Is Changing ISO Just Adjusting for Incorrect Sensor Exposure?

[toktik]

After about 20 years of not engaging in much photography, I returned to it about a year ago via Fujifilm. While refreshing my knowledge of photography, I have seen explanations that ISO is part of the "exposure triangle," but that doesn't seem correct to me.

Since exposure happens at the sensor, before ISO gain is applicable, it seems that shutter speed, aperture size, and available light are the components that impact exposure. Isn't changing ISO just a way to help compensate for over or under exposure that happens at the sensor?

From what I have learned about ISO, through reading and use, it seems to function similar to an audio amplifier while available light, shutter speed, and aperture work together to produce the music track (image) being made audible by the amplifier.

Am I misunderstanding something about the function of ISO gain?

 

[mostlyboringphotog]

I'd love for the people that seem to have a deep technical understanding of how digital sensors work to talk about practical strategies to apply the knowledge to actual shooting

As has always been true, for most digital photography, you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture). That's all there is to it.

Generally speaking, why would a person start by selecting ISO when other choices and settings have a much greater impact on the creative outcome of the shot?

Habit combined with ignorance? Or in some cases, old dogs, new tricks and no desire for optimal outcomes - good enough is good enough.

I think it's too simplistic to think that they are ignorant old dogs ;-)

Often, one is simply deciding the highest acceptable ISO even though the SS is slower than the desired speed. One constant in photography is choosing the least worst combination ;-)

A lot of people do manually select ISO first. It is usually a holdover of the way they used to do things with roll film. It is rarely an optimal way to shoot digital.

What I would ask is: why would a person use an autoexposure mode but not use Auto ISO? This is also rarely the optimal approach.

Indeed, now we can set the high limit of ISO, minimum SS and AE and Auto ISO.

Now I am waiting for the Auto EC ;-) I suppose, it's called "scene mode" :-O

 

[tokumeino]

Thanks for very thoughtful reply - the reason I make the case here is that someone may take ISO invariance to heart and "always" shoot at base ISO to "maximize DR".

I'm not sure that I've read that (but again, I didn't read everything). I think that people claiming "shoot base ISO" often make the assumption that they have f-numbers and shutter speeds small enough to "fill" the sensor.

It is obvious - for straightforward information quantization matters - that it is better to "multiply" still in the analogue (and continuous) world than in the digital (and discrete) world. But only if the "analogue" multiplication doesn't introduce some kind of noise that is more harmful than the digital information quantization issues. I beleive that can understand the question but I'm far from beeing able to answer it. Just trust people like BClaff and Illah Borg : they have more knowledge about these things than me and than you'll probabaly ever have, and they are kind people without an agenda, only trying to help and teach a bit.

Brightening in PP may not seem to increase noise much but if one is under by 4 stops, the 14bit ADC is delivering only 10bit worth of data. This lost data (tonal range) is not recoverable in PP.

It's not only about noise. Quantization issues reduce the number of possibly reprensented colours, possibly leading to some kind of "posterization" when pushing shadows. Deep shadows gradients are invisible to the human eye if you don't push them. While they remain deep, having 16 (4 bits) or 64 (6 bits) possible values to represent an anyway invisible gradation, is a non-issue in practice. But if you increase the brightness up to a point where these deep shadows become clearly visible for a human eye, then you'll have a 16 levels gradation vs a 64 levels gradation. So you are right, it makes sense to apply analogue gain in a first place, so as to lower this issue. But only if the analogue gain doesn't deterioerate the signal too much. And as far as I understand, that is what about grown ups are talking about. Am I wrong ?

For ETTR or Auto ISO? Personally, I use matrix metering with A priority and minimum shutter speed with Auto ISO. I could not easily adjust ETTR raw for more saturated color. The noise is distant second to the sharpness in my image evaluation ;-) YMMV, of course.

Indded, evaluating highlight clipping with cooked JPEG histograms is not the easiest part. There has been a nice post here several months ago, with good advices that I follow. I'll try to find it again or look at my camera settings. I prefer to use clipping warnings rather than histograms. The point is to find a JPG preset which most of the time starts showing clipping highlights a bit before the limit, so your RAW file won't be actually clipped.

 

[Bill Ferris]

I'd love for the people that seem to have a deep technical understanding of how digital sensors work to talk about practical strategies to apply the knowledge to actual shooting

As has always been true, for most digital photography, you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture). That's all there is to it.

Generally speaking, why would a person start by selecting ISO when other choices and settings have a much greater impact on the creative outcome of the shot?

If I may, this happens when from experience, I need ISO "N" to get the SS and DOF to get the sharp photo that I want, for the indoor event where the flash is not allowed...

