December 18, 2014, 02:50:41 AM

Author Topic: ISO 50  (Read 40378 times)

sanj

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Re: ISO 50
« Reply #75 on: January 21, 2013, 10:46:23 AM »
Here is the dumbest question in the thread, but since I am the one that made the test, could I please ask what is HTP?

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Re: ISO 50
« Reply #75 on: January 21, 2013, 10:46:23 AM »

TrumpetPower!

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Re: ISO 50
« Reply #76 on: January 21, 2013, 10:48:18 AM »
The important part where you're talking straight past each other seems to be this:
If you let the camera chose exposure, it will overexpose in ISO50
In manual exposure mode it makes no or very little difference between ISO50 and ISO100
None of you seem to disagree on this, so I fail to see the problem.

TheSuede, your two-line summary is correct (if, naturally, a bit oversimplified), but that's not what Mikael has been writing. And when neuroanatomist and I have been writing that, Mikael has been arguing that we're worng.

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neuroanatomist

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Re: ISO 50
« Reply #77 on: January 21, 2013, 10:50:48 AM »
My point was in reference to using ISO 50 and ISO 100 at the same aperture/shutter speed - in that case, there's no difference in the highlights in the RAW image files, and ISO 50 would not blow any more highlights (the meter would show a stop of underexposure). In your example, you are talking about changing aperture or shutter speed when going from ISO 100 to ISO 50 to maintain a metered exposure. It's that change in aperture/shutter that's causing the blown highlights, not setting ISO 50, per se.

@sanj - the reason this is relevant, at least to me, is that many times (usually, in fact), I choose aperture and shutter speed for a reason. Aperture is chosen for the desired DoF so I can include/exclude what I want to be sharp in the image, shutter speed is chosen to stop or show motion, as desired.  If those values are dependent variables (fixed for the shot, ISO becomes the independent variable.  In that case, there is no benefit to ISO 50, which was my point.  Of course, given the constraints of my personal range of acceptable ISO values (which is pretty broad on the 1D X!), I still cannot achieve the exposure I want, then I decide where to compromise.

But the key point is that ISO 50 offers no benefit, and can in some cases result in a detriment - therefore, I see no reason to use it.
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agierke

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Re: ISO 50
« Reply #78 on: January 21, 2013, 10:51:34 AM »
i would like to now see Neoro and Mikael do a rendition of "Who's on First".
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TrumpetPower!

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Re: ISO 50
« Reply #79 on: January 21, 2013, 10:53:18 AM »
Here is the dumbest question in the thread, but since I am the one that made the test, could I please ask what is HTP?

Not dumb at all. I've tried to expand the acronym at appropriate places, but obviously not everywhere it's appropriate.

HTP stands for Highlight Tone Priority. Rather than ISO 100 being the lowest selectable ISO, ISO 200 becomes the lowest selectable ISO. The camera actually exposes at ISO 100 but applies a tone curve to brighten the image. This has the end result of recording more highlight detail, but there's no such thing as a free lunch, so you get nosier shadows at the same time. If you're shooting raw, you can achieve the exact same result by shooting at ISO 100, underexposing by a stop, and brightening the image by a stop in post-production (with, of course, highlight compression to avoid digital clipping).

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neuroanatomist

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Re: ISO 50
« Reply #80 on: January 21, 2013, 10:53:28 AM »
Here is the dumbest question in the thread, but since I am the one that made the test, could I please ask what is HTP?

LOL. Earlier, it was suggested that it means 'Half The Photons' - that seems to be Mikael's definition, anyway.

Really, it's Highlight Tone Priority, which is a way to preserve one stop of highlights in an image (at the cost of an extra stop of noise in the shadows). 
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TrumpetPower!

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Re: ISO 50
« Reply #81 on: January 21, 2013, 10:56:44 AM »
But the key point is that ISO 50 offers no benefit, and can in some cases result in a detriment - therefore, I see no reason to use it.

I, too, have no use for ISO 50 nor HTP. But I would find it useful if I were shooting JPEGs with no intention of post-processing, or if I were planning on doing ETTR or ETTL (respective) exposure and wanted a closer preview image on the back of the camera -- say, if a clueless client were looking over my shoulder.

