So, being recently in receipt of the Olympus PEN E-P5, I decided to compare its noise characteristics throughout its ISO range, which extends from 200 to 25,600 plus a "pulled" ISO 100 that is useful on particularly bright days. I put this up against the Olympus PEN E-PL2, which has settings for ISO from 200 to 6400.
The only reason to increase ISO is to capture more light, and the major penalty one pays is in noise. Digital cameras have come a long way in this regard, with major improvements even within the PEN line, from the first model, the E-P1 (July 2009), through the E-PL2 (January 2011), and finally to the current flagship E-P5 (June 2013). Nonetheless, when testing it is important to shoot RAW images, in order to avoid all in-camera processing, which often includes noise reduction even when all such settings appear to be turned off.
I shot RAW images with the camera on a tripod, using a two second timer to prevent shake. I used the Olympus 45mm f/1.8 lens, an excellent performer, at an optimal f/4. I focused on the same spot on my bookshelf for the first frame, and then turned off AF so the focal distance would be identical for subsequent exposures. I let the cameras choose optimal white balance and did no processing in Lightroom (aside from automatic camera adjustments the software itself makes for each model).
I cropped each image to 300 pixels square, and arrayed them for ease of comparison. The full size image is available on Flickr, so you can make your own judgements.
The first thing to notice is that the crops are not identical between cameras. This is a result of the fact the E-PL2 has a 12 megapixel sensor while the E-P5 has a newer 16 mp sensor. There is also a difference in the white balance. Though consistent within each camera, there is a slight orange cast to the E-PL2 shots, and a slight green cast to the E-P5. The light was dim in the room, and I didn't take special notice, so I cannot say which is the more true to life. In any case, since the cast is consistent it would be easy to correct. And also easy to get right in the first place if one set the white balance to suit the scene.
Next, some general observations. The E-P5 images have significantly more detail, which cannot be the result of the greater number of sensor pixels, since I have cropped the images so that the number of pixels in each square is identical. The images also have greater contrast, easily seen in the lettering. Perhaps this is the result of Olympus removing the anti-aliasing filter. Perhaps it is an improved image processing engine. In any case, it's a welcome result!
The focus on the E-P5 is spot on, whereas it looks as though the E-PL2 was slightly off. Indeed, the newer model cameras are advertised as having an improved AF engine. Besides this, the E-P5 has dramatically increased the number of focus points from 11 to 35, and has a new facility for smaller, more precise, AF points.
Now, on to the effect of increasing ISO. I am quite used to shooting with the E-PL2 up to ISO 800, which comes clean with a little noise reduction, so long as one gets the exposure right. (I should note that correct exposure becomes ever-more important as ISO increases, since under-exposed regions will show increasing noise.) It is even reasonable to shoot at ISO 1600, depending on the scene. After this the noise becomes objectionable.
ISO 800 on the E-P5 seems to have even less noise than ISO 400 on the older camera. ISO 1600 looks very usable. ISO 3200 is only slightly worse than ISO 800 on the E-PL2. I would judge there to be almost a two stop advantage until ISO 6400, which seems to be the last usable setting. But then again, I won't know until I process real-world images and see how the noise patterns interact with the various noise reduction algorithms I have at my disposal. (I have three, from three different vendors.)
In the meantime I will shoot at up to ISO 3200 if the sitution requires it. And since I am often called upon to document events in dim theatres, this will happen with a certain regularity. Above is the full image at ISO 3200, so you can see for yourself.
In my opinion this is an amazing result for a "small" sensor camera. Sensor technology has advanced incredibly in the last four years.