ISO 50

[J.R.]

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?

 

[sanj]

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.]

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.

 

[RLPhoto]

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

 

[Jesse]

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.

 

[J.R.]

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

 

[Sporgon]

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

 

[Loswr]

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.

 

[TrumpetPower!]

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&

 

[phoenix]

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.

 

[Loswr]

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.

 

[TrumpetPower!]

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&

 

[Marsu42]

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

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]

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.

 

[TrumpetPower!]

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.

Cheers,

b&

 

[Marsu42]

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!]

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.

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.

(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&

 

[Hobby Shooter]

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!]

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]

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