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Understanding SL2 and SL2-S ISO settings


kobra

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Hi all, 

Rather than taking away from "Robbs Leica Adventures" thread, I've started a new one here. In the thread by @robb, @hansvons posted some very detailed comments on ISO and I would like to know more. 

I hope it's ok that I quoted this part of the post here:

"(The SL2-S sensor is invariant from EI 100-3200. Only after that signal amplifying kicks in. In that bracket, one only moves the middle grey up and down, or to say it in analogue film terms, the sensor's ISO is fixed, and you over or underexpose the film all else equal as you would with exposure compensation or a different EI rating compared to the box speed.

ISO 50, on the other hand, is out of that bracket and inevitably leads to overexposure and thus needs exposure compensation of -1 stop to bring it back. In other words, with ISO 50, one is sacrificing one stop of DR on the highlights end for nought, as ISO 50 offers no advantage over ISO 100 in terms of noise. But even with ISO 100, one has no leeway on the highlights end, and accidental overexposure will inevitably lead to lost information in the highlights. That's why I find using the base ISO (here ISO 100) as a safe haven problematic, as the term "base" is misleading.

I prefer ISO 800 for the most motives because I like the texture I get without losing the SL2-S' rich colour in the shadows. If lustre and cleanliness to the max are the goals, ISO 200 fits. Otherwise, ISO 400 gives you a balanced, full DR from the shadows to the highlights with the best signal-to-noise ratio in high-contrast environments such as a city park on a sunny day.

The question remains: what is the SL2-S "box speed"?  From my experience, that's somewhere between ISO 400 and ISO 800. From there, an informed decision can be made about which EI suits the targeted result best.)"

BTW, wonderful and detailed explanation @hansvons!

Here are some followup questions (I admit to not fully understanding ISO invariance):

-I've always thought that "base ISO" (ie 100) gives the widest DR, but it seems ISO 200 or 400 may be better? 

-how does ISO 200 give more lustre and cleanliness than ISO 100? Is it because it may be easier to preserve the highlights? 

-what is "box speed"? 

-what differences might there be between the ISO performance of the SL2 vs the SL2-S? (in the context of the above "lower ISO" ranges; I know high ISO favors the SL2-S)

My apologies if this is covered in another thread, but I've not found the above gems when I searched. 

Thanks in advance for helping me understand this better!

Brad

Edited by kobra
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I assume that we are talking about still photography.

SL2-S is invariant from ISO 800 - ISO 102400 as its dual conversion gain kicks in after ISO 400 (see this graph, flat ranges mean invariance). Nothing happens after 3200 (PDR graph). 

Different story with the SL2 sensor.

Shooting at ISO 800 loses you two stops of potential DR. It is the same as shooting with an m43 sensor instead of an FF sensor.

Best IQ can be reached at ISO 100 (least noise). Maybe more careful metering is needed at ISO 100, but that is why the camera has a histogram and blinkies.

"Box speed" (manufacturers recommended film shooting ISO) has no meaning in digital cameras. Native ISO is the most important criterion.

Edited by SrMi
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2 hours ago, kobra said:

-I've always thought that "base ISO" (ie 100) gives the widest DR, but it seems ISO 200 or 400 may be better? 

It does, and so does ISO 200 and down to ISO 3200 because the sensor is invariant until ISO 3200. However, as you dial up the ISO, you move down the middle grey (roughly the skin tone of a lightly sun-tanned caucasian skin). At some ISO, say ISO 800, middle grey isn't put where middle grey in a perfect world for that particular sensor would sit but a tad below, and as you increase the ISO, middle grey becomes darker and darker. To compensate for that, the camera raises the brightness/exposure associated with that ISO setting, and noise will become more and more visible.

2 hours ago, kobra said:

-how does ISO 200 give more lustre and cleanliness than ISO 100? Is it because it may be easier to preserve the highlights? 

Apologies. It doesn't if correctly exposed. But yes, as you said, at ISO 200, you have one-stop leeway for protecting your highlights. And as highlights can be seen as a complex amalgam of 3-4 stops on the bright end, as we want a gentle roll-off to full white that desaturates delicately, preserving as much information as possible makes sense.

 

2 hours ago, kobra said:

-what is "box speed"? 

Manufacturers write the EI on their film boxes, eg ISO400 for Kodak Tri-X. That's why people call it box speed. What you do with that information is entirely up to you. Most photographers tend to overexpose the film by choosing a lower EI (exposure index).

