The ins and outs of "special" (extended, LOW) ISOs on MFT bodies

Started Feb 3, 2014 | Discussions thread
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Anders W Forum Pro • Posts: 21,465
The ins and outs of "special" (extended, LOW) ISOs on MFT bodies

The ins and outs of "special" (extended, LOW) ISOs on MFT bodies

Recent MFT bodies from Olympus as well as Panasonic come with ISOs that are "special" in one regard or another. On the Olympus side, this is true about the E-P5, the E-M1, and now also the E-M5 if equipped with the new firmware (version 2.0) announced the other day. On the Panasonic side, it is true about the GH3, the GX7, and the GM1. Olympus calls the special ISOs "LOW" whereas Panasonic refers to them as "extended". One reason for this difference in terminology is that on Olympus bodies, these special ISOs are available only below the lowest normal ISO whereas on the Panasonic body on which extended ISOs were first introduced, the GH3, they are available above as well as below the normal range.

Since these special ISOs tend to raise a lot of questions (as exemplified by the recent discussion, for example here, about the new ISO LOW on the E-M5) and since these questions are usually not sufficiently well answered by the documentation provided by the manufacturers, I thought it would be a good idea to summarize the answers some of us have come up with when trying to find out what is actually going on.

Real and fake ISOs

To set the stage for that summary, it is a good idea to begin by considering what normally happens when you switch from one ISO to another, say from ISO 400 to ISO 200 (the lowest normal ISO on all the bodies mentioned above). These changes are twofold. First, the way the body meters is changed so as to suggest more exposure for any given light level. If, for example, the meter suggested an exposure of f/4 and 1/200 s at ISO 400, it would instead suggest an exposure of f/4 and 1/100 s at ISO 200. Second, the body reduces the gain (or amplification) applied when the signals (the electric charges) stored on the sensor after exposure are transformed into RAW data values. For example, a signal that would have resulted in a value of 200 at ISO 400 instead results in a value of 100 at ISO 200. If a shot at ISO 200 is given one stop more exposure than ISO 400, as suggested by the meter, the on-sensor signal is doubled, which in turn means that the value stored in the RAW file will be 200 in both cases.

When only the first of these two changes (the metering) takes place, not the second (the RAW-level gain), the ISO in question is commonly referred to as a "fake" as opposed to a "real" ISO. The reason for the appearance of that terminology is of course that nothing of importance has changed from a RAW-shooter's point of view. While the meter works differently when you switch to a fake issue, that doesn't really change anything as long as you are aware of what is going on. As a photographer, you rather than the camera decide (or at least should decide) the exposure and the meter is but an adviser. The adviser will change the advice by one stop and you can adjust for that as you see fit.

When the gain in RAW does not change in the manner described above but the metering does, another modification is instead applied to have the jpegs come out at the expected brightness for those who continue to rely on the meter's advice. The omitted modification of the RAW-level gain is replaced by a corresponding modification in the conversion of the RAW data to a jpeg image. The latter modification is referred to as a change of tone curve. The tone curve is shifted up or down to adjust the overall brightness of the jpeg. In addition to that, the shape of the tone curve is often adjusted a bit to account for the fact that the clipping point of the sensor (if you expose according to the meter) has moved up or down.

With these basic facts in mind, let us now consider what is going on with the individual Olympus and Panasonic bodies we are interested in.

ISO LOW on the Olympus E-P5 and E-M1

When you change from ISO 200 to ISO LOW on the E-P5 or E-M1, the RAW-level gain remains unchanged. Metering is adjusted to ISO 100. The tone curve is shifted and its shape changed. The change in the shape of the tone curve is described here for the E-P5 and here for the E-M1 (see bottom of page in both cases).

The implication is that for a RAW shooter, ISO LOW is the same as ISO 200. The only reason a RAW shooter might have to prefer ISO LOW to ISO 200 for a certain shot is that it reduces the brightness (with exposure held constant) of the OOC jpeg available for in-camera review, which may sometimes (but not always) be an advantage. For those who shoot OOC jpegs and follow the meter's advice with regard to exposure, ISO LOW might be preferable to ISO 200 if the light level permits additional exposure and the scene is such that there is little risk of highlight clipping or if such clipping is of little importance.

ISO LOW on the Olympus E-M5

When firmware 2.0 appeared, and an ISO LOW setting introduced on the E-M5 too, nearly everyone (myself included) initially believed that it would behave in exactly the same way as that on the E-P5 and E-M1. Surprisingly, however, such did not turn out to be the case. The small but critical difference compared to the E-P5 and E-M1 is that the RAW-level gain is not kept unchanged at ISO LOW relative to ISO 200. Instead, it is increased by 1/3 EV so that from a RAW-shooter's point of view, ISO LOW is effectively the same as ISO 250. Exactly why Olympus chose to increase the RAW-level gain rather than keep it constant remains unclear.

