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Forum: General Photography 09-04-2018, 10:54 AM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
I quoted the exact text that gave me that impression. Go back and look.

I didn't actually think you didn't know about exposure compensation, but the garbage you were spewing about having to do settings at one ISO then change to another ISO to take the picture just had me in disbelief.



I have always tried to use the lowest ISO that works with the shutter speed and aperture that I think the shot needs. If the picture was too dark I'd raise the ISO. ISO invariance means I don't have to be so worried about the ISO at the time of taking the picture, as long as I'm willing to work a little with the RAW - the picture will come out the same.

I've found that camera metering is inconsistent enough that blown highlights are always a possibility, even when I'm being careful to avoid them.



ISO invariance doesn't mean a flat curve - it means one that is straight with a slope of -1.0. Raising the ISO by 1 EV should lower the DR by exactly 1 EV. Or looking at it the other way around, lowering the ISO should raise the DR by the exact same amount, until you reach the base ISO. If a camera achieves that it is truly ISO invariant.

Just so we're clear, I'm not arguing that the perfectly ISO invariant camera exists - it doesn't. But I know now that many modern cameras are close enough that underexposing by a stop or two by using a lower ISO than I would have in the past isn't going to ruin my pictures.
Forum: General Photography 09-04-2018, 09:42 AM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
Have you really never heard of exposure compensation? Just set it to -2.0 and be done with it. Just don't combine that with auto ISO!

The whole idea is to give yourself more latitude with blown highlights. If you've never accidentally blown highlights, then you're lucky - this discussion isn't for you.

For me personally I'm just going to be less afraid to underexpose in the future, as long as I'm at a low ISO I know it isn't going to make my picture much noisier.
Forum: General Photography 09-03-2018, 08:35 AM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
But sometimes your settings are such that at base ISO, the image is too dark. So the question then becomes, raise the ISO or lighten it in post? ISO invariance says there will be no difference.
Forum: General Photography 08-30-2018, 01:22 PM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
Sometimes if your aperture and shutter speed are constrained by other considerations, it would be nice to have a very low ISO. You can usually use a ND filter, but it's not as convenient.
Forum: General Photography 08-29-2018, 06:02 PM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
No, specifically not expose to the right. Suppose that to expose to the right, you determined that you needed F/8, 1/250s, and ISO 1600. Instead you would shoot F/8, 1/250s, ISO 400. The JPEG would be dark but recognizable, and the RAW could be pulled up in post processing. If for example there were some bright clouds in the scene, they would have gotten blown out at ISO 1600 but fully preserved at ISO 400. If your camera is ISO invariant the resulting noise in the picture would be the same under both scenarios.
Forum: General Photography 08-29-2018, 02:48 PM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
Underexposing at low ISO gives you one definite benefit: it increases the amount that you can recover the highlights.

If you have a camera with the accelerator doing noise reduction, lowering the ISO is the only way to turn that off. It's up to you if you consider that a good thing or a bad thing.

Maybe what you want is not to lower the ISO all the way to base, but just to underexpose by a stop or two. The preview will still be usable.
Forum: General Photography 08-29-2018, 08:45 AM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
Just to make sure I wasn't speaking out of the wrong orifice, I went looking for an article on the subject. Here's a good one, complete with sunset pictures.

ISO Invariance Explained - Photography Life
Forum: General Photography 08-29-2018, 08:18 AM  
"I had a dream." What determines the base ISO of a digital sensor?
Posted By Mark Ransom
Replies: 75
Views: 4,459
I would agree with your first sentence - the base ISO is the point where you've achieved the maximum dynamic range, which is determined by the signal/noise ratio. Assuming the noise floor is constant, it would be the point where you have the maximum amount of signal before overloading the pixel cells. That would certainly be a technical limitation of the sensor.

My understanding of ISO invariance is different, it's all about increasing ISO. It means that increasing the ISO is no different than simply turning up the brightness during the conversion of the RAW image.
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