Depth of Field

wide_dof1

This lesson we will try to answer the question “what is depth of field and why is it hiding in my camera”?  Consolation-LakeLet’s start out by giving two examples of pictures with different depths of field.  In this scenic you can see that almost all the elements from close to far away appear sharp to your eye and you are kind of drawn into the scene starting at the foreground rock all the way back to distant mountains.

narrow_dof1

Mountain-Bluebird-May-2007-0111

Now look at this bird perched on a branch. The bird is very sharp to the eye, but the background has a pleasing blurred affect to it to keep your point of interest on the bird.

You can see that two very different visual experiences are displayed by these images and complimentary drawings. The scenic is a sweeping vista in front of you, and your eye explores all the elements in the image from close up to far away.  This is an image with a large  DOF.  In the second image your eye is drawn to a small area in the image that demands your attention.  Everything in front of and beyond that point is rendered out of focus. This image has a shallow  DOF. So a simple definition of  DOF  might be how much of the image appears in sharp focus.

So what causes this?  Well in photography crowds this is caused by something called “circles of confusion” (not to be mistaken with that circle of lost teenagers that hangs around your house eating all your food).  As light travels through the lens it is transmitted as tiny circles of light.

COF

At the point they are focussed, the sensor in this case, they appear as a point of light and therefore appear in sharp focus.  However as you get farther away from the point of focus the circles become larger and your eye no longer sees them as a point, but rather out of focus overlapping “donuts”.  So therefore the image away from the focus point begins to blur. The farther away from the point of focus the more blurred the image appears.  The range in distance that they are still acceptably sharp is known a the DOF. If you have a chance to look through a mirror reflex lens you can see an exaggeration of this.  Out of focus highlights are rendered as donuts that are quite visible on pictures.  Telephoto mirror reflex lenses may be much cheaper to make and buy, but the way they accentuate out of focus highlights is a deal breaker for most photographers.

Lenses and DOF

Actual DOF is very similar whether you are using a telephoto or a wide angle lens.  However a wide angle lens appears to have much more DOF than the telephoto lens.  This is because the view from the wide angle is much more stretched out over the scene with a more gradual increase in size of the circles of confusion.  The telephoto, which tends to compress the scene, appears to have a much quicker transition in the size of the circles.

balls

 

 

 

A simple way to visualize this is to get 2 objects exactly the same and put one 5 feet in front of you and the other one 15 feet in front of you. When you stand in front of them and look out over them they appear as having quite a distance between them, and the one closest to you appears larger than the one at 15 feet.   This is how your wide angle lens sees this scene.  Now take the same 2 objects and put them out about 100 feet away from you with the same distance between them.  Suddenly it’s hard to tell how far apart they are and they look as though they are right side by side and almost the same size.  This is how your telephoto lens would see the same scene (disregarding the magnification).

 

 

 

 

Wide angle and macro photography

Two areas where you generally want increase in DOF are macro and scenic photography.  This is accomplished by stopping down your lens. Light travels through the lens on less of an angle when stopped down.  This gives an increase in the linear distance that there are smaller circles of confusion and therefore a longer distance that appears sharp in the resulting image.  Sensor size also comes into play here.  The smaller sensor has greater DOF than a full frame sensor.

DOFapeturef16

This image demonstrates the concept of stopping your lens down for increased DOF.

diffraction

You hit the point of diminishing returns at some point in time though by something called diffraction.

Diffraction is basically a spread of light after it exits the sensor side of the aperture. This causes the resulting image to lose sharpness after stopping down beyond a certain F-Stop.  This may vary from lens to lens, but generally stopping down to about F-16 gives acceptable results.

lensbarrel

Old manual lens barrels used to have a DOF scale on them so you could estimate how much of the scene would be in focus.  If you will look at the following image you can see that if you focus on the center line that with increasing F-stop the distance between the points that will appear sharp increases.

dofscale63

These two line scale line drawings (not to proper scale, but strictly to illustrate the concept)  demonstrate the relationship between F stop and DOF at different apertures.

apeture

For example if you chose F 6.3 for your F-stop the resulting image will appear sharp and in focus from about 11 to 30 feet.  If you increase the F- stop to F-16 it will be in focus from about 3 to 60 feet.

dofscale16

This is a very  important concept in macro photography as well where you want both the eyes and tongue of that western diamondback rattlesnake you have your face five inches from in sharp focus so you don’t have to go back and do it again.

