High ISO Questions

[smoke665]

the "light sensitivity of the sensor" isn't altered.

I was under the impression that there was an increase in sensitivity on the new sensors somehow in their ability to translate and transfer the analog voltage from each pixel. From what I've read of that it appears there are limitations in future technology. Have you read or know anything about the research into memory chips as the the sensor where light photons actually alter the bits. Because it can theoretically achieve imaging with 100 times as many pixels on a chip of the same area as a conventional CMOS sensor, an inexpensive gigapixel camera could be a reality.

 

[greybeard]

For those who've weighed in that they are using the higher ISO settings, have you noticed any significant differences in noise between focal lenght and/other distance to the subject?

No I haven't noticed any difference from 24-120. or 70-300

 

[greybeard]

One big thing I think these cameras that can operate at such high ISO's allows us to do is use slower lenses in situations where in the past we need f/2.8 or faster. I've been able to use my 24-120 f/4 and my 70-300 f/4.5-5.6 in situations where in the past they would have been worthless.

Shot at 24mm f/4 ISO 7000

Bella Watching the computer by TOM STRAIGHT, on Flickr

 

[Ysarex]

This depends on the sensor. Some sensors don't do any (or at least not much) "upstream" amplification and rely entirely on "downstream" amplification. Downstream amplification results in a loss of DR (1 stop of DR is lost for every 1 stop of ISO boost).

If the sensor does "upstream" amplification, then that happens before the analog to digital conversion (ADC) and you can get some ISO gain without much of a loss in DR.

This has me a little concerned. DR is a function of exposure period. The engineering implementation in the camera that processes the sensor signal can employ a boost to the analog signal prior to ADC, digital scaling in the ADC and/or a hybrid combination of both. Regardless of implementation the method used to process the signal will do a better or worse job of retaining the sensor data but not create data. Exposure creates data. The light sensitivity of the sensor is in no way altered by the methodology engineered into the system to process the sensor output. A full sensor exposure = maximum DR. Any reduction in exposure = reduction in DR.

Joe

Pass the popcorn.

Joe, part of this is correct... in that the "light sensitivity of the sensor" isn't altered. However... the dynamic range of the image saved to your memory card can be altered by boosting ISO. But this happens as a result of what happens to the data after it leaves the sensor. You certainly can lose DR by increasing ISO (there's lots of data to back this up.)

No argument there.

What caught my attention was your comment, "....you can get some ISO gain without much of a loss in DR." That raised a flag for me thinking there might be an implication that sensitivity was being changed. But your affirmation that the light sensitivity of the sensor isn't (can't be) altered makes me happy.

I'm wondering what you mean by "DR is a function of exposure period." Can you elaborate on what you mean by that?

What I mean is that in very simple terms, since the light sensitivity of the sensor is fixed and unalterable, then the data you capture is ultimately a function of the data you expose for.

I understand that when the engineers test a sensor they expose it to a DR range that exceeds both upper and lower sensor limits. Then depending on how ISO brightening is implemented they measure the DR loss per stop of ISO brightening. I don't know for sure but I'll suspect that in the D3100 that you noted the DR loss of less than a stop per stop of exposure change at the lower ISO values is the result of a hybrid (analog gain/digital scaling) method similar to what's used in my old Fuji. Bill Claff identifies the DR change between 1/3 to 1/2 stop per exposure stop. That's the engineers.

We're not engineers. I think we use cameras to take photographs. And assuming we're photographers and not fauxtographers (more popcorn!!) let's consider this scenario: You're using a Nikon D3100 set to ISO 100. You've tested your camera and know it's limits and you meter a landscape with puffy white clouds as the brightest diffuse highlight in the scene. You place that diffuse highlight at the sensor threshhold (where it belongs). At ISO 100 you will be able to record X DR (the sensor's maximum). But oh no! you want 1 f/stop more DOF and can't afford to drop the shutter speed any further so you make the f/stop change and set the ISO to 200. How much DR did you just lose? X - .5 stop or X - 1 stop.

Instead of the Nikon D3100 consider instead the Simga Merrills. They use no variable gain at all. Therefore they have the same DR at all ISO values? The engineers would say yes. As a photographer I'd say you get the DR you expose for.

Joe