Does the camera ISO change exposure?

bassman

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There are a couple of terminology quirks in this wonderful activity of ours that periodically irk me.

/rant on

One is the so-called “exposure triangle” of aperture, shutter speed and ISO. In reality, these three settings can be used to change the brightness of the resulting image, assuming the light reflecting (or emanating) from the subject is held constant. I’m not sure how the term “triangle” actually applies. AFAICT, the three work along entirely different dimensions, while triangles exist in exactly two dimensions (Although they obviously have three sides and angles).
- Aperture controls the amount of light which passes thru the lens and the DoF by varying the size of the opening
- Shutter speed controls the amount of time that light is allowed to hit the sensor, and also can affect blur of moving objects
- ISO controls the amplification of the signal which results from the light collected by the sensor, which affects both (jpeg) image brightness and noise. Raw files have no brightness, because they are not images. In fact, ISO is only defined for jpegs. And it is defined in terms of the setting required to achieve a certain brightness in the jpeg image for a given light value exposed for so many seconds at a given aperture.

As was pointed out above, the word “exposure” is seriously overloaded. If I were teaching a class, I would talk about image brightness control, and I would include the light hitting the subject as one of four elements. After all, even when shooting with available light, one may have the choice to be in the shade or direct sun; backlit, side lit or front lit; etc. Many cameras have built-in flashes that can add light to some subjects.

The second one is the incessant use of focal length as a proxy for angle of view. Why is it so hard to say 47 degrees when talking about a m43 25mm lens, or a 50mm 135 film lens? You then eliminate all the nonsense about so-called equivalence, and perhaps the endless posts from innocent newbees wondering if their 50mm Minolta lens is “really” 100mm when adapted, etc.?

The third is “Zoom with your feet”. This is of course nonsense; the perspective of your image is determined by the camera position relative to the subject position. Changing your position by “zooming with your feet” by definition changes the perspective, and therefore your image. “Zooming” is really optical cropping of the image from changing the angle of view that the lens covers, by changing its focal length. It does nothing to change your perspective. And while We’re talking about “zoom”, telephoto lenses are not “zoom” lenses. I’ve heard many people use the latter to imply the former (“that bird is really far away, I need a zoom lens to capture the image).

/rant off
 

PakkyT

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The second one is the incessant use of focal length as a proxy for angle of view. Why is it so hard to say 47 degrees when talking about a m43 25mm lens, or a 50mm 135 film lens? You then eliminate all the nonsense about so-called equivalence, and perhaps the endless posts from innocent newbees wondering if their 50mm Minolta lens is “really” 100mm when adapted, etc.?
Because people don't usually know what the degrees of view are for their lenses. However every manufacturer prints the focal length right on them so even if you don't remember your focal length, you can just look at what it says. Until they start printing the degrees of view, focal length works fine.

Edited to add: also keep in mind that FoV only applies to the lens coupled with a specific sized sensor. Focal length is strictly a characteristic of the lens no matter which system you put it on. Where as FoV is completely dependent on what the image coming out of the back of the lens shines on.


And while We’re talking about “zoom”, telephoto lenses are not “zoom” lenses.
They can be. A telephoto is not necessarily a zoom lens and a zoom lens is not necessarily a telephoto. However either one can be the other.
 
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bassman

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Because people don't usually know what the degrees of view are for their lenses. However every manufacturer prints the focal length right on them so even if you don't remember your focal length, you can just look at what it says. Until they start printing the degrees of view, focal length works fine.

Edited to add: also keep in mind that FoV only applies to the lens coupled with a specific sized sensor. Focal length is strictly a characteristic of the lens no matter which system you put it on. Where as FoV is completely dependent on what the image coming out of the back of the lens shines on.




They can be. A telephoto is not necessarily a zoom lens and a zoom lens is not necessarily a telephoto. However either one can be the other.

Because people don't usually know what the degrees of view are for their lenses. However every manufacturer prints the focal length right on them so even if you don't remember your focal length, you can just look at what it says. Until they start printing the degrees of view, focal length works fine.

Edited to add: also keep in mind that FoV only applies to the lens coupled with a specific sized sensor. Focal length is strictly a characteristic of the lens no matter which system you put it on. Where as FoV is completely dependent on what the image coming out of the back of the lens shines on.
Well, yeah. But much of the conversation is around the FoV. For example, “what lenses should I take to italy?” “You need a wide angle; perhaps ~75 degrees for your system.” “I have an E-M1.” “Well, then any of the 12mm-xx zooms would be great.” I see people (including me) writing things like 50mm-e all the time, when they mean a 47 degree view, or what a 50mm lens on a so-called full frame camera would see. Since “equivalence” itself is a highly ambiguous term, I‘d like to see the conversation changed to discuss what’s actually being discussed! Fat chance.

