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Olympus OM-D E-M1 The first Micro Four Thirds camera that offers phase detect focusing so you can use Four Thirds DSLR lenses normally as well a Micro Four Thirds lenses.

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Old 20th March 2014
photo_owl photo_owl is offline
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Re: How do I over or under expose with Auto ISO

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Originally Posted by Ross the fiddler View Post
Not related to the specific issue, but my wife came home frustrated with the teaching staff (at school) because they weren't reading the details given.

I can understand the disappointment (or rather, frustration) in not having this particular specific function as I can see it would have its uses. Maybe a bit more pressure on Olympus & it might be included in some future models & maybe, just maybe, added to some existing models, but I am dreaming now. One ideal way to include ISO compensation would be to have the + & - adjustment in the SCP under the ISO box (like flash compensation is), but now I'm really dreaming.
don't stop dreaming Ross!

personally I would have thought that simply using the read dial (thumb) whilst the EV button is pressed and held (forefinger) would be the simplest, and most consistent, implementation.

I have tried to explain the rational to Oly individuals but it's been bad timing! We have progress with the E-M1 but it could be better.
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  #17  
Old 20th March 2014
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Re: How do I over or under expose with Auto ISO

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Originally Posted by photo_owl View Post
don't stop dreaming Ross!

personally I would have thought that simply using the read dial (thumb) whilst the EV button is pressed and held (forefinger) would be the simplest, and most consistent, implementation.

I have tried to explain the rational to Oly individuals but it's been bad timing! We have progress with the E-M1 but it could be better.
In other words, a programmable Fn that could be applied to the various function option keys as "Auto ISO Compensation (during Manual use)" (where AE compensation takes care of PAS use) & even if they don't include it on any other model, it should at least be an option on the top E-M1 model though (& then trickle down the models maybe).
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Old 21st March 2014
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Re: How do I over or under expose with Auto ISO

Hi there Everyone!

Please tell me if I am missing something here, but given that the shutter speed and the aperture are being set, the auto iso is only used for the jpeg image, and the RAW file would be the same regardless of what iso setting was chosen - with only the initial exposure setting when the RAW file is opened changing. Would the simplest work-around not be to shoot in RAW and batch develop the pictures in several batches depending on how much under or over exposing they need?

I know this wouldn't work if you needed to take a large quantity of shots in a very short time (as writing RAW and jpeg takes longer) but provided you are not filling your buffer this should be usable. Several people on the forum have suggested using GIMP with the batch processing add in BIMP to do this - I haven't tried it myself.

Hope this helps,

Ralph.
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Old 21st March 2014
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Re: How do I over or under expose with Auto ISO

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Originally Posted by Ralph Harwood View Post
Hi there Everyone!

Please tell me if I am missing something here, but given that the shutter speed and the aperture are being set, the auto iso is only used for the jpeg image, and the RAW file would be the same regardless of what iso setting was chosen - with only the initial exposure setting when the RAW file is opened changing. Would the simplest work-around not be to shoot in RAW and batch develop the pictures in several batches depending on how much under or over exposing they need?

I know this wouldn't work if you needed to take a large quantity of shots in a very short time (as writing RAW and jpeg takes longer) but provided you are not filling your buffer this should be usable. Several people on the forum have suggested using GIMP with the batch processing add in BIMP to do this - I haven't tried it myself.

Hope this helps,

Ralph.
It's possible to change White Balance from the RAW file, but not ISO as that is a hardware (with firmware control) decision in the camera at the time of taking the exposure. It's as fixed as shutter speed & aperture are in the RAW information. Yes, you can adjust the post compensation in RAW but not like the camera would (as the camera changes one or more of the three parameters, shutter speed, aperture & ISO) & all that the post adjustments do is shift the window of dynamic range of the photo to the extent that is available with the saved RAW information (which is usually greater in the latest sensors, but not as much in the earlier Panasonic sensors) or compressing it (& bending it with the tone curve etc.). Sorry for using a layman's description for the above.
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Old 21st March 2014
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Re: How do I over or under expose with Auto ISO

Hi there Ross!

My understanding was that the RAW file was simply the information from the sensor - ie a 14-bit number for each photosite on the sensor. The ISO setting then decides how to use that number to give a jpeg image.

I suppose one way to test this would be to take two photos with the aperture and shutter speeds the same but with very different iso settings (maybe 200 and 3200) and then make the correct exposure change when processing the RAW (my brain is a little fried but I make it +4eV on the ISO 200 image) and compare the two images - on my E-30 they would both be a bit noisy if the ISO 3200 was correctly exposed.

I may try this later.

Cheers,

Ralph.
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Old 21st March 2014
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Re: How do I over or under expose with Auto ISO

Pretty sure there is a hardware gain adjustment for higher ISOs.

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Old 21st March 2014
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Re: How do I over or under expose with Auto ISO

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Originally Posted by Ralph Harwood View Post

My understanding was that the RAW file was simply the information from the sensor - ie a 14-bit number for each photosite on the sensor. The ISO setting then decides how to use that number to give a jpeg image.
my understanding is that these are the right processes but the wrong order

ISO sets the sensor gain for the capture then the jpeg engine renders the image as instructed by the user (default settings, modes etc)
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Old 21st March 2014
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Re: How do I over or under expose with Auto ISO

Given the level of misunderstanding and misinformation this thread needs moving to the classic forum.
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Old 22nd March 2014
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Re: How do I over or under expose with Auto ISO

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Originally Posted by Ralph Harwood View Post
Hi there Ross!

