Let's start with a little experiment: try taking a drawing sheet and a paper handkerchief, a felt-tip pen with a very big tip and one with a very thin tip, no matter the color.
Now do these little "tests":
First experiment: take the bigger felt-tip pen and lay it for 2 seconds first on the drawing sheet and later on the handkerchief. On which paper is the bigger mark? obviously on the handkerchief that absorbs more ink than the normal paper.
Second experiment: On the same sheet (the drawing one) lay the both felt-tip pens for two seconds. Which one leaves the bigger mark? Obviously the bigger one.
Last experiment: take the handkerchief and the bigger felt-tip pen; lay it for 5 seconds and then lay it again for a brief second fraction. Which one is the biggest? The one made laying the pen on the paper for 5 seconds.
Summing up, if you want to get a big mark you have to use a more absorbing paper, a pen with a bigger tip and you have to lay it for a longer time. Vice versa, if you want a smaller mark you have to use a less absorbing paper a pen with a thinner tip and lay it for a shorter time.
Now presume for the ink to be light. A photo it's made of light, setting the camera we decide how much light will hit the sensor or the film. What we called "mark" before is, in truth the picture that will result from the shot. Let's follow the tests made before in the same order:
- ISO: the ISO indicate how sensitive to light is the sensor or the film. The lower the ISO, the minor the sensitivity (like the drawing sheet), while the higher the number the more sensitive to light will be (like the handkerchief). Then, using low ISO we'll have smaller marks (less light will expose the frame), using high ISO we'll have bigger marks (more light shall be recorded on the frame).
- APERTURES: the aperture is the set of sheets that you can find behind the lens. Try to remove the lens and watch through the glass, on the bottom you'll find a lever, try moving it while watching and you'll see the sheets opening and closing. This is the aperture. The more open the sheets the more light will pass through (bigger tip pen) the closer the sheets the less light will pass (thin tip pen). Then, to follow our previous example, to make more light pass and obtain a bigger mark we have to open the aperture, to have a smaller mark we'll have to close it. How? It's all depending on our camera. The old cameras make you open and close directly from the lens, the modern cameras make you open it from themselves. The values start from small numbers, almost always with comma, to higher numbers. Smaller numbers open the aperture, bigger numbers close it: an aperture at 2,8 will be more open (big tip, more light) and one at 22 will be closer (small tip, less light).
- TIMES: In this case the example is pretty similar to the test, when speaking of times we mean for how much time we are letting the light enter the camera, for how much time we are letting it hit the sensor or the film. The longer it is the more the light will be recorded on the picture (bigger mark). It’s measured in seconds, and usually a pose of one second is pretty long. For a picture in normal lighting conditions the times are definitely shorter, about fractions of a second. For example a time of 1/125 means that we've set a shutter speed of 1/125 of a second. On the time ferrule on your camera you may find 60, 125, 250, 500, they aren't times in seconds, but in fractions of a second (then 1/60, 1/125, 1/250...). Summarizing to make more light enter we should use more sensitive films or in case of digital photography raise the ISO, open the aperture (low numbers) and set longer times, vice versa if you want to let less light enter.
This part was written by Diagaz.
Let's examine more in details the concept just shown.
Depth of field and aperture
While focusing a scene only a single level will really be sharp, it’s an area of a pretty variable size, called depth of field; simply called DoF it’s the space that appears sharp ahead and behind our subject. But pay attention, remember that actually only a plane is really on focus!
The DoF depends on three factors:
- Opening of the aperture.
- Focal length
- Distance between the subject and the lens.
To closer apertures corresponds a higher depth of field, while for wider apertures as f1.4 (or also f2.8) corresponds a lower depth of field, the wider the aperture the bigger the light beam that draws the image on the sensor, vice versa, the closer the aperture the thinner, and then more accurate, the beam that will draw our image, that is why we'll have a higher sharpness in front and behind our subject. Attention, with too closed apertures you may run into a particular phenomenon, the diffraction.
