The desire to get to the photos are not all equally dramatic, of course, should be considered as quite natural. First, because the human eye. Secondly, it gives snapshot additional artistic expression.
This article shall dwell on physical laws describing the background blur photographic lenses. I must say, it will focus only on the degree of blur. Subtle aspects of drawing and plastic optical systems are not considered here.
You first see this formula? Do not know what the range of blur? Got more questions? In this case, refer to the literature. In extreme cases, you can read my other text, entitled “Depth of field. Famous and not so “.
Express the diameter of the circle blur terms of the other quantities:
c = (M2 f L) / (N (f + L M))
Now, this formula shows, in a spot of diameter will be transformed into a point located behind the object and located him at a distance of L. Formula approximate and is valid under the same assumptions as the original.
After some transformations, it can lead to mind:
c = c ‘(1 / (1 + (f / (ML )))), where c’ = (Mf) / N
or
c = c ‘(1 / (1 + (d / L)))
What conclusions can be drawn from this formula? Most significant, in my opinion, are as follows:
1. The first conclusion fairly trivial: the more distant background of the object, the more it is diffused. However, this does not mean that the infinitely distant background is blurred forever. Limit the blurring does not exceed c ‘= (Mf) / N.
In other words, the quantity c ’shows a spot with a diameter would be blurred on the image point at infinity. All that is closer to be blurred to a lesser extent.
When it comes to shooting in one scale, then the limit blurring is directly proportional to the focal length lens. If you are shooting with one and the same distance d and at different scales, then c ‘will be the square of the focal length.
2. In any case, the degree of background blur is inversely proportional to the aperture value. That is, changing the aperture by one step, we change the degree of blurring in 1,4 times.
3. Suppose there are two lenses with different focal lengths. What aperture should be put on them that they are equally blurred background, separated from the object at a distance L?
In general the formula corresponding to this question is as follows:
N2 / N1 = (f2 / f1) ((f1 + M L) / (f2 + M L)).
For an infinitely distant background is somewhat simplified formula:
N2 / N1 = (f2 / f1).
Consider a few examples.
EXAMPLE 1. Shooting is a scale 0,05. What aperture should be set at 135 mm lens that he blurs the infinitely distant background as well as the lens 50 mm aperture, 4? Answer: 11 (10.8).
EXAMPLE 2. Suppose that under the same conditions it comes to the ground, remote from the object at 1 pm Then, for the sameness of blurring the background necessary to put on the lens 135 mm. aperture 5.6 (5.8).
4. Consider a hypothetical portrait photography with M = 0,05. In the first case we use the lens 135 mm in the exposed diaphragm 8. In the second case we use the lens of 50 mm for N = 4,5. First of all, we note that under these conditions, depth of field will be for the first case twice. But despite this great depth of field, lens 135 mm still infinitely distant blur the background more (c ‘will be 0.84 mm against 0.56 mm for lens 50 mm). It is the predictable result.
But when it comes to the degree of background blur, removed at 1 m, the result may seem at first sight paradoxical. Short-focus lens blur nearby background is stronger. The diameter of the circle blur amount to 0,28 mm for the focal length of 50 mm and 0,23 mm – 135 mm.
5. What kind of background can be considered infinitely remote, that to assess the spreading use the simplified formula?
From the basic formula is clear that when L = 9 d the degree of background blur amount to 90% of the maximum value determined by the value of c ‘. Please note that we have received score depends only on the distance of focus and does not depend on aperture or the focal length. In other words, for any lens Infinity begins with a distance of 10 d, if the origin to the camera.
Of course, there are no hard and fast rules on this account does not exist. Everyone can create their own criteria on the basis of the above formula.
But the main conclusion, of course, is: more need to remove and gain practical experience. The vast majority of photographers never even heard of such formulas, not to mention the fact that they use. That does not prevent them to create beautiful photographs.
If the dear reader a taste of what to confide harmony with algebra, the conclusion of the proposed use a simple calculator program that is created on the basis of the file MS Excel. Entering the initial data, we can easily obtain the results on the calculation of the depth of field and blur the background. If desired, anyone can create their own version of the calculator.