2 hours ago, kobra said:

-what differences might there be between the ISO performance of the SL2 vs the SL2-S? (in the context of the above "lower ISO" ranges; I know high ISO favors the SL2-S)

Cannot say a thing about the SL2. Don't own one.

2 hours ago, kobra said:

Here are some followup questions (I admit to not fully understanding ISO invariance):

When talking about ISO in a RAW-based photography workflow with invariant sensors, it makes sense to understand it as a brightness/exposure setting in the RAW converter. Invariant in this context means that the ISO setting does not influence the signal in terms of amplifying as you dial up the ISO value. In the SL2-S context, the sensor is invariant until ISO 3200, as my testing revealed. At ISO 6400, amplification kicks in, and the noise looks different.

Edited by hansvons
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In the SL2, and with almost every camera I have ever tried, the native ISO gives the best results. For me that means the cleanest files, best color and widest dynamic range. For the SL2, I have found that to be ISO 100. As for which ISO to use, I pretty much stick with that. Looking at Lightroom, 91% of my photos are ISO 800 or below, and 66% are at ISO 100. Most of the others in that range are only when I have had auto ISO on for some reason or if I went to ISO 50 to have a wider aperture or slower shutter speed. One of the advantages of the stabilization is that it allows you to use a lower ISO, as long as movement in the scene is not a major concern. For people doing video with log profiles, there can be some different criteria, but in my experience the files start to look worse very quickly after leaving base ISO...color depth starts to diminish, shadow noise starts to go up, and these effects are all the more noticeable if you have to push the file at all in processing. The files can still look very good, but if you really look carefully, they always seem to lose something. I do not have the SL2S, however, so I cannot comment on that specifically.

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1 hour ago, SrMi said:

I assume that we are talking about still photography.

SL2-S is invariant from ISO 800 - ISO 102400 as its dual conversion gain kicks in after ISO 400 (see this graph, flat ranges mean invariance). Nothing happens after 3200 (PDR graph). 

Different story with the SL2 sensor.

Shooting at ISO 800 loses you two stops of potential DR. It is the same as shooting with an m43 sensor instead of an FF sensor.

Best IQ can be reached at ISO 100 (least noise). Maybe more careful metering is needed at ISO 100, but that is why the camera has a histogram and blinkies.

"Box speed" (manufacturers recommended film shooting ISO) has no meaning in digital cameras. Native ISO is the most important criterion.

@SrMi- thanks; yes, I'm only interested in still photography.

Can I get you to explain further? For example, on the graphs I added the SL2 alongside the SL2-S to compare and didn't see what you were explaining... 

-On the first graph (shadow improvement), it seems the SL2 is identical to the SL2-S at ISO800 and reads slightly lower at 1600 (does that mean better for SL2 reading 0.83EV vs 1.02EV for SL2-S, or am I reading correctly?). In other testing, such as at reidreviews, it is clear that by ISO3200+ the SL2-S starts showing superior results, but the SL2-S certainly did not show clean files all the way up to ISO102400. So, again, I'm not sure how to interpret this graph against real world results. 

-On the second graph, the DR of the SL2 stays about 1 stop worse than the SL2-S; and I have seen that one before. Again it shows that 1 stop less trend to ISO1600, then the graph shows "up triangles" for the SL2. 

BTW, I don't know what the graphs mean when showing the "up triangles". The chart says "up triangles indicate scaling"; but what does that mean? (and the SL2 shows up triangles for both graphs, by ISO3200 and greater). 

So, what do you mean when you said about the SL2 -- "Shooting at ISO 800 loses you two stops of potential DR. It is the same as shooting with an m43 sensor instead of an FF sensor." Again, I don't understand how the graphs show that; to me, for ISO800 both graphs show little difference between the two, and when I added m43 cameras, only the very latest sensors are as good or better than the SL2, and those of the same generation were considerably worse. 

Please help me understand further, and thank you in advance! 

Brad

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23 minutes ago, kobra said:

-On the first graph (shadow improvement), it seems the SL2 is identical to the SL2-S at ISO800 and reads slightly lower at 1600 (does that mean better for SL2 reading 0.83EV vs 1.02EV for SL2-S, or am I reading correctly?). In other testing, such as at reidreviews, it is clear that by ISO3200+ the SL2-S starts showing superior results, but the SL2-S certainly did not show clean files all the way up to ISO102400. So, again, I'm not sure how to interpret this graph against real world results. 

