Aperture: Using The Aperture To Focus Your Viewer

In the articles Stops: What Are All Those Numbers and Exposure Triangle: How To Make Those Nutty Numbers Work we learned how the aperture is one of the ways we control light, but that isn't all it can do.

The aperture has a creative function as well, it can control the depth of field.  The depth of field is how much is in focus.  Now don't get me wrong, only the distance you are focused at will be absolute tack sharp focus, but the aperture along with focal length determines how quickly objects go out of focus as they get nearer or further than the focused distance.

Camera: Nikon D80      

Lens: VR 18-55mm f/3.5-5.6G

Focal Length: 55mm

Shutter: 1/60

Aperture: f/11

ISO: 100

Flash: SB-600

Great for us because we can use that to creatively modify a picture.  Lets look at the picture at the right.  This is one of my prized possessions, my Petri f/1.9 rangefinder camera.  Here we can see that focus is is perfect on the focus ring, but decent focus is maintained throughout, only getting soft on the nearest part of the lens but not so soft that you cannot read the shutter speed numbers.  But lets say we wanted to focus our viewers attention on the focus ring, maybe we are giving a lesson and the reader may be confused if they see the numbers on the shutter speed ring.  What we can do then is open up the aperture from f/11 to f/5.6 to reduce the depth of field.

Camera: Nikon D80      

Lens: VR 18-55mm f/3.5-5.6G

Focal Length: 55mm

Shutter: 1/60

Aperture: f/5.6

ISO: 100

Flash: SB-600

Here we have reduced the depth of field dramatically.  The focus ring is still in sharp focus, but the shutter speed numbers have been all but obliterated.  Even the knurled silver aperture ring just this side of the large black focus ring has softened as well as the leatherette covering which is now only sharply textured near the focus ring and softened toward the perimeter of the photo.

As you increase in aperture size this effect gets even more and more pronounce with f/1.4 lenses getting a depth of field that may only be a few millimeters deep.

For the artist, this article is complete, it is really all you need to know.  Practicing and knowing how and when to use short depths of field, as well as long ones will help you make your art more interesting and convey a specific message or thought.  If you are a tech head, by all means continue on...

Now as I mentioned before, there is really only one distance of sharp focus, everything outside of that point has some sort of circle of confusion.  So what is a circle of confusion, well, lets say we have a white wall with a 1 micron dot on it and we shoot it with a macro lens at 1:1.  At the precise focus distance, there will be a 1 micron dot on our sensor.  But as we move from that perfect focus distance that dot will grow since that pinpoint of light will no longer make a pin point, but a confused circle.  The further from the perfect focus distance we are, the larger that circle gets.

Now how quickly that circle grows as we move from perfect focus is dependent on the aperture and focal length.  As the aperture gets larger, or the focal length gets longer, the circle of confusion will grow quicker.  As the aperture gets smaller, or the focal length gets shorter, the circle of confusion will grow more slowly.

A thing of note here is that the circle of confusion is only affected by focal length and aperture.  So lets say you take the exact same picture but with three different cameras.  First a DX sensor camera with a 35mm lens, second a FX sensor camera with a 50mm lens, and lastly a medium format camera with an 85mm lens. All of these combinations will produce roughly the same framing with the same angle of view.  But, if we sot them all at f/2 the depth of field among them would be vastly different.  The DX with the 35mm will have a huge depth with almost everything in focus, while the medium format with the 85mm lens will have almost nothing in focus but the actual focus depth.

So if you are selecting a lens specifically for short depth of field, keep in mind that as your focal length grows there is less of a need for big f/1.4 apertures.  Also it is of note that as you combine large apertures and long focal lengths the unnecessary accuracy of your focus goes way up and proper AF calibration will become paramount.  Even a 50mm f/1.4 will show minute inaccuracies, so the wonderful 85mm f/1.4 will require even more accurate focusing.

Hope this helps you enjoy your shooting and gets you what you want.

Thanks for reading!