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Stops

 

 

Shutter speed: 1/125 - 1/90 - 1/60 - 1/45 - 1/30

Aperture: f/8 - f/9.5 - f/11 - f/13 - f/16

ISO: 200 - 280 - 400 - 560 - 800

 

Take picture!

 

A "stop" in photography is used to describe a relative difference in the brightness of light. Doubling the amount of light means a stop brighter, half the amount of light means a stop darker. When refering to altering the aperture, the terms "stopping down" and "opening up" are often used, where stopping down means a smaller aperture (higher f/number) and opening up means a larger aperture (smaller f/number).

 

• For the shutter speed, every stop represents a factor 2. An exposure time of 2 seconds gives double the amount of light compared to an exposure time of 1 second.

• For the aperture, every stop represents a factor 1.4 (= √2) and, as a result, a factor of 2 means a difference of 2 stops. The amount of light which passes through an aperture is directly related to its area, which in turn is related to the square of the diameter. So by going from f/8 to f/5.6, you increase the exposure by 1 stop. Going from f/8 to f/4 increases the exposure by 2 stops.

• For the ISO value, every stop represents a factor 2. An ISO value of 200 is twice as sensitive as an ISO value of 100.*

 

I've tried to clarify it with the example on the right. In reality this picture was taken with completely different settings, but let's suppose that it was taken at f/11, 1/60 and ISO 400. By changing the settings you can see what that does to the exposure, and by clicking on "Take picture!" you can see what the picture would look like with those settings.

 

Now, in reality it's not that easy, because we can't just change these settings without interfering with other factors. Changing the shutter speed can result in moving parts in the picture, or the shutter speed may become too long to be able to take the picture handheld. Changing the aperture changes the depth of field, and changing the ISO has an impact on both noise and dynamic range! All these things have to be taken into account when choosing the right settings, so there are lots of things to think about....

 

* Strictly speaking, ISO values are not part of the exposure since it has no influence on the light which is captured by the sensor. When keeping the exposure constant (the shutter speed and aperture settings), changing the ISO will only change the brightness of the resulting photo, but this is done electronically after the picture has been captured by the sensor.

 

Shutter

 

The shutter is the most important way to adjust the exposure over a wide range. The principle behind most shutters is a pair of curtains blocking the sensor from incoming light as long as they are closed. When pressing the button the first curtain will open, then after a while, the second curtain will close the shutter again. For long exposures, the time which it takes the curtains to cross the frame is irrelevant, but for shorter exposures it will matter. There are three different situations, depending on the chosen shutter speed. So imagine a hypothetical shutter where the curtains need one second to cross the entire frame, then the three different scenarios will be like this (in real life, most shutters only need less then about 1/250th of a second to do this):

I) Shutter speeds larger then one second: If we take a shutter speed of three seconds, then it will take one second for the first curtain to reach the other side. Then, the shutter will remain open for two seconds, after which the second curtain will start to close.

II) Shutter speed of one second: In this case, the second curtain starts closing as soon as the first curtain has reached the other side.

III) Shutter speeds shorter than one second: In the case of for example a shutter speed of 1/5th of a second, the first curtain will open, and 1/5th of a second later, the second will already start closing.

I

II

III

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A curiosity with especially the last case is that, when photographing a really fast moving object (moving in horizontal direction), it will be at a different place in the beginning when the first part of the sensor is exposed, than when the last part of the sensor is exposed. On the right is an example of that, taken at 1/320 while moving the camera from left to right, which means that the curtains went from the bottom to the top of the picture. While the horizontal lines are still perfectly horizontal, the vertical lines are clearly at an angle.

 

Another result of this is that a flash cannot be used when the shutter speed is shorter than the curtain speed. A burst from a flash is many times shorter than the curtain speed, which means that when the flash fires, only the part exposed to the sensor at that moment will be bright, while the rest will be dark. This shutter speed is the so called "flash sync speed" and is about 1/250 for many cameras.

 

 

 

 

However, on modern cameras there is a function called "high speed flash sync" which enables pictures to be taken with shutter speeds shorter than the flash sync speed while using a flash. The principle is that the flash will not give one single burst, but it will give several bursts over a time equal to the chosen shutter speed, which gives equal brightness across the whole picture.

