WDCash Loc: Milford, Delaware, USA

Recient conversations and articles about the new Sony A9iii have me wondering if I understand the meaning of ISO and spicificly "base ISO" as it relates to modern digital cameras.
First off
Iso, from film days, must have been an standardized across all film manufacturers. (?) As I understand it, ISO had to do with a films sensitivity to light which was based, I think, on the "grain size ?" of the silver halide crystals. Larger crystals captured more light and also led to the Grainess of higher iso films.

In digital photography the ISO is an amplification of the light signal captured by the sensor.
The sensor captures whatever photons it captures but the electronics in the camera amplify (gain) the signal as we turn up the ISO.
I'm sure I'm going to be seriously corrected on all of this, which is why I'm asking.
Here is where the confusion for me starts
If ISO is a carry over term from film, what exactly is a "Base ISO" of a particular camera. Spicificly,
Why does Nikon use a base ISO of 64, Canon 100, Olympus 200 and Now Sony a9ii 240? And I may be wrong about some of these Base ISOs but I hope you get my question.
To the photographer, what difference do these numbers really make?
Am I to understand that Nikon's ISO 64 is its cleanest, least distorted but also less light sensitive then say Canons ISO 100? That Olympus and now Sony are using sensors that are more sensitive (their base sensitivity), collecting more light comparatively at their respective base ISO and producing their cleanest "recording" of that collection?
Please explaine.

Longshadow Loc: Audubon, PA, United States

Not sure, but I think Base ISO for a camera is the sensitivity setting where the camera results are optimum.

CHG_CANON Loc: the Windy City

Bill - all digital cameras capture at their own 'base ISO'. Most cameras this is ISO-100, some might be higher, such as ISO-125, some all the way to ISO-200. The image is then amplified (or downgraded) to the ISO specified by the exposure setting, either the human or the camera's calculation. The numbers below the base ISO are just more trickery of digital photography, not the 'base ISO' of the camera. How each camera does this is proprietary. The only issue is the result maps to the ISO international standard -- International Organization for Standardization, a nongovernmental organization that publishes standards.

So, the base ISO is the unamplified sensitivity of your camera’s sensor. Your reference to the lower values, below ISO-100 are not the base, unless confirmed by the manufacturer. ISO-080, 60, 50 are 'expanded' values. These expanded values reduce the dynamic range capability of the camera, just a values higher than the camera's normal range, such as higher than ISO-3200, check your individual user manual. When you use a lower ISO setting than your base (i.e. ISO 50 instead of ISO 100) such as to achieve a slower shutter speed, your camera sensor will digitally expose your image at ISO 100, and then the processor will reduce that exposure to simulate what it would look like at ISO-050.

The result? You can potentially lose contrast and detail in the highlights and reduce your overall dynamic range since the image is purposefully overexposed. Highlights that are on the cusp of being clipped at ISO-100, when you meter the scene, will likely be blown-out from the exposure being brought down.

Ysarex Loc: St. Louis

ISO is not defined as gain or amplification. We don't want to say ISO is amplification. Amplification of the sensor signal is one of the methods (not the only method) used to implement ISO. Saying ISO is amplification is using a common implementation method as a definition.

ISO is a specific method for determining the lightness of the final output image (JPEG) based on a measured exposure.

For any given camera model base ISO is the lowest (non-extended) ISO value possible for that camera.

That's a design choice on the part of each camera manufacturer. You'll find for example that two different brand cameras are both using the same Sony built sensor and yet they set different base ISO values for their cameras. They may use different ISO standards (see next paragraph). Odds are if you ask them why you'll get back an answer along the lines of "that's proprietary."

NOTE also that the current ISO standard allows the camera manufacturers to choose from one of two different ISO definitions (presumably they were not able to otherwise agree). Some of our cameras use the SOS (Standard Output Sensitivity) ISO standard while others use the REI (Relative Exposure Index) ISO standard. They tell you which one they're using in the EXIF data. My Fuji and Leica cameras are SOS while my Nikon and Canon cameras are REI. The two different standards do not directly agree -- they're close but typically you can expect an SOS camera to generate a slightly darker output image at the same ISO value and same exposure.

Sensor's don't have assigned ISO values. The ISO value set on your camera does not apply to the camera's sensor it applies to the output image created by the camera -- typically a JPEG. Both the SOS and REI ISO standards used by our cameras apply only to the camera generated output image.

WDCash Loc: Milford, Delaware, USA

Thanks very much, the fog is clearing

Ysarex Loc: St. Louis

ISO 64 on my Z7 is base. ISO 32 is an "expanded" value.

Except when they don't -- see: https://www.photonstophotos.net/Charts/PDR.htm#Nikon%20Z%206

Bohica Loc: SE Coast of NC

STILL MAKES NO SENSE TO ME BUT i'M A 50+ FILM GUY

WDCash Loc: Milford, Delaware, USA

Yeaaaa, Note I said the fog is clearing, not cleared.

Longshadow Loc: Audubon, PA, United States

https://helpguide.sony.net/di/pp/v1/en/contents/TP1000756719.html

https://www.picturecorrect.com/understanding-native-base-iso-on-your-camera/

Sounds like the "optimum" setting to me, least noise.

User ID

SOON ENUF YOU MUST DIE, OR WORSE, AND THEN ISO WONT MATTER FOR YOU ANYMORE. "50+" ??? "FILM GUY" ??? CERTAIN DOOM !!!

DavidM Loc: New Orleans, LA

Here's an excellent video explaining ISO and it's use..

https://www.youtube.com/watch?v=sOdlDyolhr0

DaveO Loc: Northeast CT

Thanks for the link!

chrisg-optical Loc: New York, NY

ISO is a tricky subject, especially when comparing today's ISO to yesterday's ISO/ASA standards (which is only an approximation). Shooting with ISO 100 film may give different results than ISO 100 on digital camera, at the same shutter and f stop settings, same lighting. So for film, the optimal exposure might be + or - stop difference from the same shot on digital. This is due to the response characteristics of film to the various colors or tonalities. Even comparing various digital cameras will yield surprising differences even with matched settings for exposure, lighting, WB, etc. The Northrups made a video on YT about this some time ago, comparing several digital cameras at the same ISO and exposure settings under the same lighting conditions. The results were surprising as you would think ISO is a standard like measuring temperature or pressure.

As for "base ISO" that is just the ISO at which the sensor reads the pixel signal without any *additional* amplification. Of course signals can be attenuated too (negative gain) so some cameras can dip below their normal base ISO to a degree. When a signal is amplified so is the noise which is ALWAYS present in this universe. Hence higher ISO means more noise without signal treatment.

A. T.

I knew all along you were one if those egg head geniuses......you just proved it 😉.

brentrh Loc: Deltona, FL

ISO with film was set for film you loaded in your camera you set it and then never needed to change it. Depending on what you were photographing I learned when photographing photographing Rolex 24 race at night changing iso would give you better results but you had to use the change for full roll and hope you guessed right. With digital you have the freedom to change iso and take test shots to achieve results you desire