Thursday, 19 January 2012


Getting to know your camera

Exercise  - Focal length and angle of view

The angle of view in a photo is directly related to the focal length of the lens being used, which is in turn roughly proportional to the distance between the film plane in the camera and the lens element where the light entering the lens crosses.  Simply, focal length is the distance between the sensor, or film, and the lens. The diagram I have found online and included below gives a fair explanation of how I understand the relationship between focal length and angle of view to work.
For this exercise I was asked to take 3 photos.  The first was to be taken at a focal length where what I saw through the viewfinder matched in size what I could see through my other eye.  In other words to find the focal length of the lens which gave a 'normal' viewing angle.  The second and third shots were to be at a telephoto focal length and a wide angle focal length.

'Normal' focal length found to be 53mm 
Telephoto - 135mm 
Wide Angle - 18mm

Next we were asked to print the photos onto 8x10 paper.  Instead I displayed them full-screen on my laptop as this is roughly the same size.  Then we had to move each image to a position between the position where we captured the image and the subject, where if our eye was in the same postion as when the image was taken, the size of the real objects and the reproduced images would be proportionally the same to our eye.  Then measure this distance.

'Standard' focal length (53mm)        - 600mm
Wide angle (18mm)                       - 1400mm (this was just infront of the objects I was photographing)
Telephoto (135mm)                       - 190mm

If I had used a lens with a focal length longer than 135mm, would the image have needed to be placed beyond the actual subject to produce a proportionally sized equivalent to my eye?  I expect it would have.  The 600mm distance was also by far the most comfortable distance to focus on the image from, roughly an arms length away, and is the obvious decent you would stand at when viewing a similar sized image in a gallery.

However, this exercise has left me a little confused and quite interested, for a reason that I am struggling to get my head around.  My camera uses a sensor smaller than the 35mm or 'full-frame' size, with a crop factor of what I believe to be around x1.5.  If this is the case, how can my zoom lens set at 53mm provide a 'standard' focal length?  Have they altered the markings on the side of my lens to compensate for this?  To confirm I decided to attach what I know (in full-frame sensor terms) is a 50mm prime lens - this should have an equivalent focal length of roughly 75mm (50mm x 1.5) on my camera - and to find out if this had a similar angle of view to the zoom lens set at 53mm.  I found it produces almost exactly the same angle of view.

I was expecting to find a standard focal length of roughly 35mm for this camera (as 35.33mm x 1.5 = 53), not the 53mm length I have found.  The 50mm prime lens is primarily designed as a standard focal length lens for use on 'full-frame' bodies, and is therefore said to be slightly telephoto when the crop factor of a smaller sensor is applied.  The results of this exercise have not really shown this to be true.  Is what people believe to be a standard focal length actually slightly wide-angle?  Perhaps I have I misunderstood this exercise somewhere.  Is it possible that the definition of a 'standard' focal length lens is open to interpretation?

No comments:

Post a Comment