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Accuracy of ISO readings

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Steppenwolf
10 Dec 2012 - 2:02 PM


Quote: And the step from one EV to the next is double the amount of light, but a single numerical value, so if you plot the EV number against actual light measurement, it will be a curve.
If you plot EV against ISO, it will be a curve.
If you plot light measurement against ISO, it will be linear.

We should be careful about being inaccurate in what we are actually saying.

What I said - to initiate this garbage - was "if a manufacturer displays an actual ISO2300 as ISO3200, that's about a 40% error". There is nothing "inaccurate" about that. Nick_W just conflated ISO with EV/stops, which are not linear.

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RogBrown
RogBrown  73006 forum posts England10 Constructive Critique Points
10 Dec 2012 - 2:09 PM


Quote: I didn't say it wasn't. Read my post.

I didn't say you said it wasn't.
When you said
Quote: I see the DXO test says that what the V1 calls ISO3200 is actually ISO2300, which is about 40% out

Nick_w said

Quote: No, that's only about 1/2 a stop - its not a linear scale

which I took to mean he was saying it was 1/2 a stop NOT 40% but I see I may have read it wrong.
Anyway I'm bored with this argument now. Tongue

ChrisV
ChrisV  7786 forum posts United Kingdom26 Constructive Critique Points
10 Dec 2012 - 3:56 PM

I know DPreview often report on variation between quoted and actual ISO, but I've never seen an instance where it measured more than about a third of a stop at base ISO. Given that you're talking about a halving of the light with each whole increase in sensitivity, that amount of variation is never going to alter at any given value [ie if it's 33% at ISO 100 it will still be 33% at ISO 3200]. Even if these variations between two systems were out in opposite directions that would account for rather less than one full step difference - and that would appear to be at the lower end of the likely.

What is I think far more serious is some manufacturers and models quoting ISO capabilities which are clearly beyond their useful limit. This used to be the preserve of compacts producing results that looked like water colours produced by the myopic, but now is also generously available from the flagship marques of Canon and Nikon.

If you wanted to produce a speckled, grainy impression of a black cat in a coal cellar you could always do that with a camera that has an ISO 'limit' of 52k and whack up the underexposed result by a couple of stops in Lightroom/Aperture and voila - you have an ISO 204800 beast. Or not.

Paul Morgan
Paul Morgan e2 Member 1315350 forum postsPaul Morgan vcard England6 Constructive Critique Points
10 Dec 2012 - 4:15 PM


Quote: Try using a light meter and strobes to set the scene and dial in the meter readings to each cam, it wont be as far apart as your test shows. If it was i would have all sorts of problems when im running studio sessions at my local camera club, with the amount of different cameras that are used. There is some that will under or over expose but only by about 1/3rd

Sounds about right, the meter can be calibrated to the camera, but you not going to do this with every camera, you will be there all night Smile

http://www.frankdoorhof.com/site/2011/08/calibrating-the-light-meter-some-quick-...

Last Modified By Paul Morgan at 10 Dec 2012 - 4:20 PM
thewilliam
10 Dec 2012 - 6:16 PM


Quote: the meter can be calibrated to the camera, but you not going to do this with every camera, you will be there all night Smile


Each camera and light meter only needs to be calibrated once and then you can be properly set up for several years or until you notice a problem. No true professional would dream of using a camera for work until it had been properly tested and calibrated.

Occasionally, equipment can misbehave straight of the box but this is is rare with good kit. It's happened once to me with a Hasselblad lens and to my old housemaster with a Leica lens so no marque is immune!

Last Modified By thewilliam at 10 Dec 2012 - 6:17 PM
carphunter
10 Dec 2012 - 6:25 PM

Need help please looking to get a lens to go with my canon 500d for taking photo' of wildlife, anybody got any suggestions please.

mikehit
mikehit  56454 forum posts United Kingdom9 Constructive Critique Points
10 Dec 2012 - 6:28 PM


Quote: Need help please looking to get a lens to go with my canon 500d for taking photo' of wildlife, anybody got any suggestions please.

I suggest you start another thread for that question, carphunter.

Paul Morgan
Paul Morgan e2 Member 1315350 forum postsPaul Morgan vcard England6 Constructive Critique Points
10 Dec 2012 - 6:37 PM


Quote: What is I think far more serious is some manufacturers and models quoting ISO capabilities which are clearly beyond their useful limit

Your useful limit might be different to mine Smile

mikehit
mikehit  56454 forum posts United Kingdom9 Constructive Critique Points
10 Dec 2012 - 7:03 PM

There has been suspicion for some time that some manufacturers massage ISO data to make their camera specs read better, but how much is testing regime, and how much is deliberte is hard to judge. The OM-D is said to be a full stop out compared to other marques

http://forum.dxomark.com/index.php/topic,875.0.html

Aparently the manufacturer can choose one of several ways of defining ISO
http://en.wikipedia.org/wiki/Film_speed

So where do we go...?

User_Removed
10 Dec 2012 - 10:28 PM

An interesting discussion, guys.

I was tempted to switch from a Nikon D300 which was, and still is, a superb camera for the enthusiastic amateur, to a D3s partly because I wanted to try FX format but also, to quite a considerable extent, because of the lure of the ISO 102,400 possibilities.