Why not simply choose the SS and f-stop, directly? Those are settings that have a direct impact on the creative outcome of the photo.

 

[Bill Ferris]

I'd love for the people that seem to have a deep technical understanding of how digital sensors work to talk about practical strategies to apply the knowledge to actual shooting

As has always been true, for most digital photography, you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture). That's all there is to it.

Generally speaking, why would a person start by selecting ISO when other choices and settings have a much greater impact on the creative outcome of the shot?

Habit combined with ignorance? Or in some cases, old dogs, new tricks and no desire for optimal outcomes - good enough is good enough.

I think it's too simplistic to think that they are ignorant old dogs ;-)

Often, one is simply deciding the highest acceptable ISO even though the SS is slower than the desired speed. One constant in photography is choosing the least worst combination ;-)

While compromise is sometimes necessary, why not begin by selecting what one considers optimal f-stop and shutter speed? Then, reference the in-camera metering to select an ISO or allow the camera to choose through auto ISO.

If enough light will be captured to make a quality image, there's no need to adjust. If ISO indicates a shortage of light, adjust or adapt as needed.

 

[mostlyboringphotog]

I'd love for the people that seem to have a deep technical understanding of how digital sensors work to talk about practical strategies to apply the knowledge to actual shooting

As has always been true, for most digital photography, you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture). That's all there is to it.

Generally speaking, why would a person start by selecting ISO when other choices and settings have a much greater impact on the creative outcome of the shot?

If I may, this happens when from experience, I need ISO "N" to get the SS and DOF to get the sharp photo that I want, for the indoor event where the flash is not allowed...

Why not simply choose the SS and f-stop, directly? Those are settings that have a direct impact on the creative outcome of the photo.

I don't mean to advocate select ISO first - indeed, it should be the last.

In practice, tho, I have already selected F#, SS and ISO that I want and just setting the camera up and often it ends up ISO being the first because it takes a menu set up and then view through the view finder while dialing in the F# or SS or both ;-)

And then as I shoot, I may make changes to the setting depending on what attribute to emphasize.

I used to be orthodox about setting the ISO last but after an illumination discussion with old dogs, I realized that it's not out of ignorance nor just an old habit.

Anyways, Auto ISO makes that a bit of moot, also.

Not to mention, setting the ISO to base is technically setting the ISO first ;-)

 

[Iliah Borg]

I used to be orthodox about setting the ISO last but after an illumination discussion with old dogs, I realized that it's not out of ignorance nor just an old habit.

Hm. Part of preparing for a shot was to consider the light that will be available, shutter speed and aperture needed, and than to decide on the film to load and to plan the film development and mark the film rolls accordingly, before even loading film.

That's "ISO last".

 

[mostlyboringphotog]

I used to be orthodox about setting the ISO last but after an illumination discussion with old dogs, I realized that it's not out of ignorance nor just an old habit.

Hm. Part of preparing for a shot was to consider the light that will be available, shutter speed and aperture needed, and than to decide on the film to load and to plan the film development and mark the film rolls accordingly, before even loading film.

That's "ISO last".

Indeed. Good point, tho, you may load the film first before you set F# ;-)

 

[Bill Ferris]

I've not read all the posts of this very interesting thread involving great and informed contributers.

From a number of posts, am I wrong when I find that the controversy materialize in practice in the following question : ETTR or High ISO ? provided :

• High ISO (with analogue gain) lowers information quantization issues by augmenting values earlier in the overall processing pipeline, in an analogue way (continuous rather than discrete way). But..
• High ISO does reduce Dynamic Range, as charts show

It's the limited volume of light that reduces dynamic range; not the ISO.

In the below chart comparing the dynamic range of the iPhone 7 and X-T3, the Fuji's dynamic range matches the smartphone's only when the volume of light delivered to the APS-C sensor matches that of the much smaller iPhone sensor at full saturation.

IMVHO, many questions here resume in that ones :

• Does ETTR make sense at a higher than base ISO ?
• If there is not enough light to "saturate" the sensor given minimal SS and maximal aperture of the lens, then what's the point of ETTR ?

Keeping in mind the whole purpose of ETTR is to maximize the volume of light delivered to the sensor, I would argue that within the limitations of the scene brightness, f-stop and shutter speed needed to realize the photographer's creative vision, it always makes sense to maximize the light delivered to the sensor.

The more "information/data" you have to work with, the better the resulting image. This approach does not always correspond to a histogram display indicating a majority of pixels ETR. But again, the purpose of ETTR is not to achieve a certain histogram profile; it is to maximize the light delivered to the sensor.