Fun fact: back in the day, when 8x10” plates were the “Standard”, there were “half plates” (4x5”) and “quarter plate” (3x4”). Nobody talked about equivalence, but they did understand that a “normal” lens was about 300mm on full plate, but only about 150mm on a half plate.


They can be. A telephoto is not necessarily a zoom lens and a zoom lens is not necessarily a telephoto. However either one can be the other.
Of course. But my point was that many people seem to confuse the two descriptions, making zoom out to mean a telephoto.
 

BPCS

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ISO in digital affects brightness or darkness of a JPG. The RAW file gets the same light regardless of the ISO setting, for a given f stop and shutter speed setting. F stop and shutter speed control how much light is hitting the sensor. Exposing using f stop and/ or shutter speed, at the sensors base ISO, with ETTR but not clipping highlights, is theoretically getting the most out of a sensor.
A larger sensor collects a greater total amount of light by area for the same f stop and shutter speed of a smaller sensor. A larger sensor thus has less noise because less amplification is required... The SNR is better. Think... 4 solar panels gather more than 1 solar panel although in the same light for the same time.
Sometimes, when we use Auto ISO with exposure compensation dial, and there is enough light to keep the shutter speed well above our defined slowest limit, true over or under exposure is achieved by letting in a different amount of light. If we hit say 1/60 as our slowest limit and the aperture is already wide open, and then use exposure compensation, a plus 1 exp comp is just making the ISO rise a stop to lighten the image... no different to boosting the RAW in post a stop.
 
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Excellent points BPCS, one would think we wrote my articles together. :)

"A larger sensor thus has less noise because less amplification is required.."

The only part I don't fully support, is the real importance of sensor size. One must keep in mind the large sensor is better, collects more light - bla bla bla, was developed as marketing narrative to support Canon's full-frame sensor strategy, as the way forward to save DSLR's when the market trend was mirrorless...

So how dare I say this? It's simple. If I am a Klima advocate, I will build a narrative that opening my motorcar window has a huge advantage cooling the car interior and that an aircon is not really that great. Add a few emotional aspects like a little dog enjoying the wind blowing through the car window and I will build huge support...

Sony was the one that completely destroyed the sensor size narrative when they launched the A7 III. Like the MKII, both use 24MP full-frame sensors. But the MKIII has all the differences and significant improvements full-frame advocates liked us to believe sensor size would offer us...

So yes the sensor size narrative (open window) does have tiny merits, its real impact in terms of the bigger picture is not even worth mentioning, the only real benefit it had, it was a great marketing success... :) If people only realized how much of their daily lives are shaped by clever marketing narratives... There is always some interest group behind a trend... Think of a new normal trend...

So what made the real difference between the A7 III and the A7 II? Go see my articles on marketing & social media and ETTR, I discussed this in length...
 
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Stanga

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Cutting through all the fluff and pseudo science, it might be far better to understand the real science behind sensor size.
Each pixel is a storage device that collects light falling on it. From there the term CCD. The larger each pixel is, the more charge it can store.
A more familiar charge storage device that we can refer to for a clearer understanding is a capacitor. For any given capacitance value, the larger the body of the capacitor, the more electrical energy that can be stored and released. So the maximum capacity is tied to the size of the component. On a side note, there are now super capacitors that have solved some of the problems related to the size versus storage ability.
Coming back to a camera sensor with its array of individual charge coupled cells (whether 20 million, 40 million, etc.), the larger the surface area for the same amount of cells, the more energy that can be stored in each cell. And that energy denotes a light and dynamic range value. That value determines how dark or bright the light falling on it is. The larger each pixel cell is, the more energy it can store, which means the larger the dynamic range would be between the darkest and lightest light value stored on the complete sensor.
 
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ISO in digital affects brightness or darkness of a JPG. The RAW file gets the same light regardless of the ISO setting, for a given f stop and shutter speed setting. F stop and shutter speed control how much light is hitting the sensor. Exposing using f stop and/ or shutter speed, at the sensors base ISO, with ETTR but not clipping highlights, is theoretically getting the most out of a sensor.
A larger sensor collects a greater total amount of light by area for the same f stop and shutter speed of a smaller sensor. A larger sensor thus has less noise because less amplification is required... The SNR is better. Think... 4 solar panels gather more than 1 solar panel although in the same light for the same time.
Sometimes, when we use Auto ISO with exposure compensation dial, and there is enough light to keep the shutter speed well above our defined slowest limit, true over or under exposure is achieved by letting in a different amount of light. If we hit say 1/60 as our slowest limit and the aperture is already wide open, and then use exposure compensation, a plus 1 exp comp is just making the ISO rise a stop to lighten the image... no different to boosting the RAW in post a stop.
A7 II is a full-frame sensor (Not much different to the EM1 II)
A7 III is a full-frame sensor (huge improvements)
A7 II & III has, all the same, you describe above - find the differences, read my posts carefully, and be specific... :)
 

cjoliprsf

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However, it may change the way the camera deals with that exposure, depending on the camera.
This conversation is very interesting, and made me understand something fundamental!

