How to EFTTR (expose further to the right), part 2

Started Mar 22, 2013 | Discussions thread
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Anders W
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How to EFTTR (expose further to the right), part 2
Mar 22, 2013

What can you do to maximize the image quality your camera can give you, i.e., maximize things like SNR (signal-to-noise ratio) and DR (dynamic range) without the loss of detail implied by noise reduction?

If light is abundant relative to your f-stop and shutter-speed needs and you are a RAW shooter, one answer to that question is to set your camera to base ISO and then try to ETTR (expose to the right). By ETTR I mean an exposure such that the brightest highlights that you care to preserve are just at the clipping point of the sensor but not beyond.

In some cases, when the subject is perfectly static, it is possible to reach beyond the quality that a single ETTR exposure can give you by taking multiple shots (at different levels of exposure if possible) and then merging and aligning them into a single image. In some cases, when the DR of the scene is very high and the subject not static, it might also be preferable to expose beyond ETTR, that is, sacrifice some of the highlights that you would ideally like to preserve in order to keep shadow noise at bay.

But is there something else you can do to maximize image quality when light is abundant for your base-ISO ETTR needs and the subject is non-static or you simply don't want to take the trouble to merge and align multiple shots?

Well, as seasoned ETTR shooters already know, the ETTR criterion in practice implies that you expose the green channel up to the clipping point. When that happens, the red and blue channels are typically only about halfway toward the clipping point. So one conceivable answer to my question would be to find a way to expose not only the green channel but also the red and blue channels up to full saturation.

How can we do that? About half a year ago, it struck me that a magenta filter might do the trick. Such a filter blocks green so that more light can be given to the red and blue channels without having the green channel clip. So I found a used Tiffen CC30M filter on ebay in the filter size I am trying to standardize on (55 mm) and started to experiment.

Did it work? Yes and no. As shown in this prior thread about the subject

http://forums.dpreview.com/forums/post/42322633

the filter had the right effect in the sense of allowing me to increase the strength of the red and blue channels without green-channel clipping. However, the effect was too small. The increase in the red and blue channels was only about 1/3 EV rather than the roughly one EV there was room for. After a bit of searching, I managed to find the strongest magenta filter available, at least in the form a normal screw-in filter, a Formatt CC70M (no 55 mm unfortunately so I had to settle for 58 mm) and ordered one at B&H (who sells it for a better price, USD 67.50 plus tax, than the manufacturer in UK, which charges GBP 95).

By combining the CC30M I already had with the new CC70M I had a total filter strength of CC100M and, as expected, this did suffice to bring the red and blue channel up to parity or nearly so with green in ordinary daylight.

So does it help improve image quality and if so by how much? Well see for yourself by inspecting the test images shot with my E-M5 below. As a first stab, I simply took the highest DR scene I happened to have close by and exposed it according to the ETTR criterion. I should mention here that the point of these test images is neither to show a realistic application nor to show that the filter moves us across some particular threshold between bad and good. For example, the scene is perfectly static so I could have done better by merging and aligning multiple exposures and would have sacrificed some of the highlights if I wanted the best compromise in a single shot. But the images let us have a look at how much of a difference the filter makes and that's good enough for me.

The first of the three images shows what the scene looks like when I first open it in LR 4.3 with everything at default. The second and third shows 100 percent crops from the shot without filter and with filter after the following adjustments aside from WB:

Exposure 1.79 without filter, 1.67 with (I used this as the final parameter to equalize the brightness)

Highlights -100

Shadows +100

Whites +69

Lateral CA corrected

As you can see, I gave the shot with filter 2.3 EV more exposure, so shooting with the filter effectively brings base ISO on the E-M5 down from 107 (in DxO terms) to 25. It's a bit akin to shooting with good ol' Kodachrome 25, except for some important facts: With MFT I have two stops more DoF at the same f-stop, primes that work fine wide open, and IBIS. On top of that one wouldn't have strong reasons to use the filter unless there is strong contrast and thus at least some significant light to be kept below the clipping point. In practice this means that the filter should fit the bill pretty frequently.

Some channel statistics for the two shots I present (average ADU level with number of pixels clipped in parentheses):

Without filter: R 27.9 (0), G 58.4 (1k), B 34.9 (0), G2 58.5 (1k)

With filter: R 51.5 (441), G 59.7 (1k), B 60.0 (3k), G2 59.9 (1k)

Now the test shots. Please click on "view original size" to see them properly. See any difference between the crop with and the one without filter? If so which?

Shot without filter, LR 4.3 defaults

Shot without filter, PP as described above

Shot with filter, PP as described above

 Anders W's gear list:Anders W's gear list
Panasonic Lumix DMC-G1 Olympus OM-D E-M5 Panasonic Lumix G Vario 14-45mm F3.5-5.6 ASPH OIS Panasonic Lumix G Vario 7-14mm F4 ASPH Panasonic Lumix G 20mm F1.7 ASPH +18 more
Panasonic Lumix DMC-G2
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