Once in a year opportunity : ) .... messed up. : (

Started 9 months ago | Discussions thread
GeorgianBay1939
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Re: Once in a year opportunity : ) .... messed up. : (
In reply to clengman, 9 months ago

clengman wrote:

GeorgianBay1939 wrote:

clengman wrote:

GeorgianBay1939 wrote:

clengman wrote:

It blows my mind that more people aren't following the point you've tried to make about exposing for the highlights in this shot rather than for the overall scene.

Mine too. But I am not nearly as experienced a photog as you, and thought that I didn't explain it well enough.

Thanks but I'd hardly call myself an experienced photographer. I think we're coming from a similar place. I've been an enthusiastic (but certainly no better than mediocre) amateur for a couple years. I'm also pretty interested in the technical aspects of photography and I've got enough of a background in math and engineering to follow a lot of the information that the bona fide technical whizzes provide regularly.

Yep, similar experience and background. I am also indebted to the whizzes here for lots of help.

It's clear that, like you're trying to point out, you could have used a fast lens, fast shutter and base ISO to freeze motion AND expose the lights on the train at close to the clipping point.

Right. It took me a little thinking to realize that.

The question for me is whether you would have had enough good noise-free detail in the shadows to provide the context that you want in addition to the bright lights on the train. I'm certain that my e-pl1 wouldn't have done a very good job with it. I know the new sensors are much better. Are they really that much better?

That was my question too. I looked at the converted image in Lightroom with no shadow adjustment and realized that the snow and rails in front of the train had lots of exposure to give good brightness (admittedly at ISO 800, an analogue gain of 2 stops ). The image I posted was virtually unchanged with the shadow slider. I did pull down the White and Highlight sliders to reduce the blowing/clipping of the coloured lights.

Here is a screen shot of the unadulterated Lightroom version:

( I have to use DNG because I haven't yet upgraded LR to 5.3, which I need for GX7 .RW2 files.)

I left the black and highlight clipping indicators on for the above screen shot. A lot of the red, blue and some green lights show red due to the indicators.... and the headlights of course.

Here is the RawDigger rendering :

The clipping indicators are turned off. 45k overexposed, 10M underexposed

So about 6M are salvageable. All that snow/rail in front of the loco is fine, as it is. No boosting is needed.

Your question relates to how many stops can I recover without excessive noise (when I reduce 1)exposure by a few stops and 2) analog gain by a few stops.

Gain first:

Yeah, I reduce gain by two stops by decreasing ISO (from 800 to 200), but (I THINK) that I get ~1.5 back from the DR as a function of ISO curve in DxOMark. This one:

DR, in EV, as a function of measured ISO.

So the above deals with the ISO issue. I think. Can you confirm this, please?

I'm afraid I can't. I understand in general terms that increasing pre-ADU gain can potentially improve SNR relative to a case in which the same increase in brightness is applied to the digitized signal. The improved SNR comes with a decrease in DR, but I really don't know how to quantify that trade-off. I think you're right that since you have ample light in the highlight portions of the image and you want to prevent clipping, using base ISO is the way to go, but I think that also means that you will pay a penalty in increased noise in the shadows even before you factor in a decrease in exposure.

YEah, the trick is to quantify the effects of the trade-off. I tend to be a bit geeky also but can't come up with some simple rules of thumb.

My understanding is that both read noise and photon shot noise are present prior to any analog amplification step.

same here.

Noise introduced after analog amplification but prior to discretization by the ADU is still a form of shot noise, but it is not photon shot noise.

.... to digitization by the ADC ... is what I think you mean.

Isn't that (analogue) amplifier noise part of read noise?

I realize that I very badly misunderstood some of what I had read quickly just before I posted this response.

This would be the portion of the total noise whose relative contribution to SNR can be decreased by increasing the analog gain.

????

Yeah, I wasn't sure this would read well. I was trying to paraphrase something I read earlier today that made intuitive sense to me, but may, once again, be entirely misguided. The idea was that there is a portion of the total noise that is present prior to analogue amplification and a portion of the total noise that is introduced after amplification and before digitization.

This makes sense ... gonna have to confirm with Martinec's "SN and Exposure Decisions" , which I am rereading and trying to understand more fully.

Applying analog amplification to the signal multiplies both the initial signal and the noise present at the input of the amplifier, but doesn't effect the noise introduced after amplification.

Yes. But how much noise is added after amplification. I thought that the process of Analogue to Digital conversion added some sort of noise (digitization??) but not significantly. Just a guess on my part.