Understanding the answers, just adapt your policy depending of the shot.

I basically do as Doug writes : "you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture)" (just taking care of the 800 ISO thing duer to bi-amp) and I only need 3 ISO settings :

• if light is abundant enough provided my desired SS and aperture, then if possible I use base ISO 200 and ETTR (usually giving overecposed JPEGS but correctly exposed RAW files, technically speaking of course)
• if light is not so abundant, then I use Auto ISO. I have 2 Auto ISO settings (one from ISO 200 to 640 and one over 800)

I'm not sure you do as Doug writes or even that Doug does as he wrote. The first bullet point, above, suggests you have a desired f-stop and shutter speed in mind, and that you use ISO as an indicator of the volume of light delivered to the sensor. My guess is that Doug does much the same. It's a sensible approach given that exposure settings tend to have more direct impact on one's ability to realize a creative vision for a photograph.

However, the statement quoted and italicized, above, is written to give priority to choosing an ISO, first, and then selecting f-stop and shutter speed to optimize exposure. I suspect that was not the impression Doug intended to give.

In most situations, it's an approach that makes little sense. ISO, in and of itself, is not an end. We don't select ISO as a desired outcome; at least, not in the same way we select a desired depth of field (f-stop) or a desired extent to which motion is frozen (shutter speed). These are outcomes that contribute to realizing an envisioned image. ISO is not.

--
Bill Ferris Photography
Flagstaff, AZ

http://www.billferris.photoshelter.com

 

[Iliah Borg]

I used to be orthodox about setting the ISO last but after an illumination discussion with old dogs, I realized that it's not out of ignorance nor just an old habit.

Hm. Part of preparing for a shot was to consider the light that will be available, shutter speed and aperture needed, and than to decide on the film to load and to plan the film development and mark the film rolls accordingly, before even loading film.

That's "ISO last".

Indeed. Good point, tho, you may load the film first before you set F# ;-)

It's all about ranges and latitude.

 

[bclaff]

...

In the below chart comparing the dynamic range of the iPhone 7 and X-T3, the Fuji's dynamic range matches the smartphone's only when the volume of light delivered to the APS-C sensor matches that of the much smaller iPhone sensor at full saturation.

...

You can zoom in and then use the menu by the link button get get a much more readable "screenshot".

;-)

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

 

[Doug Pardee]

As has always been true, for most digital photography, you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture). That's all there is to it.

Generally speaking, why would a person start by selecting ISO when other choices and settings have a much greater impact on the creative outcome of the shot?

Some people might for convenience sake, especially if they don't have auto-ISO. But my comment specifically said, "that gives you acceptable exposure settings." Meaning, select ISO after considering exposure.

 

[tokumeino]

It's the limited volume of light that reduces dynamic range; not the ISO.

What I had in mind was : if a light volume "fills" the sensor, then rising ISO lowers DR. You are right.

Keeping in mind the whole purpose of ETTR is to maximize the volume of light delivered to the sensor, I would argue that within the limitations of the scene brightness, f-stop and shutter speed needed to realize the photographer's creative vision, it always makes sense to maximize the light delivered to the sensor.

I fully agree and I throught that I was expressing that.

I basically do as Doug writes : "you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture)" (just taking care of the 800 ISO thing duer to bi-amp) and I only need 3 ISO settings :

• if light is abundant enough provided my desired SS and aperture, then if possible I use base ISO 200 and ETTR (usually giving overecposed JPEGS but correctly exposed RAW files, technically speaking of course)
• if light is not so abundant, then I use Auto ISO. I have 2 Auto ISO settings (one from ISO 200 to 640 and one over 800)

I'm not sure you do as Doug writes or even that Doug does as he wrote. The first bullet point, above, suggests you have a desired f-stop and shutter speed in mind, and that you use ISO as an indicator of the volume of light delivered to the sensor. My guess is that Doug does much the same. It's a sensible approach given that exposure settings tend to have more direct impact on one's ability to realize a creative vision for a photograph.

However, the statement quoted and italicized, above, is written to give priority to choosing an ISO, first, and then selecting f-stop and shutter speed to optimize exposure. I suspect that was not the impression Doug intended to give.

Sorry. It must be because I'm not native but I think that I don't understand properly.

What I mean is :

• if I know I don't need a shutter speed higher than 1/125, then why going faster, since I want to maximize light ?
• if I know that I have what I want in focus with f/5.6, why closing down further, since I want to maximize light ?

