October 21, 2014, 07:14:34 AM

Author Topic: ISO 50  (Read 37757 times)

TrumpetPower!

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Re: ISO 50
« Reply #15 on: January 19, 2013, 07:35:35 PM »
so regarding of the subject (motive with no high lights = motive with a small DR) you can overexpose and get a benefit of the over exposure in the shadows

That's great information, thanks! I'll certainly use that once I've got a 6d and am doing tripod macro focus stacks with low dr objects.

If you're going to the trouble of focus stacking...well, first, with macro work, there's often all sorts of specular highlights with lots of colorful interference patterns and other things going on, such that you might have a great deal more dynamic range in the scene that you want to preserve than you initially realize.

But, back to point...if you're focus stacking, there's no reason you can't either do "standard" ETTR (if it truly is a scene with a limited dynamic range) or HDR (if necessary). You've already got a workflow that involves lots of scripted steps; what's one more step to script?

I would note, though, that the exposure adjustments for either ETTR or the type of not-tonemapped HDR you'd want are really best done in the camera's native linear space before any sort of gamma or other tone curve is applied. I don't know if ACR / Lightroom does that, but most tools based off DCRAW do. I'd especially recommend Raw Photo Processor: http://www.raw-photo-processor.com/

If your tool of choice doesn't do exposure adjustments in linear raw space, then you're best off nailing exposure in camera, and ISO 50 therefore becomes (in that situation) a better bet than ETTR.

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b&

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Re: ISO 50
« Reply #15 on: January 19, 2013, 07:35:35 PM »

Marsu42

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Re: ISO 50
« Reply #16 on: January 19, 2013, 08:05:10 PM »
Thanks for the further explanation, I have to admit I didn't think this far :-o

Btw I always try to get around using hdr for focus stacking because using more than 100 shutter cycles for one resulting pictures is really something I only like to do if I know the shot will be stellar.

If you're going to the trouble of focus stacking...well, first, with macro work, there's often all sorts of specular highlights
Agreed, but there are also some low contrast scenes esp. in evening hours, I've shot enough stacks to be able to tell by now.

I don't know if ACR / Lightroom does that, but most tools based off DCRAW do. I'd especially recommend Raw Photo Processor

I'm afraid I'm running good ol' Windows so that tool would be only available in a MacOSX vm and I doubt it'd be worth the hassle, esp. since I'd end up having a demosaiced 100mb tif instead of a 20mb raw dng.

But I'm hopeful that in LR's raw processing adjusting the exposure doesn't collide with tone curves either applied manually or via picture styles.

If your tool of choice doesn't do exposure adjustments in linear raw space, then you're best off nailing exposure in camera, and ISO 50 therefore becomes (in that situation) a better bet than ETTR.

At least with the 60d I'm usually doing ettr and highlight recovery in Lightroom to get more shadow resolution. Since I haven't got a 6d with iso50 (yet) I'm unable to do a test the difference, but if I understand you correctly you're saying that if the postprocessing software is capable of operating in raw space iso50 should be equal to ettr @iso100?

TrumpetPower!

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Re: ISO 50
« Reply #17 on: January 19, 2013, 08:24:44 PM »
I don't know if ACR / Lightroom does that, but most tools based off DCRAW do. I'd especially recommend Raw Photo Processor

I'm afraid I'm running good ol' Windows so that tool would be only available in a MacOSX vm and I doubt it'd be worth the hassle, esp. since I'd end up having a demosaiced 100mb tif instead of a 20mb raw dng.

Well, there're Windows apps that use DCRAW -- plus DCRAW itself, if you're not afraid of the command line. But that still leaves you with a TIFF instead of a raw DNG, of course.

Quote
But I'm hopeful that in LR's raw processing adjusting the exposure doesn't collide with tone curves either applied manually or via picture styles.

Yes, one would hope so. In the past, I know it was emphatically not the case, but that was many moons ago...no idea if they've fixed it.

It'd be pretty simple to test. Do a three-shot bracket. Apply a corresponding exposure compensation to both the over- and under-exposed shots to normalize them to the middle shot -- that is, if you shot at 0, +1, and -1, then expose for 0, -1, and +1 (precisely by the numbers; don't eyeball it). Then compare all three. If they look identical (except for shadow noise and up to a stop of highlights blowing early), then they've fixed that problem. If there's any visible difference between the three, then they haven't.

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(I)f I understand you correctly you're saying that if the postprocessing software is capable of operating in raw space iso50 should be equal to ettr @iso100?

The data recorded by the sensor (and, presumably, written to the raw file) is identical for ISO 50 and ISO 100; all that's changed is the meter is told to overexpose by a stop and the raw processing engine is told to underexpose by a stop. So, yes, if you use the same shutter and aperture, ISO 50 is the same as ISO 100 with one stop of digital underexposure (again, assuming the digital underexposure is done properly, in the camera's linear raw space before any other adjustments).

