Having read the information in the OCA manual on how lenses in digital cameras vary from, what was previously, the norm of 35mm I looked in the Nikon manual for the specification for my camera. It notes that the a 35mm lens is typically 36 x 24mm but the D5100 is 23.6 x 15.6mm. It also told me that to convert the focal length of photographs taken with my Nikon to 35mm I should multiply the focal length by 1.5.
I carried out the exercise describe in the manual with a Nikon 5100 and 2 lenses:
a 18-55mm and a 70-300mm zoom lens.
I took my first photograph with the lens focused to 55mm as this brought the object to roughly the same size within the viewfinder as they actually were.
The second photograph was shot with lens set to 18mm as this is the smallest lens range I have at present.
The third photograph was taken using my zoom lens with the focal length set to 300mm. For this shot I had to move my camera further back from the point where I had taken the first 2 shots as at this point the shot was out of focus.
I then printed them off on A4 paper. Beginning with the first photograph I held the photograph in front of me and found that when I held it arms length, roughly 60cm, it appeared to be of the same size.
With the wider angle shot, I had to hold the photograph just a few cm in front of my eyes to make it the same size as the original, not a pleasant way to view a photograph.
As I explained previously, to photograph the shot at my widest focal point, 300mm, I had to reposition the camera and also persevere for some time with the focus to capture the shot. I had to walk backwards, away from the photograph to point almost 7.5 m away! Again, not a particularly pleasant way to view a photograph.
This exercise did illustrate to me the concept of ‘a comfortable viewing distance’. It was also helpful as it allowed me practical experience with working with my newly acquired camera and lenses.