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Author Topic: Why no low ISO DSLRs?  (Read 1496 times)

sanjosedave

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Why no low ISO DSLRs?
« on: March 30, 2012, 03:51:07 PM »
In my film days I used to shoot Kodachrome 25 or 64, and low ISO Kodacolor, mostly landscapes. I liked the fine detail.

The threads always talk about high ISO and low noise, but, what about low ISO and no noise? I, and, probably others, are willing to use a tripod and shoot at low ISO.

But, then, my understanding of digital vs film is just beginning. I use a G11 and a 60D

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Why no low ISO DSLRs?
« on: March 30, 2012, 03:51:07 PM »

drmikeinpdx

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Re: Why no low ISO DSLRs?
« Reply #1 on: March 30, 2012, 04:02:33 PM »
That would be a nice capability to have.

I was doing a studio shoot yesterday and wanted to open up my prime lens to creat shallow DOF, but even with my Alien Bees turned all the way down I was still at about f 4.0 - my 5D classic goes down to ISO 50, but I could have used 25 or even 12 ISO.

I've seen photographers use neutral density filters in that situation.  Guess that is what I'll try next, but it would be great to have the lower ISOs available.

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takoman46

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Re: Why no low ISO DSLRs?
« Reply #2 on: March 30, 2012, 04:33:45 PM »
I think much of the development has been focused on high ISO and low noise because the general problem is that noise increases as ISO increases.  At low ISO, while there may still be some noise (especially at longer exposures), the amount of noise is still much less relative to higher ISO. However, I know nothing about designing and engineering an image sensor but it does make me wonder why ISO performance and noise does not improve across low to high ISOs respectively? If these high ISOs are getting much cleaner and more useable, then wouldn't lower ISOs also improve somewhat?

sulla

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Re: Why no low ISO DSLRs?
« Reply #3 on: March 30, 2012, 04:38:23 PM »
I thought about that myself long ago. Without knowing for sure, this is what I came up with:

I believe it has to do with electrically saturating the sensor:
  • Turning down ISO essentially means to reduce the analog amplification of op-amps that amplify the signal coming from the sensor AFTER it is read out from the sensor and before it is fed into ADCs.
  • Now, a sensor has a given quantum efficiency, largely determined by the material it is built from. This means that x% of photons (I have no idea, perhaps 40-60%??) will each kick one electron out of a pixel via the photoelectric effect, thereby charging the pixel. More light, more charge per pixel. The quantum efficiency is a constant that can not be changed in camera.
  • If a pixel is charged too much, electrons will start jumping from adjacent pixels thereby sprading the charge of the ("right") pixel to neighbouring ("wrong") pixels. The result will be that a pixel will go into saturation and neighbouring pixels will charge up instead.
  • Essentially, there is a maximum amount of light a pixel can take before it saturates.
  • If you increase the light shining on a pixel (by opening the aperture to decrease DOF or lengthening the exposure time to create motion blur of e.g. a waterfall) your sensor will go into saturation at some point.
  • To avoid saturation you need to limit the number of photons shining on each pixel. ND filters do precisely this, but turning down ISO doesn't.

So, me thinks that this is the reason why there is no such setting as ISO 12 on a camera. Only NDs permit to use long exposure times in bright light. Thus I went and bought a really strong ND (6 stops), as one or two stops seemed to be too ineffective to me.

epsiloneri

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Re: Why no low ISO DSLRs?
« Reply #4 on: March 30, 2012, 04:46:11 PM »
Low ISO is limited by the number of photo-electrons the sensor is able to store in a single exposure. This "electron well" is not easy to dig deeper, it scales mostly with the sensor area. That's the most important reason larger-area sensors can give you lower noise / better dynamic range. The drawback is of course that large sensors are expensive.

Another way to increase the signal-to-noise to expose multiple times and then average the results (assuming still subjects).

using a neutral density filter does not help the noise, it merely slows down the collection of photons. This can be useful for various reasons, but it does not help noise.

Please find a more detailed discussion in this thread:
http://www.canonrumors.com/forum/index.php/topic,4010.0.html

flanniganj

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Re: Why no low ISO DSLRs?
« Reply #5 on: March 30, 2012, 05:03:40 PM »
I seem to recall reading on these forums a while back that ISO 100 is the sweet spot and that while some cameras have an ISO 50 option (expandable, at least) they don't go further than that because you are doing something that while it is possible to do in the camera, doesn't yield satisfying results. ISO noise increases lower than ISO 100 in digital cameras, which is both contrary to logical reason compared to film cameras and contrary to the results you want to use the lower ISOs for.

Not sure how accurate that statement is, but it might explain it. Perhaps ISO 24 for example would be akin to an H1 or an H2 on the high end (but for the low end) in terms of IQ loss.

yunusoglu

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Re: Why no low ISO DSLRs?
« Reply #6 on: March 30, 2012, 05:09:44 PM »
In my film days I used to shoot Kodachrome 25 or 64, and low ISO Kodacolor, mostly landscapes. I liked the fine detail.

The threads always talk about high ISO and low noise, but, what about low ISO and no noise? I, and, probably others, are willing to use a tripod and shoot at low ISO.

But, then, my understanding of digital vs film is just beginning. I use a G11 and a 60D

We seem to be living in an era where people want to shoot 10+ fps at candle light... The last few weeks have witnessed discussions where people looking for fine detail and high dynamic-range were made fun of...

Good luck in your search! Tell me if you find anything...

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Re: Why no low ISO DSLRs?
« Reply #6 on: March 30, 2012, 05:09:44 PM »