ISO discussion, film photography compared to digital photography Locked

Started Aug 6, 2017 | Discussions
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TimmyD11 Forum Member • Posts: 88
ISO discussion, film photography compared to digital photography

So in film photography you set the ISO for what ISO the film was, 800 film being more sensitive to light than 100 for example...

How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).

Dutch Newchurch
Dutch Newchurch Senior Member • Posts: 5,096
Re: ISO discussion, film photography compared to digital photography

Popcorn, anyone?

(Timmy, it's a good question.  I'm not sure I want to know enough about photoelectronics to know the full answer.

All I know is that I can turn up the ISO, shot by shot, in a way that I never could with film. And, when I turn it up, the noise tends to have an adverse effect on the image.

There have been many discussions about how, and why, on here.  They sometimes get quite heated.

I hope you get a helpful and comprehensible answer.)

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Dutch
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Ysarex
Ysarex Senior Member • Posts: 1,699
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

So in film photography you set the ISO for what ISO the film was, 800 film being more sensitive to light than 100 for example...

How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).

The light sensitivity of the sensor is fixed when manufactured and can't change (troll alert: didn't mention dual gain sensors). So very physically, the sensor in your camera has one fixed light sensitivity ------- period. It's common usage to suggests a digital camera can change light sensitivity, but that's sloppy language and sloppy thinking. In fact it can't.

The ability to change ISO then with your digital camera is the ability to apply less exposure to the sensor when you have to take a photo in low light and then brighten the output to normalize the photo. This is done both electronically (boost the analog sensor signal) and digitally (multiply numbers) in different cameras with the specific method(s) being an engineering choice of the specific camera manufacturer.

The noise we see in low light, high ISO photos is primarily a function then of the reduced exposure of the sensor. The less exposure the sensor gets the noisier the finished image once it's been brightened up to normal.

yardcoyote Forum Pro • Posts: 10,388
Re: ISO discussion, film photography compared to digital photography

I'm with you, Dutch. I am happy to have shot by shot adjustable ISO, I'm getting better at learning how to use it, and I'm not all that terribly interested in how it works.

(And manual+Auto ISO is awesome.  Just sayin'.)

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Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 12,450
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

So in film photography you set the ISO for what ISO the film was, 800 film being more sensitive to light than 100 for example...

How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).

A sensor doesn't become more sensitive to light when varying the ISO. In some respects it's like increasing the volume on a speaker. Every manufacturer implements ISO differently, even between models, and so exactly what happens during an ISO boost will differ in detail, and so getting personal knowledge of a camera's performance at various ISO settings can be useful.

There are various ways of estimating a sensor's base ISO, and one of the most photographically useful methods is by measuring the amount of exposure that will saturate the sensor, the point where you'll capture just white: under this measure, light meters are often calibrated to expose three stops below this limit. With modern high-end cameras this still leaves plenty of image data for shadow detail, but not so much headroom (although it's perfectly fine for flat-lit subjects with a slight amount of specular reflections). With older cameras, this was a reasonable value, as there was less shadow detail possible due to excessive noise.

https://en.wikipedia.org/wiki/Film_speed

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Iliah Borg Forum Pro • Posts: 24,590
look at Kodak P3200

TimmyD11 wrote:

So in film photography you set the ISO for what ISO the film was, 800 film being more sensitive to light than 100 for example...

No. Take Kodak P3200 for example. This film was developed to be used starting at ISO 400 exposure index and up to ISO 6400. Same as with digital photography, the film speed depends on processing (development). The letter 'P' stands for push. The "actual" emulsion "sensitivity" for this film is equivalent to ISO 1000-800, once again, depending on what developer was used.

"KODAK PROFESSIONAL T-MAX P3200 Film is specially designed to be used as a multi-speed film. The speed you use depends on your application; make tests to determine the appropriate speed." - http://www.kodak.com/global/en/professional/support/techPubs/f32/f32d.jhtml#98958

Sensitivity in terms of ISO speed is not applicable to unprocessed images (latent image on film, raw image data for digital). The speed in both cases is determined in processing and presentation. Make the monitor brighter, and you pushed the image a stop or two.

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Iliah Borg Forum Pro • Posts: 24,590
Re: ISO discussion, film photography compared to digital photography

Ysarex wrote:

TimmyD11 wrote:

So in film photography you set the ISO for what ISO the film was, 800 film being more sensitive to light than 100 for example...

How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).

