Why Focal Lenght doesn't matter - A Prime Lens Travel Challenge at Berlin

Started Oct 22, 2017 | Discussions thread
Truman Prevatt
Truman Prevatt Veteran Member • Posts: 8,156
Re: Why Focal Lenght doesn't matter - A Prime Lens Travel Challenge at Berlin

Alan Brown wrote:

Truman Prevatt wrote:

Alan Brown wrote:


Of course projective geometry led to the concept of a projective space which led to the whole concept that led to the development to the subject area known as algebraic geometry which is one of the more abstract areas of mathematics. But at its nature it was to study how we view perspective.

In reality each focal length of a lens maps a three dimension world onto a slice in a two dimensional projective space. The resulting projective properties are unique and determined by the focal length. A camera projects and captures a slice of space and time. The important thing is not the scene as it exist but the projection onto the sensor and subsequent print. A person that seems a photograph in a gallery is drawn to the image as it is display on the print - not to the original scene. Each of my lenses perform a different projective transform on the scene and that difference shows up on the print. Some people like the perspective of a 35 mm (FF) while others prefer the 50 mm (FF) while some prefer the slightly flatten perspective of a 75 (FF).

If you had 'some prefer the FOV' I would have agreed but perspective is a function of subject to distance. Not the lens.

Actually projection and perspective is defined by the angle of view of the lens. Run the following experiment. Take a subject and fill the frame with that subject. Take a shot. Do the same for different focal lengths.

I don't need to run an experiment: You have to 'move' to do this.. thus changing the perspective which is a function of distance, not the lens.... If you have a longer fL lens you must be further away to get the same subject size but it is not the lens' function that changes perspective, but the distance too the subject: as I stated.

But if you don't move you do not have the same fore ground image now do you. Here is a better example.


Woman the foreground subject fills the frame but my oh my - look at the difference in perspective that the different focal lengths project!

A set of images where one never moved but changed lenses would be pretty boring now wouldn't they. Perspective is about the relationship between different objects within the image not about your feet being nailed to the concrete ;-).

The subject is the same size in all the images. However, the background behind the subject is vastly different because the projective geometric properties of the lenses are different and unique.



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