Zach Spencer said:
The answer above is a rather smart-a@@ evasion of a dead-simple question.
The question was: "
I was wondering if 1)an 18 mm dx lens on a dx camera would give
2) the same field of view as
3)an 18 mm fx lens on a fx camera"
The answer is a flat, "NO". On FX, an 18mm lens is a VERY wide-angle lens...for most of my lifetime, an 18mm lens on a 24mm x 36mm film camera would have been classed as an ULTRA wide-angle lens. An FX sensor measures almost identically to 24 x 36mm in both dimensions.
On the smaller DX or APS-C size image sensor used in most affordable d-slr cameras, an 18mm lens has a field of view that is approximately equal to a lens of 27mm to 28mm focal length when used on the traditional 24x36mm capture area of 35mm film in the long-used 135mm format, which in today's digital SLR era is called FX by Nikon, and FF or full-frame digital by users who shoot Canon or Sony digital cameras.
In terms of angle of view: data from http://www.tawbaware.com/maxlyons/calc.htm
18mm focal length lens on DX camera with 1.5x FOV factor FOV in degrees: horizontal 67.4 degrees, vertical 47.9 degrees, diagonal 77.4 degrees
18mm focal length on FX camera with 1.0 FOV factor (null) in degrees: horizontal 90.0 degrees, vertical 67.4 degrees, diagonal 110.5 degrees.
This is a HUGE difference, by every metric.
Zach Spencer said:
The problem with all this 'crop factor' and 'equivalent field of view' is that, while it made sense when digital first hit the market, it's obsolete now.
Back in 'the day' when 35mm film was the norm for most amateurs, everyone pretty much knew how focal length and FOV were related. A 28mm had the same FOV whether you shot Nikon, Canon, Pentax, Minolta, or whatever. Only those who dabbled in other formats needed to 'do the math' to make such conversions. This was the province of pros and semi-pros. The average shooter didn't need to understand how focal length and film format resulted in a given field of view.
Then along came digital with its smaller sensor, since marketing a 'full-frame' sensor would have been fiscal suicide. So the manufacturers came up with a simple solution.... 'Equivalent Field of View". Now, old-school 35mm film shooters could easily convert their existing focal lengths and have a reasonable idea what they would get with their fancy-dancy new digital camera. Your trusty 28mm now had the 'equivalent field of view' to a 40mm now.
But now the problem has been overcomplicated with the proliferation of 'full-frame' bodies. This is causing the 'crop-factor' idea to implode. A simple, elegant and easy-to-remember solution of converting an entire flock of shooters from film to crop sensors has now become mired in an endless morass of ill-fated explanations, half-truths and sheer misunderstandings by a new generation buying into the market. The result is a black hole in the ability of some to even make sense of it since the logic of the 'crop factor' has preceded them.
My thinking is to simply abandon the idea of 'equivalent FOV' altogether. If one buys a crop body and shoots a 28mm, they get used to a certain FOV. They should learn that upping to a full-frame camera will garner a wider FOV with the same 28mm lens (assuming it will cover the sensor). Conversely, one starting out with a full-frame will eventually figure out that a 28mm will give them a 'certain' FOV. But putting that same lens on a crop body will change the FOV. In essence, make the user figure it out.
Harsh, I know. But since the current state of the situation merely churns out new user after new user walking out of the camera stores with a deer-in-the-headlights looks, there has to be a better system than just tossing out 'equivelant FOV' in the specs.