The light sensitivity of the sensor is fixed when manufactured and can't change (troll alert: didn't mention dual gain sensors).

The reason not to mention those is that they do not change sensitivity, they change responsivity

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D Cox Forum Pro • Posts: 20,902
Re: ISO discussion, film photography compared to digital photography

If you put a 400 ASA film in the camera and set the meter to 800, it will underexpose the pictures by two stops. You can compensate for this by increasing the time in the developer, or using a faster acting developer, but noise levels will increase.

Raising the ISO in a digital camera is similar to increasing the developer time. Almost all sensors are really ISO 100. Using a higher number will cause the camera to underexpose and then (to some extent) compensate.

D Cox Forum Pro • Posts: 20,902
Re: ISO discussion, film photography compared to digital photography

Dutch Newchurch wrote:

Popcorn, anyone?

(Timmy, it's a good question. I'm not sure I want to know enough about photoelectronics to know the full answer.

All I know is that I can turn up the ISO, shot by shot, in a way that I never could with film. And, when I turn it up, the noise tends to have an adverse effect on the image.

You could do the same with film, but it can't be taken so far.

Now that we can scan film in and use computer processing, it is possible to get usable images from underexposed shots that would not have been usable in the darkroom. But it is still limited. Sensors are simply better than film -- they have much less built in noise.

There have been many discussions about how, and why, on here. They sometimes get quite heated.

I hope you get a helpful and comprehensible answer.)

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Dutch
forestmoonstudio.co.uk
Photography is about light, not light-proof boxes.

Eoanthropus Regular Member • Posts: 240
Re: ISO discussion, film photography compared to digital photography

D Cox wrote:

If you put a 400 ASA film in the camera and set the meter to 800, it will underexpose the pictures by two stops. You can compensate for this by increasing the time in the developer, or using a faster acting developer, but noise levels will increase.

You did mean 1 stop.

Raising the ISO in a digital camera is similar to increasing the developer time. Almost all sensors are really ISO 100. Using a higher number will cause the camera to underexpose and then (to some extent) compensate.

Iliah Borg Forum Pro • Posts: 24,590
Re: ISO discussion, film photography compared to digital photography

D Cox wrote:

If you put a 400 ASA film in the camera and set the meter to 800, it will underexpose the pictures by two stops.

1 stop, and there is REI and EI.

You can compensate for this by increasing the time in the developer, or using a faster acting developer, but noise levels will increase.

Noise level and graininess follow different mechanisms.

Raising the ISO in a digital camera is similar to increasing the developer time.

More or less.

Almost all sensors are really ISO 100.

Sensor sensitivity / responsivity parameters are not rated in ISO speed units. ISO speed is valid only for processed images, ready for presentation.

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FingerPainter Senior Member • Posts: 6,513
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

...

How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO?

The sensor doesn't change its sensitivity to light when the ISO setting is changed. The sensor reports the same voltage for a given exposure regardless of ISO setting. The exposure is the amount of light hitting the sensor per unit area. The exposure is determined by the scene luminance, the aperture setting and the shutter setting. Note that the exposure is not how light or dark your image is.

When light hits a photosite on the sensor, some of that light is converted to an electrical charge. The voltage of that charge is then read off the sensor, and converted to a digital number in an image file. The more light that is captured, the greater the charge. The higher the voltage, the larger the digital number that is stored. The higher the digital number, the brighter that pixel is.

Mostly*, the ISO setting affects what happens after the voltage level is read off the sensor. At base ISO level, the camera will record in the image file a digital number proportionate to the voltage read off the sensor. At a higher ISO setting, the number that gets recorded for a given voltage is increased proportionate to the ISO setting. So if the ISO setting is twice the base ISO level, then the number that gets recorded for a given exposure is twice what it would have been at base ISO.

Cameras use different methods to produce the larger number yielded by using an ISO setting above the base level. In some cases, the camera will apply analog voltage gain to the voltage reported by the sensor, before it is read by the analog to digital converter (ADC) that produces the digital number. In other cases it will perform a digital multiplication or bit shift of the value produced by the ADC. These two methods have different side effects with regard to the Signal to Noise Ratio (SNR).

To understand how ISO setting relates to noise in an image, it helps to understang what noise is and where it comes from. Noise in a digital image is variation in pixel values from what is expected (usually difference from neighbouring pixels). The visibility of noise depends on the Signal to Noise Ratio. The higher the SNR, the less visible will be the noise.