 

Apart from just using the shutter speed for adjusting the exposure, it is also very useful to create effects caused by long exposures. For example, a stream of water will be captured "motionless" with a short shutter speed, but a long shutter speed will blur the water, creating "movement" in the water, like the examples on the left.

 

 

 

 

Tripod

 

For me, a tripod is without doubt one of the most important pieces of equipment as it enables the use of the full range of shutter speeds without any limitation (which is why I put it directly after the part about the shutter). Nowadays, I almost feel naked when I am not carrying my tripod with me, even though it means carrying a couple of extra kilograms on my shoulders....

 

Unfortunately, not any tripod will do, just take a look at the picture on the right, which was taken with a tripod that I bought while having a very limited budget. Watching the lunar eclipse was really nice, but getting the slides back from the lab was a huge disappointment and I found out once again why it was necessary to buy a good tripod. And this picture is actually the best one from all the pictures I took that night, so it's clear that this was no award winning tripod, especially since there was hardly any wind! 8,5 years later I did the same thing with a good tripod, and the difference is pretty obvious!

 

 

 

 

 

But after bying a real tripod I was amazed at how much better my pictures became. For me, there are several reasons why pictures will improve when using a good tripod:

• Obviously, sharpness of the pictures increases due to reduced camera shaking.

• Because setting up the tripod takes some time and is a bit of a hassle, I think twice before taking a shot. This results in me taking less uninteresting pictures that are likely to be discarded anyway.

• If I do take the picture, it takes more time doing so, which makes me think more carefully about the shot and choosing the right settings more accurately.

• Using a tripod means I'm no longer limited to the "reciprocal rule", a shutter speed of 1/10 or slower is no longer an obstacle, which gives more freedom in choosing your settings.

• Then there are some extra advantages, like the fact that working with gradual ND filters is a lot easier with a tripod.

 

If you want to make your tripod even more sturdy, you can weigh it down with something heavy, like the camera bag on the left.

 

But wait, there's even more! A tripod can also be used for other useful purposes! It can hold your cup of tea when it's winter and there is no other place to put it because of all the snow. Or, in combination with a hiking pole and a tree/rock/tent poles, it can make a nice drying rack during long hikes.

 

 

Tripod - mirror lock up and shutter delay

 

When using a tripod, it is a good idea to use either mirror lock up or shutter delay (if your camera is equipped with it of course). First of all, when pressing the button to take the picture you can create a lot of camera shake, resulting in a blurry picture. But another reason is the fact that, when you take the picture, the mirror inside of your camera moves up, which also can cause some camera shake. Note that this camera shake will only last for a short period of time, so this is especially important for pictures taken with shutter speeds between about 1/50th and 2 seconds (although these values depend a lot on the focal length you're using). If you are using shorter shutter speeds, than the camera shake will likely not be visible because of the short shutter speed. And if you are using a longer exposure, then the duration of the camera shake will be insignificant compared to the exposure time, and won't be visible either.

 

When using mirror lock up, the mirror will move up when the button is pressed, but the shutter won't open until the button is pressed one more time. Obviously, you need to work with a cable release, otherwise you will still introduce camera shake by pressing the button!

Shutter delay is basically according to the same principle. After pressing the button, the camera will move the mirror up, and automatically open the shutter after a chosen period (like, for example, one second).

 

Both these methods will result in sharper pictures, like in the example above on the right. This was taken at 200 mm and with a shutter speed of 0.3 seconds. The moon is clearly blurred in the first case, but activating the shutter delay gives a sharp moon! In this case, the camera shake was caused by my pressing the button.

 

Aperture

 

The aperture is the lens diaphragm that regulates how much light is transmitted to the sensor. It is usually a construction of several blades as can be seen in the example on the right, where f/4 is wide open.

But more important than regulating the amount of light, the aperture is crucial for controlling the depth of field. A small aperture gives a large depth of field, whereas a large aperture gives a narrow depth of field. So if the depth of field is important for your picture then the aperture should be used to get the right depth of field and the shutter speed should be adjusted accordingly. If you don't care about the depth of field and shutter speed matters more to you, then using the aperture for changing the exposure is an option.

