Leica SL Sensor Measurements Published at PhotonsToPhotos

Started Mar 2, 2017 | Discussions thread
Jared_Willson Contributing Member • Posts: 542
Re: Leica SL Sensor Measurements Published at PhotonsToPhotos

vbd70 wrote:

bclaff wrote:

vbd70 wrote:

bclaff wrote:

jng204 wrote:

Thank you for this information Bill.

Always a pleasure.

For studio use, what is the best ISO to select for maximum DR? ISO 50?

Yes.

What is the native ISO?

Trick question. With dual conversion gain you have two native ISOs.
In this case ISO 50 and ISO 200.

Regards,

Thank you Bill, most interesting. I always thought that ISO 100 was the native ISO for the SL, but it appears I was wrong.

You're not alone but I have no idea where the ISO 100 idea came from.
Even DxOMark shows the base as ISO 50 :

In your opinion, for long exposures (5+ minutes), which ISO would be better for less noise? Thanks in advance, best

Is this for astro-photography?
In any case the SL is very ISO Invariant from ISO 200 up :

So it depends on whether you're willing to trade highlight clipping for just a small amount of extra shadow recovery.
I would think ISO 200 to ISO 800 would be best.

Regards,

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Bill ( Your trusted source for independent sensor data at http://www.PhotonsToPhotos.net )

Thank you Bill! No, it's not for astrophotography, it is for long exposure landscape photography - expressive use of long exposure, let's say, something like this:

I have been using ISO 100 so far, but I can see artefacts (little dots) on exposures of 10+ minutes. I think I'll go back to ISO 50 and see if this makes any difference. Best,

Vieri

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Vieri Bottazzini, Leica Ambassador, ABIPP EP
https://vieribottazzini.com
https://vieribottazziniworkshops.com

I am an astrophotographer so know a fair amount about long exposures.  Here is where the little dots come from:

In a long exposure, there are some noise elements that start to make a difference in your exposure that simply don't exist in a shorter exposure.  The first of these is thermal noise.  Random motions in the silicon substrate of the chip cause signal to accumulate on the CMOS sensor that does not come from light.  This is why cameras do long exposure noise reduction--they subtract a second exposure with the shutter closed in order to reduce thermal noise.  It works pretty well, but it's not perfect.  On the Leica SL you can't turn it off or on--it simply runs whenever the exposure is greater than or equal to one second.  There are two problems with this simple dark frame subtraction.  The first is that the signal varies a little bit from one exposure to the next--it would actually be better to subtract an average of several (or even a few dozen) dark frames.  But nobody is willing to wait for that.  Astronomers make "master darks" during daylight hours and use those, but you can't do that with a normal camera.  The second issue is that the amount of thermal noise is dependent on the temperature of the chip, and the chip tends to heat up a bit during a long exposure.  Surprisingly, it's a fairly big deal.  A couple of degrees can make a noticeable difference.  The end result is that for a multi-minute exposure, the dark frame taken by the camera is often at a slightly higher temperature than the light frame of your actual image.  The subtraction of the dark frame is therefore less than perfect, leaving speckles behind.

There is a second source of noise in a long exposure at night, and this one comes from outside the camera.  Cosmic rays are routinely picked up by the camera as white speckles.  I know this sounds ridiculous, but it's absolutely true.  You can often see them as tiny little streaks, perhaps a couple or three pixels long.  These are from cosmic ray strikes during the exposure.  If your speckles are little streaks rather than a single pixel, it may well be from cosmic rays.  Normally you need a cooled astronomy camera to notice these since they are generally swamped by things like thermal noise in a digital SLR or mirrorless camera.

There is a third source as well that can be significant in some cameras.  The glass used in the cover slip over the CCD has radioactive iodine in it.  This creates beta decay that will also show up in a long exposure at night.  If you are seeing little streaks of perhaps eight or ten pixels in length that curve--like the beginning of a spiral pattern--that is likely from radioactive iodine in the glass over the CCD.  Again, it would be rare for this to be an issue with a digital SLR--this noise is usually drowned out by other sources of noise.

What you are seeing is mostly thermal noise that can't quite be corrected by the single dark frame with some smaller contributions from cosmic rays and radioactive decay.  Are you getting white speckles or black ones?  If it's black ones, it's likely that the dark frame is slightly over-correcting since the sensor will be hotter during the dark frame than it is during the actual exposure.

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 Jared Willson

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