What is it called when an object appears to be the same size with the eye and through the camera viewfinder,?

Question

I have a Canon 18-135 lens. I am looking into the camera viewfinder with my right eye and not through the viewfinder with my left. I start from 18mm and keep zooming in till I see a particular object as the same size with both the eyes.

I read the focal length to be just above 50 mm (say, 55mm). What is special about this 55mm with reference to the object I was focussing on?

Answer

There's a specification on (d)SLR bodies called viewfinder magnification; this refers to how large an object appears in the viewfinder when a 50mm lens is mounted and focused at infinity.

On mid-range DSLRs, which typically have around 0.95x magnification, an object will appear to be life-sized at 52.6mm. With entry-level DSLRs, you might have around 0.8x magnification, so you'd have to zoom in to 62.5mm to get a 1:1 magnification. I'm betting your camera has somewhere around 0.9 to 0.95x magnification.

This has very little to do with the object, except that the front of the camera is slightly closer to the object. With far-away subjects (landscapes, etc), the distance between your eyes and the front of your camera won't matter much, but close subjects will appear larger to the camera than to your eye even with a 1:1 magnification.

Check more discussion of this question.