Digital Camera's Native ISO

[mouse]

G-
Love your graphs.  But they would be much more helpful (especially to the uninitiated) if the variables were more clearly defined.  For example, in the above does the abscissa (EV) include ISO as well as actual exposure?  Does the heavy black line then imply response at constant exposure? (EJ Martin's definition: exposure does not include ISO).   And if so, why label the axis as "EV"?   Sorry if I'm being a little dense.

[Graystar]

...exposure does not include ISO

Photometric exposure doesn't include ISO.  Photographic exposure does include ISO.

Photometric exposure is about light.
Photographic exposure is about the response to light.  You can't have a photograph without something responding to light.  ISO defines that response.

Since the function of ISO and the function of software based Exposure Compensation is similar, it's also appropriate to speak of software based EC as adjusting photographic exposure.

[bjanes]

Photometric exposure doesn't include ISO.  Photographic exposure does include ISO.

Photometric exposure is about light.
Photographic exposure is about the response to light.  You can't have a photograph without something responding to light.  ISO defines that response.

Since the function of ISO and the function of software based Exposure Compensation is similar, it's also appropriate to speak of software based EC as adjusting photographic exposure.

Exposure is measured in lux seconds. Digital sensors can be rated by the ISO 12232 saturation standard, where the ISO (S sat) = 78/Hsat. Hsat is the exposure in lux seconds necessary to saturate the sensor. If you change the "exposure" in the raw converter, you really have not changed the number of lux seconds reaching the sensor. I don't see your point.

Regards,

Bill

[Graystar]

Exposure is measured in lux seconds. Digital sensors can be rated by the ISO 12232 saturation standard, where the ISO (S sat) = 78/Hsat. Hsat is the exposure in lux seconds necessary to saturate the sensor. If you change the "exposure" in the raw converter, you really have not changed the number of lux seconds reaching the sensor. I don't see your point.

My point is described clearly in my post.

[mouse]

Photometric exposure doesn't include ISO.  Photographic exposure does include ISO.

A rose by any other name.  Good grief, I don't feel even a bit qualified to participate in this discussion.  All I wished to know is if the abscissa of Guillermo's graphs are "photometric" or "photographic" exposure.

[Guillermo Luijk]

Yes, I understand the numbers...I do believe that the read-noise issue is real.  I simply question whether it's visible (in a pixel-peeping sort of way, mind you) in actual photographic works.

Of course that will depend on the level of demand of your application. It is not the same to use an image to build a 50x50 icon for the web (where even the worst compact camera will suffice), than to print a large copy. The extra cleaneness because of cleverly using ISO will be there for you in case you need it.

what reason is there for not using the higher ISO setting that gets the midtones at the normal, convenient level?

If there are absolutely no highlights that could get clipped, then I agree with you. There is no reason not to use higher ISO settings and I would use them. But as soon as there is some highlight source that could get clipped, its surrounding area will be better represented (softer transitions) the lower the ISO setting.

This is just a simulation, but represents the kind of unpleasant colours due to partial channel saturation thay may occur because of RAW clipping:



Another example (this one is real) in reply #3 of this thread.

A rose by any other name.  Good grief, I don't feel even a bit qualified to participate in this discussion.  All I wished to know is if the abscissa of Guillermo's graphs are "photometric" or "photographic" exposure.

The abscissa is the RAW exposure measured with respect to sensor saturation (0EV), so it includes the effect of ISO (what you call photographic exposure). That is why there is a plot for every ISO. The black thick lines represent how SNR improves if you keep aperture/shutter and start to push ISO.

[mouse]

Thanks Guillermo
That's very clear.

[BJL]

In reply to me, Guillermo quite rightly said:

If there are absolutely no highlights that could get clipped, then I agree with you. There is no reason not to use higher ISO settings and I would use them. But as soon as there is some highlight source that could get clipped, its surrounding area will be better represented (softer transitions) the lower the ISO setting.

Yes, and that suggests a strategy for dealing with the strange asymmetry of digital camera response that does not exist with film: that midtones are typically placed only about three stops from the top, while often being far more than three stops above the noise floor. It might be nice for higher exposure index ("ISO") settings to only amplify part way, enough to control read noise well, leaving increasing highlight headroom between mid-tones and maximum level, and noting this in the meta-data. That last step would negate my complaint about extra post-processing.

[madmanchan]

Some cameras do similar things, e.g., underexpose by 1 stop and indicate so in metadata, so raw conversion software and "push" digitally afterwards.  Canon's Highlight Tone Priority, Pentax's Dynamic Range Expansion, Samsung's Smart Range, and Fuji's Dynamic Range Priority (some cameras, some modes) are all examples of this strategy.

[BJL]

Some cameras do similar things, e.g., underexpose by 1 stop and indicate so in metadata, so raw conversion software and "push" afterwards.

Thanks; that is good to know And one extra stop (so a bit over four stops of headroom?) is probably enough to deal with all but some rather exteme and unusual cases, where the metered light level is below about 5% of the brightest diffuse highlights. I would guess that those exteme cases need special attention anyway, like metering on the highlights and then choosing an ISO speed setting low enough to keep them in bounds.