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Today's e-tip:

How your Lens and Aperture
affect Sharpness


 
How your Lens and Aperture affect Sharpness

The Aperture you use on your digital camera affects several things. Most obviously, it affects "depth of field" - how much of your image appears sharp (if you need a refresher on Depth of Field, check out our e-Tip on the subject). However, the aperture also affects how sharp the overall image is. And, based on our experience, there's a lot of misunderstanding about this topic.


Depth of Field
When you take a picture, the aperture (among other factors) affects how much of the image appears to be in focus. As you can see by the diagram below, the smaller the aperture (e.g,, f16), the greater the depth of field.


DOF


But, Depth of field does NOT translate to image sharpness.



How the Aperture affects Sharpness
Light travels in "waves", and these waves can be modified in our cameras by the mechanical aperture (typically "blades" that form the aperture opening). When these light waves are modified, it's called "Diffraction". (if you want the REAL TECHNICAL explanation, check out this page on Wikipedia)

Suffice it to say that when you use a small aperture, you sacrifice overall image sharpness due to diffraction.

Here are some examples of laser-printed text that I shot. These have had no image reduction or enlargement and they were taken from the RAW images with NO adjustments other than white balance. Be sure to click on the image to see the 100% version.




These were all taken with a Nikon D800 camera.

The 4 top images were taken with a very high-quality lens - a Nikon 105mm/f2.8. You can see how the image is sharp at f2.8 and f8, but starts to get soft at f16, and is pretty bad at f36.

The bottom 3 images were taken with a medium-quality lens - a "do it all" zoom - the Nikon 28-300mm/f3.5-5.6. You can see how at the maximum aperture of f5.3 (this lens has a variable maximum aperture based on the zoom focal length), the image is pretty "soft", as is the shot taken at f36. The shot taken at f10, however, is very good.


Every lens will produce slightly different results. Some are sharper when shot wide-open, as the top set of images shows, but other lenses will be very soft (typically less expensive zoom lenses) when shot wide-open, as the bottom set of images shows. But ALL will produce pretty poor results when very small apertures are used (as both sets show). Almost ALL consumer lenses will produce their sharpest results when somewhere in the middle.


Sensor and Pixel Size
The sensor's pixel size also affects diffraction, with the most diffraction occuring in smaller sensors with smaller pixels.


Selecting the sharpest Aperture
The sharpest aperture of each lens is different, but selecting the sharpest aperture is often a matter of just selecting an aperture about 2-3 f-stops from the maximum. Thus, if you have a maximum aperture of f2.8, you'd select f5.6 or f8 (f2.8 to f4 is one stop; f4 to f5.6 is 2 f-stops; and f5.6 to f8 is  3 f-stops). With an f4 lens, you'd pick f8 or f11. With a f5.6 lens, you'd pick f11 or f16.


Name your Poison
Unless  you have a specific reason to use a large or small aperture, you're going to want to select one that's about 2 to 3 f-stops from the maxium. Of course, you may want to control depth of field, and that may be more important than the overall sharpness. You get to name your poison - you get EITHER depth of field controls OR overall image sharpness, not both! Shooting in "available light" is also a challange, as you may need to shoot wide open.

Conditions dictate what you need to do, but knowing the impact of your choices allows you to make informed choices.



Happy shooting!!
 

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