Tuesday, 14 December 2010

Project: Getting to Know your Camera

Exercise: Focal Length and Angle of View (Field of View)

Objective: This exercise will demonstrate that the field of view narrows and distant objects appear larger in the frame as the focal length of the lens is increased.

Before starting this exercise, I searched the internet for information about the sensor size of my camera. it has an image sensor approx 24x16mm. Using the guidelines from the project text I calculated that the focal length of the 'standard' lens should be in the region of 30 - 40mm for this format. However, when I undertook the exercise keeping both eyes open and adjusting  the zoom until both images were the same size, the focal length scale on the 18-70mm zoom consistently showed 50mm+. After posting a question in the OCA Flickr group, (Can I believe my eyes?) I was assured by Rob and DaveB that they had experienced a similar effect. We all agreed that I was looking too deeply into the technical aspects of the exercise.

Using the 18-70mm zoom, I took the required photographs and noted that the focal length of the zoom was set at around 50mm when the viewfinder image in my right eye appeared the same size as the scene viewed with my left eye. Here are the photographs, taken near the junction of Bordon High Street with Chalet Hill. (For historical interest, to see a picture taken from a similar viewpoint in 1919 click here )

Zoom lens set at 50mm ('standard') f8 @ 1/125s AF on

Zoom lens set at 18mm (wide angle) f7.1 @ 1/200s AF on

Zoom Lens set at 70mm (telephoto) f5.6 @ 1/400s AF on

Upon returning to the viewpoint with the prints I had made, I found  that the 50mm focal length image could be viewed comfortably at  28 - 30cm from my eyes with the print matching the dimensions of the scene and the 70mm focal length image at 60 - 63cm i.e. arms length. The 18mm focal length image however, could not be viewed within my arms length, the central feature and even the foreground features (the manhole cover and the fence panel) were smaller than the actual scene even at arms length. If I had an assistant to hold the image, I would have expected it to be viewable  to scale possibly in excess of a metre from my eyes.

What have I learned?

As the focal length of the lens is increased, the field of view narrows. I have learned that the focal length of this 18-70mm zoom lens which matches the field of view of my eyes, is around 50mm on the zoom scale. When  the minimum focal length is chosen (18mm) there is some pincushion distortion at the edges of the frame. With the modest zoom range at the telephoto end (35-70mm), there is no noticable shortening of perspective.

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