These are my most important picture settings. From that point, it is boring to dial ISO so I just let Auto ISO decide, after I've set SS and F (note that by "setting", I don't necessarily mean directly "dial" : it may involve auto-SS or auto-F and exposure compensation dialing, eventually). Auto ISO accuracy doesn't matter a lot to me anyway, since sensors are close to invariant within ISO-ranges, so I don't agonise about it and let the machine do.

Actually, this is not perfectly right : when outdoor in full light, I know in advance that base ISO will be OK. So yes in that case, I technically dial ISO200 first, and then ETTR when I have time. But before dialing ISO200, I first had to know that what I want to capture will not require a super fast SS. So in my mind, I decide SS and/or F first, actually, regardless of the order I dial.

So I set 2 use cases : a) when ambiant light is obvioulsy abundant for what I have in mind, then I dial ISO first (to base ISO) and when I'm not sure about the light for what I have in mind, then set the machine so that it decides in AutoISO after I'm done with exposure.

Isn't that what Doug states ? In the italicized sentence, Doug obviously decides SS and aperture first. Am I wrong ?

In most situations, it's an approach that makes little sense. ISO, in and of itself, is not an end. We don't select ISO as a desired outcome; at least, not in the same way we select a desired depth of field (f-stop) or a desired extent to which motion is frozen (shutter speed). These are outcomes that contribute to realizing an envisioned image. ISO is not.

I think we agree, and the way I read Doug, that's just what he states. But my english is not the best so I'm perhaps completely wrong. If such, appologies.

 

[Bill Ferris]

...

In the below chart comparing the dynamic range of the iPhone 7 and X-T3, the Fuji's dynamic range matches the smartphone's only when the volume of light delivered to the APS-C sensor matches that of the much smaller iPhone sensor at full saturation.

...

You can zoom in and then use the menu by the link button get get a much more readable "screenshot".

;-)

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

Thank you! One of these days I'll actually figure out how to use all the tools available on your site

--
Bill Ferris Photography
Flagstaff, AZ

http://www.billferris.photoshelter.com

 

[bclaff]

...

Thank you! One of these days I'll actually figure out how to use all the tools available on your site

You might want to follow the Chart Help button.
Lots of useful tips that apply to all the charts.

 

[Ysarex]

I'd love for the people that seem to have a deep technical understanding of how digital sensors work to talk about practical strategies to apply the knowledge to actual shooting

As has always been true, for most digital photography, you select the lowest ISO that gives you acceptable exposure settings (shutter speed and aperture). That's all there is to it.

Generally speaking, why would a person start by selecting ISO when other choices and settings have a much greater impact on the creative outcome of the shot?

Habit combined with ignorance? Or in some cases, old dogs, new tricks and no desire for optimal outcomes - good enough is good enough.

I think it's too simplistic to think that they are ignorant old dogs ;-)

Often, one is simply deciding the highest acceptable ISO even though the SS is slower than the desired speed. One constant in photography is choosing the least worst combination ;-)

While compromise is sometimes necessary, why not begin by selecting what one considers optimal f-stop and shutter speed? Then, reference the in-camera metering to select an ISO or allow the camera to choose through auto ISO.

If enough light will be captured to make a quality image, there's no need to adjust. If ISO indicates a shortage of light, adjust or adapt as needed.

I take nearly all my photos at base ISO and I choose that first not that I'm thinking about ISO. I leave my cameras in a default set condition. Before I turn a camera off I set it back to my default for that camera so it will be there when I turn it on. One of my camera defaults is:

Expose the sensor to full capacity for the best possible photo. That's a default for me in every photo that I take. Is there some reason not to want best? ISO must be left at base in order to realize that default. So I'm not choosing ISO first I'm choosing full capacity sensor exposure first as a default.

Now of course SS and f/stop effect the photo and are equally important and when the time comes to take a photo an exposure has to be set. There's a reason I have three tripods and a monopod. SS allows for a wide range of choices that otherwise have no visible effect on the photo unless action/movement is involved. I have the luxury in what I do to rarely photograph action. So I can usually select from a dozen+ different SS all of which give me the same visible result. But just one ISO change compromises my default for full sensor exposure. As soon as the ISO is raised above base the option for full sensor exposure is off the table.

We all take snapshots as well as more serious photos. For me the definition of a more relaxed snapshot is not bothering with the tripod and then if I'm indoors under threat of camera shake I raise the ISO without compunction.

Much of this discussion revolves around practice in that we're all involved in different types of photography. My practice is to take photos in which a SS choice over an 8 stop range has no visible effect on nearly all of my photos and the possibility of camera shake is solved by my tripods. I'm well aware that other photographers do other types of work and so no general rule can really apply -- make the best compromise for the type of photos you're taking.