9. The recording medium: camera ISO, megapixels and sensors
When it comes to image quality, the first thing many new photographers think about is megapixels. But there’s much more to image quality than just the number of pixels, with sensor type (and size) and ISO also playing key roles.
© Karl Taylor
An image is recorded when light passes through the lens and reaches the recording medium. Through composition and framing (which you’ll learn about in the next chapter), and also by adjusting the aperture and shutter speed, we can control how this image looks. What we cannot completely control is the image quality.
That’s because image quality is mostly determined by factors such as the sensor itself, the resolution (number of megapixels) and ISO.
If you want to understand image quality, you need to understand each of these factors.
The recording medium
© Karl Taylor Education
Historically, the recording medium in photography was film. Today, it is usually a digital sensor.
Camera sensor types
The two main sensor types are CCD (Charge-Coupled Device) sensors and CMOS (Complementary Metal-Oxide Semiconductor) sensors.
Until recently, CCD sensors were the most commonly used type of sensor due to their excellent image quality, dynamic range and noise control. However, as technology has progressed, CMOS sensors have taken over.
Generally speaking, larger sensors offer higher quality, while smaller sensors provide a more budget-friendly option, while still delivering great results.
Camera sensor sizes (or formats)
Maybe you’ve heard of full-frame, crop-sensor and medium-format cameras. But what’s the difference?
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Crop-sensor cameras, also referred to as APS-C, come with the smallest sensors. With sensor dimensions of approximately 23.5mm x 15.6mm (compared to 36mm x 24mm full-frame sensors), these cameras are smaller, lighter and more affordable than their full-frame counterparts.
Full-frame sensors do not have a pre-determined crop. Because they are larger than crop sensors, they offer better image quality and enhanced performance in low-light conditions.
The largest sensors are found in so-called medium-format cameras. The benefit of these 53.4mm x 40mm sensors is that they allow for much higher resolution compared to cameras with smaller sensors. Mostly used by professionals, these cameras offer very high quality results, but are also more expensive than crop-sensor or full-frame cameras.
Megapixels and resolution
Often used interchangeably, the terms ‘megapixels’ and ‘resolution' actually mean two very different things. Understanding the difference is crucial if you want to get the best possible results out of your camera.
Megapixels
An example of a 100 megapixel image. © Karl Taylor Education
One megapixel is made up of a million pixels. This means that a 24-megapixel camera will record an image that is made up of 24 million pixels, while a 16-megapixel camera will record an image with only 16 million pixels. Each individual pixel contains information that contributes to the image as a whole.
When it comes to image quality, the number of megapixels is not the only thing that matters. Also important is the size of the photosites. Photosites, which record pixels, are measured in microns (µm). Their size is largely determined by the sensor size.
Cameras with smaller photosites may perform worse in low-light conditions. They may also suffer more diffraction when you’re shooting at small apertures. Larger photosites allow for a larger transitional tonal value, greater tonal accuracy and better colour accuracy.
Resolution
Contrary to popular belief, ‘resolution’ does not simply refer to the number of megapixels. Instead, it refers to how clearly the medium can capture and record detail.
For example, by simply using two different versions of the same lens on the same camera, we could change the resolution. An image shot with an older lens will have a lower resolution than the same image shot with a newer model lens with a better optical design. The same number of megapixels will be recorded (because it is the same camera), but the newer lens design will likely give better contrast, colour fidelity and overall sharpness.
ISO in photography
ISO measures how sensitive the recording medium is to light. Just as film comes with different sensitivities, digital cameras can be made more or less sensitive to light by adjusting the ISO setting.
Higher ISOs mean greater sensitivity to light, while lower ISOs mean less. Though increased sensitivity may sound good in theory, higher ISOs can lead to lower image quality. This is typically due to ‘noise’, especially in the shadow tones.
© Karl Taylor Education
Like shutter speed and aperture, camera ISO can also impact the exposure of an image. However, it should not be used as a tool to do this, except as a last resort.
Although it can be very useful in low-light conditions, the ISO setting should not be relied on due to its impact on image quality. It is far better to get your exposure correct using shutter speed and aperture where possible.
JPEG vs RAW
Also important for image quality the file format you shoot in: JPEG or RAW.
Although both file types contain the same number of pixels, RAW files store far more information within those pixels than JPEG images do. This means RAW files give us far more control in the post-production stage, which can be very useful if you want to make changes to your pictures after shooting.
One of the main drawbacks of JPEG files is compression, which can sometimes result in ‘pixel clumping’. This occurs when pixels of a similar tone are grouped together. Although clumping may not initially be apparent, it becomes more obvious and problematic as soon as we start to adjust colour and exposure in post-production.
Although JPEGs may not allow us to extract as much colour detail as RAW files (especially in highlight or shadow areas), they are still popular among photographers – such as wedding or sports photographers – shooting large quantities of images in a session. As you may have guessed, this is because, due to their compression, JPEG files are much smaller. This means you can fit many more JPEGs on a memory card than you can RAW files.
In the RAW file, there is no pixel clumping, even when adjusting the highlights and shadows.
When we adjust the highlights and shadows, pixel clumping starts to become visible in the JPEG file.
Though all of these factors contribute to image quality, you don’t have to shoot with the camera with the largest sensor, with the most megapixels, at the lowest camera ISO. In other words, don’t be put off if you don’t have a fancy camera!
The fact is, most of the cameras on the market today are of exceptionally high quality. And besides, equipment is only one part of the photography puzzle. With the right knowledge, you can create amazing images with any camera.
WATCH NEXT: Class 10: Composition and Subject
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