By setting the camera on manual or in aperture priority you have the chance to raise or lower the DoF. If, for example, we set f22 we'll have a considerable DoF.
If, for example, we set f22 we'll have a considerable DoF. we'll have substantially a bigger portion of the sharp area in front and behind the focusing plane, such opening is used generally for landscape photography.
By letting the aperture unchanged and keeping the same camera-subject distance and raising the focal length, the DoF decrease, while the higher the distance between the lens and the subject, the wider the DoF, vice versa, the closer you get to the subject, the least DoF you'll obtain.
That's why for landscape is better to use wide-angle lenses, so to have everything on focus (but also to catch a bigger portion of landscape) and why telephoto lenses are used for portraits to easily blur the background. Many people assert that the DoF is higher in digital compared to film, this is partially true and it’s applied only for cameras with sensors smaller than the full frame.
With a reflex provided with an APS-C sensor, smaller than a full frame, is substantially performed a cut-out of the center of the picture, with the result that the focal will appear more marked, here comes into play a multiplier factor of the focal length that for an APS-C is about of 1,5, for example a 35mm for the APS-C will give a frame equal to a 80mm full frame, the DoF then will remain the same because tied only to the aperture and the lens focal length.
Here there are some shots made using different apertures and focal lengths to make you understand how it varies the DoF (the pictures in this example have been
realized by Davidd):
As you have just seen in the example pictures, to move the subject away from the background it’s better to use a wider aperture.
One of the question that I'm being asked more frequently, especially by new amateur photographers, is which is the better aperture, the answer is simple, it all depends on the result that you want to obtain, for the street photography I use medium apertures that should provide me with a higher image quality, that's because with medium apertures as f11 you employ just the central part of the lens that is the one with the higher quality and avoid the risk of running into diffraction.
SHUTTER TIMES
They are another parameter with which we can dose the light, thanks to the shutter we can decide the time of the exposure; with an equal aperture, a slower time will let more light pass through than a faster one, shutter speed and aperture opening regulate the exact amount of light that reaches the sensor.
These times are indicated in second fractions, here I’ll show you some:
1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - 1/1000 - 1/2000 - 1/4000 - 1/8000
Other than these the cameras allow to use intermediate fractions, se we may have a time of 1/200 and one of 1/800.
Every SLR allows to use the B (bulb) pose that allows very long times (seconds, minutes, hours), the B pose is useful in those photographic genres as the night photography. When you set the B pose the shutter is opened by pressing the shutter button and is closed when is the button is released, it exist also the T pose that works in a similar way, by pressing the shutter release the exposure starts and pressing it again it ends, in both cases you risk to move the camera, that's why it's better to use the remote control or the self-timer.
The shutter times are divided in slow and fast; we may consider slow those times equal or longer than 1/60, we consider fast those shorter than 1/60, it all depends by the focal length used, by the speed of the photographed subject and the distance from it, with a telephoto 1/60 is generally slow and we'll have blurred pictures if the subject is moving, while with an ultra-wide-angle 1/60 may work well to freeze the movement.
If we have to take a picture of a static subject and we can't raise the ISO or change the aperture opening, the only way is to lengthen times avoiding the blurred caused by the vibrations of our hand by using a tripod.
The right amount of light is given by a certain combination of time and aperture, corresponding to a certain shutter time. The EV exposition values indicate the lighting of the framed subject through a scale of numbers, the table above shows the combinations time-aperture and the corresponding EV.
Let's see more in details what is EV.
EV means exposition value, it’s related to lighting of the scene and the sensor's ISO sensitivity, in short the EV may be considered the unit of measurement of light.
When the light of an EV varies, to compensate you have to change your shutter time by one or by one stop on the aperture, then if the light increase of a single EV it means that, to reach the correct lighting intensity on the sensor you have to close the aperture of one stop or set a faster shutter time. But the time and the aperture don't change just depending on the light intensity but also depending on the ISO sensitivity you set.
