Full frame? Or crop size sensor? Top Digital Imaging know how

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Image sensor.

In the previous issue we looked at mirrorless full-frame cameras and compared them with the more familiar DSLR cameras. With Nikon and Canon recently joining Sony’s Alpha series in the mirrorless world, full-frame cameras are in the headlines again. The question is, do you really need a full-frame camera? And how do they compare to crop-sensor cameras? That’s the topic of this article.

Image 1 – Example of a 2/3″ Sensor with its Imaging Circle.

The full-frame format refers to a 36×24 mm sized sensor which was adapted from the common 135 film format and introduced by Kodak in 1934.

Just to make things clear from the outset: the more light the sensor can capture, the better the image quality. If you want the very best, you need to go full-frame.

But why stop there? There are medium format cameras out there with even larger sensors.

When assessing image quality, we are looking at a moving target. We can only compare current cameras because each new generation of sensor comes with significant improvements.

Case in point, consider Canon’s first full-frame camera, the EOS-1Ds from 2002. It sported 11 MPixels and an ISO range of 100-1250. Back then this was a serious professional camera, but any of today’s cell phone cameras would put it to shame.

Collecting Photons and MPixels

Apart from sensor size another important factor affecting image quality is the number of pixels on a given sensor. A full-frame and a crop-sensor can both accommodate the same number of pixels, say 25 MPixels.

Each pixel can be thought of as a little bucket collecting photons of light.

All else being equal, the larger the bucket, the higher the sensitivity and the lower the ‘noise’. Larger numbers of pixels also bring an improved dynamic range and higher colour depth. I said all else being equal because, as we just noted, sensor technology keeps improving, their light detecting cells as well as the surrounding circuitry, amplifiers and processors. So we can only compare current camera models.

Table 1 – Some common sensor sizes.

A certain number of MPixels is needed to resolve image detail and for printing to a given print size. In this article I will consider only the sensor dimensions, not the pixel count. That will be the topic of another article, but keep in mind that the overall number of pixels and the pixel density do affect image quality.

Sensor sizes

The Table below shows some examples. Sensor size is often referred to with a “type” designation using imperial fractions such as 2/3” (inch) or 1/1.7”, which are larger than the actual sensor diameters. These units go back to the TV camera tubes from the 1950s. The sensor size is about two thirds the size of the imaging circle. Image 1 gives you an example of a 2/3” sensor. Note that most compact cameras have an aspect ratio of 4:3, whereas the full-frame sensor has a 3:2 ratio.

Below I list only some of the most common sensor sizes and you can look up the dimensions for your particular camera and see where it fits into the Table. Image 2 gives you a graphical comparison.

When comparing cameras and lenses the ‘crop factor’ is important. Some people call this the “focal length conversion factor” but it is a crop factor because the part of the image circle outside the sensor gets cropped. Image 2 shows how a smaller sensor basically crops the image from a full-frame sensor. It is easy to calculate by dividing 43.3 by the sensor diameter. For example, a 1/1.7” sensor with a diagonal of 9.4 mm will give you a 43.3/9.3 = 4.7 crop factor. This sensor would turn a 50 mm lens into a 230 mm tele lens.

Image 2 – Comparing some common sensor sizes.

Pros and cons of a crop-sensor camera

The main advantage of cameras with smaller sensors is their reduced bulk and weight. The lenses only need to project a smaller image circle and can also be designed a lot smaller. Needless to say, the reduced bulk of a camera plus lenses also eases the stress on your wallet.

The wildlife photographer will appreciate the longer reach of common tele lenses. For example, a 200mm lens on a Four Thirds camera (which still has a reasonably large sensor) will turn into a 400mm lens. However, you also need to multiply the maximum aperture by the crop factor as well. Your f/4 lens will slow down to f/8.

This brings us to the question of whether to use full-frame lenses on crop-sensor cameras. Many people think a lens costing many times more must also give them the better results, but it often turns out that the cheap kit lens designed for the smaller sensor will give you sharper images! You can check out the image quality to expect from any lens/camera combination at https://www.dxomark.com/Lenses/Ratings

Depth of field

Depth of field (DOF) is a hotly debated topic. Most photographers, including professionals, will tell you to get a full-frame camera because its larger sensor can create a shallower DOF. This is not strictly true since only focal length, distance to the object, and aperture affect DOF, not the size of the sensor.

In practice, with a larger sensor and the same lens, you need to move closer to your subject to maintain the same frame, thereby getting a shallower DOF.

Cell phone cameras with their teeny-weeny sensor go to some elaborate software tricks to get their background blurred. So, there is some truth that larger sensors make it easier to isolate your subject.

A full-frame camera will always capture more light than its smaller siblings. The question is, will you notice the difference?

Thanks to the high quality of today’s sensors it is quite likely you will only notice an improvement in image quality under very low light situations!

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