Monday 27 July 2009

On aperture

My website: Wedding Photographers in Kent

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Part 1 of a question from Stephen today.

Hi David,

I'm a regular reader of your blog - which is fantastic BTW - and had a discussion over the weekend on various photographic matters. I thought you would be the perfect person to enlighten me and perhaps a blog post might come of it?

Firstly lets take the subject of lens aperture. If you take a 24-70mm f/2.8 L lens and set it to 70mm at f/8, would that be the same size aperture, and therefore capture the same amount of light as a 70-200mm f/2.8 L IS USM set at 70mm at f/8? Now when I thought about it I did wonder whether manufacturers had a 'standard' aperture size to conform to thereby making any lens set at f/8 equal?

Thanks in anticipation,


Hi Stephen,

Thanks for the feedback.

Aperture is a unitless measure (rather than a size) - it's the focal length of the lens divided by the diameter of the diaphragm. It correlates with light-gathering ability.

As you change the focal length on your zoom lens, the diaphragm has to change to maintain a constant aperture. Not all lenses do this equally well though - L-series Canon lenses vary by only 1/3 of a stop over the focal length range which is pretty good.

f/8 at 70mm on both 24-70 lens and 70-200mm lenses means they will have the same diaphragm diameter. They won't be letting in precisely the same amount of light however. It will be close but lenses vary in their ability to transmit light - it depends upon the number of elements within the lens, the albedos of the materials used and probably lots of other factors too.



My website: Kent wedding photographers

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