DxOMark Mobile Report: Nokia Lumia 925

At a quick glance the Nokia Lumia 925's camera specification appears to be identical to its predecessor's, the Lumia 920, but if you look again you'll see that there are a few small but potentially important differences. The lens gains an extra element with a now 6-element design, a dual-LED flash element is added and the camera also gains an ISO 3200 setting. According to Nokia the image processing has also been significantly improved, to offer both improved detail in good light and better quality in low light. (We recently pitted all three Lumia 900 series models — the 920, 925 and 928 — against one another in our Nokia smartphone shootout.)

The Lumia 925 is PureView branded but doesn't use the Lumia 1020's large sensor and pixel-binning algorithms. Instead it features a conventional 8.7 MP CMOS sensor, a F2.0 lens and optical image stabilization. Its aluminum-edge body comes with a polycarbonate back and integrated antenna. On the front you'll find a piece of curved Gorilla Glass 2. At 4.5 inches the screen itself is the same size as that of the Lumia 920 and 1020. The device also features wireless charging (with an accessory cover) and LTE connectivity.

With the introduction of the Lumia 1020 and its large 1/1.5-inch 41MP sensor the Lumia 925 has lost its status as Nokia's flagship smartphone but given its street price is currently almost $250 lower than the 1020's it can still be a compelling device for the price-conscious mobile photographer. Read on to find out how it performed in DxO's comprehensive studio tests.

Nokia Lumia 925 headline specification:

  • 8.7 MP CMOS sensor
  • F2.0 lens
  • Optical Image Stabilization
  • Windows Phone 8
  • 4.5" AMOLED screen, 1280x768 pixels resolution
  • 1.5 GHz Dual-core Snapdragon processor
  • 1GB RAM
  • 16GB internal memory
  • 2000 mAh battery
  • Wireless charging via accessory cover
  • LTE

Summary

With a DxOMark Mobile score of 73 the Nokia Lumia 925 finds itself on the 6th position on DxO's smartphone ranking. It scored just one point less than its bigger brother, the Lumia 1020, and is trailing four points behind the current top-rated camera phone, the Nokia 808.  

The DxOMark team report that the Nokia Lumia 925 captures images with "good overall exposure" and that "in extreme low light conditions, exposure stays good when other camera phones fail". The image output shows "very low noise levels with no chroma component" and the flash performance is very good, with "good exposure, color, white balance, texture and noise". 

On the downside: Images show "noticeable color shading with indoor lighting and sometimes outdoor, low-contrast details is badly preserved, especially in low light and in low light conditions, exposure time is too long which may cause motion blur." The 925 occasionally also struggles with white balance which "is sometimes blueish outdoors or with fluorescent lighting."

In video mode, DxOMark's engineers reported that the Nokia Lumia 925 offers "good texture reproduction" and that the "video stabilization is reliable in both bright and low light". On the downside there are "instabilities in white balance when changes occur in light conditions or scene content, unpleasant sudden corrections in video stabilization during walking motion and visible color fringing".

Still Photography

Color, Exposure and Contrast

The DxOMark team found that the Nokia Lumia 925's images show "good overall exposure and -pleasant colors outdoors". "In extreme low light, the exposure stays good thanks to longer exposure time". However, there is also "strong color shading with some illuminants (e.g. fluorescent lightings), the white balance is sometimes blueish outdoors" and shows "some variation with indoor lighting".

Overall DxOMark awarded the Nokia Lumia 925 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 in low light*
  • 3.5 out of 5 for Color shading in bright light*
  • 3.0 out of 5 for Color Rendering in low light
  • 4.5 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 925 image output shows "very low noise levels, even in low light conditions, thanks to longer exposure time and no chroma noise". On the other hand "low-contrast detail is badly preserved, especially in low light and in low light conditions, exposure time is too long (1/4s compared to 1/15s for other camera phones at 10 lux), which can cause motion blur".

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.
Texture acutance increases with light levels and is slightly better under low tungsten light than daylight balanced sources. 
In terms of texture acutance the Nokia Lumia 925 is on a similar level as the Samsung Galaxy S4 and iPhone 5s in low light but cannot quite keep up with the S4 at 100 and 700 Lux. The Lumia 1020 with its large sensor and downsampling technology is a notch above the competition at all light levels.

