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mangrove 发表于 2012-3-9 09:36 
问题是，结合上面的图：
注意表格下面的小字，容许弥散圆大小和传感器和最终印张大小和观察距离都有关系。35mm放大到7寸，放大约5倍，观察距离20-30厘米，也就是视角45度左右，以1/1000算，就是0.045度，约为人眼的极限。如果观察距离增加，那么容许弥散圆就可以大一些。这就是大幅宣传海报远看是高清，近看就可以看到像素。
所以同意60楼所说。
具体可看Langford's Basic Photography 9th Ed, p55-56. 摘录如下：
How depth of field works
To understand why the aperture affects depth of field, we need to look again at how a lens
focuses an image point at one distance only, depending on how far the lens is from the
subject. Other parts of the subject nearer or farther from the lens come to focus farther
away or nearer, forming discs instead of points of light. They are known as circles of confusion.
Large circles of confusion, overlapping (Figure 2.17) give a blurred image. However, provided
the circles are relatively small they can appear sharp, since our eyes have limited resolving
power.
When viewing a final print you rate an image acceptably sharp even when tiny discs
are present instead of dots. The upper limit to what most people accept as sharp is taken
to be 0.25 mm diameter on the final print. (The same applies to the dot pitch on a computer
screen.) Lens manufacturers for 35 mm format cameras assume that if 25 × 20 cm (10 × 8 inch)
enlargements are made (film image magnified ×8) to this standard then the largest acceptable
circle of confusion on film is 0.25 divided by 8 = 0.03 mm.
By accepting discs up to this size as sharp, subjects slightly nearer and farther away than
the subject actually in focus start to look in focus too. And if the lens aperture is made smaller
all the cones of light become narrower, so that images of subjects even nearer and farther are
brought into the zone of acceptable sharp focus. Depth of field has increased.
Again if you move farther back from the subject or change to a shorter focal length lens, the
positions of sharp focus for images of nearest and farthest subject parts bunch closer together.
Their circles of confusion become smaller, again improving depth of field.
Remember that you produce greatest depth of field when:
● f-number is high (the lens is stopped down)
● subject is distant
● focal length is short.
With subjects beyond about ten focal lengths from the lens, depth of field extends farther
behind the subject than towards the lens. Hence the photographer’s saying: ‘focus one third in’,
meaning focus on part of the scene one third inside the depth of field required. With close-up
work, however, depth of field extends more equally before and behind the focused subject
distance. |
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楼主:jasonliu234
发表于 2012-3-10 20:13
