Sony A7RII: LENR vs. 14 bits

The rather cryptic title of this blog post probably needs some explanation. LENR stands for Long Exposure Noise Reduction. Nearly all current digital cameras offer this feature, sometimes you can switch it off and sometimes not.

Images of digital cameras always have different kinds of noise. Longer exposure times, higher ISO, higher ambient temperatures, and some post-processing actions amplify noise. One method to reduce the noise of long exposure images is the dark-frame subtraction. The idea behind this is, that some noise, the so-called fixed pattern noise, will be identical from shot to shot (under the same circumstances). Therefore directly after an image has been captured a second image is shot with the shutter closed. This image contains nothing but noise. Afterward, the noise (of the second image) is subtracted from the first image to reduce the fixed-pattern noise. This can either happen directly in the camera or in post-processing with tools like Adobe Photoshop. The above-mentioned LENR function does exactly this.

Normally this in-camera function has just one disadvantage. The waiting time before you can shoot the next image will be doubled. For example, if you shoot a long exposure image of 30 sec. and LENR is set to ON, the camera will automatically shoot another dark-frame image directly after the first image with an exposure time of another 30 sec. This means you have to wait 60 sec. before you are able to shoot another image. This is the case with nearly all digital cameras and not only with the A7RII. But with the Sony A7RII, there is another aspect to consider.

Normally RAW images of the Sony A7RII are recorded with 14 bits per pixel (although in this 14 bits mode only 13 bits contain real information). But in certain shooting modes, the camera reduces the recording to 12 bits (see Sony Help Guide).

The following settings will lead to a reduction to 12 bits:

  • bulb mode (this means all exposures > 30 sec.)
  • continuous shooting mode
  • silent shooting mode
  • long exposure noise reduction LENR = ON

Don’t ask me what’s the reason behind this. My guess is, that it has something to do with the limited read-out and processing speed of the A7RII electronics. Since I mainly shoot landscapes and very seldom use exposure times >30 sec. I have no problems avoiding the first three modes. But in the past I very often used LENR.

According to some published tests, the reduction from 14 bits to 12 bits can lead to about 1 f-stop more noise and 1 f-stop less dynamic range (see here and here).

The question is: is it better for maximum image quality to stay in 14 bits mode and avoid LENR or to activate LENR and live with 12 bits? And under which circumstances should one choose which option? Since I found no clear answer to this question on the web I decided to do my own tests.

The following factors will clearly have an influence on the results besides LENR on/off and 14 bits vs. 12 bits:

  • ambient temperature
  • exposure time
  • ISO
  • exposure correction in post-processing
  • LENR

It’s difficult for me to control the ambient temperature. My tests were all done at a constant ambient temperature of 22°C. At very low or very high temperatures the results can (and very likely will) be completely different.

Regarding exposure time I decided to do all tests with 15 sec.. For landscape photography, this is often the best compromise both for long exposure images of waterfalls and for milky way/aurora images. My guess is, that there will be no visible difference if you use 30 sec. instead of 15 sec. and for exposure times > 30 sec. the camera will always be in 12 bits mode independent of the LENR setting.

I chose two different ISO settings: ISO 100 and ISO 1600. ISO 100 is the base ISO of this camera with the highest image quality. I will probably use this value for 95% of my images. ISO 1600 is my default value for milky way/aurora images. I rarely use ISO values in between these two.

If you underexpose an image and correct this in post-processing this will lead to significantly more noise. But often this is unavoidable. If you want to shoot for example an image of the milky way or starry sky and still want to see something of the foreground you have to significantly lighten the foreground in post-processing. Otherwise the stars would be overexposed (or the foreground would be completely black). Alternatively, you could combine two different exposures for the ground and the sky in PP. But since I try to do everything in LR, I always try to capture everything in one image. For this test, I decided to either leave the exposure in PP unaltered or lighten the image by 3 stops with the exposure control in LR to simulate the processing of typical nightscape images.

And of course, all images were shot twice with LENR=OFF and LENR=ON.

Here are the other settings used on this test: Sony A7RII with Leica 3.4/135 Apo on Novoflex adapter, Gitzo tripod with Burzynski head, EFC, uncompressed RAW, self-timer, IBIS=off, WB=daylight, WB corrected in LR with pipette tool on the white background, in-camera-lens-corrections=off, LR lens-profile=off, LR sharpening 50/0.7/100/0, LR contrast +30, clarity +30, all other LR settings = default.

These rather aggressive LR settings for sharpening, contrast, and clarity are not my default settings for landscape images but rather the worst-case settings regarding noise which I use sometimes for low contrast images.

The test subject was my second A7RII with one of the Loxia lenses in front of a white background.

test subject

Here are the results:

ISO 100, 15 sec., f/8, LENR=OFF, 100% crop:

LENR=OFF, f/8, ISO 100, +-0

ISO 100, 15 sec., f/8, LENR=ON, 100% crop:

LENR=ON, f/8, ISO 100, +-0

ISO 100, 15 sec., f/11 + polfilter, +3 exposure control in LR, LENR=OFF, 100% crop:

LENR=OFF, f/11+POL, ISO 100, -3

ISO 100, 15 sec., f/11 + polfilter, +3 exposure control in LR, LENR=ON, 100% crop:

LENR=ON, f/11+POL, ISO 100, -3

ISO 1600, 15 sec., f/5.6 + ND 1.8 filter, LENR=OFF, 100% crop:

LENR=OFF, f/5.6+ND1.8, ISO 1600, +-0

ISO 1600, 15 sec., f/5.6 + ND 1.8 filter, LENR=ON, 100% crop:

LENR=ON, f/5.6+ND1.8, ISO 1600, +-0

ISO 1600, 15 sec., f/5.6 + ND 1.8 filter, +3 exposure control in LR, LENR=OFF, 100% crop:

LENR=OFF, f/5.6+ND1.8, ISO 1600, -2

ISO 1600, 15 sec., f/5.6 + ND 1.8 filter, +3 exposure control in LR, LENR=ON, 100% crop:

LENR=ON, f/5.6+ND1.8, ISO 1600, -2

At ISO 100 the difference between the two settings is very small, even with +3 exposure control in PP. But the 14 bits images with LENR=OFF look slightly better to me with less obtrusive noise and better details. For my long exposure shots at ISO 100, I will, therefore, use LENR=OFF.

At ISO 1600 LENR=ON leads to less obtrusive noise and especially fewer red dots/artifacts with +3 exposure control in PP. I will, therefore, switch the LENR to ON for nightscape, aurora and milky way images shot at or around ISO 1600.

Remember that these results and the recommended settings are only valid for ambient temperatures around 20°C. If you plan to shoot in very hot or very cold environments you should do your own tests.

2 thoughts

  1. Interesting study!

    Mirrorless cameras are actually at a disadvantage compared to SLRs since their Sensors are “on” much longer and thus heat considerably. As Roger Clark pointed out: “Low dark current is a key factor in long exposure low light photography. Dark current doubles every few degrees increase in temperature. Typically, the doubling in CMOS sensors is every 5 to 6 degrees Centigrade.”


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