Forum General Photography ISO Context
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Uwe Winefeld 20.12.2022, 15:01 #1

ISO Context

ISO
Hello,
I have a question and would like to thank you in advance for your response. The higher the ISO value, the more sensitive the sensor becomes to light, and the shutter speed also gets shorter, reducing the risk of camera shake.
Now, with higher ISO settings, the image also gets brighter, which would seem to mean that the shutter speed should be longer. It’s probably a simple explanation—thanks again in advance for any tips.
Best regards, Fotomotivjäger
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Andreas F 20.12.2022, 19:21 #2
Hi Uwe,
I can’t see the question directly right now, but I think I understand what it’s about.
Your seemingly logical chain of thought has a flaw at the end.
But let’s take it step by step.
“The higher the ISO value, the more sensitive the sensor becomes to light, and the shutter speed gets shorter, reducing the risk of camera shake.”
That sounds right at first glance, but it’s not entirely accurate.
It’s true that increasing the ASA/ISO value makes the sensor more light-sensitive, whether it’s a digital or analog sensor.
However, the conclusion that this automatically means a shorter shutter speed and less chance of blur isn’t necessarily correct. To your statement above, I’d probably respond briefly with “It depends.” What does it depend on? The exposure value. The exposure value is the key factor here. Your statement only holds true if the exposure value stays the same. We’re dealing with the triangle of aperture/shutter speed/sensitivity, which in practice always revolves around the exposure value.
Your second sentence seems to contain a simple misunderstanding.
“Now, with higher ISO settings, the image gets brighter, which would mean the shutter speed should be longer.”
With the same exposure value and unchanged aperture, the shutter speed should actually get shorter, not longer, when sensitivity increases.
There are indeed some quirks with analog sensors that don’t apply to digital ones. In those cases, the calculated shutter speed might need to be adjusted. This is due to the “Schwarzschild effect,” which occurs with exposure times longer than 1 second and shorter than 1/1000 of a second. Digital cameras don’t experience this, and it would be particularly interesting for modern cameras with true electronic shutters capable of speeds up to 1/180,000 second (no typo there ;)).
I hope this answers your question. If not, feel free to ask more. Maybe one of the other forum members can provide an even more precise explanation.
  Best regards, Andreas
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M B 20.12.2022, 19:26 #3
Dear Uwe, If I’m not mistaken, a sensor’s light sensitivity is always constant. For most cameras, the base ISO is 100, sometimes 200. When you increase the ISO setting on the camera, it amplifies the light signals, which then leads to what’s called 'noise.' If you raise the ISO while keeping the aperture and shutter speed the same, the exposure increases and the photo becomes brighter. ISO, aperture, and shutter speed together determine the exposure. If you want to shorten the shutter speed—for example, to avoid camera shake or to freeze fast motion—you’ll need to either open the aperture wider (a lower f-number means a wider aperture) or increase the ISO to maintain the same exposure level.
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Wilfried Voßen 22.12.2022, 07:22 #4
@Andreas F Thanks for the great explanation, Andreas – I still have a lot to learn from you.
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Andreas F 22.12.2022, 10:23 #5
@Wilfried Voßen

Thanks for the kind words. I always hope that I explain things clearly enough, especially when I’m trying to summarize the key points briefly.
 
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Uwe Winefeld 23.12.2022, 22:45 #6
@Andreas F thanks for the explanations.
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Uwe Winefeld 23.12.2022, 22:46 #7
@Maria Bürstmayr Thanks for the explanations
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Andreas F 24.12.2022, 09:11 #8
@Uwe Winefeld

You're welcome! I hope it was clear. If not, feel free to ask any questions.

Best regards, Andreas
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Erich Kremer 08.10.2025, 23:29 #9
It could also be added that doubling the ISO (ASA) corresponds to increasing the aperture by one stop or doubling the shutter speed. However, this knowledge isn’t necessarily essential today, but it was important back in the days of analog film. Back then, a film rated at 400 ASA was already considered highly sensitive. Pushing it to 1600 ASA was a "risky" endeavor that required skill to master.
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Gert Trumpold 14.11.2025, 19:44 #10
@Uwe Winefeld Good evening Uwe, I've added a "cheat sheet" for you on the photos explaining the relationships between the camera settings. Hope it doesn't get deleted. Best regards, Gert
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Gert Trumpold 14.11.2025, 19:49 #11
@Uwe Winefeld
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Gert Trumpold 14.11.2025, 19:50 #12
Under Special Techniques.
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J F 14.11.2025, 20:08 #13


It's true that a higher ASA/ISO value makes the sensor more light-sensitive,

 That's actually not correct!

ASA/ISO originated in the era of analog film, where the film’s light sensitivity was indeed defined by the ASA/ISO rating.

What we refer to as ISO on a digital camera is actually the gain, meaning the amplification of the signal. The sensors themselves have a fixed sensitivity. In good lighting conditions, enough photons hit the sensor, so you can keep the ISO low because when reading the analog sensor (which, contrary to popular belief, is not digital) and then digitizing the signal, the useful signal is strong enough to be accurately digitized.

In low light, fewer photons reach the sensor, resulting in a weaker signal that can’t be cleanly processed during digitization. To compensate, the analog signal is amplified before digitization by increasing the ISO value. However, the downside is that this also amplifies the sensor’s electronic noise and photon noise, leading to grainy images. Photon noise is unavoidable, but electronic noise has been significantly reduced over time, which is why modern cameras produce less noise than older models.

It’s similar to turning up the volume knob. If the music is very quiet, you can barely hear it, so you turn up the volume and suddenly everything sounds fine again.

The term ISO on a digital camera is really misleading and was introduced by marketing experts. It’s just as misleading as the term “full-frame.” That simply refers to a sensor the size of old 35mm film. There are no “half” sensors.
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J F 14.11.2025, 20:14 #14

If I’m not mistaken, a sensor’s light sensitivity is always the same. For most cameras, the base ISO is 100, sometimes 200.

The first statement is correct. However, the base ISO for most cameras is actually ISO 200. That doesn’t mean much, though, because ISO 200 on a Canon doesn’t necessarily equate to ISO 200 on a Nikon. This applies to all camera brands. Ultimately, it’s just a matter of the amplification factor.
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Erich Kremer 14.11.2025, 20:20 #15
Guys, buy an old photography textbook from the 60s or 70s. Basic knowledge is essential, even in the digital age. 

I recommend the paperbacks by Andreas Feininger. And above all:  
Photography: Basics, Techniques, Practice. Fischer Paperback.
by Kurt Dieter Solf 

The "Solf" also includes a bar chart and a Siemens star.
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Erich Kremer 14.11.2025, 20:20 #16
I don't recommend recent photography textbooks; they're too flashy and lack substance.

 
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Jörg Hartmann 16.09.2026, 08:32 #17
I have only been in the club since August 26, so this is my first comment on the topic. I want to explain the phenomenon with an example. Given is the aperture-shutter speed combination f8 and 1/200s. If the motif is not sufficiently exposed with this combination, I increase the ISO value. If it then happens that I have to change the shutter speed to 1/250s, I increase the ISO value again. Was this explanation helpful?
Regards, Jörg
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