Long lenses and depth of field.

 Have a look at the two images above.  They are shot from the same angle at approximately the same time, within a few seconds of each other.  The exposure value on both is the same, although the aperture, shutter speed, and point of focus are different.  The effect on the image of these changes is obvious.

Depth of field is affected by a large number of factors, but the main ones that the photographer should be aware of include focal length, aperture, and point of focus.  Long lenses (telephotos) have notoriously narrow depths of field.  As focal lengths increase depth of field decreases; they have an inverse relationship.  Aperture and depth of field also have an inverse relationship, depending on how you look at it.  Larger f/numbers equate with smaller aperture sizes.

The above photo was taken with a 500 mm lens.  Since it was shot on an APS-C sensor camera, the relative focal length is 750 mm.  The image on the left was shot at f/5.6, which for this lens is wide open.  The combination of a 500 (750) mm lens and aperture of f/5.6 means that the depth of field is very narrow.  If you pay close attention to the grass in front of the dog's ball and just at its hind end, you will see that blur starts to reduce clarity.  The depth of field is less than a meter.  

In the first image, the second dog is significantly beyond the lenses' field depth and its image is blurred.  There are benefits to this in that the viewer's eye is naturally drawn to the in-focus dog.  If your intention was, however, to get both dogs in focus, something has to change.

Most of my long lens photos are taken using wide-open apertures; the reason for this is simple - shutter speed is allowed to be at its maximum value given whatever the ISO is set at.  Since I do a lot of wildlife photography where a high shutter speed is important, this makes a lot of sense.  When the subjects are cooperating, however, this can be easily changed.

The image on the right was taken with a smaller aperture.  In this case, I kept the shutter speed the same (1/320) but increased the ISO to 1000 from 200.  This allowed me to set the aperture to f/16 which increased depth of field.  That wasn't the only change made though.

An important parameter in working with depth of field is to consider the point of focus.  For any given aperture there is a certain depth of field which is affected by focal length and point of focus as well.  If you measure the in-focus distances before and after the focus points, you will discover that it is approximately a 1/3:2/3 ratio.  That is 1/3 of the distance is in focus in front of the focus point and 2/3 is sharp behind the focus point.

In order to capture the image with both dogs in focus, I had to move the point of focus just behind the first dog.  This, in conjunction with the smaller aperture, allowed both dogs to be clear.  This is the main reason why I suggest using single point focus (S) instead of automatic (A) focus modes.  In full auto the first dog's face would be the focus point rendering the second dog out of focus, regardless of the aperture.

Now, this seems like a lot to balance all at once.  For the novice, it can be overwhelming.  The only way to get around this is to play with your equipment, learn the basics of photography, and apply that knowledge to your craft as you gain experience.  This, however, is also how you improve in anything you set your mind to.  Is it worth the effort to learn?  That depends, of course, on where your passions lie.

Thanks for reading.   www.ericspix.com