The only information I use from that graph is where the camera is ISO invariant and at what point dual conversion gain kicks in. Both cameras are ISO invariant at ISO 800 and above.

24 minutes ago, kobra said:

-On the second graph, the DR of the SL2 stays about 1 stop worse than the SL2-S; and I have seen that one before. Again it shows that 1 stop less trend to ISO1600, then the graph shows "up triangles" for the SL2. 

SL2-S has a newer sensor that performs better at higher ISOs (less noise), hence the difference.

25 minutes ago, kobra said:

BTW, I don't know what the graphs mean when showing the "up triangles". The chart says "up triangles indicate scaling"; but what does that mean? (and the SL2 shows up triangles for both graphs, by ISO3200 and greater). 

AFAIK, scaling means that the values are increased by an algorithm, not by the ISO circuit. It likely means that internal ISO is capped at 1600. I would avoid ISOs above 1600 and brighten the image in the post instead (raw).

27 minutes ago, kobra said:

So, what do you mean when you said about the SL2 -- "Shooting at ISO 800 loses you two stops of potential DR. It is the same as shooting with an m43 sensor instead of an FF sensor." Again, I don't understand how the graphs show that; to me, for ISO800 both graphs show little difference between the two, and when I added m43 cameras, only the very latest sensors are as good or better than the SL2, and those of the same generation were considerably worse. 

My comment was very simplified.
Using the same technology, m43 sensors have about 2 stops less DR, as the DR is mainly determined by sensor size. 
At ISO 800, SL2-S has a PDR of 9.16, which is a bit less than max PDR of OM-1. This means that shooting OM-1 at best ISO would give you less noise (better IQ) than when shooting SL2-S at ISO 800.
Again, that is simplified, and there are other factors in play. My point was that one is throwing away the camera's potential if one never shoots below ISO 800.

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15 hours ago, Stuart Richardson said:

In the SL2, and with almost every camera I have ever tried, the native ISO gives the best results.

Yes - if the goal is the lowest noise floor at the largest possible DR.

There are tons of reasons to want that. But the question remains, what is the native ISO, and how does it differ from the base ISO? I don't find an answer, as with digital, you cannot pinpoint the ISO as you can do with film with a densitometer. You can define a sensor's digital native ISO by noise (somewhat open to interpretation and preference) or by saturation, meaning at what speed is the sensor fully saturated. The latter, often, is the base ISO, which is for the SL2-S ISO 100. However, as described above, in real-world situations, the base ISO bares the risk of over-saturation of the sensor, meaning losing information in the highlights.  

In my experience, at least with the SL2-S, you'll gain hardly any noise when using 1-2 stops higher ISOs than the base ISO but achieve the security that you will retain information in the highlights and put skin tones in a secure exposure bracket. That's why I choose ISO 400-800 (I often photograph people in uncontrolled environments).

As a side note, the Arri Alexa's base ISO is ISO 200. But most cinematographers tend to shoot it at ISO 800. And that is what Arri themselves recommend. Why is that? Because, at ISO 800, middle grey (a slightly tanned caucasian skin tone) sits right at the centre of the camera's DR, meaning there's an equal amount of stops from middle grey to the top and the bottom. In cinematography, skin tones are an anchor that defines lighting, exposure and ISO choices. For a landscape photographer, that may be very different.

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43 minutes ago, hansvons said:

Yes - if the goal is the lowest noise floor at the largest possible DR.

There are tons of reasons to want that. But the question remains, what is the native ISO, and how does it differ from the base ISO? I don't find an answer, as with digital, you cannot pinpoint the ISO as you can do with film with a densitometer. You can define a sensor's digital native ISO by noise (somewhat open to interpretation and preference) or by saturation, meaning at what speed is the sensor fully saturated. The latter, often, is the base ISO, which is for the SL2-S ISO 100. However, as described above, in real-world situations, the base ISO bares the risk of over-saturation of the sensor, meaning losing information in the highlights.  

In my experience, at least with the SL2-S, you'll gain hardly any noise when using 1-2 stops higher ISOs than the base ISO but achieve the security that you will retain information in the highlights and put skin tones in a secure exposure bracket. That's why I choose ISO 400-800 (I often photograph people in uncontrolled environments).