(There might be some subtle advantage to doing it in-camera with ISO 50...Chuck Westfall could shed some light on that. But, if there is, the effect would be very, very subtle.)

Cheers,

b&

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Re: ISO 50
« Reply #18 on: January 19, 2013, 08:58:47 PM »
so regarding of the subject (motive with no high lights = motive with a small DR) you can overexpose and get a benefit of the over exposure in the shadows

That's great information, thanks! I'll certainly use that once I've got a 6d and am doing tripod macro focus stacks with low dr objects.
I do that regularly with my 5D3 to work around the shadow noise. Results are normally great.

Mikael, question to you, do I understand correct that you mean this is best for low DR pictures ie flatter? I learned through reading a couple of articles to also use this in more high DR situations like taking a street picture with the sun low. I have no experience of not being able to recover details from overexposed highlights. Is there something I'm missing here?

TrumpetPower!

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Re: ISO 50
« Reply #19 on: January 19, 2013, 09:27:16 PM »
no they are not
At 100iso  with the same metering the   sensor charge  is under 100 %  and at the read out =before overload and  clipping
with 50iso your double the time  or open up the lens 1 stop and therefore blow one stop of highlight.

As I've mentioned, as far as the sensor is concerned, they are the same. 1/400 @ f/8 @ ISO 100 results in the same raw file as 1/400 @ f/8 @ ISO 50.

The difference is that the meter will read a stop slower and the raw processing engine will apply a stop of digital underexposure to compensate.

But that's all to do with the meter and the post-exposure workflow. The actual exposure of the sensor and what it records is identical.

Similarly, with highlight tone priority turned on, you again get the exact same raw file at 1/400 @ f/8 @ ISO 200/HTP, but the meter reads a stop faster and the raw processing engine applies a stop of digital overexposure. You gain a stop of headroom at the expense of a stop more noise in the shadows -- but, once again, this is all done by starting with the exact same exposure recorded by the sensor, just with shifting meter readings and post-exposure processing metadata instructions.

(Again, all with the caveat that there may be a bit of electronic jiu-jitsu to marginally help achieve slightly better results, and that your raw processing software absolutely must be doing the exposure compensation in the camera's linear raw space before any other adjustments.)

Cheers,

b&

pwp

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Re: ISO 50
« Reply #20 on: January 19, 2013, 11:34:01 PM »
50 iso vs ND filters? I think other posters have made it clear that 50 iso is a compromise at the best of times. If your project requires a slowdown, ND is the preferred route.

-PW

TrumpetPower!

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Re: ISO 50
« Reply #21 on: January 20, 2013, 10:57:04 AM »
The sensor does not care about iso, iso step is added after the readout.

Actually, that's not true -- and, presumably, at the heart of your misunderstanding.

Analog electronic (not digital) amplification / gain is applied to all ISO settings above the base ISO. It's like turning up the volume knob on your stereo. The readout is performed after that gain has been applied. And, just as your amplifier will start to produce more and more ugly-sounding distortion the louder and louder you crank that knob, your camera will produce more and more noise the higher you boost the ISO.

ISO 50 and highlight tone priority are two special cases. The exact same analog signal amplification (i.e., none) is applied with all three exposures: ISO 50, ISO 100, and ISO 200 w/ HTP. If you use the same shutter speed and aperture, you'll get the exact same raw file in all three cases. What changes is first the camera's metering system and second the digital (not analog) post-processing. That post-processing is quite simple, really...in the case of ISO 50, all the digital values from the initial sensor readout are exactly halved; with HTP, they're exactly doubled. (And, of course, with ISO 100, they're left as-is.) That's all simple integer math, too -- nothing fancy.

Hope that clarifies things somewhat....

Cheers,

b&

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Re: ISO 50
« Reply #21 on: January 20, 2013, 10:57:04 AM »

neuroanatomist

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Re: ISO 50
« Reply #22 on: January 20, 2013, 12:19:40 PM »
the only purpose the sensor has is to collecting photons
agree?

Disagree - it does more.  The photon wells collect photons, there is a lot of additional circuitry on a sensor besides just the photon wells.

TrumpetPower is absolutely correct in his statement that, "ISO 50 and highlight tone priority are two special cases. The exact same analog signal amplification (i.e., none) is applied with all three exposures: ISO 50, ISO 100, and ISO 200 w/ HTP. If you use the same shutter speed and aperture, you'll get the exact same raw file in all three cases." (well, almost correct - the base ISO isn't exactly 100 for all sensors, so even at ISO 100, some gain may be applied - but it is the same gain in all three cases).  He's also correct about ISO 50 being a linear 50% reduction, but incorrect about HTP being a linear doubling (HTP processing is application of a tone curve to boost the shadows and midtones but not the highlights).
 
HTP. it is a halving of infaling light= halving of the amount of charged  electrons= go from 100 to 200 iso and make a head room, the analog signal is different = 100iso and 200 iso
Agree?

