ISO discussion, digital SLR compared to film SLR

[TimmyD11]

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).

 

[fmw]

It amplifies the electrical signal. Something like turning up the volume of your stereo system. You learn to use it by using it. As you increase the signal (ISO) you increase noise in the same manner that your stereo increases audio distortion. The noise is easily visible in your editing software. Just test and learn how much difference in noise you accept in a given environment.

 

[Braineack]

this is hands-down my favorite video for explaining ISO and the effects of it on your image:

 

[Gary A.]

What Fred said ... in other words 'Gain'. As Fred noted, it isn't magical or free ... the trade off for Gain is similar to pushing film, the more you push the greater the grain and contrast. The more you increase the amplification of the signal in the camera, the greater the noise (looks like grain) and contrast.

My first dSLR was a Canon 20D. To my tastes ISO 1600 was barely acceptable. My latest camera, a Fuji XT2, ISO 6400 is quite acceptable to my eye.

 

[KmH]

That video is horrible.

The sensitivity to light of the image sensor never changes.

Image sensors and pixels do not work the way he says they do.
Pixels do not detect color at all. Pixels only detect luminosity.

The interference part is also BS.
When he starts spewing the BS about dark current he lost me and I stopped watching the video.

Whatever signal (an analog voltage) a pixel developed during an exposure is amplified (gain) according to the ISO setting. After amplification the voltage is converted to a digital number.
The millions of pixel digital numbers get recorded on the memory card.

Back in the day ISO was standardized for film and digital.
International Organization for Standardization - Wikipedia
Back in the day we had ASA numbers for film.

Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to measure the signal gain of digital imaging systems.

American Standards Association (ASA)

 

[Peeb]

Not sure about 'horrible' but if sensors don't work the way the video describes - how do they work (sincere question- not rhetorical)

Your reference to luminosity lost me.

EDIT: Scott cleared it up for me- I was confused how colors were detected and identified if not by the sensor and I'm now told that they bayer filter does this. Awesome!

 

[Overread]

To know how to use it its simple really. As said earlier the higher the ISO value you select the greater impact the light has on the exposure.

Increase the ISO by one stop and the exposure increases by one stop.

The trade off is that higher ISOs generate more digital noise so a very high ISO will be very "nosey" - this impacts detail and clarity. The appearance is somewhat different to grain on film, but essentially its similar in effect.

Note that this "noise" can also be seen if you take a dark photo and then brighten it in editing. As a result an underexposed photo that you brighten will actually show more noise than if you'd used a higher ISO at the time and exposed it correctly. Note that by correctly this refers to the "expose to the right" theory of exposure where by you're aiming to capture as much light data as possible without overexposing the photo. It is an idealist concept and practical real world shooting will often not let you achieve it.

Note also that in recent years there have been some gains in sensor technology and some sensors now offer very little practical degradation of the image at higher ISOs; as a result some cameras are getting to a point where they are far more free with what ISO you want to pick. Nikon and Sony have pushed this technology (well sony really - nikon bought their sensors); which gives them an edge over Canon and many other brands.




If you really want to learn how sensor work electronically - be careful. It's an area rather like light and physics were the average photographer knows enough to shoot; but hasn't really got much if any proper clue as to what is really going on. They cobble it together from forum posts and websites; where often cutting complex subjects into simple terms adjusts the theory to be more digestible, but not always factually correct.

 

[SCraig]

"Horrible" seems a bit harsh. If sensors don't work the way the video describes - how do they work (sincere question- not rhetorical)

Your reference to luminosity lost me.

Luminosity is the level or quantity of light hitting the sensor, or to simplify how "Bright" it is. Colors are determined by the Bayer Filter in front of the filter. The photosites of the sensor convert the quantity of light to an analog voltage. It is then amplified based on the setting of the ISO. The ISO (actually a bias voltage chosen based on the ISO setting) controls the amount of amplification provided to the voltage from the sensor; the lower the ISO setting the less the amount of amplification, the higher the ISO setting the higher the level of amplification.

As has been stated, all electronic circuits generate "Noise" (much like seeing "Snow" on old-school analog televisions). As the level of amplification increases the ratio of the noise level to the desired signal increases as well, hence the term "Signal to Noise Ratio". A high SNR means that the level of the desired signal is much higher than the level of the noise so the overall signal will be very "Clean" or the noise is very low in comparison.

 

[TimmyD11]

So, as a photographer using daylight wouldn't I pretty much always have my camera set to 100?

And maybe 400 very early in the morning or late in the evening?

But for all intents and purposes wouldn't a photographer using natural daylight pretty much always keep his ISO set to 100?