As a side note, the Arri Alexa's base ISO is ISO 200. But most cinematographers tend to shoot it at ISO 800. And that is what Arri themselves recommend. Why is that? Because, at ISO 800, middle grey (a slightly tanned caucasian skin tone) sits right at the centre of the camera's DR, meaning there's an equal amount of stops from middle grey to the top and the bottom. In cinematography, skin tones are an anchor that defines lighting, exposure and ISO choices. For a landscape photographer, that may be very different.

Again, many thanks for sharing this, very helpful!

One note on the base ISO; on the reidreviews.com site, Sean Reid ran extensive tests to compare highlight headroom on the SL, SL2 and SL2-S. His real world controlled image tests showed that the original SL does have a "native ISO" close to 50, whereas the SL2 and SL2-S have a native ISO of 100. His tests further showed, for the SL2 and SL2-S, that ISO 50 is a "pull" and will clip the highlights sooner than using ISO100 then pushing the exposure up in post. In fact, using ISO50 and pushing even 1/3 stops resulted in clipping, whereas the ISO100 image could stand much more pushing in post (up to 2 stops or more depending on color channel). Spoiler, as to highlight clipping, sometimes the SL2 was as good or better than the SL2-S, even though the SL2-S does maintain about 1+ stop advantage in the shadows. More details at www.reidreviews.com about his methodologies and results; it is a subscription site, but very reasonable and full of tests on subjects like this. 

Summary, to me - using ISO50 would leave almost no room to recover highlights; that actually is not a surprise to me and I don't think I've ever used it other than by mistake. However, the surprise to me is that ISO100 also is more sensitive to highlight clipping than higher ISO's up to 800. I think I will make an auto ISO setting up to 800 and observe my results. 

Thanks again for your posts and helping me understand this better. 

Brad

 

Edited by kobra
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1 hour ago, kobra said:

Spoiler, as to highlight clipping, sometimes the SL2 was as good or better than the SL2-S, even though the SL2-S does maintain about 1+ stop advantage in the shadows.

I'm guessing that means the middle grey wasn't at the centre of one of the two cameras' DR.  If the SL2-S would clip highlights at the same ISO as the SLS but retain one more stop in the shadows, it would have one stop more DR.

According to Reid's findings, that also means that the base ISO of the SL2-S is approx one stop higher. Or that ISO 100 means for the SL2 something different than for the SL2-S.  

Often ISOs are different from one camera to another. The Red's Dragon sensor was a stop darker than Arri's Alev 2 sensor at ISO 800, as the middle grey sat at ISO 320 at the centre when metered properly compared to the Alexa, which showed a real-life ISO of 800. Sometimes stiff competition makes people pretty up themselves.

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Thanks for the info.  I will try to shoot at iso 100 rather than 50 with the SL2 if I have the other settings working in the range I prefer.  Only issue where I may not have this ideally is with flash.  With the SL2-s I will try iso 100 or 200.  

Bottom line is that we have to shoot sometimes at other iso's.  So we will just have to solve each solution that best fits the situation.  I don't plan on worrying too much about hitting best iso for each image.  Bigger fish to fry as they say.  My bigger worry is shooting too high an iso so that the grain / noise and details turn to crap.  The lower ranges, I can deal with their quirks.

Robb

 

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2 hours ago, robb said:

 I don't plan on worrying too much about hitting best iso for each image.  Bigger fish to fry as they say.

Absolutely. It's a matter of preference. If one prefers a clean image, go with a lower ISO, but I'd avoid clipping highlights. And if you are looking for a more organic look that likes some texture and wants to contain skin reflections, a higher ISO, say ISO 400- 800, may be meaningful for convenience (I'm talking about the SL2-S). And if the environment is dark and moody, ISO 3200 might be the best choice. 

 

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9 hours ago, hansvons said:

However, as described above, in real-world situations, the base ISO bares the risk of over-saturation of the sensor, meaning losing information in the highlights.  

That could happen if you blindly follow automatic metering. If you expose so that the highlights are not clipped, you will retain information in the highlights.
Furthermore, if clipping at base ISO is a concern, set -1.5EC permanently and still have a better IQ than at ISO 800.

 

10 hours ago, hansvons said:

In my experience, at least with the SL2-S, you'll gain hardly any noise when using 1-2 stops higher ISOs than the base ISO but achieve the security that you will retain information in the highlights and put skin tones in a secure exposure bracket. That's why I choose ISO 400-800 (I often photograph people in uncontrolled environments).

