Diffraction Unsharpness

[boBQuincy]

This is an esoteric subject, but one that is for real and can affect your images.

When a lens is stopped down diffraction starts to cause unsharpness. It takes until about f/11 or more to become apparent but by f/22 it can be a real problem.

I ran into this when taking photos of waterfalls. I wanted a slow shutter speed for that "dreamy" effect but even at ISO 100 it required a polarizer and f/22. The resulting images are really nice but sharpness suffered compared to the ones at f/8.

http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml

Anyway, it's not a disaster but is something to consider when you are adjusting settings.

What made me think of this was the weekly contest...macro, which usually requires small apertures for any reasonable depth of field.

 

[JR6ooo4]

A ND (nuetral density) filter would help? Except for DOF...

Mikeeee

 

[mhutchinson]

Bob,

You might want to try bumping your ISO up a little. I had read somewhere that the sweet spot for my Canon 20D seems to be 200, before that I was trying to push a lot at 100.

Mike

 

[boBQuincy]

A ND (nuetral density) filter would help? Except for DOF...

Mikeeee

I think you are right, the ND filter may be the only way to get the slower shutter speeds without using such small apertures. Of course I will need a multi-coated one!

I wonder how much trouble it would be for Canon to include ISO 50 and 25? It's always easier to reduce the signal than it is to boost it so it should not be difficult, but there is probably not much call for it either.

 

[Kelly Grannell]

I wonder how much trouble it would be for Canon to include ISO 50 and 25? It's always easier to reduce the signal than it is to boost it so it should not be difficult, but there is probably not much call for it either.

Now that I have my 17-55 f/2.8 IS, I really want ISO 25 for my 30D so I don't have to buy ND filters.