Art of Photography

Rob Townsend


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Exercise – Focal length and angle of view

Finally getting started with the actual coursework. The first exercise under ‘Getting to know your camera’ is about focal length and angle of view.

Brief:

Take three photographs, one at a focal length that gives a ‘standard’ (equivalent to human eye) view, one at a wide angle and one at a telephoto zoom length. Print each one at A4 size. Hold them up at eye level to measure the distance from eye to photo where the image is the same size as the actual subject. Observe the differences between the photos taken at the varying focal lengths.

Equipment:

Canon EOS 650D with EF 24-105mm f/4.0 L IS USM lens

Method:

I found this slightly confusing at first but worked it out after checking how other people had done it (and some had the same initial confusion as me, which made me feel better). I set up the camera on a tripod on a low wall in the garden and pointed the camera at a terracotta garden ornament, positioned at the foot of a tree with a few daffodils for background detail.

As instructed I worked out what 50mm equivalent would be on my Canon crop-sensor camera – a crop factor of 1.6x means that I should select 31.25mm (this was the part that had me briefly confused; makes sense now though). As it turned out, the EXIF data said I actually took the shot at 32mm, which is pretty close.

Then I took the wide angle shot at 24mm (widest I could go as this is the only zoom lens I own, for now anyway!) and the telephoto shot at 105mm. Printed them out on A4 as instructed and then did the measuring part. I needed help from my long-suffering assistant* (*wife) for the telephoto shot as my arms weren’t long enough. Then I did some mathematical checking against the theory and was reasonably pleased that it stacked up!

Results:

32mm

32mm

This was supposed to correspond to the ‘standard’ 50mm field of view which on my crop sensor should have been 31.25mm (close enough). this gave a corrected focal length of 51.2mm.

To see the image and the subject at the same size I had to have this 350mm away from my eyes.

24mm

24mm

This is the widest view, 24mm or 38.4mm equivalent when corrected for the crop factor. Not too different to the first one really. Wish I had kept my old Tamron 18-50mm…

To see the image and the subject at the same size I had to have this 290mm away from my eyes.

105mm

105mm

This is the furthest extremity of my zoom lens, 105mm. Or corrected for the crop factor, equivalent to 168mm.

To see the image and the subject at the same size I had to have this 1150mm away from my eyes.

Some maths:

I did some maths to check whether my findings were in line with focal length physics (with thanks to Martin Proctor, who inspired me to dig out the formula). Simply put, dividing the (corrected) focal length by 50 (as 50mm is closest to a 1:1 subject:image view) gives the magnification factor.

So: 24mm (corrected: 38.4mm) should give a magnification of 0.768x and an expected viewing distance of around 269mm compared to my 32mm (51.2mm) distance – I measured 290mm, so not far off.

Similarly, 105mm (corrected: 168mm) should give a magnification of 3.36x and an expected viewing distance of around 1176mm – I measured 1150mm, again pretty close. I’m putting any slight variances down to the gusty weather and me not wearing my glasses!

What I’ve learned:

After many years I think I’m finally getting my head how focal length affects an image. The concept of the ‘standard’ view makes a lot of sense now. I wish I’d had a wider lens range for this exercise but I think I’ve managed to demonstrate the principles enough for me to grasp the key points.

References:

Korpella, R. (2013) How to convert focal length to magnification [online]. eHow.com. Available from: http://www.ehow.com/how_6823034_convert-focal-length-magnification.html [Accessed 21 April 2013]

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