Detail of a spider web, photographed using an iso value of 3200. Photography is all about light. To get the exposure of our picture right, we need to balance the aperture size and shutter speed. This can become tricky in low light situations. If both aperture and shutter speed don’t suffice to get the exposure right, we can increase the ISO value. The ISO value is a measure for the sensitivity of the sensor. ISO stands for International Standards Organization.

Aperture

The aperture is the opening, iris or diaphragm after the lens. It controls how much light will fall on the sensor. With adjustable blades this aperture can be arranged in a limited number of sizes. These sizes are such that every next smaller opening halves the amount of light falling on the sensor. We measure the aperture openings as fractions of the focal distance of the objective.

Three apertures for a 50mm objective. From left to right: f/2.8, f/5.6 and f/22.

So we can use the aperture size as a control for the exposure of our picture. We can also use it to select which part of our picture will be in focus.

Shutter speed

The shutter speed (which isn’t a speed but an exposure time) determines how long we will let light fall on the sensor. Also the shutter speed can be chosen as one of a limited series. The different speeds are such that every next smaller number halves the amount of light falling on the sensor. So, we see something in common with the aperture sizes we can choose from.

Balancing aperture size and shutter speed

With every decrease in aperture size we halve the amount of light falling on the sensor. With every increase in shutter speed we double the amount of light falling on the sensor. So, this means that we can compensate a change in aperture size with a change in shutter speed. This is illustrated in the next figure. We use a seesaw as an analogy.

Seesaw analogy of the balance between aperture size and shutter speed.

If we move to the next lower exposure time, we need to move to the next higher value for x in the 1/x shutter time. So, the exposure time decreases and therefore the shutter speed increases. (Moving upward in the column on the right increases the shutter speed and lowers the exposure time).

To keep the amount of light falling on the sensor the same, we now have to increase the aperture size to the next level. This means that we have decrease y in the f/y aperture size.

The easy way to deal with this (throwing physics over board) is:

A faster shutter speed (larger numbers in the shutter speed scale), leads to a lower aperture number.

Every change in stops (up or down) in aperture or shutter speed needs to be compensated by an equal mount of stops in the other in the opposite direction.

ISO

So, in principle, for every situation you can find a number of aperture and shutter speed combinations that ensure a correct exposure. For photographing fast moving objects or persons (sports) you can start with a fast shutter speed to ‘freeze’ motion and adapt the aperture to that. For isolating persons or objects from the background you can start with selecting the aperture and adapt the shutter speed. The shutter speed should not go beyond where you can still take sharp shots handheld. This will bring you into problems under low light conditions.

Fortunately we have another dial at our disposal. That’s the sensor sensitivity, measured in ISO. The sensor of our camera registers the amount of light falling on it and translates that in voltages and currents. The camera processes these voltages and currents to deliver the picture we will finally see on the screen. If the voltages and currents are low – as in low light conditions – they can be amplified.

The base ISO value for most cameras is ISO 100. For every doubling of the ISO number (amplification) the captured light amount doubles. So, we can use the ISO dial to balance the aperture size or the shutter speed just as we balanced the latter two. In the seesaw analogy this will look like this

Seesaw analogy for balancing shutter speed, aperture size and sensor sensitivity (ISO).

Thus, to compensate for a higher shutter speed (shorter exposure time) we can increase the ISO value. To compensate for a higher aperture value (smaller aperture size) we can also compensate with a higher ISO value.

Shutter speed, aperture and ISO

To summarize, for the same exposure:

shutter speed up –> aperture number down and/or ISO number up
shutter speed down –> aperture number up and/or ISO number down
aperture number up –> shutter speed down and/or ISO number up
aperture number down –> shutter speed up and/or ISO number down

So, everything is OK now? We can take pictures in all light situations?

Well …, yes and no.

Noise

Just as for audio, amplification introduces noise. This noise is visible in a picture as a random variation of brightness and/or color. It resembles the grain in pictures from negative film. We generally experience noise as more ugly though. An example is shown below.

