There has been a little speculation that the increase in noise observed in the ISO 100 long exposure noise reduction (LENR) images in yesterday’s post might be wholly or in part the result of the a7RII’s dropping into 12-bit precision when LENR is active.
Fortunately, there’s a way to test that. Yesterday’s exposures were 30 seconds long, made in single shot mode with the camera timing the exposure. That results in 13-bit non-LENR and 12-bit LENR images. In bulb mode, the camera also drops into 12-bit mode (and performs some spatial filtering to boot.
So, if we compare two 30 second dark-field bulb exposures, one with LENR and one without, we’ll have two 12-bit images to compare, and we can see if the LENR still hurts the standard deviation.
A 400×400 central ample without LENR:
And with LENR:
You can see the LENR exposure his higher “sidebands”, indicating more noise, although in both cases, the noise is close to the least significant bit of the 12-bit representation, indicating possilble insufficient dither.
Looking at log representations shows LENR at work. First, the non-LENR image:
There is indication of a few hot pixels in the tail to the right.
Now with LENR:
The tails are gone, but at the expense of increased spread to the histogram.
Now let’s look at a few histograms of the entire image. The danger of doing this is that we could be looking at pattern read noise, but you could argue that LENR is supposed to reduce that, too. The advantage is that, instead of a measly 160,000 pixels in the sample, there will be more than 42 million.
Linear histogram, no LENR:
And with LENR:
The same spreading occurs as in the smaller sample.
Now with log scaling of the vertical axis. First with no LENR:
With the larger population, the hot pixel tail is more apparent.
And with LENR turned on.
The tail is gone, but at the expense of a larger standard deviation.
Let’s put this noise in perspective. It’s so small encoded with 12-bit precision that LENR is unnecessary, even if it had no negative side effects. On the other hand, the damage done by LENR on these images in minimal, and probably inconsequential, once you’ve accepted the drop back to 12-bit precision that comes with bulb mode.
But with long exposures, LENR is a pain to deal with in the field. Best to leave if off at low ISOs. Even at high ISOs you should think twice about using it, especially since Sony starts using it a shutter speeds that are too short.