In comparing, let's say Canon's 600D and 100D (which are by explanation from my store guy, practically the same camera) I noticed that they have different ISO sensitivity declarations.

Since, they seem to have the same sensor, what influences in the camera body that they have different ISO, important for when shooting in areas of low light?


4 Answers 4


If you're are talking about the range of ISO sensitivities available then it's simply a choice by camera manufacturers about how much gain (amplification) is applied to the sensor data. The highest settings are usually implemented in software by just doubling all the numbers after analogue to digital conversion, so it's entirely arbitrary how high they can be made to go.

The Canon 600D goes from ISO 100 to ISO 6400, whereas the newer 100D goes from ISO 100 to ISO 12800.

Lots of people claim that Canon have been using the "same 18MP sensor" in all APS-C models since the 7D. This conjures up images of a warehouse full of 18MP chips manufactured in 2007 that Canon keeps dipping into with each new release. In fact each sensor is different, but the differences are incremental based on the same basic design, hence the difference in ISO range (and other factors) between the 100D and 600D.

It's tempting to say that as sensors get better at handing noise, manufactures have increased the allowable range accordingly. But ISO ranges have expanded far faster than camera performance. The 20D only went up to ISO 1600, it's 2013 descendant the 70D goes a whopping three stops (x8) higher.

In essence it's become an arms race between manufacturers as the higher setting tend to suggest a camera is better in low light. Both the 100D and 600D offer an "expanded" ISO setting of 12800 for the 600D and 25600 for the 100D. It's hidden behind a setting so manufactures can claim a wide range, whilst at the same time preventing anyone from using it due to the very poor quality of detail captured with this setting.

  • \$\begingroup\$ +1 The 100D has had 2 years (maybe 3) of small incremental improvements in manufacturing over the 600D across the whole system. The 100D is also almost 200g lighter for example. \$\endgroup\$ Commented Jan 22, 2014 at 10:55
  • \$\begingroup\$ This reminds me of how car manufacturers long ago would scale the speedometer to totally unrealistic top end values to give the impression the car was fast. \$\endgroup\$ Commented Jan 24, 2014 at 13:09
  • \$\begingroup\$ @OlinLathrop That's a good analogy! \$\endgroup\$
    – Matt Grum
    Commented Jan 24, 2014 at 13:27

The cynical answer is "whatever the marketing department at the camera manufacturer decides". This isn't entirely fair - it could be that the newer model (the 100D) has a better JPEG engine, so ISO 25600 becomes "useable" on the 100D whereas it wasn't on the 600D. There's nothing fundamental which limits the gain which can be applied to the sensor output. Of course, if you're shooting in RAW then the differences in JPEG engine are moot.

Note that the above doesn't apply to the low end of the ISO range - that's limited by the number of photons that an individual photosite can collect. While some cameras have "extended ISO" at the low end of the range, that's done by shooting at the lowest base ISO and then underexposing in the image processing pipeline rather than changing the analog gain from the sensor - and this does mean you lose dynamic range.

  • \$\begingroup\$ What does the "JPEG engine" do with ISO? The JPEG encoding is deterministic per the standard, so different processors and software supposedly emit the same JPEG when receiving the same raw input. Do you mean image processing block or pipeline? \$\endgroup\$
    – TFuto
    Commented Jan 23, 2014 at 20:55
  • 1
    \$\begingroup\$ By "JPEG engine" I mean the entire process of converting a the RAW sensor data (capitals deliberate) to a JPEG - white balance, noise reduction, sharpening, everything else. \$\endgroup\$
    – Philip Kendall
    Commented Jan 24, 2014 at 0:11

out of the hardware itself, "same" sensor could have around it several new device (like cpu to treat the info and softaware used into) and that does not alwayas appear in commercial spec.

Changing the capacity of cpu allow a more complexe treatment in same amount of time like noise reduction or HDR creation. Also, the software (firmware in this case) could also be enhance or use other algorythm allowing some modification not possible with earlier version.

so there is lot more modification with the new body and it allow to do some direct thing that are, before, only available via external processing from RAW format.

sensitivity especially in jpeg shoot is a mix between raw info and treatment of this where 1) the amplification of the signal is set at shoot time and 2) post processing occur before writing in jpg. High iso are mainly there to permit to shoot a photo (even with lot of noise) and marketing of manufacturer "set" an acceptable quality to present a higher iso shooter than concurrent but nearly every time the last 2 selection aren't very interesting in most situation (this is always subjective)


They do not supposedly have the same sensor. Go to DXOMark, click the grey "Compare up to 3 items" button, and you will see that there is a bit of difference.

E.g. click on "Measurements", "ISO Sensitivity". Those are measurements against the standard ISO sensitivities.

More importantly "SNR 18%" shows that the 600D is a little bit less noisy than the 100D except at ISO 100.

Dynamic range: 600D is the winner, color sensitivity: at low ISO, there is a match, but higher ISO, the 100D is winning.

The differences are not big though. For me dynamic range and color depth is important, and also noise performance, so I would go with the 600D. Others consider different parameters. These cameras are pretty close. Even the DXOMark scores line up pretty well.

However, I do think they are different sensors.

UPDATE: And just read that the sensor size is a bit different, so I am right :-) (5208 x 3476 vs. 5344 x 3516).


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