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Author Topic: ISO 50  (Read 31207 times)

J.R.

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ISO 50
« on: January 19, 2013, 11:44:34 AM »
I was just going through some EOS reading material and came across this regarding ISO 50 expansion ... "There will be approximately one less stop of DR in the highlights at ISO 50". So if I am reading this correctly ISO 50 is just empty night showing a "correct" exposure without any use ...

Is my understanding correct or am I missing something?
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ISO 50
« on: January 19, 2013, 11:44:34 AM »

sanj

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Re: ISO 50
« Reply #1 on: January 19, 2013, 11:55:37 AM »
No JR.
I find 50 asa to be 'smoother' than 100.
And YES it seems like DR is reduced when shooting at 50, but if on a tripod you could bracket and merge in PS.
I so prefer to use ISO 50 when my camera is on tripod or when I have enough light.

Perhaps you should make a test and judge for yourself. If you do, I would love to know your thoughts. Please send me a personal message as I might miss your reply here.

This is of interest to me.

Best...

J.R.

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Re: ISO 50
« Reply #2 on: January 19, 2013, 12:06:13 PM »
Thanks Sanjay (I checked out your website :))

I've used it only when I want to slow things down and it was not possible otherwise. Because it is "expansion", i believe (unless proven otherwise) that there must be a compromise somewhere. I generally use the NDX rather than the ISO 50.

i must say I have noted very little difference except low recovery from the highlights. I think I might as well do a test! ... Will get back to you on PM.

Cheers ... J.R.
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RLPhoto

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Re: ISO 50
« Reply #3 on: January 19, 2013, 12:51:10 PM »
I used ISO 50 a lot in my 5Dc because of how super smooth the files looked. The 5d3, not so much.
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Jesse

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Re: ISO 50
« Reply #4 on: January 19, 2013, 12:52:07 PM »
There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.
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J.R.

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Re: ISO 50
« Reply #5 on: January 19, 2013, 01:07:54 PM »
There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.

I used ISO 50 a lot in my 5Dc because of how super smooth the files looked. The 5d3, not so much.

So basically my hunch appears to be right ... It's useless except for reporting the correct exposure
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Sporgon

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Re: ISO 50
« Reply #6 on: January 19, 2013, 01:11:48 PM »
There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.

I'm not sure it's that simple. Why do you get less highlight range ?

If you have to under expose more to hold highlight, is the shadow recovery improved enough to give more benefit from just using ISO 100 and greater exposure ?

I too though files were better on 5Dc at 50, but it may be psychological   :)

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Re: ISO 50
« Reply #6 on: January 19, 2013, 01:11:48 PM »

neuroanatomist

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Re: ISO 50
« Reply #7 on: January 19, 2013, 01:29:07 PM »
There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.

Exactly.  There's no real benefit to ISO 50, except perhaps convenience if you're shooting in Av mode and want a stop slower shutter speed.  But if a highlight would be blown at ISO 100 with a given aperture/shutter combo, it'll be just as blown at ISO 50.
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ilkersen

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Re: ISO 50
« Reply #8 on: January 19, 2013, 02:29:54 PM »
Exactly.  There's no real benefit to ISO 50, except perhaps convenience if you're shooting in Av mode and want a stop slower shutter speed.  But if a highlight would be blown at ISO 100 with a given aperture/shutter combo, it'll be just as blown at ISO 50.

Let's say I have a "right exposed" image at ISO 100, and I go down to ISO 50 in AV mode.  Shutter speed doubles.  If the sensor was acting as if at ISO 100, it would saturate at the longer exposure and I would get blown out highlights at 50 (maybe more towards gray than white), but not at 100.  I'm not entirely convinced this is how it works.  I wish I knew where and how the expansion is executed in the camera. 

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Re: ISO 50
« Reply #9 on: January 19, 2013, 03:22:16 PM »
There might be a bit of analog jiu-jitsu contributing to the mix, but, with that caveat, the linear data recorded at the sensor with a shot at, say f/8 @ 1/400 @ ISO 100 ("sunny f/16") will be identical to the same shot at f/8 @ 1/400 @ ISO 50. At some point in the processing chain, however, the values in the ISO 50 file will get halved (by simple arithmetic, not electronic amplification), resulting in an image one stop darker than the ISO 100 file. That will reduce noise overall. However, the sensor is still saturating at the exact same point. The net effect is that a pixel that the sensor recorded as, say 256, is being rendered as 128...and that there's nothing in the original data that gets mapped between 128 and 256.

