AG Photo luxury
Luminosity (F-number)
Introduction
We will begin by taking about another characteristic that is fundamental for a lens: its brightness or luminosity. The luminosity “F” of a lens determines the quantity of light, expressed in LUX, which passes through it. The lower the F-value the greater the luminosity will be. The luminosity is in fact a relationship between two variables and the formula that identifies it is: F = FL/d. In this formula, “FL” represents the focal length expressed in millimeters and “d” is the diameter of the maximum opening of the diaphragm expressed in millimeters.
Let us look at an example. If you had a lens with a focal length of 50mm and the maximum diameter of the diaphragm was 11mm, its luminosity would be F/4.5 whereas if the diameter were 25mm, it would have a luminosity of about F/2. From this we can understand that, by having a greater luminosity, we can lower the focal length or increase the diameter of the diaphragm and of the entire lens. By increasing the diameter of the lens, however, we also increase the diameter of the glass and this makes the price of the lens go up enormously. Therefore, at the same brightness, a lens with a short focal length is smaller. If the luminosity is weaker, we will have a lens that is smaller and cheaper. The luminosity is generally indicated by a number preceded by an F/ (among the characteristics of a lens). In the case of the zoom lens, we can find either one or two numbers after the F/. I give you another example: a zoom lens with a focal length of 18-70mm could have an F-number of 4-6.5 (F/4-6.5).
When two numbers appear after the F/, it means that the brightness varies as we vary the focal length. Therefore, using a focal length of 18mm corresponds to the brightness of F/4 while a 70mm focal length gives us a brightness of F/6.5. In the middle, all the other luminosity values are possible. But the same lens could show the caption F/4, and it would mean that it has a constant brightness along the entire focal range equal to 4. A minor difference in this number can greatly change the price of the lens.
01 In this figure I tried to represent graphically the formula F = LF/d where the circles represent the diaphragm in its maximum opening. In this figure we can see that "d" is contained six times in focal length and therefore we would have F=6.
Focal length
Maximum diameter of the diaphragm
In practice
The luminosity of a lens is very important, because it determines the quantity of light that it is able to capture. Translated into practical terms, if we want to take a photograph at night, the problem that we face is since it is dark, we have to increase the exposure times (during the day this would not be necessary) to impress the sensor sufficiently and not have photos that are too dark. We need a certain amount time for the light to hit the sensor in sufficient quantity and need to keep the camera sufficiently still; otherwise this will cause the photo to become blurry even if the subject is stationary.
The person becomes an important factor, because involuntary movement of our hand and body produce a noticeable movement in the span of time equal to 1/13 of a second or more (with the stabilizer active). I wrote 1/13th of a second because it is the limit of my own hand, which (when the time is equal to or greater), the photo will be blurry. I know people with still hands that have this limit adjusted down to 1/8 of a second, but very often the photo seems beautiful until they discover that there is some micro-blurriness present. By micro-blurriness I mean an effect that is noticeable on smaller details. Having a lens with a greater luminosity means being able to work with lower exposure times by having the same amount of light enter. If I have a lens with a luminosity of F/4, and I find myself taking night photographs, it will happen that I will have to use an exposure time of 1/8 or also 1/6 of a second. In that case, I will be forced to use a tripod that will immobilize the camera and also when it would be necessary greater exposure times.
If in the same conditions the lens had a luminosity of F/2.8, I could create the same photo with exposure times equal to 1/13 of a second, for example. With this shutter speed I know that my hand does not produce movement, and I would not need a tripod. Below you can see a practical example of what has been stated above.
Introduction
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How it is made a Reflex camera
Photographic lenses
Equipment and accessories
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Buyer’s guide
02 Non-luminous lens, freehand, exposure time A.
03 Non-luminous lens, freehand, exposure time >A.
04 Bright lens, freehand, exposure time A.
05 Tripod, exposure time >>>A.
In figure 02, we can see an example of a photo shot freehand with a less luminous lens but with an exposure time that helped me to avoid not ending up with a blurry photo. The photo is sharp but very dark. In figure 03, instead, we used a longer exposure time (even if only slightly), and the resulting photo is brighter but visibly blurry. In figure 04, we can see the same photo shot with a bright lens, using the same exposure time of figure 02. The photo is not blurry anymore and is also bright.
Now you can understand how important the luminosity of a lens is even if this is not the only important aspect.
Below, in figure 05, I show the same photo shot with a tripod and with impossible exposure times with the human hand.