Full Frame/FX, APS-C/DX, 4/3, CX, digicam; how to decide which camera to use for different tasks.
Recent developments in the market have presented camera buyers with some complex choices, most of which centre upon the size of the camera’s sensor. Smartphones are replacing cameras at the bottom end of the digicam market, forcing manufacturers to provide larger sensors and longer lenses. ‘Mirrorless’ interchangeable-lens compact system cameras (CSC) are bridging the gap between digicams and DSLRs and replacing the latter for many photographic travellers.
Meanwhile, just about everyone owns at least one imaging device, even if it’s only a smartphone. Photo enthusiasts often own several cameras with different sensor sizes. To help you decide on the size of the sensor in your next camera, this feature looks at issues related to image sensor size.
The sensors in high-end digicams (the only potentially profitable sector of this category) are increasing in size, although these cameras still struggle to compete against CSCs. Fujifilm was the first manufacturer to use a DSLR-sized sensor in a compact camera, announcing its revolutionary X100 model to great acclaim at Photokina 2010.Sony followed suit and has been the most innovative manufacturer of fixed-lens compact cameras, introducing models with 36 x 24mm and 12.8 x 9.6 mm sensors from mid-2012.
In the CSC category, Olympus and Panasonic will stick with M4/3. Canon, Nikon, Pentax, Samsung and Sony will continue supporting APS-C for their DSLR cameras, with Canon, Samsung and Sony using them in CSC models as well. Nikon seems set to stick with CX for its CSCs.
‘Full frame’ sensors are entering the CSC sector, courtesy of new Alpha models released by Sony in late 2103. This format is also trickling down into the enthusiast DSLR category as economies of scale bring sensor prices down. However, cameras with larger sensors continue to command premium prices.
The Current Market
Canon, Nikon and Sony currently offer two sensor sizes within the same system, which can greatly complicate lens buying decisions. Pentax has interchangeable-lens cameras with radically different sensor sizes and lens mounts. Samsung has adopted APS-C for its interchangeable-lens cameras, while Ricoh has interchangeable sensor+lens modules.
The death knell for the 4/3 format was struck with the announcement by Olympus in late 2013 that no further camera bodies would be made. Both Panasonic and Olympus are concentrating on developing cameras for the Micro Four Thirds (M4/3) format and the latest Olympus camera, the OM-D E-M1, has a hybrid AF system that enables older 4/3 lenses to be used with full autofocusing. Third-party manufacturers are producing adaptors that enable lenses designed for one camera system to be used on another, further muddying the waters.
Large vs Small Sensor?
All else being equal, larger sensors will deliver better image quality than small ones. Unfortunately, larger sensors not only increase the size and overall weight of the equipment but also push up its cost ““ sometimes substantially. Choose a camera with a large sensor if you want:
- Higher maximum resolution with current sensor technology.
- Greater control over depth of field, using wide-aperture lenses.
- Lower noise at the base ISO value.
- Brighter viewfinder images for easier manual focusing at equivalent depth of field.
In contrast, small sensors provide the following advantages:
- Lower cost sensors and, therefore, cheaper cameras and lenses.
- Shorter and lighter lenses.
Buyers of interchangeable-lens cameras can currently choose from five sensor sizes, the main features of which are outlined below.
FX or Full Frame
Measuring approximately 36 x 24 mm, these sensors are the same size as a 35mm film frame, which means your lenses will cover the same focal lengths as they did back in the days of film (and as listed in their specifications). Roughly 30% larger than the next size down, they can support larger photosites for the specified resolution. Larger photosites collect more light, which means a higher signal-to-noise ratio and better image quality.
On the downside, you need a bigger camera body to support the larger sensor ““ and large lenses as well. Both cost more to make and, therefore, are more expensive to buy.
APS-C or DX
This sensor size derives from a now-obsolete film format that measured 25.1 x 16.7 mm and had the same 3:2 aspect ratio as 35mm film. Different manufacturers have introduced their own variants, so sensor sizes in this category range from 20.7 x 13.8 mm to 28.7 x 19.1 mm.
Each variant results in slightly different angles of view from lenses for the same focal length. Lenses cover a much narrower angle of view compared to 35 mm film, which is indicated by their crop factor. Nikon, Pentax, Samsung and Sony interchangeable-lens cameras have a crop factor of 1.5x (as do some of the Ricoh GXR modules), while Canon’s crop factor is 1.6x. Aside from their influence on lens mounts, these differences are negligible.
Four Thirds (4/3) and Micro Four Thirds (M4/3)
Introduced by Olympus and Kodak a decade ago, Four Thirds has been designed from the ground up to be entirely digital and open, permitting the interchange of lenses and bodies from different manufacturers. Its sensors have an imaging area of 17.3 x 13.0 mm and a 4:3 aspect ratio plus a 2x crop factor.
Today, this format is dominated by the Micro Four Thirds (M4/3) design, which enables even smaller camera bodies and lenses to be produced. Removing the reflex viewing system means users have to rely on a live preview is shown on either the camera’s LCD monitor or via an electronic viewfinder. Autofocus is via contrast-detection using the image sensor, as in digicams.
CX (Nikon 1)
Announced in September 2011, the Nikon 1 system steps down in sensor size to 13.2 x 8.8 mm, this time with a 3:2 aspect ratio and a 2.7x crop factor. Despite the smaller sensor, the two Nikon 1 bodies and four lenses released so far are much the same size as their M4/3 equivalents.