So if you compare two images with equal brightness, one underexposed at base ISO and pushed using amplification of the digitized signal only, and one using the same exposure, but with analog amplification applied prior to digitization (taken at higher ISO), it appears to me that in the first case you are multiplying all the noise introduced at all phases of the signal processing chain. In the second case you are multiplying only the noise that existed in the signal ahead of the first stage of amplification. This just seemed to make sense as an explanation of why one might want to use analog amplification in the camera rather than using base ISO and pushing in PP.

good argument IF there is significant noise added after amplification.

This has a lot to do with my confused rambling about photon shot noise and read noise.

Mine too. I will have to spend a few hours reading Martinec. Have you found other reliable explanations of how to optimize ISO settings. There is a lot of BS on the internet due to lots of misunderstanding the differences between Exposure and Brightness ........ etc.

Is this electronic shot noise most apparent in better exposed parts of the image like photon shot noise, or is it distributed throughout the image? This is something I haven't gotten my head around

So, I was trying to come up with a difference in the type of noise introduced before the amplifier and the type of noise introduced following the amplifier, but now it seems to me that there really is no difference. There are sources of read noise and of shot noise both before and after the amplifier.

Isn't the shot noise fixed basically be the statistics of the incoming light? ... and therefore is fixed for the whole process?

Aren't we primarily concerned with read noise (that, I think, occurs before the ADC).

Additionally to answer my own question, I think that shot noise (whether it is optical or electronic) must always follow the same dynamics i.e. the magnitude of the shot noise increases with the square root of the magnitude of the signal, so it will always be more evident in the better exposed parts of the photo.

Yes, but who cares? You are not boosting that part in post processing.

I realize that the above has very little bearing on the questions you've asked. Just trying to think through some of this stuff "out loud" as it were.

It IS helpful because it gets me to try to think this thing through.

I only have one more thought related to ISO and noise performance. You've written a number of times that you'd have preferred to use ISO 200 for greater DR and lower read noise, but from what I see in the link below, (and what I thought I understood from reading prior discussions) read noise decreases as ISO increases. Have I misunderstood this as well?

Hmmmm. Now I you've really got me thinking. I will have to go back there!!!

I think that you are referring to this at http://www.dpreview.com/forums/post/52505814

Yes, so the artist in you has to decide case by case: is the lost DR worth the cleaner shadow SNR? In some cases you'll already be as wide open as you can go with the slowest shutter speed that will not cause undesirable blur and at base ISO a quick glance at the histogram will show you that the brightest highlight that you'd like to retain detail in is a couple of stops short of The Right - in which case feel free to increase ISO a couple of stops and pocket the cleaner shadows. (With an ISOless camera in this situation you would have had the option to go either way with virtually the same shadow noise)

In other cases, in the exact same situation, that highlight may already be bumping its head against The Right by ISO200, in which case you would stop there.

I think that Jack is saying that, if I am in no danger of risking clipped highlights,  I can increase ISO (at the "artistic" EV) a "couple of stops and pocket the cleaner shadows". So I have to agree with you.

I don't know if this

read noise decreases as ISO increases.

is so universal as to lead to the tactic of reducing read noise by increasing ISO until the highlights are clipped?   In other words set the EV using artistic reasons (DoF, Motion Blur) and increase ISO until the rights side of the histogram clips.

How does this relate to Dynamic Range??  Does that determine if I even have the option of the above --- if highlight clipping is critical to me?  In other words is the luminance of the highlights so high that my artistic settings (EV) still will cause clipping, even at BASE ISO?   In that case I would leave the setting at BASE and either shorten the shutter interval or close the f/ down.   If I were on a tripod and WANTED some motion blur in the train lights, I would stop the f/down and set the ss to cause the right amount of blur.

More to ponder.

Tom

Mine either. I thought I had it withJack Hogan's handholding ... but I don't yet think that I have internalized it. And this old head ain't what it used be when it comes to hanging on to that stuff.

.

The other issue deals with exposure.

Lets say that I dropped the exposure by (EV) 3 stops from what was taken. Would I be able to recover that (in the snow above.) using the Shadow slider in Lightroom?

I don't know how to calculate this but I suspect, from other stuff that I played with that I could probably recover 3 or 4 stops, maybe more. And it is on this point that I look to you competent folks for some good advice.

I hope to see some additional input as well.

I hope so too. See http://www.dpreview.com/forums/post/52636367

As I write this I can think of a test to make some night.... take repeated shots of illuminated ground/snow at varying EV.... for different ISOs ... and see what I can recover in Lightroom. That way I can confirm my conclusions re the ISO issue above. And I can see how many stops (EV) I can recover in Lightroom using the Shadow slider. While I'm at it I will have to find some sort of specular lights / dark area to see how my metering really works.

Thank you very much for working through my stuff and (hopefully) helping with some critique/suggestions re above.

Wish I could be of more help. I'm trying to learn, myself. Thanks for the thought experiment!

Tom

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