DxOMark Mobile Report: Nokia Lumia 920

Summary

With a DxOMark Mobile score of 66 the Nokia Lumia 920 is occupying a space at the lower end of the DxO smartphone ranking. Its score is the same as the HTC 8X Windows phone, but it cannot keep up with the best in class such as the Nokia 808, Apple iPhone 5 or Samsung Galaxy S3. 

The DxOMark team report that the Lumia 920 captures images with good overall exposure, shows good contrast even in difficult situations and very low noise, even under extreme low-light conditions. The latter is achieved by using exposure times about four times longer than most smartphones under similar conditions. Optics stabilization then compensates - at least partly - for the expected motion blur.

On the downside: "texture and sharpness are poor, even under bright light," "white balance is unstable," "colors are slightly too saturated" and "autofocus is neither stable nor repeatable for objects closer than 2m."

In video mode, DxOMark's engineers reported that the Lumia 920 offers "efficient stabilization in low light and bright light for moderate motion" and low noise. On the other hand "autofocus is inaccurate, even in bright light, white balance is unstable, even with constant lighting" and "Jello effect and rotation are not corrected, making stabilization performance only average for stronger motion."

Dr. Frédéric Guichard, DxO Labs’ Chief Scientific Officer, said: '“The Lumia 920 outperforms the highly-regarded Nokia 808 Pureview for video but for stills it misses the mark slightly. Exposure and contrast are generally good even in difficult lighting, and with very low noise levels that are mainly a result of the longer exposure times made possible by stabilization. The main drawbacks are disappointing autofocus accuracy, poor sharpness and some lack of texture.”

Still Photography

Color, Exposure and Contrast

The DxOMark team found that the Lumia 920's "images usually have good contrast even in difficult situations", with good overall exposure. However, DxO found "strong color shading with fluorescent illumnants" and "slightly too saturated colors." While noise levels are relatively low in low light, it was noted that "in extreme low-light conditions exposure time is about four times longer than for most smartphones" and that "the optical stabilization does not compensate for all the subsequent motion blur." Because of this DxOMark scored the Nokia Lumia 920 at 68/100 for color in bright light and 58/100 in low light.

Overall DxOMark awarded the Nokia Lumia 920 scores of

  • 4.5 out of 5 for Exposure
  • 3.0 out of 5 for White Balance accuracy
  • 2.5 out of 5 for Color shading*
  • 3.0 out of 5 for Color Rendering in low light
  • 3.5 out of 5 for Color Rendering in bright light

*Color Shading is the nasty habit cellphone cameras have of rendering different areas of the frame with different color shifts, resulting in pictures with, for example, pinkish centers and greenish corners.

Noise and Details

DxOMark's engineers reported that the Nokia Lumia 920 images show "very low noise even in low light thanks to longer exposure time" but that "images are not very textured" and "many details are blurry,  even outdoors."

Texture Acutance

Texture Acutance is a way of measuring the ability of a camera to capture images that preserve fine details, particularly the kind of low contrast detail (textures such as fine foliage, hair, fur) that can be blurred away by noise reduction or obliterated by excessive sharpening.

Sharpness is an important part of the quality of an image, but while it is easy to look at an image and decide visually whether it's sharp or not, the objective measurement of sharpness is less straightforward.

An image can be defined as "sharp" if its edges are sharp and if fine details are visible, but in-camera processing means it's possible to have one of these (sharp edges) but not the other (fine details). Conventional MTF measurements tell us how sharp an edge is, but have drawbacks when it comes to measuring fine detail preservation. Image processing algorithms can detect edges and enhance their sharpness, but they can also find homogeneous areas and smooth them out, to reduce noise.

Texture Acutance, on the other hand, can qualify sharpness in terms of preservation of fine details, without being fooled by edge enhancement algorithms.

A detail of  target made of a dead leaves pattern, designed to measure Texture Acutance. It is obtained by drawing random shapes that occlude each other in the plane, like dead leaves falling from a tree. The statistics of this model follow the distribution of the same statistics in natural images.

In this example from a DSLR without edge enhancement sharpness seems equal on edge and on texture. Many details are visible in the texture.

In this second example edges have been digitally enhanced, and the edge looks over-sharp, with visible processing halos ('ringing'). On the texture part, many details have disappeared.

At first sight, the images from these two cameras may appear equally sharp. A sharpness measurement on edges will indeed confirm this impression, and will even show that the second camera is sharper. But a closer examination of low contrasted textures shows that the first camera has a better preservation of fine details than the second. The purpose of the Texture Acutance measurement is to qualify this difference.

Note: Acutance is a single value metric calculated from a MTF result. Acutance is used to assess the sharpness of an image as viewed by the human visual system, and is dependent on the viewing conditions (size of image, size of screen or print, viewing distance). Only the values of texture acutance are given here. The measurements are expressed as a percentage of the theoretical maximum for the chosen viewing condition. The higher the score, the more details can be seen in an image. 

For all DxOMark Mobile data presented on connect.dpreview.com we're showing only the '8MP equivalent' values, which gives us a level playing field for comparison between phone cameras with different megapixel values by normalizing all to 8MP, suitable for fairly large prints. DxOMark also offer this data for lower resolution use-cases (web and on-screen). For more information on DxOMark's testing methodology and Acutance measurements please visit the website at www.dxomark.com.
 In good light, texture acutance is very slightly better in daylight than under artificial light sources.
 In lower light the Nokia 808 with its PureView technology is the clear leader here. In bright light the iPhone 5 and Samsung Galaxy S3 come closer to the 808 but the 920 trails behind the competition at pretty much all light levels. Only in low light is it on the same level as the Galaxy S3.

