Friday, August 22, 2008

Photography tips: the sweet spot

Sweet spot, you may have heard that word before. You may even have been checking some of your pictures for it. You didn't findt it, did you? Sweet spot is not in the picture. It relates to the optimal aperture of a lens.

The term 'optimal' is a bit misleading here, because the optimal aperture depends on the type of picture you are trying to make. If you want a shallow depth-of-field, your aperture has to be larger than in the case you want a deep field. In low light situations, a large aperture (small f-number) is often the only option. But apart from these considerations, you may be looking for the maximum resolution (i.e. sharpness) of your lens. The aperture where the overall resolution reaches its maximum value is called the sweet spot. It is nice to know the sweet spot of your lens by heart, so that you can use it if you end up in a situation where you can. Shooting landscape photos in broad daylight would be such a situation.

Finding the sweet spot
But how do you find the sweet spot? It is not in the manual of the lens. It is actually quite simple. Find a review of your lens that contains a laboratory resolution test at different apertures. Photozone has them for all the lenses they have reviewed. Then check the resolution graph (or table) for the highest value and find the aperture that produces it. You have now found the sweet spot. Note that some lenses have different sweet spots for centre and border, and zoom lenses may have different sweet spots for different focal lengths. Then it takes some common decide to decide.
An example
Here's a quick example from one of my favorite lenses, the Canon EF 35/2. The figure (click on it to see it in it's context) shows that center sharpness is maximum at f/5.6, and border sharpness at f/8. That is not a clear verdict as to what the sweet spot is, but since border sharpness is below centre sharpness, I would say overall sharpness is best at f/8.

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