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Re: ISO 50
« Reply #81 on: January 21, 2013, 10:56:44 AM »

TheSuede

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Re: ISO 50
« Reply #82 on: January 21, 2013, 11:12:18 AM »
Regardless of camera brand I often recommend using 'HTP' or whatever acronym a particular brand has chosen to signify a more gentle contrast curve towards the clipped highlights when shooting outdoors in sunny weather. -But for jpg shooters only!.

Often there's something more involved in the altered process too, like a different color profile, local contrast enhancements and/or other 'optimizations' and so on. And I do dislike the often overly contrasty rendition that most in-camera jpg engines use. Using HTP in stead of just lowering the 'contrast' setting does in that case often have several benefits.

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Re: ISO 50
« Reply #83 on: January 21, 2013, 11:19:01 AM »
Regardless of camera brand I often recommend using 'HTP' or whatever acronym a particular brand has chosen to signify a more gentle contrast curve towards the clipped highlights when shooting outdoors in sunny weather.

When I got my 60d I was shooting htp all the time in sunny weather, but stopped doing so because it increases the sensor's main weakness: high noise levels after raising shadows.

I admit clipped highlights are the worst thing that can happen, but it's closely followed by shadow noise - so today I only enable htp for jpeg and when I know the scene has a tendency to clip hightlights and I won't have to raise shadows in post a lot.

sanj

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Re: ISO 50
« Reply #84 on: January 21, 2013, 11:29:08 AM »
Regardless of camera brand I often recommend using 'HTP' or whatever acronym a particular brand has chosen to signify a more gentle contrast curve towards the clipped highlights when shooting outdoors in sunny weather.

When I got my 60d I was shooting htp all the time in sunny weather, but stopped doing so because it increases the sensor's main weakness: high noise levels after raising shadows.

I admit clipped highlights are the worst thing that can happen, but it's closely followed by shadow noise - so today I only enable htp for jpeg and when I know the scene has a tendency to clip hightlights and I won't have to raise shadows in post a lot.

Me same

TrumpetPower!

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Re: ISO 50
« Reply #85 on: January 21, 2013, 11:38:28 AM »
Regardless of camera brand I often recommend using 'HTP' or whatever acronym a particular brand has chosen to signify a more gentle contrast curve towards the clipped highlights when shooting outdoors in sunny weather. -But for jpg shooters only!.

There's some slightly better and simpler advice you could offer JPEG shooters: do whatever you have to to make the image on the back of the camera look the way you want it to. Those on-camera knobs are the only ones you have to fiddle, so fiddle with them until you're happy with the result.

And HTP is an excellent in-camera implementation of "ETTL" (expose-to-the-left) post-processing, and perfect for situations where you care about highlights and won't see noise in shadows. Similarly, ISO 50 does a great job of doing exactly what ETTR does, when it's shadow noise you're worried about and don't care if the highlights blow. If you're shooting JPEG, you should know what each does and when to use either -- as well as when not to use them.

But, if you're shooting raw (and, generally, you should be if your post processing is much more than picking keepers), you should also understand what HTP and ISO 50 are doing, and why you're better off doing what they do yourself. Maybe it's an especially contrasty scene, and a single stop of ETTL isn't enough to keep those highlights you're so interested in, so you'll want to underexpose not by one stop but by two stops, for example.

Cheers,

b&

neuroanatomist

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Re: ISO 50
« Reply #86 on: January 21, 2013, 11:48:09 AM »
Maybe it's an especially contrasty scene, and a single stop of ETTL isn't enough to keep those highlights you're so interested in, so you'll want to underexpose not by one stop but by two stops, for example.

I eagerly await Mikael's comment that in such a situation, one should instead use a D800 where the motive can be to underexpose by four stops and allow bringing up of shadows without motive of banding and noise.  I understand the technique works very well for sunrises, sunsets, cluttered sheds, and barbecues with QPcards on them.

EDIT: too late, Mikael beat me to the punch.  ???
« Last Edit: January 21, 2013, 11:50:10 AM by neuroanatomist »
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Aglet

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Re: ISO 50
« Reply #87 on: January 21, 2013, 01:22:10 PM »
LOL. Earlier, it was suggested that it means 'Half The Photons' - that seems to be Mikael's definition, anyway.

 'Half The Photons' is correct IF you concede that enabling HTP raised the effective ISO amplification +1EV from whatever ISO you were at before activating HTP.
so the resultant metering of course causes a -1 EV exposure - and HALF THE PHOTONS! :)

Did you not understand what Michael meant vs what he may have typed or did you just feel the urge to have a battle from the same side of the argument?