Example of noise. Camera: Nikon D200. f/5.3, 1/60 sec. ISO 1000. Detail 100%.

Noise is more visible in dark areas than in light areas. It also depends very much on the camera. Camera manufacturers have done a great job in improving the noise reduction. With modern cameras we can now photograph in low light situations where we could not, not so long ago.

To illustrate this, the next figures show 100% details of a spider web photographed with a Nikon D7200 camera for different ISO settings.

Examples

We start with ISO 200. This is not the lowest value possible, but going lower did not give me a less noisy picture.

ISO 200. f=200mm, f/5.6, 1/50 sec.

Next, we move to a value that still produces a near-noiseless picture. All intermediate values do not show any change.

ISO 800. f=200mm, f/5.6, 1/200 sec.

Then we move to the first value where noise is visibly introduced.

ISO 3200. f=200mm, f/5.6, 1/640 sec.

We double the value and see an increase of noise.

ISO 6400. f=200mm, f/5.6, 1/1250 sec.

Doubling again further increases the noise.

ISO 12800. f=200mm, f/5.6, 1/2500 sec.

The final value results in the following amount of noise.

ISO 25600. f=200mm. f/5.6, 1/5000 sec.

If we compare the noise behavior of the D7200 with that of the D200 we see how noise reduction has improved over the years. The noise at ISO 25600 looks bad, but we have to consider that we are looking at a detail on 100% enlargement. Therefore, as a last example, we show the whole pictures side-to-side at ISO 200 and at ISO 25600.

two spider web pictures for different ISO values.

Complete spider web picture. Left: ISO 200. Right: ISO 25600.

We see that for small prints the difference is very hard (if at all) to see. But then, this is for the state of the art at the writing of this post. If you have an older camera, the noise may become visible at a much lower value.

How to work with aperture, shutter speed and ISO in practice

There may be different ways to work with aperture, shutter speed and ISO settings. The way I work with the settings that control the exposure works well for me. It is based on working with an older (more noisy) camera:

I start with defining the aperture to create the DOF I want.
Then, looking at the light meter in the viewfinder (I hate live view) I choose the correct shutter speed.
If I end up with a shutter speed that makes the success of taking handheld shots questionable, I increase the ISO value.

Of course you could let the camera make all these decisions for you. Cameras have become very smart and -in general – will choose an excellent setting. However, by using Manual mode you will have full control. This can help you in tricky light situations and will give you more creative means. For example, sometimes you want to have movement in your picture.

The way I started with photography was using Automatic mode. I let the camera decide on aperture, shutter speed and ISO value. Very soon I switched off the automatic ISO setting, since it gave me too often too noisy pictures (using a Nikon D70S and later a Nikon D200). Then I moved to Aperture Priority. In this setting (again with a fixed ISO value) I choose the aperture and let the camera decide on the shutter speed. Finally I moved from there to Manual mode. I’m only using the light meter and the auto focus from the arsenal of automated functions. All steps were rather natural. Only the last step – going to Manual – brought me out of my comfort zone. But with a little perseverance I learned to get used to it and now I like the full control it gained me.

Categories: Tutorials

Tags: apertureexposureISOshutter speed

Tutorials

Rule of Thirds: What is it and Where Does it Come From?

Atalanta butterfly on ivy flower When you are interested in photography you will start reading books on the subject or browsing the web. Eventually you will encounter the rule of thirds. Or maybe, when playing around with your camera settings, you found this grid you can overlay on your screen or viewfinder. You may have wondered about the use of it. Both have to do with picture composition and creating appealing pictures.

What is this rule of thirds?

For applying the rule of thirds you are supposed to divide the screen/picture in nine equal segments by dividing the horizontal and vertical sides in three equal parts. Then, if you place your main subject on the intersection of any of the horizontal and vertical lines, you will have created a composition that is appealing to most human beings. In the examples underneath I show two examples where I first placed the main subject in the centre and then moved it to one or two of the intersection points.

Atalanta butterfly on ivy flower before and after applying rule of thirds

Kitchen sink before and after applying rule of thirds

Use of the rule of thirds

For most people the pictures on the right are more appealing than the ones on the left. How come? It all has to do with the ‘divine proportion’ or divine section, golden ratio, or golden cut.

