Nex-7 vs EM5 Shutter Speed comparision at ISO 200

Started Apr 3, 2013 | Discussions thread
Detail Man
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Addendum: Nex-7 vs EM5 Shutter Speed comparision at ISO 200
In reply to Detail Man, Apr 4, 2013

Detail Man wrote:

Regarding the different Shutter Speeds (E-M5=30, NEX-7=40) leading to approximately the same result, note that the two cameras have different sRGB JPG tone-curve transfer-functions.

Have a look at the top graph (titled Cameras Compared, Dynamic range comparison) here:

http://www.dpreview.com/reviews/sonynex7/17

The NEX-7 plot already exists (Green, with "DRO Off", as appropriate). Select the E-M5 for one of the other tone-curve transfer-function plot (set to to "Gradation Auto", as appropriate), and select "No Camera" in order to disable the other two plots so just the E-M5 and NEX-7 plots are displayed.

The vertical scale-line labelled "0" shows you where middle grey (sRGB=118) exists. Project a horizontal line from where the NEX-7 tone-curve transfer-function intersects that vertical line. That horizontal line represents a middle grey output level (sRGB=118). Note that the E-M5 tone-curve transfer-function intersects that horizontal line at an input illumination level that is around 2/3 EV lower than is the case with the NEX-7 tone-curve transfer-function.

This would indicate that the E-M5 Shutter Speed might be around 2/3 stop slower in order to produce the same JPG sRGB output levels. In your comparison above, the cameras metered so that the E-M5 Shutter Speed was 1/3 stop slower produced approximately the same levels.

The image-file meta-data of the original crops lists the ISO specification method of the E-M5 to be SOS, and the ISO specification method of the NEX-7 to be REI (which is not constrained to the SOS procedure's use of an output of sRGB=118 for a given scene luminance level).

I looked in the DPReview NEX-7 review for a statement regarding how their own measurement of ISO Sensitivity (using a SOS-related procedure), but found no such statement in the review. The reason for that may well be because Sony uses REI (not SOS) to specify the NEX-7 ISO Sensitivity.

Due to the use of different ISO Sensitivity rating standards used for the two cameras (where REI is quite arbitrary in its requirements), it is not necessarily surprising that the Shutter Speeds that you found differed from the 2/3 stop slower that appears to be expected from the DPReview published JPG tone-curve transfer-functions for the E-M5 (and were 1/3 stop slower, as you found).

Actually, (despite the SOS/REI difference), assuming that DPReview's published JPG tone-curve transfer-functions are fully applicable to the particular shooting settings used on both cameras when recording as they may affect camera metering (the E-M5 was set to "Gradation Auto", and the NEX-7 was set to "DRO Off"), one would (then, in those specific conditions) expect the E-M5 to have a 2/3 stop slower Shutter Speed.

Other relevant factors may be that both cameras were using Spot Metering (might be some differences in what they metered, due to slightly different postions, and/or different sizes of the Spot Metering areas existing), and the T-stops of the two lenses used may differ somewhat.

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