The Myth About Depth of Field

Click on image to enlarge.

Have you ever spoken to a fellow shooter about Depth of Field (DoF)?  Have you ever been told you must have expensive, "fast glass" (a lens with a large maximum aperture - like F2.8 or F1.4) to get shallow DoF?  Well, it all depends on what you're shooting and what your subject is.  If you were to shoot someone's portrait using almost any lens and had enough empty space between the subject and the background you could shoot closed down to the minimum aperture (F 16 or F22) and get an acceptably out of focus background.  (It might have to be miles away and your subject at the minimum focusing distance of the lens, but you could do it.)  Take a look at today's image.  Before reading on, take a shot (guess) at what aperture might have been used.  Once you've made up your mind, hit the "Read More".

If you know what you're looking at you should have seen the clues to let you know what's going on.  There's not Adobe Photoshop (PS) trickery going on.  The image was "developed" straight out of the camera adjusting only color aspects of the shot.  Not purposely adding any blur to the background.

The first thing to notice is how big the flower is in relationship to the frame.  (It was cropped down to an eight by ten aspect ratio, but no pixels came off the top or bottom of the frame.)  So, it is an intentional shot. 

The second clue would be that the front and rear leaves are both in acceptable focus.  That's a biggie.

Third is the fact that there is no doubt about the separation between the subject and the background.  There's nothing to attract your eye in the background and cause you to loose attention from what you are supposed to be looking at.

Back to one.  I was standing just outside the minimum focusing distance of a 70 - 300 mm lens, racked out at 300 mm.

Two: The DoF at that distance couldn't have been more than a couple inches.

Three: There was no need to worry about the distance to the background because I "scouted" the area around the flower (by eye, I didn't wander anywhere) until I visually cleared any obstructions from the composition.  I have another shot where I wasn't so careful and got what looks like a pair of a certain "mouse ears" out of focus behind a different flower.

So, what's the deal?  It's F16 at 1/160th of a second with an ISO of 800.  The flower was probably 4' away and the grass in the background was probably 20 - 25' further back.  It wasn't shot with "fast glass".  It didn't need to be.  Since I was at F16, having a fast, expensive lens wouldn't have helped me "in this situation".  The place for fast glass is when you're shooting a two thirds shot of someone and you can't get enough distance between the person and the background.

One of the reasons sports shooters us fast glass is because of the distances they're shooting at.  Like half way across a football field or similar.  Fast glass gives them the ability to throw the people in the stands out of focus when the main subject is sixty feet away.  Every lens has it's place and what you're shooting makes a huge difference in how fast a lens you need to get where you're going.