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Author Topic: DSLR ISO range  (Read 5194 times)

NigelC

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DSLR ISO range
« on: July 10, 2008, 10:48:10 am »

In the never ending rumour threads about the spectfication of yet to be announced DSLRs, higher ISO settings with reduced noise are often requested, but what about the usefulness of very low settings. There are times when I want to show movement in water/grasses etc but not have to use the smallest aperture with consequent diffraction effects. It would be more convenient to have access to 6ASA or 12ASA rather than have to mess around with an ND filter.

Is it technically possible to achieve this? The 50ASA  setting on my canon 5D is believed to slightly reduce image quality (otherwise why make it a selectable option rather than on the main menu)
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Panopeeper

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DSLR ISO range
« Reply #1 on: July 10, 2008, 11:17:25 am »

A lower ISO setting means higher exposure, i.e. more photons collected - but where to put them? The well capcity of the sensor is a given; if you go lower than the "native" ISO, then sensor saturation (clipping on raw level) is pre-programmed.
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Gabor

Er1kksen

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DSLR ISO range
« Reply #2 on: July 13, 2008, 11:13:09 am »

I wonder if it's possible to design sensors with higher or lower base ISOs, say a sensor with a native ISO of 12 which would then have settings of 12, 25, 50, 100, and 200. There would be some appeal to tripod-landscape shooters looking for longer exposures and excellent IQ on the tripod. Then again, noise would be ridiculous for ISO 200. A few might buy it but I suspect any company to manufacture it would take a definite loss.

Similarly, what about a sensor with a native ISO of, say, 800 or 1600? 6400 would be an entirely normal ISO setting with such a sensor. I suspect a camera with a sensor like that would have much greater market appeal, and say if the camera slipped a ND filter over the sensor (the way the sigma SD14 mechanically can move the AA and cutoff filters from in front of the sensor) to provide lower sensitivities? I'd still prefer a camera with the sensitivities we have, but if it's possible, I suspect there would be a pretty strong market.
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BernardLanguillier

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DSLR ISO range
« Reply #3 on: July 15, 2008, 12:03:43 am »

Quote
In the never ending rumour threads about the spectfication of yet to be announced DSLRs, higher ISO settings with reduced noise are often requested, but what about the usefulness of very low settings. There are times when I want to show movement in water/grasses etc but not have to use the smallest aperture with consequent diffraction effects. It would be more convenient to have access to 6ASA or 12ASA rather than have to mess around with an ND filter.
[a href=\"index.php?act=findpost&pid=206994\"][{POST_SNAPBACK}][/a]

The Kodak SLR/n used to have an ISO 6 setting that was extremely popular. The images were totally noise free.

I don't know exactly how they were doing it, but it was working very nicely for sure.

Cheers,
Bernard

grepmat

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DSLR ISO range
« Reply #4 on: July 15, 2008, 01:29:09 am »

Engineers have developed many different techniques to allow what is in effect lower ISO in extreme conditions. For example, one frequently examined technique is to have a pixel with a bilinear response. It would have normal sensitivity for most of its dynamic range, but as the amount of light begins to approach the pixel's "well capacity," the pixel would switch to a second, more gradual response curve (via a technical trick) that would prevent saturation in much stronger light.

Another trick is to use a logarithmic response curve, which accomplishes the same thing but without a sharp discontinuity (but with an over-all non-linear response).

People have also made chips with what are essentially neutral-density filters built in on sub-pixels, which can be used when the primary pixels are saturated.

Again, there are many tricks for doing this. However, for general photography, the "keep it simple, stupid" approach seems to be best. Therefore, you rarely see these tricks actually used. The major exception is the so-called "super ccd" approach, with a sub-pixel per pixel that retains responsivity in high light. This comes with a trade-off in resolution, however, and so it remains a niche product.

Cheers.
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grepmat

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DSLR ISO range
« Reply #5 on: July 15, 2008, 01:32:17 am »

P.S., I should add that the biggest problem in sensors is not having too much light (requiring low ISO), but having too little, hence requiring low noise at higher ISO. All of the tricks I mentioned only address the "too much light" problem. It's easy to waste light. It's VERY hard to not waste it!
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Plekto

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DSLR ISO range
« Reply #6 on: July 15, 2008, 04:09:07 pm »

Basically this is why MF digital backs do so well - they have "fatter" pixels and much larger sensors.  So they can soak in more light before they start to get saturated(and also take less light as well to get a useable image)

Many of the tiny/less than full frame sensor models are closer to ISO200-400 in actual sensitivity, and use software and other tricks to increase their results.  And, like film, most sensor types are set up to run at one ISO well versus another.  

Fuji, for instance, is set up to do HDR and very fast ISOs best(which is really does well!), while a Foveon/Sigma sensor is pretty much set it to the lowest ISO all the time and leave it there(really is "film like").  As such, it's worthless for night and most indoor shots, but does stunning things outdoors for scenery.  CCD arrays are a bit more in the middle, IME.  They do a respectable middle range - works pretty well in most environments.  Each technology has its advantages, of course.
« Last Edit: July 15, 2008, 04:11:31 pm by Plekto »
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KiboOst

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DSLR ISO range
« Reply #7 on: July 22, 2008, 05:08:44 am »

Quote
I wonder if it's possible to design sensors with higher or lower base ISOs, say a sensor with a native ISO of 12 which would then have settings of 12, 25, 50, 100, and 200. There would be some appeal to tripod-landscape shooters looking for longer exposures and excellent IQ on the tripod.

[a href=\"index.php?act=findpost&pid=207824\"][{POST_SNAPBACK}][/a]

Just my dream ... Lower ISO with full DR would be so much usefull in studio with strobes !
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