When 85mm is Really 129mm
The 50mm prime lens not only frequently gave the sharpest details and less distortion of the form, but the larger f stops allowed me to isolate my subject as I desired. The ability to shoot in low light with a larger aperture was a bonus. As a result, I ended up with a small collection of 50mm lenses in the ƒ1.4 to ƒ1.2 range. I learned to fill the frame of my photos by shooting close, and moving.
When I purchased my Nikon D-200 complete with an 85mm ƒ1.8 prime lens, I was relieved to get some space between myself and my subject. I did not realize just how much space I was using until I took photos of people at the Tea Party.
I had an immediate advantage in that people thought I was photographing them with their sign when I was actually taking a portrait. When I wanted to get the entire person in the frame though, I had to back up to a distance that confused my subject. Even my fast fifties seemed to need more distance to frame the same subject on the D-200.
After a bit of research at Ken Rockwell's website, I found that what I was sensing was reality. The Nikon D-200 is a DX camera. That means that its sensor is 1.5 times smaller than 35mm film. That is not necessarily a bad thing. What it means is that a lens with a 50mm focal length actually reads as a 75mm lens. My 85mm prime was reading as a 129mm telephoto on the digital camera. My 70-210mm zoom lens reads as a 105-315mm lens! Ken has a page at his website explaining the crop factor when SLR lenses intended for film are used on a digital SLR, as well as a chart to convert the focal lengths.
While I will likely continue to enjoy the 85mm lens for portraiture, I am considering purchasing a 50mm ƒ1.8 prime lens for when I am taking photos of people in tighter quarters. I already have a 18-70mm auto focus zoom lens that fills that focal length, but its aperture is a variable ƒ3.5-4.5. For the isolation of my subject, I want that big hole in the back.