 

On the left are two examples showing the change in depth of field for different apertures (the first one is at a focal length of 20 mm and the second one at 200 mm). In both cases, focus was on the trees in the foreground. A large aperture (small aperture number) gives a small depth of field, whereas a small aperture (large aperture number) gives a large depth of field.

 

The aperture number is a relative figure and is obtained by dividing the focal length by the apparent diameter of the aperture (the apparent diameter is the size of the aperture as we see it when looking into the lens, which is what counts). Thus, a 100 mm lens with an aperture number of 4 has an aperture of 25 mm, whereas a 16 mm lens at f/4 has an aperture of only 4 mm. That's why it's written like f/4, f stands for focal length and the importance of this is that an aperture of f/4 gives the same amount of light, regardless of the focal length. If we would not write it like this, then we would be obliged to either learn that a 4 mm aperture at 16 mm is equal to a 25 mm aperture at 100 mm, or calculate this for every instance.

 

Focal length (mm):

Aperture number (f/):

Size of aperture (mm):

 

 

 

 

 

 

 

Aperture - diffraction

 

Diffraction is the effect of light spreading out when squeezed through a small opening, and in our case the small opening is the aperture. The diffraction causes a so called Airy disk to form, which becomes larger with increasing diffraction. Diffraction occurs at any given aperture, but for the larger apertures (small f/-numbers) it is insignificant. At smaller apertures, diffraction increases and at a certain point (when the Airy disks start to approach the size of the circle of confusion) it starts to degrade image sharpness. So if you want a large depth of field in your photo, don't just use the smallest aperture as you will end up with decreased overall sharpness due to diffraction. Calculate, or even better, test at which aperture sharpness starts to degrade due to diffraction.

f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32 - f/45 - f/64

 

Some examples of how diffraction affects sharpness are on the left, all are 100 % crops. It is clear that f/5.6 is not that sharp, but given the fact that that is with the aperture wide open, it is hardly surprising as spherical aberration is at its worst with wide open apertures (apart from spherical aberation, some chromatic aberations are also visible at this aperture). Stopping down gives a better sharpness, but at f/16, diffraction starts showing up and sharpness starts to degrade, getting a lot worse at f/45 and f/64.

 

The table below shows for various wavelengths (400, 450, 500, 550, 600 and 650 nm) wether the apertures are diffraction limited or not. If they are, it means that sharpness decreases due to diffraction at that aperture. The focal length of a lens is of no importance. A lens with a longer focal length has a larger aperture, which would decrease the diffraction, but the light has a longer way to travel from aperture to sensor, which increases the effect. These two factors more or less cancel each other out.

 

Note that the results of this calculator should be taken with a small pinch of salt. Diffraction will most of the time be hardly visible in real life for the first two or three aperture settings were diffraction is occurring according to this calculator. That's why testing your lens is the best way to see at which aperture diffraction starts to degrade image quality.

Circle of confusion (mm):

Effective aperture factor*:

 

Wavelength (nm):

f/

f/

f/

f/

f/

f/

f/

f/

f/

f/

Limit (f/):

*Only of importance when taking pictures at high magnifications. For more on this, see lenses/magnification and macro photography/magnification.

 

ISO

 

The ISO value is used for films as a measurement to describe its sensitivity. Besides an increased sensitivity, a higher ISO value also means that the grain size of the film is increased which has some influence on the pictures, becoming more grainy.

The same ISO standard has found its way into the digital cameras although it is not exactly the same thing. Basically, a digital sensor has only one real ISO value (one sensitivity), often called the base ISO or native ISO, and all other ISO values are derived from that native ISO. There is however a big difference in how the other ISO values are obtained. If we assume a native ISO of 100, then specific multiples of the native value (ie 200, 400, 800, each with one stop difference) are obtained by amplifying the analog signal gain of the sensor prior to conversion to a digital signal. The other ISO values (like 125, 160, 250, 320, 500, etc) are obtained by digital tricks with the nearest multiple after the conversion. For example, ISO 500 is in reality pushed ISO 400 while ISO 320 is pulled ISO 400, which has some consequences of course.

