Those "in-between" ISOs...

[Chuck Lee]

They yeild the same result as shooting ISO 200 +1/3 EC. You lose 1/3 stop highlights, but gain less noise. They're really the same, not better. (In a fixed exposure, ISO 160 has lower read noise in absolute ADU, but the SNR is the same, so it's not better that way.)

If they're the same then what's the problem. I actually shoot shoot ISO 160 +.3ev on my 40D so that would be the equivalent of ISO 200 +.7ev. I find the the +1/3 tweeners( 160,320,640...) better thanshooting to the right with the normal ISOs. I started this because of the dark noise histograms provide by Peter Ruevski. I discovered this plot because of a post on dpreview where the poster was concerned with the amount of noise in a ISO 250 photo he had taken. Thats' because ISO 250 is 200 -1/3ev. It's essentially under exposed ISO 200 which increases the appereance of noise in the image. In fact, it looked worse thanISO400. Ruevski's plot confirms thisfinding.As far as "dynamic range" I can still pull down 1-2ev of blown highlights with these +1/3ev "tweeners". For me it takes the guess work out of creating raws with less noise. For Daniel, it's about milking the camera for every last drop of dynamic range available. I do not purposely over expose my raws in the hopes of increasing their dynamic range. The overhead proided by the raw format simply provides a safety net in case the photo has some over exposed areas.


I understand what you are saying Daniel, but I sometimes have to wonder whether you have real world data to back up the claims you continue to make on this subject. I have not seen personally any more overhead "dynamically" with ISO 100 images than ISO 160 images in all the time I've been post proccessing 40D RAW files. Not that what you say isn't true. I just wish my 5D had as much with the normal ISOs. I am always approaching this sublect with a Missori mentalitity. "Show me" If it works better, I'll use it.


canoli, You will find that if you do some images of the sky, say in the evening, you will see a difference in noise when shooting at ISO 100vs. ISO 160. I suggest you try it for yourself. In fact, try all the ISOs and judge for yourself whether or not it is useful. As far as combing in files, who cares? Just do a level adjustment in photoshop and watch the histogram get combed.


If you want to put the camera on ISO 200 +.3ev, do it. Otherwise, you'll find ISO 160 0ev just as good and you won't have to think about it. In fact according to Ruevski you'll get 2/3ev faster shutter and a cleaner image than shooting ISO100. According to DB you'll be giving up something in terms of dynamic range. To some that is important, to othersit's not. The art of Photography, I believe, is theoffspring ofalimited dynamic range medium.And I'd rather have cleaner images than increased DR.


Great Discussion!!


Chuck

[Daniel Browning]

I find the the +1/3 tweeners ( 160,320,640...) better than shooting to the right with the normal ISOs.

The raw files have the exact same dynamic range and SNR, so if it's really any better it can only be due to issues with the raw converter, such as the problem mentioned above with DPP. Another example would be if the raw converter didn't have a good exposure compensation tool, like Adobe.

I started this because of the dark noise histograms provide by Peter Ruevski.

His read noise plots are fine, but they only account for Noise, not Signal. (By "signal", I'm referring to light, not the average level of the read noise.) ISO 160 only looks like less noise than ISO 200 because of a digital -0.33 EC. If Signal stayed the same, then ISO 160 would truly be better. But Signal doesn't stay the same: it goes down -0.33 with the noise, so the SNR stays the same.

I discovered this plot because of a post on dpreview where the poster was concerned with the amount of noise in a ISO 250 photo he had taken. Thats' because ISO 250 is 200 -1/3ev. It's essentially under exposed ISO 200 which increases the appereance of noise in the image. In fact, it looked worse than ISO400. Ruevski's plot confirms this finding.

Agreed.

I understand what you are saying Daniel, but I sometimes have to wonder whether you have real world data to back up the claims you continue to make on this subject.

I'll post a demonstration.

I have not seen personally any more overhead "dynamically" with ISO 100 images than ISO 160 images in all the time I've been post proccessing 40D RAW files.

A 1/3 stop difference in read noise is pretty hard to detect. Most of my shots aren't even within 1/3 of my ideal exposure, there just isn't time to get it perfect.

As far as combing in files, who cares? Just do a level adjustment in photoshop and watch the histogram get combed.