Similarly, if I shoot at higher ISOs, I regularly adjust metering by lowering ISO a stop or two. Important is to maximize exposure, that is where the noise comes from.

 

10 hours ago, hansvons said:

As a side note, the Arri Alexa's base ISO is ISO 200. But most cinematographers tend to shoot it at ISO 800. And that is what Arri themselves recommend. Why is that? Because, at ISO 800, middle grey (a slightly tanned caucasian skin tone) sits right at the centre of the camera's DR, meaning there's an equal amount of stops from middle grey to the top and the bottom. In cinematography, skin tones are an anchor that defines lighting, exposure and ISO choices. For a landscape photographer, that may be very different.

The techniques from video cannot be mapped to still photography.

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9 hours ago, kobra said:

Again, many thanks for sharing this, very helpful!

One note on the base ISO; on the reidreviews.com site, Sean Reid ran extensive tests to compare highlight headroom on the SL, SL2 and SL2-S. His real world controlled image tests showed that the original SL does have a "native ISO" close to 50, whereas the SL2 and SL2-S have a native ISO of 100. His tests further showed, for the SL2 and SL2-S, that ISO 50 is a "pull" and will clip the highlights sooner than using ISO100 then pushing the exposure up in post. In fact, using ISO50 and pushing even 1/3 stops resulted in clipping, whereas the ISO100 image could stand much more pushing in post (up to 2 stops or more depending on color channel). Spoiler, as to highlight clipping, sometimes the SL2 was as good or better than the SL2-S, even though the SL2-S does maintain about 1+ stop advantage in the shadows. More details at www.reidreviews.com about his methodologies and results; it is a subscription site, but very reasonable and full of tests on subjects like this. 

Summary, to me - using ISO50 would leave almost no room to recover highlights; that actually is not a surprise to me and I don't think I've ever used it other than by mistake. However, the surprise to me is that ISO100 also is more sensitive to highlight clipping than higher ISO's up to 800. I think I will make an auto ISO setting up to 800 and observe my results. 

Thanks again for your posts and helping me understand this better. 

Brad

 

Highlight clipping is a matter of metering and can always be prevented by adjusting the exposure accordingly. But yes, both Q2 and SL2 are more prone to lose highlights at ISO50 and when using fully automatic metering. If you apply appropriate corrections to prevent clipping, you will notice less noise in shadows at ISO50. I avoid ISO 50 as the benefits are small and practical negatives more significant.

Edited by SrMi
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10 hours ago, hansvons said:

I'm guessing that means the middle grey wasn't at the centre of one of the two cameras' DR.  If the SL2-S would clip highlights at the same ISO as the SLS but retain one more stop in the shadows, it would have one stop more DR.

According to Reid's findings, that also means that the base ISO of the SL2-S is approx one stop higher. Or that ISO 100 means for the SL2 something different than for the SL2-S.  

Often ISOs are different from one camera to another. The Red's Dragon sensor was a stop darker than Arri's Alev 2 sensor at ISO 800, as the middle grey sat at ISO 320 at the centre when metered properly compared to the Alexa, which showed a real-life ISO of 800. Sometimes stiff competition makes people pretty up themselves.

I don't want to quote Sean directly as it may infringe on his IP, rather direct to his site www.reidreviews.com - and I admit I am likely not interpreting as accurately as I hope, but I will try!

My comments on his comments ;)

-ISO100 is verified to be the native ISO on both models, with same test. 

-The clipping was in color channels (not mid grey), as each sensor handles color differently.

-The DR advantage of the SL2-S is in the shadows, so still will clip color highlights; sometimes better, sometimes worse than the SL2 but this depends on the color, the light, the part of the image, etc. Does not change the base ISO nor the total DR as this clipping is on specific colors not average or total. But he makes the case that a clipped color highlight will negatively impact the overall IQ of an image, so needs to be considered. 

-He did not cover dual gain ISO. 

-He continues to stress that proper exposure is the most important thing. He wishes Leica would add RGB histograms as he believes without that it is harder to achieve accurate exposure, especially when trying to maximize DR. He believes that in challenging situations an accurate RBG histogram could be worth more than an extra stop of shadow recovery. He, like others, recommends -1 EC (or more) to preserve highlights, and the extra stop of shadow recovery of the SL2-S will help here. He also uses RawDigger regularly to learn more about what his exposures really look like. 

Again, I recommend to any interested to go to Reid's site to get the details and accurate info. You can read free articles and see the entire list of articles before committing to a subscription. 

