DxOMark Mobile Report: Nokia's Lumia 925 scores nearly as high as 41MP Lumia 1020
Lars Rehm | Published: Nov 7, 2013 at 13:59 UTC22
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:
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".
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:
*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 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.
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.
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."
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.
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:
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:
Distortion and Chromatic Aberrations
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.
DxOMark scored the Nokia Lumia 925 72/100 overall for its flash performance, deducting points for strong corner shading.
Overall DxOMark Mobile score for Photo: 75 / 100
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.
Overall DxOMark Mobile score for Video: 69 / 100