The implication is that for a RAW shooter, ISO LOW is not quite the same as ISO 200. As long as you can afford to give the camera 1/3 EV more exposure, ISO 200 is preferable in terms of SNR and DR. In other regards, the situation is largely the same as for the E-P5 and E-M1 but with the risk of highlight clipping when exposing according to the meter increased by another 1/3 EV when using ISO LOW.

Low extended ISOs on the Panasonic GH3, GX7, and GM1

These three bodies all have an extended ISO of 125 (and 160) below the minimum normal ISO of 200. Unlike ISO LOW on Olympus bodies, these ISOs are in fact perfectly real. In other words, the RAW-level gain is changed in the expected way (downwards) as you go from ISO 200 to ISO 125. Since that is the case, and since ISO 125 provides higher SNR and DR in RAW than ISO 200 (per DxOMark's measurement and provided that you give the sensor the additional 2/3 EV exposure that ISO 125 allows before it clips), it remains an unsolved mystery why Panasonic did not extend the normal range of ISOs all the way down to 125.

Another unsolved mystery is that Panasonic decided to change the tone curve for ISO 125 in comparison with ISO 200 in spite of the fact that the real nature of ISO 125 would have allowed them to keep the tone curve the same. As you can see here here for the GH3, here for the GX7, and here for the GM1, ISO 125 has a steeper tone curve that makes the OOC jpegs clip earlier (if you expose according to the meter) at ISO 125 than at ISO 200. At ISO 125, the OOC jpegs additionally clip significantly before the point at which clipping begins in the RAW data.

Note that DPR is not correct when, in some of the reviews to which I link, they suggest that the RAW-level gain at ISO 125 is the same as that at ISO 200. While that is what one might have guessed, that's not how it turns out to be.

As to the practical implications, ISO 125 is preferable to ISO 200 from a RAW-shooter's point of you whenever you are in a position to give the former the 2/3 EV extra exposure that it requires in order to saturate the sensor. In order for ISO 125 to make sense for an OOC jpeg shooter, it is additionally required, just as for the Olympus bodies, that the scene is such that there is little risk of highlight clipping or such clipping is of little importance. The latter requirement could have been omitted if Panasonic had chosen to keep the tone curve the same at ISO 125 as at ISO 200. Possibly, the tone-curve adjustments available via the i.Dynamic feature of recent Panasonic bodies can be used to alleviate this problem.

What about high ISOs?

Of the bodies mentioned above, the GH3 is a bit special in that it has extended ISOs not only below but also above the normal range, which ends at ISO 12800 on the GH3. The extended ISOs are in this case fake. The same, however, is in fact true about some ISOs within the normal range as well, i.e. those above ISO 6400. The implication from a RAW-shooter's point of view is that there is no reason to crank up ISO beyond ISO 6400 but no real harm done if you nevertheless do it inasmuch as the clipping point stays unchanged.

On the other five cameras, I would think (without knowing for sure in all cases) that all ISOs toward the top of the range remain real in the sense that the RAW data values are adjusted upwards as the ISO is increased. However, I would also think (without knowing for sure in all cases), that from a certain point onwards, this is accomplished by means of digital scaling rather than by analog gain. On the E-M5, ISOs above 3200 are digitally scaled, and on the GX7 those above ISO 6400. It may well be the case that the E-P5 and the E-M1 resemble the E-M5 in this respect and that the GM1 follows in the footsteps of the GX7 (although, again, I have not taken the trouble to check).

From a RAW-shooter's point of view, there is no more reason to use these digitally scaled higher ISOs than to use their fake counterparts on the GH3. However, and in contrast to the GH3, there are in this case also reasons to actively avoid them inasmuch as they reduce the "headroom" left before the highlights clip without providing any benefit in exchange for that disadvantage.

For OOC jpeg shooters, the fake and digitally scaled ISOs at the top of the range remain as viable as any other ISO. They are simply required in order to reach the appropriate brightness if you expose according to the meter.

Comments and questions are of course welcome.

 Anders W's gear list:Anders W's gear list
Panasonic Lumix DMC-G1 Olympus OM-D E-M5 Olympus E-M1 Panasonic Lumix G Vario 14-45mm F3.5-5.6 ASPH OIS Panasonic Lumix G Vario 7-14mm F4 ASPH +28 more
Olympus E-M1 Olympus PEN E-P5 Panasonic Lumix DMC-GH3 Panasonic Lumix DMC-GM1 Panasonic Lumix DMC-GX7
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