The EV value is related to a sensitivity of 100 ISO. When the sensitivity varies you have to compensate this variation with a different combination time/aperture. For example, passing from 100 ISO to 200 ISO I'll have to halve the shutter time or the aperture opening.
To calculate EV, times and apertures, times ago was used the table you saw before, now you’ll find a built-in light meter in all the new generation cameras.
Aperture and shutter time both influence the picture in two different ways.
The pair time - aperture
1st) By modifying the amount of light that reaches the sensor the aperture changes the intensity, the shutter changes the time.
2nd) Each give to the image a different effect, the aperture modifies the DoF, while with the shutter time, when the subject is moving, according on the time set, we can freeze the movement (using fast times) or create a blurred effect (with slow times).
To clearly record the image, the sensor has to receive the right amount of light. There are time/aperture combinations for which using a short shutter time with an open aperture or a long time with a closer aperture is not influential, at least for the quantity of light that reaches the sensor.
The image-graphic above shows the ratios of doubling and halving between apertures and times, using these values you can modify the effects on the image letting always enter the same amount of light.
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| Among thees pictures there are just small variations regarding the lighting (cause of the sudden change in the natural lighting and other factors related to the camera), the only clear changes are in the DoF and in the blurred effect. |
The ISO sensitivity
When, cause of the poor lighting, it's not possible to change the shutter time or the aperture over a certain limit it comes to assist us the adjustment of the ISO sensitivity.
The photosensitivity of a sensor is defined by the ISO standard (International Standard Organization), for the film it was called ASA, and to vary it you had to change the film itself, while in digital photography you can adjust the photosensitivity to your liking, the more the technology develops, the more we can increase the ISO sensitivity without suffering the electrical noise.
For every doubling of the ISO the photosensitivity of the sensor doubles too (and vice versa by halving the ISO we halve the photosensitivity). For example at 200 ISO we can shoot with a shutter time or an aperture halved compared to those used to take the same picture at 100 ISO.
Practical example - ISO test 1
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| ISO 200 f/11 1/60sec. |
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| ISO 800 f/11 1/60sec. |
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| ISO 3200 f/11 1/60sec. |
Practical example - ISO test 2
In the first test you can notice how by changing the ISO values and keeping unchanged times and apertures the lighting in the framed scene increases. In the second test I’ll try to make you understand what’s the effect of increasing the ISO, that is the arrival of the electrical noise and the lowering of detail, the pictures in the second test were taken with the compact Canon G12.
Then which ISO values should we set?
In conditions of optimal lighting (both in exterior or in studio) it's better to leave the ISO value at the standard that is usually 100 or 200 depending on the digital camera.
Try to avoid raising the ISO and to find an acceptable solution regulating the aperture and the shutter time, if it's not possible ‘cause of a poor lighting requiring too long shutter times, then raise the sensitivity of the bare essential to obtain fast shutter times and not to wide apertures.
I'm telling you this because by raising the ISO will increase the electronic noise of the digital signal that causes the effect of digital grain and chromo noise, that are the red and blue dots, the image will be consequently less sharp. In some photographic genres, that require a high detail of the image such as the fashion photography, the ISO should never be raised.
Sometimes we may use this grain effect caused by the noise at our advantage in post-production, for example in Black and White can add something to our picture.
Warning: some cameras with a standard sensitivity of 200 ISO can shoot at 100 ISO in extended mode. At 200 ISO the sensor can work at its optimal condition, giving its best performances, the chance to lower the ISO to 100 is just an option useful to shoot in situation of high lighting conditions, when we don't want to change our time/aperture combination.
This sensitivity lowering is obtained via software, lowering the "lighting" of the image, basically it’s the same when recovering overexposed pictures with raw camera. This entails less dynamic gamma and a lower color correspondence compared to a picture correctly exposed.
Translation by
Nina Kozul