The combing is fine if you have a bunch of extra (wasted) bits. It would only be harmful if Canon was doing like Sony or Nikon and building them with just the perfect amount of precision.

If you want to put the camera on ISO 200 +.3ev, do it. Otherwise, you'll find ISO 160 0ev just as good and you won't have to think about it. In fact according to Ruevski you'll get 2/3ev faster shutter and a cleaner image than shooting ISO100.

Ruevski's chart correctly shows that the read noise in ADU is lower, but what really matters is SNR, and that is not improved over ISO 100 (in fact it's 1/3 stop worse).

And I'd rather have cleaner images than increased DR.

In this case, ISO 100 provides both.

[Daniel Browning]

I'll post a demonstration.

Until I get a round tuit, here's a chart that pretty much sums up everything about tweener ISO performance. (It's for the 5D2, and other cameras are slightly different, but it gives the idea.)


http://forums.dpreview.com/forums/read.asp?forum=1032&message=31378147

[canoli]

Thanks for your input folks, it is much appreciated.


Daniel, I was wondering if you could explain what "implement ISO as metadata" means. That's not your exact quote, but I think that was the gist of it.


Is it something you can explain to a more-or-less casual photographer? I love learning this stuff, but I'm not very well versed in all the terminology. If you can elaborate on that phrase a bit - without (if possible) getting too technical, I would really love to understand it. I assume not all of us know what that means, so maybe it'll be a worthwhile explanation for others too.


In any case, thanks again for your contributions.

[Daniel Browning]

Daniel, I was wondering if you could explain what "implement ISO as metadata" means.

White balance could be applied as a digital manipulation to the raw file before it's saved; instead, it is just "metadata", a recommendation for how to white balance the file in post. Currently, many ISO settings are applied as a digital manipulation to the raw file before it's saved. Metadata ISO would change that to be just a recommendation for how to push the file in post. It's very similar to how HTP works now.

Many camera manufacturers implement some ISO settings by digitally manipulating the raw file in the camera. This results in highlights that are blown needlessly (two full stops in the case of ISO 6400), larger file sizes (45 MB instead of 20 MB for the 5D2), and increases the precision needed to prevent quantization error.

There are no benefits to applying gain digitally in the camera, so those ISO settings should instead be implemented as metadata. Like white balance, the metadata ISO would only recommend a certain action during raw conversion.


Hope that helps.

[canoli]

Definitely helps Daniel, thanks very much.


I always thought of the ISO setting as a physical thing, sending more (or less) electrical current to the sensor, changing its sensitivity, its gain. That's what you mean when you say -

Currently, many ISO settings are applied as a digital manipulation to the raw file before it's saved.

So the capture is defined (partly) by the ISO setting - it is"hard-wired" into the result so to speak, right?


But you're saying manufacturers could forgo that practice completely and instead write the ISO setting into the metadata. Interesting.


I can't help thinking there must be some reason they went the way they did. It has to be more complicated and more expensive to build an auxilliary amp or dedicated electrical pathways to change ISO. Isn't there some benefit to having it "hard-wired" into the capture?

[Daniel Browning]

So the capture is defined (partly) by the ISO setting - it is"hard-wired" into the result so to speak, right?

Yes. Some of the ISO settings use analog gain (amplifiers), but many of them are just digital manipulations.

I can't help thinking there must be some reason they went the way they did.

I should point out that they used to have this feature on the Canon 10D. It disappeared from all later cameras.


I can think of several possible reasons:


* Because they can. Customers accept all sorts of compromises at high ISO, even ones that are completely unnecessary (like this). Many photographers never even use high ISO. Canon knows that few people use it and the ones who do just have to be willing to accept additional unnecessary compromises.

* Because of Customer Service. Some customers will have a rude awakening when they realize that Canon has been deleting 1, 2, or more stops of their highlight headroom for no reason. Even if Canon buries the metadata feature in some custom function, and provides copious documentation, many customers will get thoroughly confused about what it is or does. Misconceptions will travel rapidly on the web about it, no doubt. Even some raw processing programs might get confused by the metadata. Adobe, for example, doesn't implement HTP correctly, it just does a linear push without preserving highlights. Other converters might not apply the push at all, so customers will wonder why it's too dark.