If I understood correctly then, the ISO setting on the camera is simply a way to tell the post processing software that the picture was underexposed by so many stops, and this should be compensated for.

For example,
At ISO 100 setting, I get perfect exposure with 1/15s and F5.6.
However, I find this is too slow speed, and I don't want to open the aperture because I need the depth of field.
So I increase the ISO setting to 400, maintain F5.6 and speed becomes 1/60s. I take shot #1.

I also take shot #2, letting ISO set at 100, F5.6 and 1/60s, this being a 2-stop underexposure.

I would then get exactly the same result in post processing after boosting by 2 stops the shot #2.
It makes sense, but I had never thought about this...
 

comment23

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This conversation is very interesting, and made me understand something fundamental!

If I understood correctly then, the ISO setting on the camera is simply a way to tell the post processing software that the picture was underexposed by so many stops, and this should be compensated for.

For example,
At ISO 100 setting, I get perfect exposure with 1/15s and F5.6.
However, I find this is too slow speed, and I don't want to open the aperture because I need the depth of field.
So I increase the ISO setting to 400, maintain F5.6 and speed becomes 1/60s. I take shot #1.

I also take shot #2, letting ISO set at 100, F5.6 and 1/60s, this being a 2-stop underexposure.

I would then get exactly the same result in post processing after boosting by 2 stops the shot #2.
It makes sense, but I had never thought about this...
Yes. But not always.

:wtf: I hear you say.

I’m no expert but as far as I understand what you’ve described is ISO invariance and seems to be (approximately) the situation with the 20MP mu-43 sensors. Conversely, the 16MP mu-43 sensors do not seem to exhibit the same behaviour and are less tolerant of boosting underexposed shots at lower ISOs.
 

pdk42

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Excellent points BPCS, one would think we wrote my articles together. :)

"A larger sensor thus has less noise because less amplification is required.."

The only part I don't fully support, is the real importance of sensor size. One must keep in mind the large sensor is better, collects more light - bla bla bla, was developed as marketing narrative to support Canon's full-frame sensor strategy, as the way forward to save DSLR's when the market trend was mirrorless...

So how dare I say this? It's simple. If I am a Klima advocate, I will build a narrative that opening my motorcar window has a huge advantage cooling the car interior and that an aircon is not really that great. Add a few emotional aspects like a little dog enjoying the wind blowing through the car window and I will build huge support...

Sony was the one that completely destroyed the sensor size narrative when they launched the A7 III. Like the MKII, both use 24MP full-frame sensors. But the MKIII has all the differences and significant improvements full-frame advocates liked us to believe sensor size would offer us...

So yes the sensor size narrative (open window) does have tiny merits, its real impact in terms of the bigger picture is not even worth mentioning, the only real benefit it had, it was a great marketing success... :) If people only realized how much of their daily lives are shaped by clever marketing narratives... There is always some interest group behind a trend... Think of a new normal trend...

So what made the real difference between the A7 III and the A7 II? Go see my articles on marketing & social media and ETTR, I discussed this in length...
Errr - all other things begging equal, and at the same sensor technology generation, larger sensors will always deliver better SNR/ lower noise and better DR. That's true and it's not" marketing". Whether any given photographer *needs* a larger sensor is quite a different point and opens up plenty of room for" marketing"!
 
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Errr - all other things begging equal, and at the same sensor technology generation, larger sensors will always deliver better SNR/ lower noise and better DR. That's true and it's not" marketing". Whether any given photographer *needs* a larger sensor is quite a different point and opens up plenty of room for" marketing"!
Absolutely - that's one of the main reasons why no M43 camera will ever perform as well as the original Canon 5D. Those huge photocells are just unbeatable... Add the Canon name and tech and M43 can just as well close shop... Man, I always enjoy how you challenge me intellectually... :yahoo::drinks::daz::clapping::026::dance::laugh1::bravo-009:
 

mfturner

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I am coming into this a little late, in addition to the comment and link by @Mack (Iso is seldom just digital...), i also didn't see mention about those of us who use jpeg in camera and not raw.

If you aren't using raw, then jpeg engines bake in tone curves which leave you less information to work with in the shadows and highlights. If you are happy with jpeg in camera to save work/time in PP (as i generally am), then it's worth getting the image brightness close in camera, which means ISO setting matters. You could try using low contrast "log" tone curves like videographers, but i would then expect to lose information in the mid tones. If someone already mentioned this, sorry for the clutter.
 

CO_yeti

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Yes, ISO does effect the exposure. It is a gain function and its effect is baked into the RAW file. ISO invarient sensor or not, the mathematics of how a digital file is created show that 1 stop increase in ISO equals a 1 stop reduction in theoretical maximum DR. The highlight info lossed by increasing ISO cannot be recovered or to put it another way a tree failing in the woods always makes a sound.
 
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