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Alice Springs Photographer Blog

5D MKII and Lens Diffraction

The Canon 17-40 f/4.0L & 24-70 f/2.8L

Der-fraksh-nnn... Wah?

Thanks to Red Bubble user Peter Hammer for pointing this one out to me yesterday in reply to yesterdays post. Peter noticed that I was using an aperture of f/22 for most of my settings and pointed out to me that this was not the best for a very good reason. Diffraction, the change in the directions and intensities of a group of waves after passing by an obstacle or through an aperture whose size is approximately the same as the wavelength of the waves.

Hmm let me see if I'm getting this right. When you have a whole bunch of light collected by the front elements of the lens and squeeze it through a really small hole (f/22 aperture) it mucks up how the light comes out of the hole and ultimately how the sensor records it. This sounds like crazy talk but its true and it is the source of much web banter between people far geekier than me.

The 24-70 f/2.8L Test Shot

The 17-40 f/4.0L Test Shot

Not only does the light bend and distort but it also affects how much of the light the sensor sights can record. So on my 5D MKII with its manly 21.1Mp sensor on a 35mm format the size of the sensor sights are really small... really, really small. These small sensor sights combined with diffraction of the light though the a lens using a small aperture equals soft and washed out images, even if very slightly. That "slightly" adds up to alot when you just paid that much for a camera and damn it I want the best quality images coming out of that thing.

 

Here is a link to a detailed article about Diffraction & Photography hosted at www.cambridgeincolour.com a site that I will be looking at more over the next few days while I'm in town. In that article they have a visual example of aperture Vs pixel size and how the two interrelate. Also on the www.cambridgeincolour.com site there is a diffraction limit calculator which will help you work out how diffraction affects your camera and lens combination. Not only that they have a Depth of Field Calculator as well to help you get the best out of your chosen DoF. It's very much worth a read to get yourself up to speed with this issue that affects us all in one way or another.

 

So as a result of this article I will be most definitely changing how I take photos and opening up the aperture a little more. Below are a few sample shots I took yesterday try to demonstrate the effects of diffraction on the 5D MKII. I shot a wide range of apertures from the widest to the smallest with both my main landscape lenses the 17-40 f/4.0L and the 24-70 f2.8L set to 17 and 24 mm respectively.

The test shots are totally unprocessed and have had no in camera sharpening (picture style) applied. Shot at ISO 400 with the focus point 1/3 from the bottom of the frame.

For the horizon shots you must take into account that the daytime temperature was 42 degrees and so an amount of heat wave distortion is present. Oh and Just incase your wondering these photos are a great example of midday light here in the centre so don't make any comments about the photos as.. I already know!


17-40 Focus Point

17-40 f/4.0 Horizon

 

 

 

 

 

 


24-70 f2.8 Focus Point

24-70 f2.8 Horizon