Edge Acutance

Edge acutance is a measure of the sharpness of the edges in images captured by the phone's camera, and again we're only looking at the most demanding of the three viewing conditions that DxOMark reports on, '8MP equivalent.'
 The Nokia Lumia 920's ability to retain fine texture in images doesn't improve much from very low light (20 Lux, for example a floodlit building at night ) to 100 Lux (very dark overcast day) but increases slightly for 700 Lux (outdoors on an overcast day). Results under artificial light are very similar.
 In terms of edge acutance the Nokia Lumia 920 trails behind the competition at all light levels. In bright light the gap to the best in class decreases slightly but is still significant. 

Visual Noise

Visual Noise is a value designed to assess the noise in an image as perceived by the human visual system, depending on the viewing condition (size of image, size of screen or print, viewing distance). The measurements have no units and can be simply viewed as a weighted average of noise standard deviation for each channel in the CIE L*a*b* color space. The lower the measurement, the less noise in the image.

 Visual noise is low in low light levels and decreases to marginal levels in brighter light. Most of the measures noise is luminance grain as chroma noise is being eliminated by noise reduction.
 The results under tungsten light are very similar to the daylight measurements.
 The Nokia 808 is the winner here at low light levels but the Lumia 920 comes close at medium light levels and takes the lead by a whisker in bright light. The Samsung Galaxy S3 and iPhone 5 are noisier than the Nokias in low light but on similar levels in good light. 

Noise & Detail Perceptual scoring

DxOMark engineers don't just point camera phones at charts, they also take and analyse scores of real-world shots and score them accordingly. Their findings for the Nokia Lumia 920 were:

Natural scene

  • Texture (bright light): 2.5 out of 5
  • Texture (low light): 2.5 out of 5
  • Noise (bright light): 4.0 out of 5
  • Noise (low light) 4.0 out of 5
 Bright light sample shot
 100% crop shows good detail
 Low light (20 Lux) studio shot
 Textures lack detail
 Noise is low thanks to longer-than-usual exposure time.

Artifacts

Phone cameras, like entry-level compact cameras, tend to suffer from artifacts such as sharpening halos, color fringing, vignetting (shading) and distortion, which can impact on the visual appeal of the end result. DxOMark engineers measure and analyse a range of artifacts. Their findings after testing the Nokia Lumia 920 are shown below:

Pros:

  • n/a

Cons 

  • Sharpness is not uniform across the image field
  • Strong color fringing

Perceptual scores

  • Sharpness 3.0 out of 5
  • Color fringing 3.5 out of 5

Measured findings

  • Ringing center: 3.2%
  • Ringing corner 2.0%
  • Max geometric distortion 0.5%
  • Luminance shading 5%

Distortion & Chromatic Aberrations

 The graph shows the magnification from center to edge (with the center normalized to 1). The Nokia Lumia 920 shows a very slight complex distortion, with a maximum geometric distortion of 0.5%. You are not going to notice this in normal photography.
 The Lumia 920 shows more lateral chromatic abberations than what we've seen from most of its competitors. 

Autofocus

DxOMark also tests autofocus accuracy and reliability by measuring how much the accutance - sharpness - varies with each shot over a series of 30 exposures (defocusing then using the autofocus for each one). As with other tests these results are dependant on the viewing conditions (a little bit out of focus matters a lot less with a small web image than a full 8MP shot viewed at 100%). Using the 8MP equiv. condition the Nokia Lumia 920  results are unusually low, with an overall score of only 28/100 in bright light and 38/100 in low light.

It's worth noting that autofocus starts from a default position which is infinity in default mode and 20cm in close up mode. If the focus is not reached, the default position is chosen.

With our software version, the AF default mode is the macro mode, which can be a source of errors.

Pros: 

  • AF is fast for objects more than 1m away

Cons: 

  • Unstable AF in both bright and low light
Behaviour:

Autofocus starts from a default position which is infinity in default mode and 20cm in close up mode. If the focus is not reached, the default position is chosen. With our software version, the AF default mode is macro mode, which can be a source of errors.

 Autofocus repeatability - average acutance difference with best focus: Low light 19%, Bright light 30.63%
Flash

DxOMark scored the Nokia Lumia 920 71/100 overall for its flash performance, deducting points for occasional white balance issues in mixed light.

Pros: 

  • Good color preservation
  • Low noise
  • Exposure is stable with the flash

Cons: 

  • White balance is red when flash is mixed with an additional tungsten light source

Overall DxOMark Mobile score for Photo: 64 / 100


Video Capture

DxOMark engineers put phone cameras through a similarly gruelling set of video tests, and you can read their full findings on the DxOMark website here. We'll simply summarize for you. DxOMark found the Nokia Lumia 920's exposure and color reproduction to be good. However, the white balance is very unstable, even in daylight, image stabilization for stronger motion is only average and the autofocus severely lacks accuracy when refocusing to infinity. In this case, it starts refocusing from macro distance to infinity, with visible lens breathing. 

Pros: 

  • Efficient stabilization in low light and bright light for moderate motion
  • Good noise

Cons: 

  • Unstable white balance, even with constant lighting
  • Color shading
  • Inaccurate autofocus, even in bright light
  • Jello effect and rotation are not corrected, making stabilization performance only average for stronger motion

Overall DxOMark Mobile score for Video: 70 / 100