Tuesday, June 28, 2016

Basic Photography: The confusing world of focal length simplified

There are many confusing concepts in photography but one of the most confusing is focal length. The Websters.com definition of focal length is as follows: "The distance from the surface of a lens to the point of focus." That is easy enough but it depends on the size of the sensor or the film that is the focal point. It is in the size of the sensor or film that a source of confusion enters the conversation. The photo at the left shows a Pentax 35mm camera. I show this because 35mm film and sensors have become the standard for explaining crop factors. I always like to understand it in relationship to what the human eye sees. I can remember, back in the day with the Kodak Instamatic Cameras. I was a young adult in the late 1960's and early 1970's and the Instamatic film camera was a staple to be taken on vacations. When traveling in the mountains out west, and in Appalachia as well, I was always disappointed with any scenery photos that I took. The reason was that they never showed the depth and height of the mountains that I was looking at. I have since found out that the reason was that the focal length for those cameras was very wide angle compared with human vision.

Human vision is close to a 50mm focal length on a 35mm camera. That is likely the reason that most SLR's and other view finder 35mm cameras had a 50mm lens. These cameras most closely mimicked what human vision saw naturally. Therefore, the only camera that focal length is not a source of some confusion is the 35mm film camera and the full frame DSLR. Why am I mentioning this? Well, you need to be aware of crop factors.

As I have stated before in this blog. I own a couple Sony A3000 which are mirroless APS-C cameras which have a crop factor of 1.5. This means that I have to take the focal length of the lens and multiply it by 1.5 to get the equivalent of what a full frame camera would be like. Therefore, if I want to get close to a full frame 50mm lens view, which will mimic human vision, on my APS-C camera, I have to use a 30mm to a 35mm lens. I have a 30mm Sigma f/2.8 lens that actually mimics a 45mm lens on a full frame and it gets me pretty close to human vision. If I use a 35mm lens, i mimic 52.5mm on a full frame camera so both the 30mm and the 35mm will get me close to a 50mm image on a full frame camera. So if you want a view close to what you see with your eyes, and own a APS-C camera, then you will want to choose a 30mm - 35mm lens, or you will want to set a 18-55mm lens on approximately 32.5mm to get that view.

The one thing that you can count on is that point and shoot cameras show their focal lengths in 35mm equivalent terms. So, when they tell you that they have a zoom range from 24mm to 640mm, they are explaining that it 35mm equivalent terms. I hope that I have explained that the main thing to consider is how any focal length will compare with what you see with the naked eye.

Below are some images from my 30mm lens which comes close to human vision.



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