The divine proportion

The divine proportion is a ratio of dimensions that is encountered in nature in e.g. the positions of branches along the stem of a plant, the veins in leaves and the proportions of the different body parts of a human. Mathematically, the golden ratio determines how the lengths of two line segments relate to each other.

Diagram explaining golden ratio in one dimension

The golden ratio

If in the above figure the lengths a and b relate to each other identically as the lengths a+b and a, or put in an equation a/b = (a+b)/a, then the ratio of a and b is called the golden ratio. Working out the math yields that a/b = 1.61803…, the dots meaning that there is an infinite number of decimals.

If we now make a rectangle having horizontal size a+b as in the above line segment and vertical size a, the lengths of the sides of the rectangle have the divine proportion. Then, applying this divine proportion to the horizontal length results in a smaller (standing) rectangle that also has the divine proportion, see the figure underneath.

The golden ratio in two dimensions

We can divide the small rectangle in two smaller ones, again having the divine proportion. The dividing of the large rectangle in smaller ones can of course be done in different directions. If we do this two levels deep and connect the line segments, we get the structure as shown below. The crossings of the horizontal and vertical lines are the golden cuts.

Golden cuts

Our eyes, when looking at a scene, will by instinct scan the divine proportion positions, i.e. the crossings of the horizontal and vertical lines. Thus, positioning the subjects we photograph there will in general lead to compositions that are considered as appealing.

The easy way out

Since it is practically impossible to mentally divide a scene up in parts of which the lengths relate to each other as 1.61803… (at least it is for me and I’m a nerd), we aren’t that far off if we divide in thirds instead, see the figure underneath.

diagram explaining the connection between the golden cuts and the rule of thirds

Relation between the golden cuts and the rule of thirds

The purists among us will opt for the real golden cut. For me, the rule of thirds is good enough. And then, this ‘rule’ is not carved in stone. It’s more like a guideline. If your main object is somewhere near one of the thirds lines and / or intersections, it’s OK.

A few more examples:

Dutch windmill at sunset

Cow in the morning

Bumblebee on a cornflower

Coffee in London

The London Eye at night

Big Ben

Conclusion

One final thing: As with all rules, being too strict about it will be counterproductive. So let’s not call it ‘the rule of thirds’ anymore but rather ‘the suggestion of thirds’.

Tutorials

Focus

Choosing the Right Aperture to Create Depth Of Field

Through selecting the range of focus in your picture, you can draw attention to the elements you want to emphasize. You can accomplish this by selecting the focal point and by choosing the appropriate aperture size. Understanding Depth Of Field (DOF) is a powerful creative tool. We will explain how decreasing the aperture size may compensate – to a certain degree – being out of focus. Therefore we will go through some (very) basic physics, starting with the concept of focal distance.

Focus: The Focal Point

I guess we all remember from out childhood how to create fire with a magnifying glass on a sunny day. Therefore we hold a magnifying glass between the sun and a piece of paper. For the proper distance, we see a small, bright, spot of light. This spot may contain enough energy to eventually start burning the paper, see the figure underneath.

Magnifying glass held in in a beam of light creating a spot on a piece of paper

Focus

The magnifying glass is a curved piece of glass we call a lens. Light rays that fall on the lens will be refracted. That means that they will leave the lens traveling in a direction different from where they came from. This change of direction is different for different places on the lens. For rays coming from a very distant source, like the sun, the incoming rays are all in parallel. After passing the lens, they all go through the same spot. This spot – called focal point or focus (f) – is unique for the lens. The focus is related to the curvature of the lens, see the following figure. Note that the light ray going through the center of the lens does not change in direction.

Photograph pf a magnifying glass and a schematic of lens and light rays explaining the concept of focal point.

Parallel light rays are entering a lens from the left. After refraction they all pass through the same point. This point is called the focal point (f) of the lens.

We will use this ray tracing or geometric optics concept to explain how lenses function in creating images on a camera’s sensor.

