ISO/Sensitivity accuracy

In this section of our reviews we are measuring the actual sensitivity of each indicated ISO sensitivity. This is achieved using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV.

Like several recent Olympuses, the E-520 proved to have inconsistently indicated ISO sensitivities. In reality ISO 100 proved to be marginally more sensitive than indicated (more like ISO 125).

Indicated sensitivity	Olympus E-520 (actual sensitivity)	Canon EOS 450D (actual sensitivity)	Sony DSLR-A350 (actual sensitivity)
ISO 100	ISO 125	ISO 100	ISO 100
ISO 200	ISO 200	ISO 200	ISO 200
ISO 400	ISO 400	ISO 400	ISO 400
ISO 800	ISO 800	ISO 800	ISO 800
ISO 1600	ISO 1600	ISO 1600	ISO 1600
ISO 3200	n/a	n/a	ISO 3200

ISO Sensitivity/Noise levels

ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. This works by turning up the "volume" (gain) on the sensor's signal amplifiers (remember the sensor is an analogue device). By amplifying the signal you also amplify the noise which becomes more visible at higher ISO's. Many modern cameras also employ noise reduction and/or sharpness reduction at higher sensitivities.

To measure noise levels we take a sequence of images of a GretagMacBeth ColorChecker chart (controlled artificial daylight lighting). The exposure is matched to the ISO (ie. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.5 in this review). Click here for more information. (Note that noise values indicated on the graphs here can not be compared to those in other reviews). Room temperature is approximately 22°C (~72°F), simulated daylight lighting.

Olympus E-520 vs. Canon EOS 450D (Rebel XSi) vs. Sony A350 vs. Pentax K200D

• Olympus E-520: Olympus 50 mm F2.0 Macro lens, Aperture Priority, Manual WB,
  Default Parameters (Normal), High ISO NR (Normal), JPEG Large/Fine
   
  
• Canon EOS 450D: Canon 50 mm F1.4 lens, Aperture Priority, Manual WB,
  Default Parameters (Standard PS), High ISO NR (Default; Off), JPEG Large/Fine
  
  
• Sony DSLR-A350: Minolta 50 mm F1.4 lens, Aperture Priority, Manual WB,
  Default Parameters (Normal), JPEG Large/Fine
  
  
• Pentax K200D: Pentax FA 50 mm F1.4 lens, Aperture Priority, Manual WB,
  Default Parameters (Normal), High ISO NR (Normal), JPEG Large/Fine
  

Olympus E-520 ISO 100	Canon EOS 450D ISO 100	Sony DSLR-A350 ISO 100	Pentax K200D ISO 100
Olympus E-520 ISO 200	Canon EOS 450D ISO 200	Sony DSLR-A350 ISO 200	Pentax K200D ISO 200
Olympus E-520 ISO 400	Canon EOS 450D ISO 400	Sony DSLR-A350 ISO 400	Pentax K200D ISO 400
Olympus E-520 ISO 800	Canon EOS 450D ISO 800	Sony DSLR-A350 ISO 800	Pentax K200D ISO 800
Olympus E-520 ISO 1600	Canon EOS 450D ISO 1600	Sony DSLR-A350 ISO 1600	Pentax K200D ISO 1600

At the lowest two ISO settings, there's little to choose between these four models, in terms of noise. There are contrast differences, reflecting the different companies' approaches to image rendering, but similar levels of detail in all the images. At ISO 400, the Olympus image starts to soften noticeably, in comparison to its peers. Beyond that, the Olympus takes a hard line on noise - generating images with little chroma noise, a degree of luminance noise but very little in the way of detail, which has all been smoothed away by noise reduction.

Both the Canon and Pentax retain more detail (while taking rather different attitudes towards noise - the Pentax allowing much more luminance noise to remain, leaving a gritty, grain-like texture). The E520 offers images with little in the way of distracting color noise but, particularly at ISO 1600, the results are a touch mushy for our tastes. (It arguably does better than the Sony, which struggles with both noise and excessive noise reduction at high ISO settings).

The graphs below support this assessment, with the Olympus data scoring very well in terms of chroma and gray noise. What the numbers don't represent, of course, is the effect on image quality of the noise reduction required to give these figures. There is a trade-off being made to achieve these results but it's one the user can choose - turning down the 'noise filter' option allows more noise through by applying lighter noise reduction. We'll look at that in more depth on the next page.

Gray luminance noise graph

The E520's noise figures are very impressive though this is mainly because the camera is applying a relatively high degree of noise reduction.

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis.

Black luminance noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis.

Chroma (color) noise graph

Chroma noise reduction works pretty much the same way system as the luminance variant, the curve is being kept very flat throughout the entire sensitivity range.

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of color on the vertical axis.