ISOless sensors, read noise and photography - many questions!

Started Oct 3, 2013 | Discussions thread
gollywop
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Re: ISOless sensors, read noise and photography - many questions!
In reply to boardsy, Oct 4, 2013

boardsy wrote:

bobn2 wrote:

boardsy wrote:

Accepting the standard definition of exposure as the amount of light registered by the sensor per unit area, and dependent solely on aperture size and shutter speed (not on ISO selection, even if this is used to influence aperture size and shutter speed!), a few questions arise in my mind:

- in order to maximise exposure and DR and minimise noise, do you really need to know your sensor's read noise and ISO DR curves?

Bobn tells us here that the flat read noise characteristic of the Nikon D7000 makes it effectively ISOless:

- if the sensor is effectively ISOless then is base ISO always optimal (providing that you can see enough of the image on your camera to compose and focus in a sub-optimal exposure scenario), or are you as well off raising it in camera (providing highlights are preserved) rather than in RAW/pp?

There are other factors to take into account, in no particular order:

- on those graphs the 'base ISO' is not always too accurate, because the sensor may have been run into non-linearity.

- there are qualitative factors as well as the measure of read noise to take into account.

Sorry, this is all a bit opaque to me - what is the upshot for practical purposes?

- is it possible or likely that the camera's ISO implementation is better tuned to its sensor and RAW data than your RAW software? Aside from highlight preservation on exposure, are there other benefits to "doing ISO" in post?

It's possible. Nikon, for instance, does some subtle gain adjustments with its cameras. Presumably they wouldn't be doing them unless the engineers thought that they brought some advantage. BTW, I think it's worthwhile distinguishing between 'processing' and 'post processing'. I think the brightness setting is best done in processing (raw conversion), not post-processing.

Do you mean: processing means RAW software (ACR, LR etc), and post-processing applies to JPG (PS, GIMP etc)?

- for a given sensor may it be more effective to raise ISO to some minimum/optimal level vis-a-vis its read noise?

Yes, I think that's true. The D7000 above is better at 200 for instance.

But it has less DR & Sat Cap and equal read noise at 200 vs 100?

- how do us non-scientists interpret these sensor graphs? E.g. for the Sony NEX C3 (probably the same sensor as my F3, maybe, lol), read noise actually drops down to ISO800 ...but so does DR! Is ISO400 the best compromise here, or should best DR (at ISO200) trump lower read noise (at ISO800)? And could this non-base ISO be valid for all shooting, or just low-light where optimal exposure becomes problematic?

DR trumps lower read noise if you need it, which is one of those judgements based on experience. A rule of thumb which covers most cameras pretty well is go with ISO but stop about 2 stops faster than base (3 for an APS-C Canon, 4 for a FF one).

You mean raise ISO in-camera as required, up to 2 (APS-C), 3 (APS-C Canon), or 4 (FF) stops?

From http://www.sensorgen.info/SonyNEX-C3.html

Alan, where you set your "normal" ISO, and when you change it when using ISO-variant (or partially-ISO-variant) shooting, depend very much on your normal shooting conditions.

A good deal of my shooting, for example, tends to be outdoors in decent light with scenes that have a DR that readily pushes my E-M5 to its limits at base ISO.  Thus, I fix ISO at base 200.  I then set exposure to ETTR (which term, by the way, applies only to shooting at base ISO).

Likewise, when shooting indoor scenes with windows whose content-detail I am desirous of keeping, the scene DR far exceeds what can be recorded so as to produce a normal looking image.  Again I use base ISO and shoot ETTR to preserve the highlights.  This results in a very dark image and entails significant boosting of the shadows and midranges during processing, while compressing highlights to retain the window details -- a process known as exposure compression.  If you're using ACR, this is accomplished by bringing up the Exposure slider to achieve desired mid-tones, dropping Highlights (possibly quite significantly, even down to -100) to retain window detail, and boosting Shadows to bring up the dark areas to an acceptable and tolerable level.

So I'm usually shooting at base ISO.  When, however, shooting conditions do not allow me to ETTR, I set aperture and SS as needed to achieve a maximum exposure value and then dial in ISO as needed up to my partial-ISO-invariant limit.  In the case of my E-M5, that's ISO 800, and that looks to be the same for your C3.  If further brightening is required, I shoot dark and apply it in processing.  When applying ISO in this way, I tend to be conservative, not always brightening in-camera to the extent that would appear to be allowed. I would rather prevent any unwarranted highlight clipping in-camera and control the brightening during processing.

If your normal shooting conditions allow achieving an adequate DR at an ISO above base (and at or below the partial-ISO-invariant limit - 800 in your case), then it makes sense to set that as your usual ISO and vary it only as required for changing scene DR conditions, lower, for example, if you need more highlight headroom.  With a partially-ISO-invariant camera, you would want to use the lowest ISO (at or below the limit) that provides you with the necessary DR. And, as Bob notes, this is not always base ISO.

Processing by the way really applies to the basic operations that convert a raw file into an image, namely application of WB, demosaicing, and fundamental tonal transformations.  Some processors accomplish more operations at this stage some less. But when people talk about applying brightening during processing, they really mean setting WB and the brightening you do with the so-called exposure slider and its associates in ACR.

ACR also accomplishes some operations, like saturation, clarity, sharpening, HSL, lens corrections, that could be considered post-processing.  The one thing that is for sure, once you've rendered your raw file into a jpeg or TIFF and sent it on to an image processor like PS, anything that is done there is post-processing.

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gollywop

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