Depth of Field

Depth of field seems to be an area that causes a large amount of confusion for photographers. Understanding and properly using depth of field is a very important aspect in photography. It is a primary method of providing separation of the subject in order to focus attention and minimize distractions.

First, lets start with a couple basic terms:

Depth of Field (DOF)- This is the area in an image that is in acceptably sharp focus from front to back.
Less DOF= shallow DOF= less in focus.

Bokeh- Bokeh is NOT depth of field. Bokeh is the character of the part of an image that is not in focus. I.e. is that out of focus spot circular in shape and soft edged, or is in octagonal in shape and hard edged? In general, the more out of focus an area is the "smoother" the bokeh will be. None of this discussion is directly about "bokeh."


Now we'll address controlling DOF and Blur:

Blur- or "how out of focus something is" is controlled by several factors.
First, does the object fall within the DOF or not. It must be beyond the DOF in order to be blurred. And the further out of the DOF it is, the blurrier it will be.

DOF and Blur are both primarily affected by aperture and subject distance. And both of those are controlled by the lens selection for a given image. 
Aperture- DOF is reduced by using a larger aperture (smaller number).
Subject distance- DOF is reduced by working closer to the subject (less subject distance)

For these two reasons many make the "mistake" of choosing "normal lenses" with wide apertures; so they can work from closer distances with a very large aperture. This can be a "mistake" because a lens is never at it's sharpest at it's largest aperture and there are things not being taken into consideration. And a normal lens is, well "normal". And that can be boring.

If you use a lens with twice the focal length it will have half the field of view and require twice the working distance to capture "the same image". If you use a 50mm lens at f/4 from 10 ft it will capture "the same image" as using a 100mm lens at f/4 from 20ft, and the DOF will be very close to the same.

BUT, that doesn't tell the whole story. 
Remember that the aperture number is the ratio of the aperture diameter to the focal length. At f/4 the physical size of the aperture is 1/4 the focal length (f-stop x aperture diameter= focal length). A 50mm lens at f/4 the aperture is 12.5mm in diameter. An 100mm lens at f/4 the aperture is 25mm in diameter. Because the aperture is physically twice the diameter the out of focus areas will be more blurry for the 100mm lens, even though they are both at f/4! And the out of focus areas are twice as large. It's like zooming in on a blurry section of an image.

And there is more! If you noticed I said they would capture "the same image", and it was in quotes. The reason it is in quotes is because there are significant differences. The "image" will be the same at the subject because the field of view at that point is the same, but the field of view is NOT the same at the background. The longer lens will also capture less of the background which helps eliminate distractions by not including them in the first place.

I created this diagram to illustrate.

Because there will be less background included in the image, and what is included will be twice as blurred, the longer lens may be the better choice. In fact, you can use the longer lens at a smaller aperture, with a slightly deeper DOF for better subject sharpness, and still have the background just as blurred as using the shorter lens at a wider aperture!

Some examples: All taken at f/2.8 with a Nikon D7000 and the Nikon 24-70mm f/2.8 or the Nikon 70-200mm f/2.8 (I don't own a "nifty fifty"). The distance was set with a tape measure.

30mm at 3ft ("normal" focal length for crop body), 
Nikon 24-70mm f/2.8

60mm at 6ft, 
Nikon 24-70mm f/2.8

90mm at 10ft, Nikon 70-200mm f/2.8 (10ft was as close as I could focus so this image actually has slightly greater DOF)

120mm at 12ft, Nikon 70-200mm f/2.8

All 4 images have the woman's face approximately the same size and have approximately the same DOF (exception being the 90mm image). But you can clearly see that the longer lens has a narrower FOV beyond the subjects and creates more background blur.

Now a couple of examples of working distance changing the DOF. Both with the D7000 and lenses at f/2.8.

60mm at 3ft, Nikon 24-70mm f/2.8

200mm at 9ft, Nikon 70-200mm f/2.8

I wasn't as critical with keeping the woman's face exactly the same size, but they are close. You can clearly see the combined effects of a longer focal length and closer working distance in the last image. In fact, the woman is a bit smaller in the 200mm image than in the 60mm image. But there is much less  background and it is further out of focus. The DOF is very close for these two shots.

And now varying apertures with a longer lens:

200mm, 12ft, f/2.8

200mm, 12ft, f/4

200mm, 12ft, f/6.3

200mm, 12ft, f/8

With the longer focal length I can use a smaller aperture for a sharper subject and a bit more DOF without loosing much compared to a wider lens used at a much wider aperture. I would consider the 200mm f/6.3 image directly comparable to the 200mm 9ft f/2.8 image based upon composition and background blur. But the 200mm f/6.3 image is at a much better aperture (hard to see at these sizes and less notable on these lenses) . It also has a better DOF for the husband. The relatively small increase in distance hasn't significantly affected the composition but it has inreased the DOF as has the smaller aperture.


Depth of Field:

Part One-Using two focal lengths and the same aperture take "the same picture" from two distances. If the longer focal length lens is twice as long (i.e. 100mm vs 50mm) the working distance should be twice as far. Note how the longer lens includes less background and makes the  area that is out of focus more blurry.

Part Two- Using a single focal length vary the aperture to see the changes in DOF. You might be able to just change the aperture and use the DOF preview button on your camera. Personally I find it pretty useless so I would suggest actualy taking the pictures and reviewing them on the computer.

constant aperture zoom like the 70-200mm f/2.8 is ideal for this assignment. Another choice would be two prime lenses. 
Your kit lens zoom like the Nikon 18-105mm or Canon 18-135mm may work *if* you use the longer focal lengths with the subject inside the hyperfocal distance and the background well behind the subject. You will need to use the smaller aperture the lens defaults to at the longer setting for both shots, this will result in a deeper DOF.  

The depth of field needs to place the background out of focus. You will probably want to review the images on the computer in order to clearly see the differences. 

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