Image Projection

A magnifying glass showing trhe flipped image of a house on a far distance

Optics: A lens will flip an image top to bottom and left to right.

Have you ever wondered about the fact that far away objects when seen through a magnifying glass are always flipped upside down (and turned left to right)?

We can explain this “flipping” by using simple optics.

Optics

What we see through the lens we call the image. We may envision the creation of this image as shown in the next figure.

Schematic explanation of the forming of an image by a lens.

The creation of an image by a lens; Light rays from top, center and bottom of the object travel through the lens to form the image.

When the object under observation is at a distance S₁ from the lens, a sharp image will form – on the other side of the lens – at a distance S₂. The two distances S₁ and S₂ are related to the focal distance f through

thinlenseq

We may construct the image by drawing light rays from the source through the center of the lens to the image position. We see that in this process the image has flipped (upside down and left to right) with respect to the original.

In the Camera

If we place a camera’s sensor (or negative film) at the position of the image, we may capture a (digital) image. After processing (developing) this image, we turn it around to show the picture in the way we want it.

(The enlarged image we see from a magnifying glass is not “flipped”. This has to do with the fact that we place the original at a distance closer than f to the lens. Then the image will form on the same side of the lens, still following the earlier shown thin lens equation. A further explanation is beyond the scope of this post).

In the figure underneath we have placed the lens of the above picture in a camera. The image will project on the camera’s sensor.

Schamatic of a the functioning of a lens in a camera

The image created by the lens in a camera is projected on the sensor (or negative film).

The focusing of our camera, manual or in auto-focus, is carried out by (slightly) displacing the lens position. This happens most often by a screw-like motion as envisaged in the figure. The position towards the object we want to photograph is fixed at S₁ and so is the focal distance f in this example. This means that we have to adjust S₂, i.e. the distance of the lens to the sensor to satisfy the thin lens equation and obtain a sharp image on the sensor.

The thin lens equation also works the other way around. If we know f and S₂, the equation tells us the value of S₁. That is why we may find a distance indication on the housing of a lens that runs with the focus adjustment.

Focus and Depth Of Field (DOF)

We have described how to obtain a sharp image on the sensor. It came down to adjusting the lens position relative to the sensor plane, given the object we want to have in focus and the focal distance of the lens. But what happens with the image of objects that are not at the location S₁ but (slightly) in front of it or behind it? Before explaining these situations we first need to expand our ray-tracing model of lens, object and image.

Light rays

Thereto we look at the rays coming from the top and bottom of the object that pass through the top, center and bottom part of the lens.

Schematic explaining ray tracing for lenses

Using ray tracing to construct an image of the object in front of the lens.

The rays coming from the top we represent with solid lines, the rays coming from the bottom we represent with dashed lines. The figure shows how rays passing through the center of the lens are progressing without making a change in direction. The rays at the boundaries of the lens are making the maximum change in direction. The figure also shows that – with the image being in focus – a point from the original transfers to a corresponding point in the image.

Out of focus

Now, we place an additional object in front of the object already present. Given the position of this new (red) object and the fixed focal distance of the lens, a sharp (red) image will occur at the position shown in the figure underneath.

Using ray tracing showing the concept of in and out of focus.

The original object is projected sharply on the camera’s sensor. This is indicated in black in the figure. The sharp image of the red object is projected behind the sensor. We see that points from the original object spread out at the sensor’s position and thus will result in a blurry image. We may use the same reasoning for an object that we place behind the original object that we focus the lens at. For this situation, a sharp image will occur in front of the sensor plane. If we focus at an object, we cal the range before and after this object that is still reasonably sharp in the picture Depth Of Field (DOF).

Increasing the DOF

So what can we do to increase the DOF? In the figure above we introduced unsharpness. We have seen that this relates directly to the area over which points will spread out over the sensor. This area depends on the position of an object relative to the object in focus and the size of the lens. Size since refraction will be stronger further away from the center of the lens. Thus, to increase the sharpness we should restrict the lens area that we use, i.e. decrease the aperture size. In the figure underneath we show – in red – the non-focused object of the previous figure. To the right of this ray-tracing figure, we show the blurred projection of the image on the sensor. In blue, the projected image is shown for a restricted aperture. This aperture restriction is indicated with the thick black lines to the left of the lens.

schematic ray tracing explanation of the effect of an aperture on the functioning of a lens

The effect of an aperture on the focus of an image.

