The definition is a lens that is circa the same size as the diagonal of the sensor of the camera. This gives the pictures a normal perspective with neither exaggerated perspective like on a wide angle lens or compression of the perspective like on a telephoto lens. The perspective looks like you would see it with your eyes if you were there. How your brain constructs your world is another issue than how your eyes see the world physically, so even if a normal lens would be very close to how your eyes see the world, the resultant image might not look like how you perceive the world.
The traditional standard lens is 50 mm on full frame, 33,33 mm on APSC and 25 mm on micro four thirds. This is actually slightly longer than the diameter of the sensors on these formats, but close enough and a nice round number on full frame. I think another reason why 50 has been the standard is that it is long enough to be useful for portraits since you would have a working distance long enough to avoid perspective distortion of people’s faces, while at the same time a 50 is short enough to get quite a lot of an urban or landscape scene into the frame. It’s Cartier-Bresson’s focal length of choice and it is still popular for photojournalism, environmental portraits, full body portraits, street photography and many of us use it for landscapes and seascapes as well because of the pleasant perspective.
As mentioned before in other blog posts, I really like the normal lens perspective. Having used my Panasonic Leica 25 mm f/1.4 Summilux (50 mm full frame equivalent) quite a lot, I have come to learn more or less how much of the scene the angle of view lets me fit in a frame at various distances and this combined with the natural perspective makes it really easy to previsualize shots. However, from time to time I would like to have a slightly wider normal lens. The Panasonic Lumix G 20 mm f/1.7 (40 mm FF equivalent) is exactly that, but as mentioned before in other blog posts, the slow focus annoyed me a lot. So I have been thinking that something close to 35 mm full frame equivalent might be good as an alternative to my Summilux for a walk around lens, especially in cities with narrower streets.
As mentioned in “The mild wide angle prime dilemma for micro four thirds”, it is hard to find a lens that ticks all the boxes in that category and since I actually prefer something longer to something more wide angled, the one really good choice, the Panasonic Leica 15 mm f/1.7 Summilux is not ideal for me. I also already own the 14 mm f/2.8, so that makes it even a less good choice. I ended that blog post saying that I am considering buying the Sigma 19 mm f/2.8 DN Art lens since it is almost a normal lens, but fits almost as much as a 35 with its 38 mm full frame equivalent focal length and seems like an underpriced lens compared to many others in the mild wide angle category even if it has a narrower aperture than some of the others.
Then I started thinking. The diagonal of a micro four thirds sensor when shooting pictures in a 4:3 aspect ratio is 21 mm. However, when shooting in the 3:2 aspect ratio which I think looks better in landscape orientation, the diameter is 19 mm. (With 20 MP on the GX8, I can sacrifice a few pixels for a nicer aspect ratio. I get around 17 MP when shooting pictures in 3:2 which is still higher than on many of the older and even some slightly newer MFT cameras than my GX8. For portrait orientation, especially for upper body shots, 4:3 seems smart to fit the whole width of a person and it looks quite good in my eyes, but there is something magical about the 3:2 aspect ratio in landscape orientation that I just like.) When shooting 16:9, the diameter on MFT is 18.5 mm.
That led me to think that since I usually shoot 3:2, the Sigma 19 mm is actually a normal lens when it comes to its perspective. I have one on the way from Japanphoto.no now that I am looking forward to getting. It was so cheap that I thought I could take a chance on it, and the reviews I have read are quite positive to its image quality compared to for instance the Olympus 17 mm f/1.8 and it has a faster autofocus than the 20 mm Panasonic pancake which annoyed me a lot even if I liked its image quality. So why not take a chance on it? A wide standard suits my way of shooting really well and I miss having something a bit wider than my 25 when in narrow streets or for fitting a bit more of a landscape into the frame without having to go really wide with the 14.
On Flickr, I am a member of the “Normal lens” group which defines the normal lens as a lens between 40 and 60 mm on full frame and equivalents on other formats. So I asked if I could post pictures taken with a 19 mm in that group when it is the diameter of the sensor when I shoot 3:2 which I usually do. I got a positive answer with the caveat that this is such a special case that even if the group moderator would tolerate my pictures, he would not change the group rules. That seemed very reasonable. So now I am eagerly awaiting my 19 mm Sigma and I know that I can share my pictures in the normal lens group instead of the wide angle lens group. I actually think that for my aesthetic ideals, this lens might be nicer for me than a 35 mm equivalent. It is still a bit wider than the 25 (=50 FF) with its 38 mm full frame equivalent, but it retains the normal perspective that I like when I shoot 3:2 which I almost always do now.