This article is a part of a series about choosing a digital SLR camera:

• Choosing a Digital SLR Camera – Part 1: The basics about a Digital SLR Camera
• Choosing a Digital SLR Camera – Part 2: Resolution
• Choosing a Digital SLR Camera – Part 3: Auto Focus
• Choosing a Digital SLR Camera – Part 4: Sensor Size
• Choosing a Digital SLR Camera – Summary

Sensor Size

The topic of sensor size is closely related to resolution, ISO performance, and in some ways, focus performance as well. In the days of  film cameras, the film size dictates the format of the camera with traditional options such as 35mm film seen in most photo lab shelves; APS, a smaller, newer version of the 35mm; medium format film that ranges between 6”x4.5” to 6”x9” film; all the way to large format films and plates that can exceed 8”x11” in size.

In digital SLR photography, the standard sensor sizes available are “full-frame” 35mm equivalent, APS-C (1.5x or 1.6x crop factor), APS-H (1.3x crop factor), and the 4/3 system (2x crop factor).

The most common sensor size based on the most non-professional camera models is the APS-C format. This format still retains the 2:3 (height/length) ratio of the image of a 35mm sensor, but is physically smaller than the 35mm sensor. Which brings us to discuss the topic of crop factor.

Crop Factor

The term crop factor means the equivalent view of the image if the sensor is cropped off by a certain amount based on a full 35mm surface area.

Note: The term “magnification factor” is a common term used to describe crop factor but this is highly inaccurate as the image isn’t magnified at all as the image is merely cropped off. The apparent size of the subject is identical whether a 35mm full-frame sensor was used or a 2x crop factor sensor was used.

A smaller sensor is generally cheaper to produce and also allows the camera to be designed with smaller overall parts. This allows manufacturers to create cameras that are light and easier for the first-time DSLR owner to get used to carrying a large piece of camera equipment coming from portable cameras.

As seen in Figure 1, apart from the 2x crop factor sensor, all other sensor sizes maintain the 2:3 aspect ratio of the image in terms of height and length. 2x crop factor sensors follow a 3:4 ratio which does not fit the standard print size ratio but is usually visually more balanced when used vertically.

As mentioned, the popularity of APS-C (1.5/1.6x crop) sensors is primarily driven by cost and size. The cost of full-frame 35mm sensors are still a lot more expensive than any crop sensor cameras, often twice the cost of the most expensive crop sensors, so the APS-C format is a good compromise between image area and cost.

The APS-C sensor, however, is almost half the size of a full-frame 35mm sensor size. The reduction of sensor size affects several technical factors such as depth-of-field, image quality, resolution, auto-focus performance, viewfinder visibility, and ISO noise control. These issues are compounded even more with smaller 2x crop sensor cameras.

Effect of Sensor Size on Photographs

As mentioned previously, different sensor sizes affects the final photograph differently. We’ll discuss the following differences in the following order:

• Depth-of-Field (DOF),
• Image Quality and Resolution, and Noise Control, and
• Auto-Focus Performance and Viewfinder Visibility.

Depth-of-Field (DOF)

In simplified terms, the physical size of a camera’s sensor affects depth-of-field in an inverse manner, meaning, the smaller the sensor, the greater the depth-of-field, the larger the sensor, the smaller the depth-of-field. This inverse ratio is increased as the subject gets closer to the minimum focus distance of the lens assuming the camera is using the same lens.

As shown in Figure 1 and 2, a cropped sensor is literally a 35mm sensor chopped off to a smaller, physical size. The reduction of sensor size results to a narrower view in our viewfinder, essentially masking off all four edges of a 35mm sensor. When we mount a 50mm lens  on a 35mm full-frame camera, for example, the actual focal length is 50mm, with an effective field-of-view of a 50mm lens, as it should be.

However, when we mount the same 50mm lens on a 1.5x crop sensor camera, the reduction of sensor size results to an image that appear similar to a 75mm lens on a full-frame sensor camera (50mm x 1.5 crop factor).

If we mount the 50mm on a 2x crop sensor, the resulting image will appear to be similar to a 100mm lens on a full-frame sensor (50mm x 2). As the field-of-view narrows with a smaller sensor, the depth-of-field changes as well.