My understanding was that the RAW file was simply the information from the sensor - ie a 14-bit number for each photosite on the sensor. The ISO setting then decides how to use that number to give a jpeg image.


Ralph.
The "sensor" is a bit more complex than that.
Each pixel has a photo diode, which converts the light received into an analogue voltage and a group of transistors which amongst other things include amplification of that voltage. This is then followed by an analogue to digital converter which produces the 14 bit number. Increasing the ISO sensitivity is achieved by increasing the gain of the amplifiers before the A/D conversion. The "native" ISO sensitivity (ie. zero or minimal amplification) of the current sensors is ISO 200. Higher ISO sensitivity is achieved by increasing the amplification. All electronic circuits produce electrical "noise" at an effectively fixed level.At "native ISO" with an adequate light signal this noise is not a major problem as the signal to noise ratio is high.When the gain is increased to achieve higher sensitivity at lower light levels the signal to noise ratio is degraded and the noise is also amplified so that at higher ISO's it starts to become visible in the final image.

Regards.
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Re: How do I over or under expose with Auto ISO

Hi there Everyone!

Thanks PeterBirder - I had found a similar explanation just minutes before your post - so my understanding is now that the camera can use the iso setting to vary the bias current for the transistor on the CMOS sensor, increasing the gain (and noise!).

I now have a further question though (eek!) - once you start using the extension ISO settings is that done throught the software side post sampling or is the extension simply a level where the manufacturers would rather we didn't go in terms of analogue gain which creates ever increasing noise (higher bias currents increasing the temperature of the sensor further increasing the thermal noise).

Apologies for the questions, my curiosity gets piqued and I have to start digging!

Cheers and thanks to everyone who's pointed me in the right direction!

Ralph
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Old 22nd March 2014
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Re: How do I over or under expose with Auto ISO

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Originally Posted by Ralph Harwood View Post
Hi there Everyone!

Thanks PeterBirder - I had found a similar explanation just minutes before your post - so my understanding is now that the camera can use the iso setting to vary the bias current for the transistor on the CMOS sensor, increasing the gain (and noise!).

I now have a further question though (eek!) - once you start using the extension ISO settings is that done throught the software side post sampling or is the extension simply a level where the manufacturers would rather we didn't go in terms of analogue gain which creates ever increasing noise (higher bias currents increasing the temperature of the sensor further increasing the thermal noise).

Apologies for the questions, my curiosity gets piqued and I have to start digging!

Cheers and thanks to everyone who's pointed me in the right direction!

Ralph
Hi Ralph.
I presume the "extension ISO settings " you refer to is the recently introduced " ISO Low " function. I can only guess how this is achieved but as Olympus advise caution in its use it must potentially reduce some aspect of performance. If it reduces the gain of the amplifiers it will decrease any thermal noise not increase it. (Thermal noise is not the only noise generated in the sensor there is also Shot Noise, which you will have to look up.) I think it is more likely that it is due to the fact that the photo diode/amplifier combination (as originally designed) has a certain dynamic range and when you go below base ISO this may be affected.

I may of course be entirely wrong, I frequently am.

Regards.
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Old 19th May 2014
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Re: How do I over or under expose with Auto ISO

and the work around answer is to use Cog K - Exposure Shift.

The answer was provided in another thread.
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Old 20th May 2014
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Re: How do I over or under expose with Auto ISO

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Originally Posted by PeterBirder View Post
Hi Ralph.
I presume the "extension ISO settings " you refer to is the recently introduced " ISO Low " function. I can only guess how this is achieved but as Olympus advise caution in its use it must potentially reduce some aspect of performance. If it reduces the gain of the amplifiers it will decrease any thermal noise not increase it. (Thermal noise is not the only noise generated in the sensor there is also Shot Noise, which you will have to look up.) I think it is more likely that it is due to the fact that the photo diode/amplifier combination (as originally designed) has a certain dynamic range and when you go below base ISO this may be affected.

I may of course be entirely wrong, I frequently am.

Regards.
Hi there Peter!

I was thinking of the higher iso ratings - upwards of 6400iso I think. I thought those were done by adjusting the exposure through the RAW image rather than turning the sensor voltages up massively - the dynamic range reduces at a rate that would tally up very nicely. Thermal noise and shot noise take me back a fair long way - 20 years ago to the first year of my electronics degree. Unfortunately my current job is much broader than just electronics so a lot of it is in long term storage! Shot noise is the noise caused by natural fluctuations in the numbers of carriers (electrons, photons) of information, so high iso is dealing with low numbers of photons entering the CCD and low numbers of electrons leaving the CCD for the amplifiers. From what I can remember, digital amplification being noiseless (loose 1 significant bit for a 2x amplification) whereas the analogue amplifier would add some thermal noise in addition to the shot noise being amplified by a factor of 2 in either case.

As I say, with all this in long term storage I could have the wrong end of the stick entirely!

Cheers,

Ralph.
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