Honestly, Neuro, we appreciate your extensive technical knowledge but some of us old guys are occasionally reminded of a a couple brothers scrapping over a toy by such an extensive display of (where do I get that cool little animation for) beating your head on a brick wall.

I hope you keep up your high rate of helpful and clear advice but temper it with a bit more restraint and human understanding in such cases.  I think you might have driven Michael to drinking after this thread. ;)

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Re: ISO 50
« Reply #87 on: January 21, 2013, 01:22:10 PM »

Meh

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Re: ISO 50
« Reply #88 on: January 21, 2013, 02:03:45 PM »
LOL. Earlier, it was suggested that it means 'Half The Photons' - that seems to be Mikael's definition, anyway.

 'Half The Photons' is correct IF you concede that enabling HTP raised the effective ISO amplification +1EV from whatever ISO you were at before activating HTP.
so the resultant metering of course causes a -1 EV exposure - and HALF THE PHOTONS! :)

Did you not understand what Michael meant vs what he may have typed or did you just feel the urge to have a battle from the same side of the argument?

Honestly, Neuro, we appreciate your extensive technical knowledge but some of us old guys are occasionally reminded of a a couple brothers scrapping over a toy by such an extensive display of (where do I get that cool little animation for) beating your head on a brick wall.

I hope you keep up your high rate of helpful and clear advice but temper it with a bit more restraint and human understanding in such cases.  I think you might have driven Michael to drinking after this thread. ;)

Why would any of us concede to your explanation when that is not what HTP does.  HTP does not change the aperture or shutter speed and therefore does not change the number of photons that are incident on the sensor.

We all understand what Mikael meant because he was very clear about it.  He was also very clear about his condescending remarks such as "look at the attached link, can't you boys read" when in fact the attached article exactly refuted his statements.

neuroanatomist

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Re: ISO 50
« Reply #89 on: January 21, 2013, 02:26:03 PM »
LOL. Earlier, it was suggested that it means 'Half The Photons' - that seems to be Mikael's definition, anyway.

 'Half The Photons' is correct IF you concede that enabling HTP raised the effective ISO amplification +1EV from whatever ISO you were at before activating HTP.
so the resultant metering of course causes a -1 EV exposure - and HALF THE PHOTONS! :)

Sorry, but you seem to be falling into the same trap and suffering from the same misconceptions as Mikael.

It's only half the photons if the aperture is set a stop narrower or the shutter speed is set a stop faster. Are you suggesting that enabling HTP directly affects aperture or shutter speed? 

In fact, enabling HTP does nothing to the resultant metering - the camera meters for the ISO you select, it doesn't indicate that it's actually exposing at a 1-stop lower ISO, but it is (and that's why ISO 100 is unavailable with HTP enabled).  Try it - set ISO 200 or higher (but not a H expansion, since those are unavailable with HTP), meter a scene, then toggle HTP and see if the metering changes - does it?  Even in P Mode with Auto ISO where the camera is selecting all the parameters, enabling HTP does not change the aperture, shutter speed, or ISO selected. 

HTP doesn't raise the ISO, unless you have it set lower than ISO 200 before enabling HTP (and if you're at an H expansion ISO, enabling HTP lowers it).  I concede that in the specific case of the camera being set to ISO 100 before enabling HTP, the metered exposure will change by one stop, and IF you are in an auto exposure mode or you manually adjust exposure to compensate by one stop, then the number of photons will be halved.  Also 'halved' applies only if ISO 100 was set - if it was set to 50, 125, or 160, the change will be more or less than one stop (but not 'halved').  But that's a byproduct of how HTP works, applicable in a limited range of circumstances.  To conclude that reducing the photons by half is the mechanism by which HTP works is like concluding that a stopped analog clock is keeping correct time because you just happened to look at it at whatever time at which it was stuck.

As for misunderstanding Mikael, as I indicated earlier and Meh just wrote, that's clearly not the case.  He specifically stated, at least five times in at least four different posts, that HTP halves the number of photons and/or the amount of light hitting the sensor, and used that phrasing in describing the mechanism of HTP.  That manifestly demonstrates that his understanding of how HTP really works is seriously flawed, and if you are supporting that explanation, you are also failing to understand how HTP works.
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Re: ISO 50
« Reply #89 on: January 21, 2013, 02:26:03 PM »