Tuesday, 11 January 2011

Exercise 1 - focal length and angle of view

"Exercise 1. This should be easy" I thought when I'd finally waded through my Nikon D300 manual. Hmmm.


I thought I was familiar with the concept of focal length - most books and magazines have articles about it and I've been using DSLR's for quite a while, so the 'crop factor' for my D300 should be about 1.5x which should reflect on the results of the exercise. We were required to look at a scene through the view finder and find the point where the objects were approximately the same size as when viewed with our eyes.


I set up the shot of our funky conservatory clock expecting the result to be around 75mm (50mm x 1.5 crop factor).


The actual result of precisely 50mm confused me somewhat. Where was my crop factor?!
I then took the other shots at 18mm and 200mm to compete the exercise, with the results below:
ISO400  f/4.8@1/250  50mm

ISO400  f3.5@1/400  18mm

ISO 400  f5.6@1/160  200mm
I then printed the pictures out and found the following distances where each print appeared approximately the same size as the real objects:

50mm - 50cm
200mm - 190cm
18mm - approx 15cm (but this one was very difficult for me to verify as I'm long sighted and couldn't focus on something that close to my eye). 

Interestingly, the distance in centimetres matches or is very close to the mm value of the photograph.

After some further research on crop factors, I think I've got it clear in my mind why it didn't appear to affect the results of this exercise. The crop factor is due to the size of the camera sensor being smaller than the 35mm equivalent. If you viewed the same scene with the same lens (and focal length) on both a 35mm camera and DSLR with crop factor, the scene would not appeared 'magnified' by the DSLR. You would however, be able to see more of the scene through the 35mm viewfinder, as shown by the diagram below:


Fig 1 - comparison of 35mm and DSLR photo sizes


The large rectangle shows the size a 35mm camera would take with the smaller rectangle showing the DSLR sensor size. 
Fig 2 - 35mm photo size

Fig. 3 - DSLR photo size
The DSLR takes a smaller photo of the same scene and therefore appears to crop the sides of the 35mm equivalent when printed to either a standard size or blown up. The DSLR picture would appear 'magnified' hence the crop factor. 

The crop factor therefore only applies to the finished photograph and NOT the magnification of the view we see through the lens.

Learning outcomes
This interesting exercise not only made me research and properly understand a subject I thought I already knew, it also taught me to stop making assumptions about my knowledge!

Sources
Fig.1, Fig.2 and Fig.3 from: 
http://www.digital-slr-guide.com/crop-factor.html

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