Use of Aperture

At the right of the figure above we show how the top and bottom points of the out-of-focus object project on the sensor when we use the full aperture (red). We also show these projections when we limit the aperture in size (blue). We clearly see how decreasing the aperture – i.e. not using the whole lens surface – reduces the blur. As shown in the next figure, where we decrease the aperture even further we see how we can nearly completely compensate this blur.

graphical ray tracing explanation of the effect of a small aperture in front of a lens on the image sharpness.

The effect of a small aperture on the sharpness of an image.

Aperture in numbers

The aperture diameter size is given as a fraction of the focal length, e.g. f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, etc. This means that a 50mm lens having an aperture f/1.4 uses a diameter of 3.57cm (1,41”). The same lens having an aperture f/11 needs a diameter of 4.55mm ( 0.18”). So the higher the f-number, the smaller the opening or aperture. The sequence of f-numbers shown is such that every f-stop is a factor 1.4 or the square root of 2 times the previous one. The amount of light entering the sensor is for every f-stop a factor 2 times less than the previous one. So, the lower the f-number, the more light is entering. So, for example, a 300mm lens having an aperture size f/1.4 would require a lens diameter of 21.4cm (8.4”). For f/5.6, we only need 5.36cm (2.11”).

In Practice

Let’s see how this theory works in practice. Underneath we show three pictures of a match taken with a 50mm lens.

Three pictures of a match in and out of focus using different aperture sizes.

The effect of aperture size on picture sharpness. Left picture: f=50mm, f/4, in focus. Middle: f=50mm, f/4, out of focus/ Right: f=50mm, f/22, out of focus.

For all three pictures we fixed the camera and object positions. The picture on the left we took at f/4 using autofocus. The match is perfectly in focus. For the middle picture we kept the aperture at f/4 but we switched off autofocus. We used manual focus and turned the lens slightly out of focus. The match now looks blurred. For the picture on the right, we kept the lens position the same as for the middle picture but we decreased the aperture size to f/22. Even though we know the match not being in focus, the small aperture produces a sharp image of the match. The picture is not as good as the picture on the left but the compensation by the small aperture for the out of focus image is quite remarkable.

Examples

So, now that we understand focus and aperture we can put it in practice. Narrow aperture sizes (large f/-numbers) are used mostly in landscape photography. This is because, as we have just seen, by choosing a small aperture (large f/-number) we can ensure the largest sharpness all over the picture. Using a large aperture (small f/-number) can be used to emphasize an element in your picture since it will ‘blur out’ a distracting background. See the following two examples.

Landscape. Here we need sharpness through the whole picture. f/9, 1/80 sec, iso 100, 40mm.

A bumblebee on top of a corn flower aginst a background of colored flowers

Bumblebee. Here we have used an aperture size small enough to show the bumblebee and flower reasonably sharp, but large enough to ‘blur out’ the flowers in the background. f/4, 1/1000 sec, iso 200, 40mm.

Conclusion

By selecting a certain aperture size we can control how much of our picture will appear to be sharp. We can use this as a creative tool. For example, it can help us to emphasize elements in our picture. We can also use it to ‘blur’ out a distracting background. When we choose a certain aperture size, we need to get the exposure right with the correct shutter speed and/or ISO value.

ISO: When Shutter Speed and Aperture Are Not Enough

Published by Hubregt Visser on January 20, 2017January 20, 2017

Aperture

Shutter speed

Balancing aperture size and shutter speed

ISO

Shutter speed, aperture and ISO

Noise

Examples

How to work with aperture, shutter speed and ISO in practice

Tutorials

Rule of Thirds: What is it and Where Does it Come From?

What is this rule of thirds?

The divine proportion

The easy way out