Finally - a Possible use for "High" ISO *** with the Foveon!

*** To me, anything over 100 ISO is high.

I use my SD15 sans the UV/IR dust-cover. This affects the metering because the AE sensor below the mirror is almost blind to IR. Therefore when I meter a scene, the metering recommends an exposure as if the sensor is only getting visible light, whereas we should know that it's often getting a ton of IR into the bargain.

Ergo, it is necessary to apply negative EC to the settings so as not to over-expose the sensor. The amount of negative EC needed is dependent upon the amount of IR coming from the scene and upon occasion I have run out of adjustment at -3EV.

My un-tested thought is:

What if I were to set the ISO at say 400? This under-exposes the sensor by 2EV for visible light, but is reasonable enough when IR is included. Then I could go from -5 to +1 EV adjustment in terms of effective EC.

Any comments, especially from full-spectrum shooters?

What you want to avoid isn't just overexposing the sensor, but overexposing the data in the resulting RAW file. Since the SD15 has a variable Analog Front End, the camera is likely to compensate for the change of ISO and the end result may not be very different.

EDIT: Hypothetically speaking, have sold my SD15 and can't test...

Johan Borg wrote:

What you want to avoid isn't just overexposing the sensor, but [clamping] the data in the resulting RAW file. Since the SD15 has a variable Analog Front End, the camera is likely to compensate for the change of ISO and the end result may not be very different.

I agree, Johan.

The Programmable Gain Amplifier within the SD15's Analog Front End chips does indeed compensate for the change of ISO by causing constant ADC outputs for a given scene (assuming zero EC), unlike earlier SD cameras.

Thus, particularly when shooting IR, it is important to have an adequate downward adjustment of EC available.

EDIT: Hypothetically speaking, have sold my SD15 and can't test...

Cloudy and wet forecast today ... maybe tomorrow ...

I also feel another test coming on ... I have a radiometric meter (W/m2, not lux) and a heat-lamp. It would be a simple matter to interpose my Marumi 700nm and see the difference. Of course, the spectral radiance of a heat-lamp bears little resemblance to that of a green leaf in full sunlight - maybe I should measure that instead ...

It might work with the SD14 or the SD1M but not with any camera with the AFE.

mike earussi wrote:

It might work with the SD14 or the SD1M but not with any camera with the AFE.

Please tell us why, Mike. Remember that I do apply EC in addition to changing the ISO ...

xpatUSA wrote:

I also feel another test coming on ... I have a radiometric meter (reads in W/m2, not lux) and a heat-lamp. It would be a simple matter to interpose my Marumi 700nm and see the difference.

I tried a quick test under a domestic 5000K LED desklamp. With lamp about 10cm from the meter flat window:

Bare: 21.5 Watts/sq.meter

With 700nm IR filter: 3.5 Watts/sq.meter

With Kolari visible filter: 9 Watts/sq.meter

Posted for what it's worth ...

Of course, the spectral radiance of a heat-lamp bears little resemblance to that of a green leaf in full sunlight - maybe I should measure that instead ...

xpatUSA wrote:

I also feel another test coming on ... I have a radiometric meter (reads in W/m2, not lux) and a heat-lamp. It would be a simple matter to interpose my Marumi 700nm and see the difference.

Of course, the spectral radiance of a heat-lamp bears little resemblance to that of a green leaf in full sunlight - maybe I should measure that instead ...

Sun came out briefly today - so I rushed out with the meter and pointed it at a big green leaf, noted the reading, then interposed the Marumi:

Without Marumi: about 120 W/m2

With Marumi: about 90 W/m2.

Although the measurements were a bit wobbly (slight breeze, unsteady balance), the implication is that the "visible" part of the total leaf reflection is only approx 25% radiometrically speakin'.

That seems to confirm the need for a lot of negative EC with the SD15.

xpatUSA wrote:

mike earussi wrote:

It might work with the SD14 or the SD1M but not with any camera with the AFE.

Please tell us why, Mike. Remember that I do apply EC in addition to changing the ISO ...

Because iso is only a software tag without the AEF so there's no amplification of the signal.

mike earussi wrote:

xpatUSA wrote:

mike earussi wrote:

It might work with the SD14 or the SD1M but not with any camera with the AFE.

Please tell us why, Mike.

Because iso is only a software tag without the AEF so there's no amplification of the signal.

Thanks for the reminder that the difference between say the SD15 and the SD14 is the location of the ISO gain - before or after the ADC, respectively. Did you try any numbers?

Remember that I do apply EC in addition to changing the ISO ...

So apparently EC does not help with an SD15, hmmm ...

xpatUSA wrote:

mike earussi wrote:

xpatUSA wrote:

mike earussi wrote:

It might work with the SD14 or the SD1M but not with any camera with the AFE.

Please tell us why, Mike.

Because iso is only a software tag without the AEF so there's no amplification of the signal.

Thanks for the reminder that the difference between say the SD15 and the SD14 is the location of the ISO gain - before or after the ADC, respectively. Did you try any numbers?

No numbers, just experience. The SD14 shot at iso 50 produced absolutely no noise either in the sky or shadows. I've never seen another camera, either Foveon or Bayer that's been able to duplicate that. The SD15, utilizing the AEF, produced far more noise at all isos and less about 1/2 stop less DR.  Because of this diminished performance I never understood the rational of including it.

Remember that I do apply EC in addition to changing the ISO ...

So apparently EC does not help with an SD15, hmmm ...

mike earussi wrote:

xpatUSA wrote:

mike earussi wrote:

xpatUSA wrote:

mike earussi wrote:

It might work with the SD14 or the SD1M but not with any camera with the AFE.

Please tell us why, Mike.

Because iso is only a software tag without the AEF so there's no amplification of the signal.

Thanks for the reminder that the difference between say the SD15 and the SD14 is the location of the ISO gain - before or after the ADC, respectively. Did you try any numbers?

No numbers, just experience. The SD14 shot at iso 50 produced absolutely no noise either in the sky or shadows. I've never seen another camera, either Foveon or Bayer that's been able to duplicate that. The SD15, utilizing the AEF, produced far more noise at all isos and less about 1/2 stop less DR.

Thanks again. I'm beginning to understand your point. I wasn't thinking about noise at all in the OP, only correct exposure and how to get it.