Thanks to the gamma curve that gets applied after linear processing, the end result is that there's no data in the last stop. It therefore gets rendered as pure white -- and, thus, a loss of a stop.

You can do the exact same thing yourself, assuming your RAW processing software is capable of linear exposure adjustments.

It's potentially useful in scenes with low dynamic range, or in scenes where you don't care about highlights but do care about shadows.

The common term amongst photographers who do that sort of thing is, "ETTR." I generally strongly caution against doing that, as it's very easy to blow out the highlights, and there's so much wonderful and delicate color to be found in the highlights that is so easy to clobber. But there are certainly situations in which it can be useful.

In general, the meter in most cameras underexposes the linear data by one to two stops, and the processing pipeline applies an equal and opposite amount of digital overexposure to compensate. This is generally a very good thing, because sensors clip so readily and so unforgivingly and modern sensors have so little noise. But, yes, if you're very careful, you can make use of that "extra" headroom.

Cheers,

b&

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Re: ISO 50
« Reply #10 on: January 19, 2013, 03:31:28 PM »
you can also use it to sync your flashes at 1 stop lower aperture giving you better subject isolation. especially useful now that cameras are coming out with slower syncs.

neuroanatomist

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Re: ISO 50
« Reply #11 on: January 19, 2013, 03:55:52 PM »
Exactly.  There's no real benefit to ISO 50, except perhaps convenience if you're shooting in Av mode and want a stop slower shutter speed.  But if a highlight would be blown at ISO 100 with a given aperture/shutter combo, it'll be just as blown at ISO 50.

Let's say I have a "right exposed" image at ISO 100, and I go down to ISO 50 in AV mode.  Shutter speed doubles.  If the sensor was acting as if at ISO 100, it would saturate at the longer exposure and I would get blown out highlights at 50 (maybe more towards gray than white), but not at 100.  I'm not entirely convinced this is how it works.  I wish I knew where and how the expansion is executed in the camera.

Expanded ISO means digital gain (negative gain for ISO 50).  The exposure is at ISO 100, then pulled down a stop.  In your example, you'd lose the highlights.
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TrumpetPower!

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Re: ISO 50
« Reply #12 on: January 19, 2013, 04:33:39 PM »
Expanded ISO means digital gain (negative gain for ISO 50).  The exposure is at ISO 100, then pulled down a stop.  In your example, you'd lose the highlights.

-- assuming, of course, there are highlights to be lost.

There almost always are, which is why ETTR is generally not such a great idea. But, when there aren't, or when you truly don't care about losing them, then, yes, ISO 50 or ETTR is a sometimes-useful tool to have in the toolbox.

b&

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Re: ISO 50
« Reply #12 on: January 19, 2013, 04:33:39 PM »

Marsu42

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Re: ISO 50
« Reply #13 on: January 19, 2013, 05:33:45 PM »
well there are a benefit with Canon sensors= less noise in the shadows=richer exposed but you lose 1stop DR


Interesting, though for the 6d at least dpreview cannot see any advantage except for skipping a nd filter: http://www.dpreview.com/previews/canon-eos-6d/7

Quote
The EOS 6D's lowest standard ISO is 100 but it can offer ISO 50 as an extended mode. Essentially this is doing the opposite of Highlight Tone priority mode - it's increasing the exposure by a stop, then using a different tone curve to pull the image brightness down to compensate. However, whereas HTP mode attempts to protect the image from highlight clipping, switching to ISO 50 makes it far more likely.

Because it's all-but impossible to recover over-exposed highlights, we'd recommend not using the camera's ISO 50 unless you have a specific reason - in everyday shooting you'd generally be better off using a neutral density filter if you need the longer shutter speeds. That's not to say that it's useless though; if you're shooting under controlled lighting and can be confident of retaining highlights, it should give the best quality.

Marsu42

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Re: ISO 50
« Reply #14 on: January 19, 2013, 06:19:27 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.

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Re: ISO 50
« Reply #14 on: January 19, 2013, 06:19:27 PM »