Once in a while we get a picture that comes out blurry.
First, we must define a picture that is “not sharp”. In fact every picture is not sharp enough, for if we enlarge it again and again we will finally reach its silver halide components (the film) or the pixels that construct it (digital). So, what’s the definition of “sharp”?
In the early days of photography, all the wise men sat in council trying to determine the definition of “sharpness”. I will not discuss the definitions of the Airy Disk here and its effect on visual sharpness, and the MTF graph, and the contrast of optical elements, and the interaction with film or the digital sensor and so on – I will only talk here about the concept of sharpness.
No doubt it is quite a subjective term, which may be quantified to numbers with some effort. On average, the human eye can separate between two dots – a black one and a white one, which are distanced one minute of an arc away from each other.
One minute is 1/60 of a degree, and a complete circle has 360 degrees. This brings us to an eyesight of 6x6 – the ability to resolve a distance of 6mm from a distance of 6 meters.
This is of course an approximated mean, and one can find people that are able to resolve twice this density.
This information was put together by the group of people that sat in council in the early days of the twentieth century to define sharpness, in the following ambiguous term – if one enlarges a picture to the size of 8x10” (20x25cm), and observes it from a distance of about 25cm and it would appear sharp – it can be said that this picture is sharp.
Surprisingly enough, this index is quite similar to the resolution capability of our eye.
There may be several reasons for obtaining a picture that is not sharp:
1. The subject was not in focus and the part that we wanted to appear sharp did not come out sharp. On the other hand, other elements in the picture did come out sharp, and this will be the subject discussed in this article.
2. We did not use a good enough lens – and the optics failed us, as seen in the results.
3. We spoiled the quality of the optics by mounting a teleconverter/filter of low quality on a lens that is “not fond” of teleconverters.
4. The speed of exposure was too low, the camera moved and the picture came out blurry.
We shall now examine these causes one-by-one and see how we can cope with them.
1. The subject was out of focus – this is very common, due to a user’s mistake, or an error in the automatic focusing system.
A user’s mistake – we focused the camera manually to a point that is not our subject, perhaps because we could not precisely determine what is sharp and what is not. Sometimes this happens when there is insufficient lighting, or we are in some kind of hurry, or we just missed it due to a lack of concentration.
An error of the auto focusing system is more common nowadays. We tend to rely on the auto-focusing systems, and though they are usually precise, they too miss the main point from time to time.
How do the auto-focusing systems work? Schematically speaking, there is a system that on the one hand can analyze the information coming to it and on the other hand can activate the lens focusing mechanism.
The system that analyzes the data at the point we aimed (we aimed it!), detects the maximum contrast at this point – the greatest difference between dark and light.
When aiming the camera to a white wall, very often it gets completely mixed up.
The automatic focusing system has nothing to hang onto because everything is white. This of course is an extreme example but represents the “stupidity” of the auto-focus systems prevalent in most of our cameras.
Now, let us imagine that we want to take a picture of someone standing in front of a background of woods or trees. The trees present a better contrast for the auto-focusing system, and if we are not precise, the camera will “escape” to the tree background and our poor subject will come out all blurry.
Luckily for us, the depth-of-field formulas are included in the equation. As we all know, the depth of field is the range in which everything in front of it and everything behind it comes out sharp.
The more closed the aperture, the larger the depth of field. The equation of the depth of field includes the focal length of the lens – the wider the angle of the lens, the larger the depth of field. The third important parameter for the depth of field is the distance from which we took the picture of the subject.
It is most likely that if we take a picture of someone at a distance of 10 meters with a wide-angle lens – 24mm with, let’s say, aperture 8, it will appear sharp even if the focus “escapes” to the woods behind.
We can also suppose that if we shoot the same subject from the same distance with a 400mm lens and aperture 4 we will get a blurry subject if the auto-focus “escapes” to the woods.
What do we have to do then to keep the subject sharp?
First, we must make sure that our camera knows to hit the correct spot in auto-focus.
How can we check this? By placing a ruler on the floor and mounting the camera on the tripod so that an angle of 45 degrees is created between the camera and the ruler.
We then focus to a certain line on the ruler and check if the camera hits the correct line. The distance of shooting should be 50-100cm, with a lens that can focus at these distances.
Presuming the system (camera + lens) hits the correct spot, we will search for a contrasting element on the shooting plane, and focus the camera to that point.
We will lock the focus (most DSLR cameras include this option), recompose without changing the distance from the shooting plane, and take the picture. It takes some practice to get it right.
If we just remember that the auto-focusing system likes to lock onto a contrasting subject – this by itself will significantly improve our performance.
Nevertheless, if the camera insists on running away from the subject – we must focus manually.
We can also carry with us the hyperfocal distance table and use it. A hyperfocal distance is the distance of which if we point the camera towards it, any subject between infinity and half of this distance will come out sharp enough. Sharp enough means that the circle of confusion (blurriness) on the film or the sensor will not be larger than about 25 microns (25 thousandths of a millimeter).
The distance depends upon the focal length and the aperture.
For example, if we found that the hyperfocal length is 40 meters from the photographing point, then any subject positioned within the range between infinity and 20 meters, will come out sharp. For example, a 50mm lens with aperture 2 – its hyperfocal distance is 50 meters, meaning – anything within 25 meters and infinity will come out sharp.
Generally speaking if we sufficiently close the aperture, we can obtain a sharp background and subject (if we want to); otherwise, we should use one of the rules above.
Finally, I will present some graphs and a picture that will demonstrate all of the above.
How does the depth of field change in relation to the shooting distance?
It is easy to see that the further the subject is away, the larger the depth of field (while keeping a constant focal length and constant aperture):
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| Color Space: 65535 |
How does the depth of field change as a function of the focal length?
The longer the focal length (more of a “tele”), the shorter the depth of field (while keeping a constant aperture and shooting distance):
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| Color Space: 65535 |