It is incorrect to assume that just because the resulting field-of-view of a smaller sensor image is similar to a longer lens on a full-frame camera that the two image produce identical outputs.

The physical focal length of a lens alters the minimum focus distance, depth-of-field, and distortion between the subject and camera. So just because 50mm on a APS-C sensor appears to have the same area of coverage as a 75mm on a full-frame, we’re not getting identical images, just an apparent image size in relation to the frame.

With wider angle lenses, subject distortion is evident and the closer the subjects are to the subject, the more pronounced the distortion will appear. The distortion at 35mm on a crop sensor will be more pronounced than a 50mm on a full frame.

Image Quality, Resolution and Noise Control

If we refer back to our discussion about resolution, a larger sensor can support a larger amount of pixels compared to a cropped sensor.

As resolution increases, the amount of pixels packed together in a finite sensor surface increases as well.

Generally speaking, the more pixels are present the less sensitive each pixel is to light because each pixel is now smaller in surface area. The risk of each pixel “spilling” color data to the next pixel also increases as the signal-to-noise ratio is lower than larger pixels. Larger pixels would require a larger sensor and/or smaller amounts of individual pixels for optimal dynamic range, color rendition, and general image quality. Technology continues to push this gap smaller between small and large sensor size when it comes to overall image quality, but full-frame sensors still has an advantage as of now.

The discrepancy between physical sensor size is still big right now for crop sensors to match image quality, noise control, and resolution if the same amount of pixels are present per given area, partly because of lens limitations. As resolution increases, the higher the demand on lenses as well.

Since there is less surface area to project a full image in smaller sensors, a lens must to be able to separate details in smaller increments to retain the high amount of detail and resolution captured.

This can cause older, designed-for-film-use lenses to be less-than-ideal when used on high resolution, small sensor digital cameras. Thankfully, a healthy supply of new “digital” lenses are available and these are designed to match the resolution demands  of smaller sensors.

These new lenses often carry a focal range of that matches the traditional focal ranges of 35mm film cameras, negating the crop factor difference in terms of scene coverage.  For example, a 28-70mm zoom lens for 35mm film/sensor now has the digital equivalent lens that uses a 18-55mm focal length. The shorter focal length compensates the crop factor difference (18×1.5= 27mm/55×1.5=82mm).

The eventual bottleneck in resolution in digital photography won’t be the sensor, but the lenses’ capability to resolve details to suit high resolution camera sensors of the future.

Auto-Focus Performance and Viewfinder Visibility

The sensor itself doesn’t really have a direct relation with AF performance and viewfinder brightness, however, larger sensors require larger mirror and viewfinder prism. With a larger mirror and prism, more light is available for the user when looking through the viewfinder which allows much more accurate viewing, manual focusing, and overall usability for the photographer.

Should I consider a cropped sensor or a full-frame sensor?

The answer to that question mainly boils down to cost. A full-frame digital SLR costs a lot more than a cropped sensor digital SLR, on average, about twice the cost of the highest-tier cropped sensor camera. The larger physical size of a full-frame camera may be an issue to those who don’t want to carry a large camera as well.

The image quality and noise control benefits of a full-frame sensor is apparent to most experienced users but the main question boils down to whether the advantages are worth the price difference compared to a crop-sensor camera.

If you can afford a full-frame digital SLR, and you’re used to shooting with film cameras in the past, then the transition from film to digtal SLR would be seamless as the resulting images will look very familiar in framing. If you’re just starting out on a budget or maybe have a collection of crop-sensor optimized lenses, then it may be best to stick to cropped sensor cameras instead.

The choice between a cropped sensor and a full-frame sensor digital SLR camera should be based more on cost and size as the image quality differences are usually not as noticeable unless scrutinized extensively through test charts and large prints. The initial cost of the camera coupled with its physical size are the main disadvantage of a full-frame camera, while a crop sensor camera often will reach its image quality limitations faster than a full-frame camera as megapixel count continues to increase. There’s nothing wrong with choosing either format, both will full-frame and crop sensor cameras are capable in provide high-quality images, just pick one that suits your budget, ergonomics, and print requirements.

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