A "pulled ISO" means that the brightness is digitally decreased from the nearest "multiple ISO", so ISO 320 is ISO 400 decreased with 1/3 of a stop. This results in the darkest and noisiest 1/3 of a stop being discarded, which makes that these pulled ISO values are often regarded as the cleanest (less noise), but the trade off is that the dynamic range decreases with 1/3 of a stop compared to ISO 400. Similarly, a "pushed ISO" will generally look noisier.

 

Higher ISO values are usually regarded to give more noise, but that is not true. The real reason is that when people increase the ISO value with for example one stop, they often simultaneously decrease the exposure (shutter and aperture settings) with one stop. And that results in only half of the original amount of light hitting the sensor, which decreases the signal to noise ratio and thus gives more noise.

It may seem counterintuitive, but increasing the ISO value while maintaining the same exposure (thus resulting in a brighter picture) actually improves the signal to noise ratio, which is because higher ISO values have lower read noise levels. If an increased ISO value would simply increase the output signal, there would be no extra benefit compared to just increasing the brightness afterwards on the computer. But increasing the ISO value does not only increase the output signal, but also results in a higher signal to noise ratio, and a photo taken with a higher ISO is thus superior to a picture which has been brightened to the same brightness on the computer (more on this can be found under bits & bytes/ISO to the right).

The downside of raising the ISO value is a loss in dynamic range. By doubling the ISO value, a full stop is lost at the brightest side of the dynamic range, but on the other hand, since the read noise is lower for higher ISO values, the dynamic range is increased at the darker end. However, the gain in dynamic range is smaller than the loss and effectively the dynamic range decreases.

 

How the camera sees the world

 

Exposing is probably the most important part of photography, but also one of the trickiest since a camera is far less flexible than our eyes. Not only because the dynamic range of our eyes exceeds the dynamic range of camera sensors (film, for that matter, is of course also a type of sensor), but also because the camera has a very rigid way of measuring the exposure, which we need to compensate for.

The way a camera measures the light conditions is that it always adjusts the exposure so that it will be 18 % grey (which is mid grey for us, since we see light in a different way than cameras). The problem is that a camera has no way of deciphering a black object from a white object, so for a camera, a dark object in bright light looks the same as a light object in the shade. So regardless of the nature of the subject (dark or light), the camera will assume it is a midtone object, and expose accordingly. But only a small part of the real world is midtone so therefore it is often necessary to overexpose or underexpose.

 

For example, if you want to take a picture of a bright white object like the sunlit snow in the picture above on the right, then the camera will still assume that it is midtone and this will result in the camera choosing settings that will give an underexposed picture, with grey snow. So in order to achieve a correct exposure, we need to overexpose with a certain amount, 1 stop in this case.

The same goes for dark objects, a dark object will still be regarded as midtone and the photo will be overexposed so that the dark object ends up as a midtone object. Take for example the burnt tree on the right, if we let the camera determine the exposure, then the picture will be overexposed. But if we apply an exposure correction of -1 stop, then the exposure is fine.

 

So, although it sounds a bit counterintuitive, dark objects need to be underexposed and light objects need to be overexposed! Note that most cameras only measure luminance, and are therefore colorblind. So even though grass is green, it is often still a pretty good 18 % grey standard, depending on the light conditions.

 

Then there is also the fact that exposing on digital cameras is different than on film. If you want your pictures to be free of noise as much as possible, you'll need to "expose to the right", which is described in detail under bits & bytes/expose to the right.

 

 

Dynamic range (DR)

 

The dynamic range is the range of levels of brightness a sensor can capture and ranges from the the full well capacity of the pixel (the maximum amount of signal a pixel can detect without being saturated) to the lowest level where the signal to noise ratio is 1. It is usually given in stops or EV's (exposure values). If the dynamic range of a scene exceeds the dynamic range that the sensor is capable of capturing, then information will be lost, either on the dark side or on the light side, depending on the exposure. The combination of our eyes and our brain gives us a much larger dynamic range than cameras and one of the most challenging things in photography is to capture the scene we see with the camera.

A good way to mimic the lower dynamic range of a camera is to squint your eyes, which gives you a pretty good estimate of how the camera "sees" the scene.

On the left is an example of a scene with a dynamic range larger than the film was capable of capturing; the shadows on the left side have turned into featureless black.