Getting to Know Your Camera 2: Sensor, lens, and focal length

Before getting to the exercises in the coursebook, there is a section called Getting to Know Your Camera, in which course author Michael Freeman suggests sitting down with camera and manual and reviewing all your features and functions.  This seems like a reasonable idea, except that many of the features and functions of the camera can't be explored until you get out and use them.  Having a look through the manual, though, is useful insofar as you then know where to look when you encounter a problem. (Speaking of manuals, I've got three:  Nikon's, Thom Hogan's, and the Blue Crane reference card.)

The exercise I included in my previous post seems like a more reasonable approach to familiarizing yourself with your tech.  I want to extend that here to include the issue of focal length, which Freeman addresses separately.

Full Frame 35mm, and Nikon D5100 sensor size in red.  
Photo:  KenRockwell.com

The Nikon D5100 includes:

     • 23.6 x 15.6 mm CMOS sensor

     • Nikon DX format (1.5x FOV crop)

Because the sensor is smaller than full-frame 35mm, the lens’ stated focal length must be multiplied by 1.5 to find accurate focal length.  This is sometimes known as the Crop Factor.  It’s also known as the Multiplication Factor, as this is the function required to create an equivalent full-frame image from a photo made with such a sensor. 

So my lenses have true local lengths of:

     1.  AF-S DXNIKKOR 18-55mm f/3.5-5.6G VR          18-55mm             27-82mm

     2.  AF-S VRZoom-Nikkor 70-300mm f/4.5-5.6G        70-300mm           105-450mm

A helpful discussion on accurate focal length and crop factor can be found at Ken Rockwell's site, as well as this informative slide show and video.

Unless you are switching from a full frame camera to something of smaller size like this Nikon D5100, the implications should be negligible in your photographic practice.   

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