Noise comes from two sources. The camera adds some noise, but most noise is variation already present in the light itself even before it is captured by the camera. This naturally-occurring variation in light captured by the camera is called shot noise. Shot noise exists because of the way light is created. Photons are emitted by atoms when the energy state of the atom is increased. The rate at which the photons are emitted, and the colour of these photons is random, but the distribution of possible values follows a Poisson distribution. This means that whenever we capture light, the amount and colour of that light will vary randomly somewhat. The more light we capture, the more the variations will average out. More light is a higher signal. The SNR for shot noise is equal to the square root of the signal.

The visibility of noise depends on the SNR. The SNR for shot noise depends on the amount of light captured. The amount of light captured depends on the exposure. At low exposures, the SNR is low. At low exposures and base ISO, pixel values will be small and the image will be too dark. So at low exposures, which have low SNR, we tend to use higher ISO settings. This is one of two reason that people think high ISO causes high noise. In fact it is the low exposure that is causing the highly visible noise. The other reason is that when you use a high ISO setting in an autoexposure mode (P, A or S), or use the meter reading to guide your setting choice in M mode, using a high ISO setting will cause the camera to use (or recommend) a low exposure setting, and the low exposure causes more noise. But the ISO setting itself does not cause more or less shot noise. You can verify this by adjusting the ISO setting without letting the exposure change.

When applying extra analog voltage gain is the method the camera uses to implement a higher ISO setting, a camera that adds very little noise after the gain stage will have very little change in SNR between high and low ISO settings for a given exposure. However, if a camera adds a significant amount of noise after the gain stage, applying analog gain can Increase the SNR for camera added noise. That's because the gain increases all the signal but only some of the noise: the noise added before the gain stage. So on cameras that add a significant amount of noise after the gain stage and use analog gain to implement ISO increases, increasing ISO without decreasing exposure can reduce the visibility of noise.

*On some cameras, the sensor has an ISO level at which the capacitance of the sensor is reduced. This results in the max voltage on the sensor being reduced, but the noise floor being lowered. The result is lower camera-added noise at higher ISOs.

I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).

Dutch Newchurch
Dutch Newchurch Senior Member • Posts: 5,096
Re: ISO discussion, film photography compared to digital photography

D Cox wrote:

Dutch Newchurch wrote:

Popcorn, anyone?

(Timmy, it's a good question. I'm not sure I want to know enough about photoelectronics to know the full answer.

All I know is that I can turn up the ISO, shot by shot, in a way that I never could with film. And, when I turn it up, the noise tends to have an adverse effect on the image.

You could do the same with film, but it can't be taken so far.

Now that we can scan film in and use computer processing, it is possible to get usable images from underexposed shots that would not have been usable in the darkroom. But it is still limited. Sensors are simply better than film -- they have much less built in noise.

Thank you.

I like that explanation.

There have been many discussions about how, and why, on here. They sometimes get quite heated.

I hope you get a helpful and comprehensible answer.)

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Dutch
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OP TimmyD11 Forum Member • Posts: 88
Re: ISO discussion, film photography compared to digital photography

It all sounds so electronic theory and bone dry!

I guess it doesn't matter too much...just seems to me that without film that varies in sensitivity to light I see no reason to need an ISO setting in digital SLR photography. I would think they could have a standard ISO that never changes and you could just expose the sensor through a combination of shutter speed and aperture.

But admittedly, I don't fully understand this.

FingerPainter Senior Member • Posts: 6,513
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

It all sounds so electronic theory and bone dry!

The "why"s of it are basic physics, which is beyond the interest of many.

... just seems to me that without film that varies in sensitivity to light I see no reason to need an ISO setting in digital SLR photography.

If the camera is ISO invariant and you only shoot RAW, there may be no need for an ISO setting. However, the former is true of very few (if any) cameras, and the latter is true of only a minority of photographers.

imagine a scene is dark enough that when you use the widest aperture you are willing to use (because it is the widest aperture on the lens, or you need a certain amount of DOF, or your lens is too soft wider than a certain aperture), and the slowest shutter you are willing to use (because slower would produces unwanted subject or camera motion blur), the resultant image is darker than you want. Using a higher ISO will produce an image of the desired brightness.

I would think they could have a standard ISO that never changes and you could just expose the sensor through a combination of shutter speed and aperture.

But admittedly, I don't fully understand this.

Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 12,450
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

It all sounds so electronic theory and bone dry!

Dryness is in the presentation, not in the subject!

