ISO and Exposure - What Do They Really Mean?

TeriTerryTarry

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The SJ4000 Wi-Fi has menu settings for ISO and Exposure but what do these really mean? I know that ISO has to do with sensitivity to light - the higher the ISO the greater the sensitivity. But when you change the setting, what's happening inside the camera to make this change come about? The sensor is not something that is programmable, i.e., it always does the same thing the same way, correct? Does changing the ISO setting have something to do with the programming of the image processor? On the SJ4000 the settings are 100, 200, 400 and Auto. Is 200 twice as sensitive as 100, and 400 four times as sensitive? Are these only relative terms specific to this camera or are these actual industry standard numbers comparable across all digital cameras? Does the setting apply to recording videos and shooting photos?

When I tested exposure with my camera I found that they fell into three groups with all the photos within a group having the same exposure despite the camera having multiple settings. This is no doubt a camera defect but if it had been operating properly one would see a gradual but noticeable change with each setting. But a change in what? It can't be an aperture setting because the camera has no moving parts. Is it the equivalent of shutter speed? But action cams don't have shutters. Therefore exposure must have something to do with the programming of the signal processor, but what? On the SJ4000 the range is -2.0 to +2.0 in 1/3 increments. And again does it apply to both videos and photos?
 

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TeriTerryTarry

TeriTerryTarry

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exposure or EV setting can be used to lighten or darken the picture, @niko posted examples of that here https://dashcamtalk.com/forum/threads/understanding-of-wdr-on-off-vs-ev-exposure-value-settings.13732/#post-179228
I understand that EV can be used to lighten or darken the picture, but I'm just trying to understand how and if it has an equivalent in ordinary full-feature digital cameras and camcorders. On my digital point-and-shoot I can adjust the aperture, shutter speed and ISO but the SJCam has no shutter or aperture though the ISO can be adjusted. Maybe I'm overthinking this. Maybe the EV does nothing more than lighten or darken the picture the same as you could do in any image editing program.
 

Nigel

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Normally the ISO adjustment changes the analog amplification of the pixels before the analog to digital converter so that the analog value is in the correct range for the analog to digital converter to work well. It is like an analog volume control for a microphone.

What it actually does in any particular camera may be different, it may even be a fake adjustment, but by definition it is the sensitivity of the sensor and if it is adjustable then you should be adjusting the sensitivity of the sensor, probably before the conversion to digital.
 
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TeriTerryTarry

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Normally the ISO adjustment changes the analog amplification of the pixels before the analog to digital converter so that the analog value is in the correct range for the analog to digital converter to work well. It is like an analog volume control for a microphone.

What it actually does in any particular camera may be different, it may even be a fake adjustment, but by definition it is the sensitivity of the sensor and if it is adjustable then you should be adjusting the sensitivity of the sensor, probably before the conversion to digital.
Okay, that is something I never realized about sensors. I thought that sensors could NOT be adjusted, that they always worked the same way (photons knocking electrons into wells, whatever, I don't understand all of it) and that any and all changes were made in the image processor. In film ISO is standardized but judging from your response that's not the case with digital cameras. I also assume that this analog amplification works whether one is recording a video or shooting a photo, correct?

Thanks and cheers :)
 

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Okay, that is something I never realized about sensors. I thought that sensors could NOT be adjusted, that they always worked the same way (photons knocking electrons into wells, whatever, I don't understand all of it) and that any and all changes were made in the image processor. In film ISO is standardized but judging from your response that's not the case with digital cameras. I also assume that this analog amplification works whether one is recording a video or shooting a photo, correct?