Disagree.  When you set ISO 200 with HTP, the camera is actually exposing at ISO 100, which is why ISO 100 cannot be set with HTP on, but (incorrectly) reporting ISO 200 in the metadata and setting the HTP flag.  If you open an HTP RAW file in something like Rawnalyze, you'll see it's a stop underexposed - it's not halving the incoming light, it's reducing the analog gain applied to the same amount of light, by one stop.
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neuroanatomist

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Re: ISO 50
« Reply #23 on: January 20, 2013, 12:31:25 PM »
well then take a look what happening with HTP , 100 iso exposed as a 200iso exposure= under exposed and later adjusted .
not the same signal from the sensor ( and do nor mix apples with bananas this time)

The point is that ISO 50, ISO 100, and HTP ISO 200 are the same analog gain at the sensor, and if the aperture and shutter speed are held constant, the RAW data coming out of the ADC are the same for all three ISO settings.

regarding 50iso and 100 iso= not the same parameters regarding time

You're saying if I set my aperture and shutter in M mode, and then change from ISO 100 to ISO 50, my exposure time or aperture will change?  I'd like to see some evidence for that...
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neuroanatomist

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Re: ISO 50
« Reply #24 on: January 20, 2013, 12:36:30 PM »
Well Neuro, read again what Im answering earlier, Im going to the gym for a hour , we can discuss it later

I don't need to read the same incorrect statements again, thanks.  Enjoy your workout!
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TrumpetPower!

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Re: ISO 50
« Reply #25 on: January 20, 2013, 01:23:45 PM »
He's [...] incorrect about HTP being a linear doubling (HTP processing is application of a tone curve to boost the shadows and midtones but not the highlights).

That makes sense -- thanks for the correction. At a rough guess, starting with the original linear data I'd suggest that the HTP curve is still a linear doubling from full dark until the last stop or two of brightness, and then a smooth curve from there back to unity at the saturation point. In Photoshop, create a new curve with one point, 92 as the input and 184 as the output, for an idea of what it might look like. Then apply white balance and color correction matrix and s-curve for contrast and the rest as usual. At least, that's how I'd do it....

Cheers,

b&

neuroanatomist

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Re: ISO 50
« Reply #26 on: January 20, 2013, 02:02:14 PM »
can you boys read?
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html

it is hard  to discuss HTP etc if you do not know what it means

Can you not be insulting, boy?
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TrumpetPower!

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Re: ISO 50
« Reply #27 on: January 20, 2013, 02:15:43 PM »
can you boys read?

Yes, we can. Whether you can remains to be demonstrated. From your very link:

Quote
By the way, underexposing at lower ISO is precisely what Canon cameras do in the raw data when Highlight Tone Priority (HTP) is enabled; and what Nikon cameras do when Active D-Lighting (ADL) is enabled. Instead of using the ISO gain set by the user, the camera uses a lower ISO (but exposes with the indicated aperture and shutter speed), effectively underexposing the image; this provides more highlight headroom. In post-processing, the image data can be brought back up while preserving the highlights with a modified tone curve in higher exposure zones. The place where image quality suffers is in shadows at lower ISO, precisely as the above quantitative model predicts.

That's exactly what neuroanatomist and I have been writing all along. Instead of using the ISO gain set by the user (200), the camera uses a lower ISO (100) (but exposes with the indicated aperture and shutter speed). In other words, ISO 200 w/ HTP is exactly the same as ISO 100, but with a different tone curve.

b&

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Re: ISO 50
« Reply #27 on: January 20, 2013, 02:15:43 PM »

neuroanatomist

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Re: ISO 50
« Reply #28 on: January 20, 2013, 02:41:56 PM »
No problem admitting when I am wrong.  But I'm not.

You stated:
HTP. it is a halving of infaling light

That's flat out wrong. The light is not being halved, rather, as we and the page you linked correctly state, the gain is being halved, relative to the selected ISO value.

Further, you suggested that setting ISO 50 changes time value and/or aperture...and that's wrong, as well.
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neuroanatomist

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Re: ISO 50
« Reply #29 on: January 20, 2013, 03:19:45 PM »
HTP. it is a halving of infaling light

I think you must study the subject and understand how iso gain works together with full charge and then what it means by 200 iso= halving the number of electrons.

Which are we halving? Light or electrons? Are you going to admit being wrong about the light being halved, or just accuse others of not being able to admit when they're wrong?

Interestingly, I notice that you're consistently ignoring my question about ISO 50 changing aperture/shutter values...

last time I was answering you when you also where totally wrong I was turned off because of my language.it will not happen this time.

Questioning our ability to read, as I stated, is highly insulting, especially the fact that you took the time to edit your post to bold that remark.  It seems you have a history of insulting behavior here, which resulted in past consequences and yet continues. 
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Re: ISO 50
« Reply #29 on: January 20, 2013, 03:19:45 PM »