* Because of Management. Canon had ISO metadata in the 10D, but removed it in all later cameras. It may have been removed on purpose, by edict from Management. Software engineers wouldn't remove a feature that was doing the obviously correct thing and replace it with the obviously incorrect thing for no good reason. A good reason would be if the manager said "cripple the camera or I break-a-you-face."

* Because of Marketing. Perhaps the feature was removed because Canon wanted to save that trick for a rainy day, so they could come out with a new camera with "2 stops more dynamic range at ISO 6400!" without any development cost. If they were nefarious enough, they could sell a firmware update to all cameras going back to the 20D offering ISO 25600 as metadata, with 4 stops more dynamic range across all cameras. Sony sells $1,000 firmware upgrades on some of its high end video cameras, so there's precedent.

* Because of Engineering. It's possible that the software engineers responsible for the firmware are unaware of the issue, but given the high level of competence they've demonstrated I think that's highly unlikely. I don't think Hanlon's razor applies.

* Because of Bureaucracy. This is the most likely reason, IMHO. Many suboptimal things happen in big corporations. Maybe the 10D metadata ISO feature did not have the correct "addition of feature form" filled out, and the engineer who added it was fired because he didn't follow the correct procedure for requesting authority to add a new feature. Maybe the paperwork for removing the feature was easier than the paperwork required for leaving it in.


Who knows.

[Chuck Lee]

Ruevski's chart correctly shows that the read noise in ADU is lower, but what really matters is SNR, and that is not improved over ISO 100 (in fact it's 1/3 stop worse).

And I'd rather have cleaner images than increased DR.

In this case, ISO 100 provides both.

Not at the same shutter speed and aperture as ISO 160. You would have to shoot ISO 100 at -.7ev to get the same speed/aperture combination. I'm willing to sacrifice a whole 1/3 stop or DR for that. The real world result is that I canhandhold at1/125th rather than 1/80th shutter speedand geta cleaner and possibly sharperimage.

[canoli]

Not at the same shutter speed and aperture as ISO 160.

Hmm, so - without getting in between you and Daniel's discussion (as it's obvious you both can talk at a higher level about this than me), I think you guys did agree that some 'tweens are better than others. 1250 over 1000, 500 over 320...(okay, that 2nd one I just made up). But there is consensus on using 1250 instead of 1000 right? And somebody mentioned that 160 on the 40D was "as good as it gets [with 'tween ISOs]."


Which 'tweener ISOs are less bad and which ones should be avoided? I realize it depends upon the camera. Personally I am particularly curious about the 40D as it's my main body for now.


One more question - is any of this relevant to amateur photography? We all want to maximize our body/lens' potential, get the best images we can, but are the differences so minute that it takes 100% views on 24" displays to notice them? Or will a photo shot at one of the especially bad 'tweeners, printed 8x10, look obviously worse than the same photo shot at a native ISO?


Thanks guys!

[Daniel Browning]

Not at the same shutter speed and aperture
as ISO 160.

Agreed. I should have clarified that I meant a different exposure.

You would have to shoot ISO 100 at -.7ev to get the same
speed/aperture combination. I'm willing to sacrifice a whole 1/3 stop
or DR for that. The real world result is that I canhandhold at1/125th
rather than 1/80th shutter speedand geta cleaner and possibly
sharperimage.

Me too. I only use ISO 100 when I have plenty of exposure to spare. (1/3 stop is such a small difference anyway.)

Which 'tweener ISOs are less bad and which ones should be avoided?

The "minus 1/3" ISO (160 320 640 1250) are fine, the "plus 1/3" ISO are suboptimal (125 250 500 1000). Everything above ISO 1600 is suboptimal.

One more question - is any of this relevant to amateur photography?

It's a pretty minor effect by itself. Just like HTP is a pretty small effect, and ALO is a pretty small effect. But when you start combining all these small factors, it results in a big factor. For example, ISO 250 by itself may not increase shadow noise enough to notice. And ISO 200+HTP by itself may not either. But combine 250+HTP and the result will be much more noticeable: the shadow noise is as bad as ISO 640. Add ALO into the mix and you can get people wondering why their ISO 250 shot looks like ISO 1600.