Monday, August 14, 2006

when diffraction rears its ugly head

Reducing the aperture on your lens (setting it to a higher f/stop) increases your depth of field. Objects both far and near will be in focus. This is exactly what I needed for this dried baby durian fruit which was destined to by "dry-cut" (cut out and put on a white background). Fortunately, I also remembered there is a point where stopping down actually reduces the overall sharpness of your image and thus becomes self-defeating. Since I couldn't remember what that point was, I shot the durian at f/5.6, f/16, and f/36.

On this closeup, shot at f/5.6, the depth of field is quite shallow and parts of the durian are out of focus. This has nothing to do with diffraction, it's due to depth of field so we'll just ignore it. Let's focus on what is sharp; those parts are just as sharp as in the picture below.

This crop, shot at f/16, has a much greater depth of field. The parts that were sharp in the f/5.6 crop are still sharp, but others, that were out of focus, are now sharp as well.

Now at f/36, the picture is noticeably less sharp. It's not out of focus, the picture was shot on a tripod with a cable remote at the same focus as the other two shots. This image is suffering from diffraction. The point of today's post: remember to not go beyond f/22 unless you know what you're doing.

If you still absolutely need to know what diffraction is and how it happens, Sean T. McHugh has an amazing page on the topic.


Anonymous Mathieu said...


a. I don't have anything that will stop down that far.

b. my D70s apparently has much bigger pixels than your 350

still, I had no idea the effect could be that strong in certain conditions...

11:41 PM  
Blogger gregoire said...

well yeah, they all pretend it doesn't matter, but then you get those instances where having much bigger ones does make a difference.

11:47 PM  

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