Another way to gain more DOF is to take several images with a tripod of the same scene focusing on a slightly different point into the scene on each shot, and then combining the images only using the in focus portion of each image.  This can be done manually in photoshop or there is software that can do this for you automatically.

Telephoto Photography

Let’s go back to the bird sitting on the branch with the pleasing out of focus background.  Remember that the telephoto lens compresses the distance between the circles of confusion.  This is giving you a shallow appearing DOF.  With telephoto lenses you tend to shoot with a wider aperture to get more shutter speed as even the slightest camera/lens motion will blur the image.  This also helps give you a shallower DOF to help blur the background.  You would still stop down enough to keep the bird in focus from beak to tail feathers.  One important thing to watch for as well is how much distance there is to any object behind your subject.  If it is too close it will be more in focus than something that is farther away.  So to get that blown out background look in your images try to get into a position where the background will not have objects that will compete with your subject.  Again, sensor size will also influence how out of focus things appear.  A full frame sensor will have much more of a blurred background than a small sensor.

Hyper focal distance

Most modern cameras have a DOF program that automatically chooses the proper F-stop for a two points of focus you have chosen. I will try to explain how this was done on a manual lens just so you get the concept of what is going on inside your camera.

Going back to the previous DOF scale on the lens barrel you can see that by stopping down a lens you can get an increase in DOF.  Let’ say you are shooting a scene that has a rock at 6 feet in the foreground with a mountain range way off in the distance.  You want to get both the rock and mountains in sharp focus. You focus on some point at about 60 feet away which makes the distant mountains appear in sharp focus.  Then you stop down your lens to F16.  If you look at the barrel of the lens you see by  the aperture numbers on the DOF scale what distance range will potentially be sharp (at F16 this will be from about 3 feet to 60 feet).  However you can see you are “wasting” some of the acceptable sharpness by focussing at 60 feet.  Now your image will be sharp from about 20 feet to well beyond infinity.  This means that rock at 6 feet in the foreground will not be sharp.  In other words  your image has the ability to be sharp out to infinity and beyond.  “But there is no beyond infinity,” you say!   Exactly!  So the idea is to make use of the “wasted” sharp region by bring infinity and your maximun DOF back to line up with each other.

hyperfocal

Setting hyper focal distance is a way of manipulating your focus point to get the maximum use of the DOF range you have to work with at a given F-stop.  To accomplish this instead of using the center focus point to focus where you want focus to be, you would choose the F-stop on the DOF scale as your maximum or minimum point of focus.  If for example you wanted to ensure that the distant mountains were in focus and you were using F-16 you would turn the right hand 16 on the DOF scale to line up with the infinity point on your lens.  This mean that you are going to set the far field maximum focus point at the mountain range and the image will be in sharp focus all the way down to the 16 on the left side of the DOF scale.  This is about 5.5 feet on the following image, which hopefully includes the near objects you want in focus as well.

dofinfinity

If it doesn’t and you are more interested in having the near object in focus than the distant mountain range, you would set the 16 on left hand side of the scale to the distance of the nearest object you want in focus and let the far field be in focus as far as it can for your given aperture.  In the following image for example, you chose 2 feet as the object closest to you that you want to be sharp and in focus.  This would give you an image that stayed in acceptable focus from 2 to 48 feet.   So you can see by manipulating the distance that will be in focus from the DOF scale you can use the full range to your advantage.

dof2ft

If this is still not enough DOF to render all the parts you want be sharp, then your only other options are to stop down more for increased DOF (and suffer with some diffraction) or shoot multiple images with different focal points and combine them.  The later is always tricky if you have any moving objects such as clouds or waves in the scene as each picture will show them in a different position and make them difficult to combine.

So now hopefully you have a bit of an understanding of DOF.  If you camera does not have a DOF setting a way to get reasonable results is to stop down your lens, look through the viewfinder and focus approximately 1/3rd  of the way into the scene from the bottom.  You may have to take several images a bit on either side of that point to make sure you get acceptable results, but it works in a pinch.

I hope these simple instructions have been of help and you get out and use your camera.