I stumbled across something interesting on micro-4/3 camera forums, and thought it worth pursuing. It’s been said (here, and here, and here) that the Olympus E-M1 has a problem with long exposures. I’d noticed what looked like grain in my E-M1 images, so thought maybe it had a similar cause.
Since I’ve got an E-M1 and an E-M5 camera, and a bit of history with debugging, I thought I was well-placed to do some characterization testing. To get a fair comparison, I first ran pixel mapping on my E-M5 and E-M1 bodies (to take care of any obviously “hot” pixels on the sensors), then took dark images (with the lens cap on) under the same conditions with both bodies:
- ISO 200
- 60 second exposure
- Noise Filter set to Standard
I did this twice for each camera body — once with Noise Reduction off, and once with it on. I took all 4 RAW images, converted them to JPG using Olympus Viewer 3, then grabbed a 400×400 pixel crop out of the center of each.
Here’s how they compare:
You can see that Noise Reduction does a pretty good job on the E-M1, but still — the un-filtered image is dramatically noisier than was the case for the model’s predecessor. Not a good thing for a model being billed as Olympus’ “flagship” camera.
Meanwhile, Noise Reduction works by taking a second image of equal exposure (but with the shutter closed) and subtracting it from the first — so a 60 second exposure will be followed by 60 seconds in which you can’t do anything with the camera. Not good for astrophotography or fireworks or low-light HDR.
Given that Olympus’ cameras turn off Noise Reduction when sequential shooting is being done, I suspect this issue is also at fault for the “grain” I’ve seen in a lot of my low-light images. I take low-light photos in 3-shot bursts using low-speed sequential shooting, in order to get at least one frame unblurred by my button pushing.
Bottom line — this is a big problem for long exposures that can’t (for whatever reason) use Noise Reduction, even at low ISOs.
Update 18 November: the data, sliced in more directions…
I did a few more tests over the weekend, just on the E-M1. First, I wondered if the Noise Filter settings would make any difference in the results — so at ISO 200, taking 60 second exposures (approximately, timed by the display on the camera’s screen), here’s what things look like (all slices are 100% crops):
So, setting the Noise Filter to High takes some of the edge off — but still, not a huge improvement.
Finally, I was wondering if changing the sensor ISO had any effect on the sensor noise — so I took additional images assuming that sensor reciprocity (doubling ISO and halving exposure time) still held:
So at first blush, ISO seems to have little impact on the noise (I was expecting this noise would increase at higher ISOs), while the exposure time appears to be the dominant factor. I’ll have to run more tests to double-check this. Stay tuned…