Simply put, depth of field is the distance from the front of your image to the back of the image that will be in focus.
Look at the photo of the ant. Notice that the ant is in focus but the near leaf and the background are out of focus. The point of focus is on the ant. This photo has a shallow depth of field. By using shallow depth of field I have made the ant stand out from its surroundings.
If I would have taken this photo with a greater depth of field imagine how distracting the image would be. Instead of the focus being on the ant, the leaves in the foreground and background would distract our eyes from the subject.
This doesn’t mean that you should always use shallow depth of field. Landscape photos strongly benefit from a deep depth of field. In the case of a sprawling landscape you may benefit having everything in focus from near to far.
How much depth of field you use in a photo depends upon what you are trying to convey in the photo. Its as simple as that. The important thing is knowing that you have a choice. You can control your depth of field. Depth of filed is a creative, important tool.
CONTROLING DEPTH OF FIELD – APERTURE
A very good way to control your depth of filed is through aperture. Compare the two photos below. Which has greater depth of field?
1. This image was shot at f/3.5
2. This image was shot at f/16
f/3.5
f/16
It’s obvious; the image shot at f/16 definitely has greater depth of filed.
REMEMBER:
The smaller the aperture the greater the depth of filed. (larger f-number)
The larger the aperture, the shallower the depth of filed. (smaller f-number)
The subjects in the photos above are no more than 2 inches tall and are spaced less than an inch apart. As you can see, even when objects are spaced only an inch apart by changing the aperture we have altered the depth of field.
Depth of field gives you a margin for error when focusing, the smaller the aperture the greater the margin.
Another way to change depth of field is to change the distance on which you focus.
Lets take a look at the two photos of the torches below. Both photos where shot at f/5.6. The photos were shot with a crop camera (Pentax K10D) with a 58mm manual focus Zeiss Biotar f/2 lens attached. The crop factor is 1.5, so I times that by 58mm, which equals 87mm. (this is the actual focal length of this lens on the Pentax digital camera).
I focused on the front torch, notice how the other torches are out of focus.
In the second photo I focused on the center torch.
#1
#2
We can see a huge difference in how much is sharp from photo to photo. The farther away you focus the greater the depth of field.
The second photo shows two torches and the background in focus, while the first photo only has the front torch in focus.
This is why you should take great care when focusing on near objects. Close focusing offers less depth of field; therefore there are fewer margins for error.
So, we now have learned two ways to control depth of field:
1. Use a small aperture
2. Focus on a point farther away, or move the camera farther from the subject.
DEPTH OF FIELD SCALE
A question that may be asked is” How much will be sharp in front of the point of focus and how much will be sharp in back of the point of focus?” It is very possible that the answer is on your lens.
Take out your lens and look at the marking on the barrel. Your lens may have a depth-of-field scale as shown in the picture below. If your lens has these markings then please proceed with the following exercise. If your lens does not have these markings, please read the information provided regardless, as this is important information.
Now lets take a look at the lens below and determine exactly how this scale is set.
In the photo you will notice that the distance scale is set at 3 ft, just shy of 1 meter. The aperture scale is set at f/16.
So what does the depth-of-field scale tell us?
f/16 28mm lens
With this lens set at distance of 3 ft. an aperture setting of f/16, the depth-of –field scale tells us our depth-of-field will run from about 2 ft. to about 6ft.
How do we know this?
Just take a look at the number 16 on both sides of your depth-of-field scale. By matching the depth-of-field scale to your ft. marker you have determined the distance.
So what happens if we set our aperture to f/8 and our distance scale to 3ft.?
Give it a try, just look at the photo and determine the answer.
Look at both 8 markers on the depth-of-field scale.
The answer would be about 2 1/2 ft. to about 4 ft.
So in conclusion you should always remember:
1. The smaller the aperture the greater the depth of field.
2. At any aperture, there is about twice the depth-of-field behind the point of focus as there is in front of it.
Now lets try changing the distance at which our lens is focused. Look at the picture below; you will see the same lens focused at 10 ft. So, what is the depth of field at f/8?