Digicams
If we leave aside the rangefinder-style models with ‘full frame’, APS-C and ‘1-inch’ sensors, we’re left with digicams that cater primarily for snapshooters (although a few models provide a pocketable ‘go anywhere’ camera for serious shooters. While the sensor sizes in fixed-lens digicams have increased recently, they’re still much smaller than even the CX sensors, enabling smaller cameras to be made. Chip sizes range from 8.8 x 6.6 mm to 5.76 x 4.29 mm, with only the Pentax Q and some Ricoh GXR modules offering interchangeability. Image noise affects such small sensors, even with the most sophisticated processing chips.
Depth of Field Issues
Depth of field becomes shallower as the sensor size increases. With some subjects, such as portraits, a shallow depth of field facilitates background blurring. With others, like landscapes, you want as much of the subject to be as sharp as possible.
The influence of the sensor size explains why shots taken with small-sensor digicams are usually sharp all over, while large format cameras struggle to achieve adequate depth of field in landscapes. Depth of field control can be further constrained in smaller cameras because it is difficult to design lenses with wide maximum aperture settings throughout the zoom range.
This photograph, taken with Canon PowerShot G10, which has a 7.44 x 5.58 mm sensor and crop factor of 4.84x, shows how difficult it can be to blur backgrounds, even with long focal length and wide aperture settings. (30mm focal length – equivalent to 254mm in 35mm format – at f/4.5.)
The same subject photographed with a Canon PowerShot G1X, which has a 18.7 x 14.0 mm 1.9x crop factor. This shot illustrates how much easier it is to blur backgrounds when a shallow depth of field is desired. (60mm focal length – equivalent to 115mm in 35mm format – at the maximum available aperture of f/5.8.)
The same subject photographed with the Canon EOS 5D Mark II, which has a ‘full frame’ 36 x 24 mm sensor using a focal length of 105mm and aperture of f/4 to produce a high degree of background blurring, thanks to the larger sensor and wider maximum aperture.
The ‘Equivalence’ Factor
What happens when you want to capture the same angle of view with the same depth of field in a different sensor format? Actually, the smaller the camera’s sensor, the more difficult this becomes, regardless of whether we’re after a shallow or wide depth of field. You either run out of aperture settings or focal length options ““ or both.
Here’s how the different formats pan out when we’re after a relatively shallow depth-of-field:
– The FX/Full Frame photographer will use a 300mm lens at f/8;
– The APS-C/DX photographer will use a 200mm lens at f/5.6;
– The Canon G1X photographer will use a 162mm lens at f/4.3;
– The M4/3 photographer will use a 150mm lens at f/4;
– The CX photographer will use a 110mm lens at f/2.8;
– The digicam photographer will be at around 60mm at f/1.8 (or smaller).
A couple of problems spring to light. The Canon G1X is restricted to 60.4mm and we don’t know of any digicams with f/1.8 maximum apertures at 60mm.
Suppose we want to shoot landscapes with a wide depth-of-field:
– The FX/Full Frame photographer will use a 28mm lens at f/11;
– The APS-C/DX photographer will use an 18mm lens at f/8;
– The Canon G1X photographer will use a 15mm lens at f/5.9;
– The M4/3 photographer will use a 14mm lens at f/5.6;
– The CX photographer will use a 10mm lens at f/4;
– The digicam photographer will be at around 6mm at f/2.8 (or smaller).
One reason full frame DSLRs have become so popular with professional video shooters is that their larger sensors provide greater control over depth of field than the film cameras that were formerly used. Smaller sensor formats that can’t match this tend to be relegated to amateur video productions.
When it comes to shooting stills, it’s a case of selecting the right tool for the job. Some jobs can be done as well (or better) with a decent M4/3 camera ““ or even a camera with a smaller sensor. For others, a big sensor is the only option.
We’re left balancing several competing factors: size, weight, cost, imaging performance and equivalency with 35mm format being the most significant. If you need a compact and lightweight system, the smaller sensor formats can deliver excellent image quality, although you may be restricted when it comes to wide-angle coverage and defocusing control with tele lenses. These systems will generally be cheaper than a FX system, although by how much is open to change in the future.
Using Crop Factors
Crop factors are so named because they enable you to calculate the field of view any sensor/lens combination covers with respect to the standard 35mm field of view. DSLR photographers find them handy with bodies and lenses that can be exchanged. For example, if you have a 100mm lens designed for a Full Frame/FX body and put it on an APS-C body, it crops out the centre of the lens’s field of view, thereby extending the effective focal length by the crop factor. The angle of view becomes the same as a 150mm lens, as shown in the illustrations on this page.
A shot taken with a 105mm lens on a Full Frame/FX body.
The same shot, cropped to show the coverage if the same lens had been used on a camera with an APS-C sensor.
You might think that using a heavier and more expensive lens with a smaller sensor has few benefits. However, this isn’t the case; with most lenses, resolution falls off towards the edges of the frame. Consequently, the cropped sensor doesn’t use the lowest resolution areas in the lens. Instead, you use the sharpest parts and end up with better image quality overall.
Narrowing the angle of view of the lens also has benefits for sports and wildlife photographers because it enlarges the subject in the frame, enabling them to shoot close-ups without intruding on their subjects.
This is an excerpt from Photo Review Issue 53.
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