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:

Jared,

thank you for the comprehensive explanation - much appreciated indeed.

I see both white-ish speckles AND black, bigger ones; I thought either appeared randomly, but now I have a better understanding of the phenomenon.

Would making the back frame shorter than the actual exposure help in limiting these? Some manufacturer use to have a black frame shorter than the actual exposure, and I thought this might be the reason why.

Thanks again, best

Vieri

I haven't seen anyone scaling their dark frames, but it is certainly possible to do.  It's a little more process intensive.  Here is the standard process:

Take a light frame, for sake of argument 5 minutes duration

Take a dark frame, the same five minutes

Subtract the dark frame from the light frame.  That's your new final exposure.

Here is what you would do for a scaled dark:

Take a light frame

Take a bias frame (shortest possible exposure with the shutter closed--just read noise)

Take a shortened dark frame, say half the length of your light

Subtract the bias from the dark frame

Multiply all values in the dark frame by two

Subtract the bias from your light frame

Subtract the new 2x dark from the light

The result is your new, final exposure

Scaled darks work fairly well as long as the exposures aren't long enough for hot pixels to be clipping.

I'm not certain how best to address them aside from what you are already doing.  In astrophotography, most people choose to use master darks (which will create a better result than a single dark frame) and dither their images--each image is slightly mis-aligned from the last, and then they are averaged together using a statistical method such as "sigma clip" to completely remove hot and cold pixels.  That is technically possible with a terrestrial image as well.  You could take five separate two minute exposures or so with the tripod moved ever so slightly between frames, then use a sigma clip combine to average the frames.  Of course, Photoshop and Lightroom don't have sophisticated stacking algorithms so you'd have to use some astronomy software for that, and most astronomy software doesn't do a great job on aligning terrestrial images since it expects point sources to be available, but it is definitely doable.  I have done it occasionally.  I wouldn't recommend it, though, unless you just can't accomplish what you want to with the techniques you are already trying.

Perhaps an incremental solution would be to cut your exposure duration significantly (to lower the thermal noise), boost your ISO to compensate, then take multiple exposures and average them together in Photoshop without any re-alignment.  You wouldn't get the benefits of dithering, but you would start out with lower thermal noise.  This comes at the cost of higher read noise in your image, but I bet you could find a reasonable balance.  For example, instead of one single 8 minute exposure at ISO 100, try 8 separate one minute exposures at ISO 800 averaged in Photoshop as separate layers.  You can do this with the "smart object" tool.  That should give you an overall cleaner result without requiring any more time when shooting.  I'm not quite sure what it will do to the clouds, but it might work quite well.  Give it a shot.

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Jared Willson

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