Thanks and cheers :)
The ISO standard applies to both film and digital sensors, it just indicates the sensitivity of the sensor in standard units. A 100 ISO film has the same sensitivity as a digital sensor set to 100 ISO. The sensor works exactly the same for photos and video, although for video it normally only bothers with a 16:9 area and for higher resolution sensors may not process all pixels as doing so would be too slow for 60fps video
 
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TeriTerryTarry

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My comment about non-standard ISO in digital cameras came from a Wikipedia article that reads in part:

"The ISO standard ISO 12232:2006
[59] gives digital still camera manufacturers a choice of five different techniques for determining the exposure index rating at each sensitivity setting provided by a particular camera model. Three of the techniques in ISO 12232:2006 are carried over from the 1998 version of the standard, while two new techniques allowing for measurement of JPEG output files are introduced from CIPA DC-004.[60] Depending on the technique selected, the exposure index rating can depend on the sensor sensitivity, the sensor noise, and the appearance of the resulting image. The standard specifies the measurement of light sensitivity of the entire digital camera system and not of individual components such as digital sensors, although Kodak has reported[61] using a variation to characterize the sensitivity of two of their sensors in 2001.

"The Recommended Exposure Index (REI) technique, new in the 2006 version of the standard, allows the manufacturer to specify a camera model’s
EI choices arbitrarily. The choices are based solely on the manufacturer’s opinion of what EI values produce well-exposed sRGB images at the various sensor sensitivity settings. This is the only technique available under the standard for output formats that are not in the sRGB color space. This is also the only technique available under the standard when multi-zone metering (also called pattern metering) is used."

Note the second paragraph especially. The full article is here:

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

 

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My comment about non-standard ISO in digital cameras came from a Wikipedia article that reads in part:
...
However it is calculated, the aim is still the same, any camera set to ISO 100 and 1/60th second should produce the same image brightness so that you know what you are doing when using it. You also know that a camera that can use ISO 1000 is going to be able to work well in darker conditions than one that has a maximum of ISO 200, so you can choose your camera based on your requirements and the camera specification.

Although even that doesn't actually work perfectly since it will depend on the lens used and the sensor size and probably a few other things, but if you go into the full details as in the Wikipedia article then it gets too complicated, you don't need that level of detail in order to use the camera.
 
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TeriTerryTarry

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Thanks. You're right, I definitely don't need that level of detail to operate the camera - just curious. On my SJCam I don't know the shutter speeds so I guess it's still a bit of a crapshoot anyway.

Cheers :)
 

Nigel

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Thanks. You're right, I definitely don't need that level of detail to operate the camera - just curious. On my SJCam I don't know the shutter speeds so I guess it's still a bit of a crapshoot anyway.

Cheers :)
For photos, I think you can check the shutter speed that it used in the exif information - file properties/details, if you know what it normally does then it can help in planning a photo and maybe deciding to use an ND filter to reduce the light levels.
 
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TeriTerryTarry

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For photos, I think you can check the shutter speed that it used in the exif information - file properties/details, if you know what it normally does then it can help in planning a photo and maybe deciding to use an ND filter to reduce the light levels.
That is absolutely fantastic Nigel! I never bothered to scroll down through all the Properties information for any photo ever. How can I have missed something so basic?!

This raises an interesting question. Last night I took a series of photos with all the settings remaining constant except the image size. ISO was set at 400. However, Properties indicates the ISO varied from 339 to 286. Is it common in these inexpensive cameras for the ISO setting to be off by 15 to 29 percent? Also something weird happened with what seems to be the white balance. The two smallest photos have reasonably accurate color and the rest are all too yellow. Properties correctly indicates White Balance was set to Auto. I've attached two samples.

Thanks again Nigel. Cheers :)

EDIT: By the way, all photos indicate an F-stop of f/1.8, shutter speed of 1/30 sec, focal length 2mm and max aperture of 1.6.
 

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Nigel

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I guess that either the ISO information is wrong, or more likely it ignored your instructions - a bug in the camera. I'm not actually surprised by that. There are too many people who will buy a camera based on it having features like ISO settings, but then never realise that it isn't working, or just don't bother to complain, the manufacturer then doesn't bother with the few people that do complain.

I think the white balance turned orange because you got the lights in the image and it set the white balance off the lights because they are brighter than everything else. You could try setting a fixed white balance, otherwise try to avoid the lights getting in the image.

The focal length and aperture is fixed on an action cam, it should always be the same unless you change the lens. I doubt you actually have an f1.8 lens though, that will just be the default value that SJCAM hasn't bothered to set correctly.
 
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Thanks again Nigel. You've been very helpful. :D
 
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