I have a question regarding DX sensor and lens crop factor to calculate the angular and dimensional field of view (for horizontal and vertical only) of photos taken at specific focal lengths and distances. There seems to be a number of conversation on Nikonians and the web about the crop factor, in particular, and there are several good online calculators.
The problem is that I need to be able to calculate the FOV data offline for my research, which is often in remote locations. I have found the mathematical formulas, but none account for crop factor and for the life of me I cannot get my results to match up with any of the online calculators and it is driving me nuts. I hope one of ya'll will be able to help me out.
For reference, I am using my Nikon D300s and AF-S DX NIKKOR 18-200mm f/3.5-5.6G ED VR II. The sensor size of the D300s is 23.6 mm x 15.8 mm.
The formula to calculate angular FOV that I have is:
angle = 2arctan(image size / (2 x focal length))
At 18 mm focal length: the horiz FOV = 2arctan(23.6 mm / (2 * 18)) = 66.5 decimal degrees
the vertical FOV = 2arctan(15.8 mm / (2 * 18)) = 47.4 degrees
But, these values do not match up with the online calculators like tawbaware.com. As best as I can tell I am not accounting for the sensor crop factor, though I thought it was not needed if I was using a DX sensor and DX lens?
Can someone please tell me if my math is correct (maybe the online calculators are wrong) or tell me where to account for the crop factor in the formulas so that I get accurate results.