 

[Peeb]

"Horrible" seems a bit harsh. If sensors don't work the way the video describes - how do they work (sincere question- not rhetorical)

Your reference to luminosity lost me.

Luminosity is the level or quantity of light hitting the sensor, or to simplify how "Bright" it is. Colors are determined by the Bayer Filter in front of the filter. The photosites of the sensor convert the quantity of light to an analog voltage. It is then amplified based on the setting of the ISO. The ISO (actually a bias voltage chosen based on the ISO setting) controls the amount of amplification provided to the voltage from the sensor; the lower the ISO setting the less the amount of amplification, the higher the ISO setting the higher the level of amplification.

As has been stated, all electronic circuits generate "Noise" (much like seeing "Snow" on old-school analog televisions). As the level of amplification increases the ratio of the noise level to the desired signal increases as well, hence the term "Signal to Noise Ratio". A high SNR means that the level of the desired signal is much higher than the level of the noise so the overall signal will be very "Clean" or the noise is very low in comparison.

Thanks, Scott. OK- so the sensor detects brightness and the bayer filter detects colors (as a very rudimentary explanation).

Did not know that! I'm afraid to learn much but I'm happy to know that much.

 

[Overread]

Depends.

If you're shooting portraits of people chances are 100 might well be fine, but you'd fast want 800 or even 1600 or higher if indoors without using flash.

However if you were taking photos of action you might well need a higher ISO as even with a wide aperture there might not be enough light (and middle of the day light brings its own problems).

You've got three settings which you balance based upon your requirements and the situation. Indeed whilst keeping the ISO low is good, its generally bad advice for most new people to follow. Because, like yourself, you quickly think that you must keep it low; when in reality you might find that ISO 400 is a more practical starting point outside and even 800 on very cloudy days. You've got to be willing to react and change based on the situation.

 

[Designer]

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).

You are correct in thinking that the sensitivity of the sensor does not change.

ISO is applied gain. The processor applies gain according to the ISO setting.

The signal-to-noise ratio (SNR) is also a function of the sensor design and firmware that is designed into your camera. Some cameras have a wider dynamic range than others, and some can handle higher ISO (post-capture gain) better than others.

Here is a two-part tutorial on the topic of noise:

Digital Camera Image Noise: Concept and Types

Image Noise: Examples and Characteristics

 

[SCraig]

So, as a photographer using daylight wouldn't I pretty much always have my camera set to 100?

And maybe 400 very early in the morning or late in the evening?

But for all intents and purposes wouldn't a photographer using natural daylight pretty much always keep his ISO set to 100?

That would depend on the aperture and/or shutter that is needed for the particular shot. If you're shooting birds in flight and need somewhere around 1/2000 second shutter speed and enough aperture to give some decent depth of field then you would likely need to increase the ISO. How much would depend on the light level. On the other hand if you're shooting a landscape using a tripod then, yes, ISO 100 would likely work fine. You can't just say one particular setting will work for all circumstances. That's the reason they are all adjustable.

 

[Peeb]

Depends.

If you're shooting portraits of people chances are 100 might well be fine, but you'd fast want 800 or even 1600 or higher if indoors without using flash.

However if you were taking photos of action you might well need a higher ISO as even with a wide aperture there might not be enough light (and middle of the day light brings its own problems).

You've got three settings which you balance based upon your requirements and the situation. Indeed whilst keeping the ISO low is good, its generally bad advice for most new people to follow. Because, like yourself, you quickly think that you must keep it low; when in reality you might find that ISO 400 is a more practical starting point outside and even 800 on very cloudy days. You've got to be willing to react and change based on the situation.

Agree- in fact many birders go out with their rig set to something like: 1/1000 [or faster] plus f/8 plus auto-ISO.

The shutter speed is non-negotiable (long lenses and possibly moving subjects) and they need decent depth of field so the only variable to give is ISO. It's amazing how well modern sensors can deliver usable images at previously unimaginable ISO levels.

 

[Designer]

So, as a photographer using daylight wouldn't I pretty much always have my camera set to 100?

And maybe 400 very early in the morning or late in the evening?

But for all intents and purposes wouldn't a photographer using natural daylight pretty much always keep his ISO set to 100?

Some photographers fiddle with the setting as if it were the third leg of an "exposure triangle", but in reality, changing the ISO has no effect whatsoever on the exposure.

It does, of course, have an effect on how the processor generates the image.

If you don't want to keep adjusting the ISO, just put it on "auto". Or if you wish to get ahead of the curve when faced with a low-light situation, go ahead and turn up the ISO before shooting. As you bump up the ISO, you will eventually run up against the processor's ability to produce a clean image, and you will get artifacts in the image called "noise".