23rd May 2012, 05:27 PM
ETTR on a D90
I'm trying to work on my technique with the D90, and ETTR seems to be a little tricky.
With my old Oly I could go into spot metering and it would tell me what EV it was picking up (it had a meter and numerical feedback), so I could use that and measure from my highlights to know exactly what was going to clip in my raw file by memorizing what EV it still had good details at, but the D90 seems to be a little different.
First off, the meter only shows two stops in either direction, and it seems that I can get a fair bit back when the meter reads more than two stops over, and when the histogram shows some clipping, so I haven't really found a good, definitive way to know what is too much.
Any advice is appreciated!
24th May 2012, 06:59 AM
Re: ETTR on a D90
Assuming that you're shooting RAW ...
Spot-metering returns a value that will turn the spot-metered area into a middle gray; so if it's a highlight, you then need to increase the exposure by exactly 2 stops (assuming that it was the brightest portion of the scene that you were metering).
ETTR is mis-understood by most though; in reality, you only need to do it for scenes that have a high dynamic range. For a normal reflective scene the dynamic range will only be about 4 stops anyway - most cameras will capture around 12 so even with a 2 stop safety margin you're still about another 6 stops above the noise floor (so noise won't be a problem). The problem with ETTR is that sensor response isn't linear as you approach the saturation / clipping point - so you're likely to find that you end up getting a weird colourshift in 1 channel (depending on the colour temperature of the scene) that's VERY difficult to null in post-processing (a simple white balance won't work).
My advice is to avoid ETTR unless you have a high dynamic range scene (eg something with backlighting, but where you still want to protect / reveal foreground shadow detail). Many will "wax eloquent" about how ETTR captures the most information, but it's more theory than practice because (a) normal reflective scenes are so far inside the safety margin that visibly you won't see any difference, and (b) there's a very real downside that they don't tell you (or probably even know about).
24th May 2012, 07:14 AM
Re: ETTR on a D90
I'm really only concerned with adding another technique to my arsenal to help keep noise down in high ISO situations.
Playing around today I did see what you meant about strong colour shifting, it's pretty interesting.
24th May 2012, 10:00 AM
Re: ETTR on a D90
This might be of interest: "What's up with the histogram?"
25th May 2012, 07:01 AM
Re: ETTR on a D90
Your D90 has built-in ETTR, so I would suggest using that if conditions warrant. I'll explain...
The purpose of the technique dubbed ETTR is to increase exposure in order to increase signal and reduce noise. If you're not increasing exposure beyond what is necessary, then you're not performing ETTR.
ETTR is only valid at base ISO. If you're at ISO 800 and overexpose by two stops, then you get the same exposure as standard exposure at ISO 200. So there's no gain. It's been pointing out that certain cameras may contribute less processing noise at ISO 800 than at ISO 200. However, I've yet to see a valid example of visible improvement in the finished product. In any case, the D90 isn't such a camera. Therefore, I'll stick with the assertion that ETTR is only valid at base ISO.
The D90 has an expanded ISO range of "L1.0". When you use the expanded ISO range you are effectively performing ETTR. When you increase exposure by one stop, you lose one stop of highlight DR. That's exactly what happens when you shoot at L1.0. The only difference is that you don't have to fix the exposure yourself in post processing, and that helps with image review during shooting. So I say that if you want to use ETTR on the D90, just shoot at "L1.0".
I use L1.0 for portraits shots with my D90, as it reduces noise in the shadow area, and is especially noticeable in areas with blur. L1.0 is perfect for indoor portraits because you're in control of the lighting and you can limit the DR of the highlight range to prevent blowing highlights.
Metering on the D90 is fairly stable. Spot and CW meter 12.7% gray, which equals RGB values of 100 on the sRGB space. So if you spot meter a white, evenly lit wall and shoot it at f/11, you'll get a gray image where all the pixels are RGB 100, +/-5. You can get variations as much as +/-10 or more, depending on the lens and aperture and metering mode. Spot will get it correct in the center and the edges will be off due to fall off. CW will be better balanced with over and under exposures across the frame. Matrix is generally unpredictable (which is part of its design.)
For some reason, Nikon's default image processing tends to boost exposure too far. For example, a shot taken as described above will show RGB values of 100 in a third party RAW editor, but RGB values of 140 in ViewNX. This is part of the reason why JPEG shooters will apply -1/3 or even -2/3 of Exposure Compensation to all their images. However, if you open the image using LightRoom with a "zeroed" profile, or the free editor Raw Therapee with a "neutral" profile, you'll see the true values of the pixels. I usually use Raw Therapee for my image processing. Sometimes I'll finish in Photoshop Elements if I need to perform more complex editing, such as masking with layers.
On the D90 you get almost three stops of highlight space above 12.7% gray. So you can spot meter the brightest white highlight, increase exposure by 2.7 EV, and be pretty sure that you've got the largest exposure possible without clipping the highlights. Of course, if you're using L1.0 then make that 1.7 EV.
The viewfinder's Exposure Display has a range of +/-2 stops when using 1/3 EV steps. Switching to 1/2 EV steps will give you a range of +/-3 EV. I shoot in auto modes only, so I would simply apply an Exposure Compensation of +2.7. But you can do the same thing in manual mode. You can apply +2.7 of EC, and then center the Exposure Indicator in the Exposure Display. You basically use EC for the major portion of the correction, and then use the Exposure Display in the viewfinder to make minor adjustments of +/- 1/3 or 2/3 EV.