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 Lumia 925's ability to retain fine texture in images decreases very slightly from very low light (20 Lux, for example, a floodlit building at night ) to 100 Lux (very dark overcast day) and then increases significantly at 700 Lux (outdoors on an overcast day). 
In terms of edge acutance the Nokia Lumia 925 trails slightly behind the competition in all but the brightest conditions. 

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 levels are low and remain almost unchanged across the Lux scale. Most of the measured noise is luminance grain as chroma noise is being eliminated by noise reduction.
The Nokia Lumia 925's noise levels are very similar to the Samsung Galaxy S4 and Nokia Lumia 1020. Apple's iPhone 5s is significantly noisier at low and medium light levels. 

Noise and 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 925 were:

Natural scene

  • Texture (bright light): 4.3 out of 5
  • Texture (low light): 3.5 out of 5
  • Noise (bright light): 4.0 out of 5
  • Noise (low light) 3.8 out of 5
Bright light sample shot
100% crop
Low-contrast detail is smeared but noise levels are low.
Low light (20 Lux) studio shot
100% crop
Low-contrast detail is lost through noise reduction but noise is very well controlled.
At low light there is some visible luminance noise (grain) but no chroma noise.

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 925 are shown below:

Cons 

  • Slight fringing sometimes noticeable

Perceptual scores

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

Measured findings

  • Ringing center: 9.0%
  • Ringing corner 6.2%
  • Max geometric distortion -0.2%
  • Luminance shading 21.1%

Distortion and Chromatic Aberrations

The graph shows the magnification from center to edge (with the center normalized to 1). The Nokia Lumia 925 shows a slight pincushion distortion, which you are not going to notice in normal photography.
The Lumia 925 shows some chromatic abberations that can be noticeable under close inspection of images.

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%). The overall score for the 925 is 79/100 in bright light and 65/100 in low light.

Pros: 

  • Fast AF both in low and bright light
  • Good repeatability when taking pictures using the physical shutter button

Cons: 

  • Some AF errors when using the touch screen to focus
  • Overshooting in all situations
Autofocus repeatability - average acutance difference with best focus: low light 7.79%, bright light 6.63%

Flash

DxOMark scored the Nokia Lumia 925 72/100 overall for its flash performance, deducting points for strong corner shading. 

Pros: 

  • Good overall behavior with flash: good exposure, color rendering, white balance, texture preservation and noise levels

Cons: 

  • Strong loss of light in image corners

Overall DxOMark Mobile score for Photo: 75 / 100


Video Capture

DxOMark engineers put phone cameras through a similarly grueling 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 925 to produce good texture and offer efficient image stabilization in video mode. However, the white balance can be unreliable when the light changes, the OIS can overcorrect when walking while recording and there is some visible color fringing in the video footage. 

Pros: 

  • Good texture reproduction
  • Video stabilization is reliable in both bright and low light.

Cons: 

  • Instabilities in white balance when changes occur in light conditions or scene content
  • Unpleasant corrections in video stabilization during walking motion
  • Visible color fringing

Overall DxOMark Mobile score for Video: 69 / 100

DXOMark Mobile Score
73

DXOMark Image Quality Assessment

After the launch of the Lumia 1020 the 925 might not be Nokia's flagship smartphone anymore but at an almost $250 USD savings compared to the 1020 means it can still be a great alternative for cash-conscious mobile photographers.

The Nokia Lumia 925 captures images with good exposure, even in very low light, and low noise levels across the ISO range. Flash performance is also very good. 

On the downside images can show color casts under artificial light and low-contrast detail is lost through noise reduction, even in bright light. Extremely slow shutter speeds in low light also increase the risk of image blur through camera shake. 

In video mode the Lumia 925 captures good detail and offers very efficient image stabilization but the white balance can struggle when light conditions change, there is visible color fringing and the OIS has a tendency to overcorrect when you walk while recording a video. For a more detailed analysis, visit www.dxomark.com.

Photo Mobile Score 75   Video Mobile Score 79
Exposure and Contrast 88   Exposure and Contrast 79
Color 76   Color 76
Autofocus 72   Autofocus 62
Texture 61   Texture 78
Noise 84   Noise 59
Photo Artifacts 83   Video Artifacts 68
Flash 72   Stabilization 60