The scale tells us that the depth-of-filed runs from about 6 ft. to infinity.
The lens used for these samples is a 28mm lens. What would happen to the depth of field if we were to use a different focal length?
Lets experiment with a 50mm lens. If you have a 50mm lens take it out. Set the distance scale to 3 ft. and your aperture to f/16. Determine your depth-of-field. It should read about 2-½ ft. to about 4 ft. (Pic below)
Lets compare this reading to the picture of the 28mm lens set to the same distance and aperture. The reading of the 28mm lens gives us a depth-of –field of about 2 ft. to about 6ft. (Pic above marked f/16 28mm)
As you can see we have a greater depth of field with the 28mm lens. Check other f/stops and you will see the same results.
So, we have reached another conclusion:
3. The shorter the focal length of a lens, the greater the depth-of-field at any setting.
The longer the focal length of any lens, the less the depth-of-filed at any setting.
You may ask, “Why is this so important”?
Imagine your looking across a vast filed that stretches to a beautiful mountain scene, your puppy is playing in the field near you and you want to get the entire scene including your puppy into clear focus. You may discover that a 50mm lens may not be the best lens for this shot. As you have seen from the previous experiment, the 50mm lens may not provide enough depth-of-field at its smallest aperture for you to get your near-by puppy and the rest of the scene into clear focus. But a wide angle lens such as the 28mm may provide you with enough depth-of-field to get the job done.
If you wanted to shoot a subject such as a portrait where you isolate the head and shoulders of your model and throw everything else out of focus then you would be better off using a longer lens. It’s narrow depth-of-field can provide “selective focus” that you might not obtain with a shorter lens.
USEFUL TIPS ON DEPTH-OF-FIELD
Sharpness, what does it mean?
What do we mean by reasonably sharp? If we were to set a 50mm lens at a distance of 8ft. and an aperture of f/16, we would have a depth-of-field of about 5ft. to about 18 ft. Does this mean that an object 5 feet away or 18 feet away will be as sharply focused as an object 8 feet away?
No. Sharpness of focus is relative. In the example above objects 8 feet away are more sharply in focus than objects 5 feet or 18 feet away.
Here’s something to remember:
To be in focus, a point in the subject should appear as a point in the print when viewing film. On your monitor if you’re viewing digital images you will be looking at a pixel.
When viewing a film negative under a microscope, each precisely focused point in the negative is a tiny doughnut-shaped circle. The circle is so tiny that to the naked eye it appears to be a point. A digital print will have small square pixels.
A point in the subject that is slightly out of focus will appear under a microscope to be slightly larger. If however, it still looks like a point to the naked eye, we say it is acceptably in focus.
As we move farther and farther from our plane of focus the circles or pixels become progressively bigger and flatter until we no longer see them as points, but as tiny blurred circles or squares. We now say they are out of focus.
Lets go back to the paragraph where we were focusing on an object 8 feet away. Everything that is 8 feet away will from the camera will reproduces as tiny perfect circles or square pixels. We call this distance the plane of focus. What about the points an inch closer or farther away? They will be slightly larger and flatter but will appear as points to the naked eye.
The distance we call depth-of-field is the distance in front and in back of the plane of focus in which all points will reproduce as small enough and round enough, or square enough when referring to pixels, to be perceived as points when viewed by the naked eye.
So depth-of-field is really just a zone of acceptable sharpness.
Keep in mind; acceptable focus depends in part on the size of the print. For enlarged prints and projected slides you need very precise focusing.
ZONE FOCUSING
When shooting moving objects, you pre-set your distance scale and stop down your lens to give you the largest margin of error.
Simply pre-set your distance scale to the distance at which you will want to shoot. Whenever your subject enters these limits you are assured of a reasonably sharp image.
-------------------------------------------------------------------------------------
No comments:
Post a Comment