Male Harry Woodpecker - image cropping and shutter speed.


 Most people familiar with long lenses will undoubtedly be aware of the value of vibration reduction.  The formula for recommended minimum handheld shutter speeds is 1/focal length.  That, of course, is relative focal length and the actual focal length has to be multiplied by the crop factor in case a smaller-sensor camera is employed.  In case I lost you there, let me explain a bit before I go on.

A full-frame sensor is the same size as the negative you got when shooting with a 35mm SLR film camera.  It measures 24x36 mm.  It is the basis for considering "crop factor."  Many digital cameras have sensors smaller than this, the APS-C format is one of the most common.  It has a crop factor of 1.5 (1.6 in Canon), which means the actual focal length of any lens on the camera must be multiplied by that value to determine what the field of view would be on a full-frame camera.  So an 18 mm lens on an APS-C sensor camera would provide the same field of view as a 27 mm (18 x1.5 = 27) on a full-frame camera.  This is true for telephoto lenses as well; a 400 mm seems like a 600 mm.  Cool.

So, a 400 mm on an APS-C sensor camera behaves like a 600 mm on a full-frame camera regarding field of view.  It also means you need a minimum handheld shutter speed of 1/600th of a second to reduce blur from camera operation.  Enter vibration stabilization and that minimum drops considerably.  If your camera/lens boasts a 3-stop reduction in shutter speed, that 1/600th drops down to a paltry 1/75th (~1/80) of a second.  This means lower ISOs, the ability to use less expensive lenses (f/5.6 instead of f/4 or f/2.8 minimum apertures), and many a photographer was singing Ode to Joy with the introduction of this technology.

What no one told you was that it doesn't end there. The above is all true and wonderful, except when cropping is involved.  So, imagine that you have dutifully calculated all the necessary values and are merrily shooting away, photographing wildlife like you never could before.  With supreme confidence, you end up with terrific images, until you start cropping.  Why are the images softer than what you expected; the details just aren't there.  They seem kind of blurry like the shutter speed was too slow, but you did everything correctly.  What's going on?

The answer is cropping.  When photographing wildlife, I rarely am able to fill the screen with my quarry.  The truth is that I often have to heavily crop the image in order to end up with the shot I really was hoping for.  But cropping is like magnifying or using a longer lens, which in reality means that a higher shutter speed is needed as blur from operator movement is also magnified.  So a 25% crop is similar to using a 2x teleconverter - my 600 mm becomes a 1200 mm but now I need to use a faster shutter speed to compensate.

To summarize, when photographing anything requiring significant cropping do not rely on minimum shutter speeds.  Two or three stops above that minimum will provide better images in the long run, even if you have to use a higher ISO.  A little noise is an excellent tradeoff for reducing blur.

Thanks for reading.   www.ericspix.com







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