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AP sometimes comment on fairly big variations in ISO measurements when they do their side by side tests of cameras. I was running a few comparisons of the Nikon V1 (10-30) against my Sony A77 (16-80) by taking identical photos with each at various ISOs. I noticed that, when using the same effective focal length, same shutter speed and same aperture the selected ISO for each camera was radically different - by a factor of 2.5. So, what the V1 called ISO3200 the A77 called ISO1250, and what the V1 called ISO400 the A77 called ISO160.
I wonder if the testers take this variation into account when they comment on a camera's ISO performance. It's obviously a fairly simple way that camera manufacturers can make the high ISO performance of their cameras seem better than it is. I was initially quite surprised at how good the V1 was at ISO3200, but according the Sony measurements I would need to set the V1's ISO to ISO8000 to get a real ISO3200.
This has always been the case!
In the old days of film, a higher "box ISO" was more desirable so we were never surprised when we measured a "zone system" ISO at least 1 stop lower and sometimes a lot more.
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.
Quote: This has always been the case!
I'm sure it has. I'm just wondering if the testers correct for this - or just believe the camera's metering system. Maybe EPZ testers could comment.
Easy way to check -
Go out in bright sunshine and photograph a well illuminated scene on Manual at ISO 125, shutter 1/125th and aperture f/16. Then examine the histogram. It should indicate a reasonably correctly exposed image. If not, then one or more of your camera settings is "out". Probably ISO, but maybe also shutter speed or aperture control.
It always worked OK in the days of Verichrome Pan. 
.
Nice example of the Nikon 1 V1 ISO performance at DX0 labs that illustrates the difference between manufacturers quoted ISO and actual ISO in use.
So using a light meter which reads 100 iso, for example, the acutula ISO would different, and the camera ISO would need changing. I just wonder when using in camera metering it uses the actual ISO.
I see the DXO test says that what the V1 calls ISO3200 is actually ISO2300, which is about 40% out - still nowhere near what it seems relative to the Sony. I guess the Sony may be miscalibrated in the other direction and also the Sony lens may be losing less light. Obviously it doesn't matter but it did fool me into thinking that the V1 was much better at high ISO than it actually is.
Quote:
No, that's only about 1/2 a stop - its not a linear scale
ISO is a linear scale.
Quote: 40% IS about 1/2 a stop.
I didn't say it wasn't. Read my post.
Quote: 25, 50, 100, 200, 400, 800, 1600, 3200, 6400.
So? Each step on your (arbitrary) sequence doubles the sensitivity. That's called linearity.
And if a manufacturer displays an actual ISO2300 as ISO3200, that's about a 40% error. LIke I said, it doesn't actually matter very much, but it's a larger error than people might expect and it probably misleads testers who don't check the ISO accuracy into thinking that the camera is rather better at high ISO than it actually is.
Exposure doubles when ISO doubles - what is not linear about that?
If you plot ISO on the X axis and EV on the Y axis then it is a straight line. If you have points 1,2,3 and 4 on a graph and assign them ISO50, ISO100, ISO200, ISO800 etc and do the same for EV the no, you will not get a straight line.
http://dpanswers.com/content/tech_iso.php
Quote: The sensitivity scale defined by ISO actually defines two parallel scales, one linear (arithmetic) scale and one logarithmic scale. This is because the ISO sensitivity scale was created in 1987 by merging two older scales known as “ASA” and “DIN”. The ISO linear scale corresponds to the older “ASA” scale, and the ISO logarithmic scale corresponds to the older “DIN” scale.
When both values are written, they should be separated by a slash (/) and the logarithmic value is marked with a degree (°) symbol. Example: “ISO 200/24°”. If only one value is written, it is always the linear value. Example: “ISO 200”.
In the ISO logarithmic scale, adding 3 to the numeric value indicates a doubling in speed, while the ISO linear scale, doubling the number indicates a doubling in speed.
In practice, the ISO logarithmic scale is no longer used, and you will instead see the film or sensor speed measured in the linear values. From this point on in this article, I shall ignore the ISO logarithmic scale and only refer to the linear scale.
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.
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