) and features ED (Extra-low Dispersion) glass elements that ensure superior optical performance.
However. I don’t actually see it working as a 105-400mm lens and without a purely DX equivalent 70-300mm I cannot compare. I do however have a DX only 18-105mm and a DX only 55-200mm so I have taken some comparison shots as included below.
70mm zoom on barrel
I would have expected that for the 70-300mm lens, 70mm on the barrel should equate to 105mm equivalent and hence be the same coverage as the the 105mm zooms of the other two lenses directly below. But its not. It is identical. What gives?
105mm zoom on barrel
Again the frame coverage is identical. So. What. Gives? Am I using a wrong setting on my D5200 or what.
Hopefully I’m going to go FX format camera later this year (D750 hopefully) so it will be a bit beside the point however I’m still keen on knowing why things are not as I am expecting.
Below I have included photos through the whole zoom range from the various lenses mainly for my own purposes 🙂
andrew kenyon photography 
There us no magnification only a change in angle of view. Think of a thinner straw. At 70 its the angle of view og a 105. Dx gives slighly greater depth of field than fx does. Fx gives you a brighter view finder.
I get that but why then do people describe it as a 105-400 effective zoom on a DX camera. My understanding is that it would be – for example – a 70mm on a full sized FX sensor but as the DX sensor is smaller you only see part of the picture which would be equivalent of what you would seek with a 105mm lens. I think a diagram is needed!
It’s a marketing ploy to some degree. If you roll a piece of paper and down the tube, then role it tighter, that tells you what you need to know. The sensor does not magnify. However if you have a 14 megapixel camera and a 24 mpg camera think about the cropping possibilities. With full frame lenses you are shooting with best part of the glass no vignetting. Hope this helps.