After about 18 months it dawned on me that I could probably count the occasions that I had used anything over ISO 6400 on the fingers of one hand.

So when the D800 came out - with a much lower maximum ISO than the D3s, it didn't bother me that I'd be losing the top end of the range. In fact, at ISO 6400, the D800 seems to be better than the D3s.

Even then, though, most of the time I don't crank it up over ISO 1600 very often.

A couple of weeks ago I wanted to shoot a whole evening of after-dark photographs in a misty Venice. With the camera set at ISO 6400 I was able to shoot hand-held in even the darkest streets, canals and alleys and the image quality turned out to be more than satisfactory.

So, the point of this rambling diatribe, really, is to say that it is of no significance to me, as a keen amateur user, whether the ISO calibration on my camera is the same as on, say, a Canon 5D Mk3. What matters is that it is calibrated to give me the correct exposure in low light conditions. Here's an example at ISO 6400. If it was really ISO 5600 or ISO 7200 doesn't honestly matter to me.

venice-7.jpg

.

mikehit
mikehit  56454 forum posts United Kingdom9 Constructive Critique Points
10 Dec 2012 - 10:58 PM

That is some achievement, LF. And controlled highlights as well.

As usual the proof is in the eating - if you can get useable images in darker conditions than your previous camera, then as you say who cares what the ISO dial reads.

Paul Morgan
Paul Morgan e2 Member 1315350 forum postsPaul Morgan vcard England6 Constructive Critique Points
10 Dec 2012 - 11:01 PM


Quote: The OM-D is said to be a full stop out compared to other marques

I doubt it, think i would have noticed.

LenShepherd
LenShepherd e2 Member 62478 forum postsLenShepherd vcard United Kingdom
11 Dec 2012 - 7:58 AM

An important detail probably being overlooked is that focal lengths and apertures are not constant with probably 90% of lenses the closer you focus Sad
Many know in close focus at say 200mm; a 200mm prime , f2.8 70-200 zoom and a variable aperture zoom with 200mm have different viewfinder crops.
When this happens the physical size of the aperture blades does not change, which means the aperture used is different to the one displayed on the monitor or in the EXIF! Variations of one stop + or - stop are not uncommon - which may explain some of the readout difference between the different lenses on the 2 different systems.
In addition some metering systems vary - current Nikon matrix takes into account light reflectance under the active AF point - so a slight change in framing to a different tone can cause a change in exposure.
Getting technical one of the few types of lenses that does not change aperture relative to recorded aperture in close focus is a 50mm f1.8 (but not an f1.4). For the most accurate comparison of ISO a 50mm f1.8 is best (assuming the system includes one) used at longer focus distances, spot or centre weighted metering, viewfinder eyepiece covered.
With several recent cameras which do not have a 50mm f1.8 it can be difficult to separate ISO accuracy read out issues from the other common optical and metering variations. Provided each camera takes accurate exposures there is not a straightforward resolution of the indicated differences.

Steppenwolf
11 Dec 2012 - 8:54 AM


Quote: There has been suspicion for some time that some manufacturers massage ISO data to make their camera specs read better, but how much is testing regime, and how much is deliberte is hard to judge. The OM-D is said to be a full stop out compared to other marques

http://forum.dxomark.com/index.php/topic,875.0.html



Interesting. 1 stop is roughly the difference I'm seeing between the V1 and A77. Given that 1 stop is roughly 2 years' sensor development in terms of sensitivity, this is pretty significant. It also puts the high ISO performance of the OMD in context - unless of course the testers ignore the ISO reading that the camera is telling them and use the actual ISO reading. I don't think most of these testers have the facilities to do that though.

Last Modified By Steppenwolf at 11 Dec 2012 - 8:55 AM
samueldilworth
11 Dec 2012 - 9:20 AM

The ISO setting on a digital camera is a surprisingly confusing thing, since both the sensor’s gain and the brightness of the JPEG image can be arbitrarily varied. We’re no longer linking a given exposure to a negative density, as was the case in the good old days.

I don’t claim to wholly understand it, but the ISO 12232:2006 standard (mentioned in mikehit’s link) has all the gory details. There are at least five things a digital camera maker can legitimately call ISO 100, three of which were added in 2006.

DxOMark seems to measure something else again, roughly based on the saturation-based technique described in ISO 12232, but with additional tweaks. The explanation on their website predates the 2006 additions to the ISO standard. DxOMark’s ‘ISO’ measurement is designed to allow direct signal-to-noise ratio comparisons between different sensors, which it does well.

But. But. If you take photos with two cameras at the same exposure (shutter speed and f-number), and leave the ISO up to the camera (auto ISO), you’re really comparing metering systems rather than ISO sensitivities. (LenShepherd suggested as much above.) One camera’s matrix meter might try to retain detail in a bright sky, for example, while the other camera favours the foreground. Wildly different ISO settings (and JPEG brightnesses) could be chosen accordingly.

In truth, the exact ISO value used is much less important than whether the photo has an agreeable brightness and signal-to-noise ratio. The noise depends mostly on exposure (shutter speed and f-number, not ISO setting), and secondly on the sensor’s efficiency at converting photons to electrons. The OM-D has a very efficient Sony sensor, so it will do very well in any comparison that measures actual performance.

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