When it comes to image quality, the main thing many new photographers think about is megapixels. But in fact, image quality comes down to much more than just the number of pixels — the sensor type (and size) and ISO also play a role.
As we’ve covered in previous chapters throughout this course, a digital image is recorded when light passes through the lens and reaches the recording medium. We can control how this image looks through our composition and framing (which you’ll learn about in the next chapter) and also by adjusting the aperture and shutter speed. What we cannot (totally) control is the image quality. This is mostly determined by factors such as the sensor itself, the resolution (number of megapixels) and ISO.
To understand image quality, it’s important that you understand what each of these terms means.
The recording medium
In photography, the recording medium is the device that we capture or record an image onto. Historically, this used to be film, but nowadays it is more commonly a digital sensor.
Camera sensor types
Depending on your camera, there are different types of sensors. The two main types of sensors are CCD (Charge-Coupled Device) sensors and CMOS (Complementary Metal-Oxide Semiconductor) sensors.
CCD sensors, until recently, used to be the most commonly used type of sensor due to their superior image quality, dynamic range and noise control at the time. However, as technology has progressed, CMOS sensors have now taken over.
Generally speaking, larger sensors offer the highest quality while smaller sensors provide a more economic option, but still deliver great results.
Camera sensor sizes
Camera sensors also come in different sizes (these sensor sizes are also referred to as ‘formats’), with smaller sensor sizes offering lower image quality compared to that of larger sensors. Most commonly, you’re likely to have heard of full-frame, crop-sensor and medium format cameras. But what do each of these mean?
Crop sensor cameras, also referred to as APS-C, are the smallest of the sensor sizes. Measuring at approximately 23.5mm x 15.6mm compared to the 36mm x 24mm full-frame sensor, they are smaller, lighter and more affordable than their full-frame counterparts.
Full-frame sensors do not have a pre-determined crop, and because of their larger sensor size they offer better image quality and enhanced performance in low light conditions.
Medium format cameras offer the largest sensor of these three options. The benefit of this 53.4mm x 40mm sensor is that it allows for much higher resolution compared to cameras with smaller sensors. Mostly used by professionals, this quality does come with a much higher price tag than crop sensor or full-frame cameras.
Megapixels & Resolution
Used interchangeably, megapixels and resolution actually mean two very different things and it’s important to understand the difference if you want to get the most out of your images.
One megapixel is made up of a million pixels, which 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 of these individual pixels contain information that makes up the final image.
When determining image quality, it’s not only the number of megapixels that matters, but also the size of the photosites (these are what record pixels). Photosites are measured in microns (µm), and their size is largely determined by the sensor size. Cameras with smaller photosites may perform worse in low light conditions and also feature more diffraction when shooting at small apertures, whereas larger photosites allow for a larger transitional tonal value, greater tonal accuracy and better color accuracy.
The term resolution, although often used interchangeably with megapixels, does not simply refer to the number of megapixels. Resolution 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 model 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 have better contrast, colour fidelity and overall sharpness.
To learn more about resolution, refer to the PDF guide.
ISO measures how sensitive the recording medium is to light. The same way that film came in different sensitivities, we can adjust our cameras to be more or less sensitive to light by simply adjusting the ISO.
Higher ISO numbers are more sensitive to light, whereas lower ISO numbers will be less sensitive to light. While increased sensitivity may sound good in theory, the main drawback is that higher ISOs result in a degradation of image quality, which often appears in the form of ‘noise’, especially in the shadow tones (as you would have seen in the video).
ISO, like shutter speed and aperture, can also impact the exposure of an image. However, ISO should not be used as a tool to do this, but rather as a last resort. Although it can be very useful in low light conditions, ISO 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
Another factor that relates to image quality is the type of file format you shoot in — either JPEG or RAW.
Although both file types contain the same number of pixels, RAW images store far more information within those pixels than JPEG images do. This means we have far more control in the post production stage with a RAW image, which can be very useful if you want to make changes to your pictures after shooting.
As I explain in the video, one of the main drawbacks of JPEG files is that the compression can sometimes result in what is known as ‘pixel clumping’. This is when pixels of a similar tone are grouped together. Although this may not initially seem apparent, it does become more obvious as soon as we start to adjust colours and exposure in post production. Although JPEGs may not allow us to extract as much colour detail, especially in highlight or shadow areas, they are still a common file type for photographers who shoot high volumes of image (such as wedding or sport photography). This is because, due to their compression, JPEGs produce much smaller file sizes, allowing you to fit many more images on a memory card.
Each of these factors contribute to image quality, but it’s not to say that you have to shoot with the camera with the largest sensor, with the most megapixels and at the lowest ISO. The points above will help you understand image quality, but you shouldn’t in any way be put off if you don’t have a top end camera. Most of the cameras on the market today are of exceptionally high quality, far greater than those from the days of film - and those cameras produced some of the most iconic images of our time! It doesn't all come down to the tools. If you have the right knowledge, you can create amazing quality images with any camera.
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