Thanks again for helping me understand this better! 

Brad

 

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32 minutes ago, kobra said:

-ISO100 is verified to be the native ISO on both models, with same test. 

Jim Kasson did a detailed analysis of Q2 Monochrome's ISO 100 (same as SL2's and Q2's ISO 50) in several articles, for example:

Leica Q2 Monochrom highlight linearity at ISO 100

My net on all this is that the ISO 100 setting on the Q2 Monochrom is useful in some circumstances, and is not a fake ISO ...

... clipping at ISO 100 will occur earlier than the live histogram in the finder indicates

44 minutes ago, kobra said:

He continues to stress that proper exposure is the most important thing. He wishes Leica would add RGB histograms as he believes without that it is harder to achieve accurate exposure, especially when trying to maximize DR

RGB histogram will help a bit, but what we need to achieve best technical exposure is a raw histogram.

46 minutes ago, kobra said:

He, like others, recommends -1 EC (or more) to preserve highlights

Make sure you apply -1 EC to ISO and not to shutter speed or aperture.

P.S.: I subscribe to Sean Reid's reviews.

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2 hours ago, SrMi said:

Jim Kasson did a detailed analysis of Q2 Monochrome's ISO 100 (same as SL2's and Q2's ISO 50) in several articles, for example:

Leica Q2 Monochrom highlight linearity at ISO 100

My net on all this is that the ISO 100 setting on the Q2 Monochrom is useful in some circumstances, and is not a fake ISO ...

... clipping at ISO 100 will occur earlier than the live histogram in the finder indicates

RGB histogram will help a bit, but what we need to achieve best technical exposure is a raw histogram.

Make sure you apply -1 EC to ISO and not to shutter speed or aperture.

P.S.: I subscribe to Sean Reid's reviews.

@SrMi - thanks for the tip on applying -1 EC to ISO; I've not paid close enough attention to that in the past. 

Also, I have a decidedly laymans view of all of this as my education is in other areas, so I welcome and appreciate your comments on the other areas!

Brad

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1 hour ago, kobra said:

thanks for the tip on applying -1 EC to ISO; I've not paid close enough attention to that in the past. 

That's the same as choosing a higher ISO value of 1 stop, as the sensor is partially invariant, at least in the lower ISOs. Assuming you shoot at ISO 100 and select -1 stop exposure compensation, you will let half the light hit the sensor, which is the same as choosing ISO 200.

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14 hours ago, SrMi said:

The techniques from video cannot be mapped to still photography.

I have to disagree with that one, having shot hundreds of "cinematic" weddings and events (and who has shot lots of film and video projects). You will benefit greatly from taking a cinematic approach if you shoot in changing light. Maintaining a consistent look and narrative is more important than tweaking every file nearly to the point of breaking.

I'm sure that anyone here will understand why you don't want to apply individual correction to each and every one of 2,000+ images: it's inefficient. There is another reason that is more important and less obvious: it disrupts the narrative. You are telling a story visually, and that story has a flow. From morning to night, anticipation to elation to celebration. You rarely want to "ETTR the heck out of your images," unless that is a specific story you want to tell. Most of the time, you want your technique to be more transparent. You want images to relate to each other in an organic way, so that the relationship between images/characters/times/locations/etc. comes through.

For all of this, a video approach can be a lot more successful than a shot-by-shot approach, because it's more consistent. Tweaking every exposure means that your base exposure will vary by quite a lot, based on lighting ratios, highlights, shadows. It's better to just expose you mid-tones and skin tones consistently, and leave enough room at both ends of the tonal scale. Of course, that's not to say that you don't keep lighting ratios in mind while shooting, and correct as you go with reflectors, flash, framing, positioning your subjects, etc.

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9 hours ago, hansvons said:

That's the same as choosing a higher ISO value of 1 stop, as the sensor is partially invariant, at least in the lower ISOs. Assuming you shoot at ISO 100 and select -1 stop exposure compensation, you will let half the light hit the sensor, which is the same as choosing ISO 200.

Hmm, I'm not sure on that... I don't think ISO on digital sensors is changing how much light hits the sensor; rather it is changing how sensitive the pixels are to the light that hits them - unlike shutter or aperture that actually will increase or decrease the light hitting the sensor pixels. 

So, in your example above, if using ISO100 and -1 EC, wouldn't that force ISO50? Or maybe I'm not understanding what you meant. 

Thanks,

Brad

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