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Welcome to the third lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use.
We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor.
We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Lenses, Light and Magnification
Last week we looked at the basics of the lens. We saw how lenses bend light by slowing it down, how the angle the light enters the lens affects how much it is bent, and how we can use this property to bring light that enters a lens into focus and create a bright, clear image.
The advantage that lenses gives us over pinhole cameras is twofold: brightness and magnification.
We saw in lesson two, with our experiment with the candle and the magnifying glass (Fig 1.2.3), that all the light that entered the lens from the candle was focused into the image. If we substituted a larger lens with the same focal length, then more light would be focused, and the resulting image would appear brighter, but no bigger.
It seems logical that if you double the diameter of a lens, you’ll double the size of the image it makes, but as you can see in Fig 1.3.1 below, that’s not true.
Fig 1.3.1 Doubling the diameter of the lens halves the f-ratio (see below) and collects more light but does not change the size of the image, which is a function of focal length (also see below). Doubling the diameter actually more than doubles the brightness of the image, as the light collecting sufrace of the lens increases rapidly as the radius increases (per the formula ?r2 – pi times the radius squared).
In photography there’s a handy number used to describe the relationship between lens diameter and focal length: the “f-ratio”. Simply put the f-ratio is the focal length divided by the diameter. In Fig 1.3.1 above we have a lens with a focal length of 50mm and a diameter of 10mm. 50/10=5 which gives us an f-ratio of 1/5 or f5. If the lens was still 50mm focal length with a 20mm diameter, it would be f2.5.
The f-ratio for an SLR lens should always be written on the lens somwhere. Most compact cameras also describe the f-ratio somewhere on the body. The “shorter” the f-ratio, that is the closer it is to 1, the brighter the image the lens will produce. The term “speed” is also used to describe a lens. The word speed in this case refers to how fast the lens will allow the camera to capture an image, given the amount of light available. If the lens produces a bright image, then the shutter can be open for a shorter time to capture enough light to make an image. Thus a short f-ratio lens like f1.8 is considered a very “fast” lens, while a longer lens such as an f8 or f11 is a “slow” lens.
Recalling lesson 1, we learned that a large hole for the light to pass through makes for a brighter but less sharp image. Now that we know about f-ratios, we can connect these two facts together and understand why faster lenses have a narrower “depth of field” – the area which is in focus. We’ll talk more about this in the next lesson, but it’s helpful to connect the dots and see how all these various principles fit together.
Modern cameras allow a photographer to have some level of control over a lens’ speed by adjusting the aperture, we’ll also cover that in more detail in the next lesson.
Anyone who has played with a magnifying glass knows that, as the name suggests, lenses magnify. The amount of magnification depends on the focal length. The “longer” the lens, the more it magnifies the image. Short focal lengths have the opposite effect, reducing the size of the image.
Fig 1.3.4 All other things being equal, as the focal length of the lens increases, the relative size of the image also increases.
We saw above that f-ratio affects the image brightness. The two factors in the ratio are lens diameter and focal length. So far we have only talked about changing the lens diameter, but with greater magnification you increase the focal length, so you also increase the f-ratio. This means that the more you magnify the image, the dimmer it becomes. Most telephoto (long focal length) lenses have large f-ratios, and are therefore slow lenses. The exception of course are the hugely expensive and very heavy, giant lenses used by sports photographers. These use long focal lengths, and big diameter lenses. These telephotos are not for the casual photogrpaher!
We’ve talked about how lenses make the image bigger, and that’s certainly how it appears when you look through the viewfinder, or at the print from a telephoto lens. In reality, because most objects are distant, and the sensor is small, the vast majority of lenses produce an image which is smaller than the object itself. There are some specialist lenses, however, which do make an image larger than the subject. For this to be possible, the focal length needs to be long and the subject close. These are, of course, macro lenses.
Macro lenses will often be described by their “magnification factor”. A lens with a 1:1 magnification factor produces a projected image on the sensor which is the same as the subject. So the image of a 20mm diameter coin will span 20mm of the physical sensor, resulting in an image which will nearly fill the entire frame of a typical DSLR. A 1:1 magnification factor is usually considered the minimum for a lens to be described as a “macro” lens. Specialist macro lenses are often 1:3 or even 1:10 magnification factors, meaning that 1mm across the subject becomes 3mm or 10mm when projected onto the sensor, thus 3 or 10 times magnification.
Field of View
The final variable in this initially confusing balancing act of optics is the field of view. This refers to how much of the world the camera can see. A lens’ field of view depends on the focal length of the lens and the size of whatever the image is projected onto. In the case of digital cameras this is the sensor chip.
Fig 1.3.6 As the focal length increases, the field of view narrows and the image enlarges.
Fig 1.3.6 makes it obvious that at the wide-angle end, a slight difference in focal length translates to a large difference in field of view. The difference in field of view between a 10mm and 20mm lens is far greater than the difference between 210mm and 220mm. Some lenses can have exceptionally short focal lengths and wide fields of view, such as 12 or 8mm. These are fisheye lenses, so-called because the front of the lens bulges so much it looks like a fish’s eye. These lenses can have a 180 degree field of view, or even greater.
The size of the sensor also contributes to the field of view of a particular lens. In Fig 1.3.6 a particular sensor is shown at different focal lengths. Obviously if the sensor is smaller, it can see less of the image presented by the lens, thus the field of view is reduced and magnification is increased. This is the case for “cropped sensor” DSLRs, and compact cameras.
The “standard” frame size is 35mm, the size of a single picture on a roll of film. Cameras with this sized sensor are known as a “full frame” cameras. Large format film cameras exist with much larger film sizes, such as 150mm x 100mm. Less expensive, or earlier model DSLRs use sensors smaller than a 35mm film frame, and are referred to as cropped sensors. A typical cropped sensor may be described as a 1.6x, meaning that the apparent focal length of a particular lens is 1.6 times longer. Compact cameras use the smallest frame sizes of all, and as such require very short focal length lenses to get wide angle views.
Photography 101 – Aperture and stops.
Now that we’ve pulled together the main theory behind the lens and creating an image, we’ll next turn our attention to exposure and how we control the capture of an image. Next week will see the introduction of the exposure triangle, an explanation of “stops” and brightness levels, and a look at the first point on the triangle: aperture.