Category Archives: Ex 2.1 Focal length

Ex 2.1 – Focal length

The focal length shown in the pictures is the 35mm equivalent.  All exposures are 1/200 at f8, ISO200.  With the camera pointing almost directly into the early morning sun I was asking quite a lot from this lens, hence the flare.

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The effect of varying focal length is demonstrated in the images above.  A wider angle produces an image which includes more peripheral content whilst a ‘longer’ focal length includes less.  When compared side by side the longer length gives the impression of compressed perspective but actually the lens is not responsible.  Ignoring lens artifacts, the same image could be produced simply by cropping the wider angle exposure.

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To check this I compared the two images above.  To the left is a crop of the 24mm image cropped (approximately) to show the the same content as the 80mm shot to its right.  You wouldn’t know that one had been taken with a wide angle, the other with a longer focal length; it doesn’t alter the image, just narrows the angle of view. Other factors such as definition and lens aberrations might reveal the deception but for practical purposes they are very similar.

The question of the ‘natural’ focal length of the human eye is, I have discovered, rather more complicated than it might first appear.  The generally held view is that a 50mm (35mm equiv) lens approximates the field of vision and this is probably as far a we need to go for photographic purposes.  But even a cursory examination of the workings of the human visual system reveals its immense complexity and underlines the futility of any attempt at meaningful comparison between the camera and the eye.

Human vision is the result of real-time interpretive processing, not an instantaneous segment of time.  It is actively binocular, which allows the gathering of accurate spatial detail.  It interacts with our other senses, mainly touch and hearing, to produce an ongoing experience of ‘presence’ in our relationship with our environment, a feature which has developed largely as a survival mechanism.

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