I guess it doesn't matter too much...just seems to me that without film that varies in sensitivity to light I see no reason to need an ISO setting in digital SLR photography. I would think they could have a standard ISO that never changes and you could just expose the sensor through a combination of shutter speed and aperture.

But admittedly, I don't fully understand this.

High ISO lets you take photos at night without a tripod. That's pretty remarkable.

There are practical reasons why you can't do this without high ISO, although in theory it would be possible. But I'm pretty sure that most folks today wouldn't want 100 megabyte or larger image files.

ISO, depending on the camera, can act like an image compression scheme.

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OP TimmyD11 Forum Member • Posts: 88
Re: ISO discussion, film photography compared to digital photography

imagine a scene is dark enough that when you use the widest aperture you are willing to use (because it is the widest aperture on the lens, or you need a certain amount of DOF, or your lens is too soft wider than a certain aperture), and the slowest shutter you are willing to use (because slower would produces unwanted subject or camera motion blur), the resultant image is darker than you want. Using a higher ISO will produce an image of the desired brightness.

So a higher ISO amplifies something to allow for more brightness in digital photography, whereas in film photography the ISO, when set higher, allowed less light in because a higher ISO film was more sensitive to light? Something like that?

Dutch Newchurch
Dutch Newchurch Senior Member • Posts: 5,096
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

It all sounds so electronic theory and bone dry!

I guess it doesn't matter too much...just seems to me that without film that varies in sensitivity to light I see no reason to need an ISO setting in digital SLR photography. I would think they could have a standard ISO that never changes and you could just expose the sensor through a combination of shutter speed and aperture.

That's right.

Now, suppose the light is poor, and you are stuck with 1/2s @ f/2.0 and 100 ISO.

With film, you could replace the film with one rated at, say, 1600 ISO. That needs only 1/32 as much light as the film you've taken out (four stops). Then you could increase your shutter speed to 1/30s.

With digital, I can do much the same simply by pressing a couple of buttons. Or, as yardcoyote has already said - and this is really clever - set auto ISO, so the camera does it for me.

And, with digital, I can go up to ISO 6400 (higher, if I'm content with a 'grainy', black and white photo), and take the shutter speed up to 1/125s.

But admittedly, I don't fully understand this.

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Dutch
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D Cox Forum Pro • Posts: 20,902
Re: ISO discussion, film photography compared to digital photography

TimmyD11 wrote:

imagine a scene is dark enough that when you use the widest aperture you are willing to use (because it is the widest aperture on the lens, or you need a certain amount of DOF, or your lens is too soft wider than a certain aperture), and the slowest shutter you are willing to use (because slower would produces unwanted subject or camera motion blur), the resultant image is darker than you want. Using a higher ISO will produce an image of the desired brightness.

So a higher ISO amplifies something to allow for more brightness in digital photography, whereas in film photography the ISO, when set higher, allowed less light in because a higher ISO film was more sensitive to light? Something like that?

No, they both work the same. Higher ISO setting on the camera for the same film gives less exposure. The resulting dark photo is brightened later.

The difference is that there really are films with different sensitivities, whereas sensors are all ISO 100. In both cases you can turn a knob on the camera to set the "wrong" ISO.

D Cox Forum Pro • Posts: 20,902
Re: ISO discussion, film photography compared to digital photography

Dutch Newchurch wrote:

TimmyD11 wrote:

It all sounds so electronic theory and bone dry!

I guess it doesn't matter too much...just seems to me that without film that varies in sensitivity to light I see no reason to need an ISO setting in digital SLR photography. I would think they could have a standard ISO that never changes and you could just expose the sensor through a combination of shutter speed and aperture.

That's right.

Now, suppose the light is poor, and you are stuck with 1/2s @ f/2.0 and 100 ISO.

With film, you could replace the film with one rated at, say, 1600 ISO. That needs only 1/32 as much light as the film you've taken out (four stops). Then you could increase your shutter speed to 1/30s.

With digital, I can do much the same simply by pressing a couple of buttons. Or, as yardcoyote has already said - and this is really clever - set auto ISO, so the camera does it for me.

However, you are not "doing the same". The sensor is still an ISO 100 sensor. You are just underexposing it.

And, with digital, I can go up to ISO 6400 (higher, if I'm content with a 'grainy', black and white photo), and take the shutter speed up to 1/125s.

But admittedly, I don't fully understand this.

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Dutch
forestmoonstudio.co.uk
Photography is about light, not light-proof boxes.

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