tonybeach_1961

Optimal Data — Understanding Exposure and Noise | DSLRBodies | Thom Hogan To quote the relevant passages: Three things determine “exposure”: The brightness of the scene being photographed The aperture used The shutter speed used Later in the article Thom writes: ISO is not part of the exposure. Rather, it’s basically a form of taking a dim exposure and amplifying the data in that exposure so that it is “brighter.” Instead of using a slider in Lightroom or another program to do that, you’re telling the camera to do the same thing to the original data coming off the sensor. I won’t get into the reasons why we’d rather have the camera do that in some (most?) situations rather than using post processing software, but changing ISO does not change your exposure. The same amount of light still hits the sensor no matter what ISO you set. The sensor doesn’t get “more sensitive.” Finally, Thom writes: Note that exposure compensation does change the exposure ;~). That’s because it alters either the aperture or the shutter speed value, and that will let more or less light get to the sensor. Personally, I arrived at the same conclusions the hard way, by trial and error. My epiphany came years ago as I was testing my D70 and D200 for noise at various ISO settings and I switched from testing with incandescent lighting to using a Speedlight. The same ISO had strikingly different noise levels, which lead me to understanding the importance of the frequency of the lighting as it relates to the exposure of the individual channels (so for instance, one camera would have a better red channel response than another, and that noise would propagate into the de-mosaiced color -- e.g., noisier skies, dark tones, etcetera). Understanding that ISO is not part of exposure helps to get a better grasp of how and why to practice ETTR. When you muddy exposure with ISO you have to consciously untangle them. As I wrote above, the steps are that you first determine the optimal exposure and then you determine the optimal ISO -- and with some cameras you can actually arrive at a better result by decreasing the exposure slightly and correspondingly raising the ISO slightly. If ISO is misunderstood as being a part of exposure (as opposed to being part of the exposure process, which also includes metering) then it is difficult to grasp that lowering and raising the ISO setting on your camera can have counterintuitive results.

Exposure is just one part of the equation. The light in the scene is another part, as is the sensitivity of the sensor, as is the ISO applied, as is the calibration of the monitor or the lighting used to view a print. Conflating any of those other things with exposure just makes a mess of an otherwise clearly defined thing. You start with the best possible exposure (which is contingent on various constraints) of the scene/subject. Then you apply the best ISO for that exposure, one that doesn't blow desired highlights and raises detail to a desirable level. Then when you process the resulting file into a photograph you try to attain the best image you can given the intended viewing medium and distance. Putting exposure with ISO skips a vital step in the process and often leads to either a less than optimal exposure or using a less than optimal ISO.

Speaking of exposure, ISO is not a part of that equation, so you have a "good" exposure when you have optimally exposed the image while retaining a desired aperture and shutter speed (i.e., the only two parameters available to you in the camera to control exposure are those two). There is a difference between parameters and controls. There are a number of ways to effect metering which will change your exposure, but to say ISO is one of those is putting the cart in front of the horse. You can change the exposure by changing the EC, and if you are using Auto-ISO the ISO will change accordingly to compensate for the change in exposure. The point I'm trying to make is that if you are shooting at f/5.6, 1/2000s and ISO 400 it isn't the same exposure as shooting at f/5.6, 1/4000s and ISO 800; the latter is half the exposure with double the ISO applied to it.

I obviously disagree, so when I wrote, "...what is ignored in those comparisons is that they are all taken using correspondingly different exposures." I'm not disputing that. However, where in the article is that stated? Perhaps I missed it or it's assumed based on the two previous articles I didn't bother to read. Regarding, "Why should a beginner care?" What happens when you run out of room to change the exposure at what you consider a desirable ISO and are faced with either holding the line at a lower ISO and pushing the Raw file during conversion, or biting the bullet and raising the ISO? You will be taking the wrong approach if you have the same camera as the article's author (Sutton) and you believe that "...the higher the ISO, the more sensitive your sensor is to light, but at the cost of more noise." As can be discerned from the chart I offered, raising the ISO can actually result in less noise, and while that can be counter-intuitive it is useful to teach a beginner why that can be the case.

When using Auto-ISO with Manual Exposure mode I watch the ISO displayed in my viewfinder. I have dialed down to zero (as stated in my initial reply to this thread), and that usually affords me some headroom in the Raw files, so when I hit ISO 100 I still have a little room to work with. As for the ISO going "too high," that's just another constraint along with not having enough ability to use a wider aperture or a slower shutter speed. I sometimes do use your approach, but I generally prefer mine. I don't see myself flirting with "disaster" when taking my approach and don't see how taking your approach would be any less of an issue since you end up with a shutter speed that's too slow versus when the light is too low; OTOH, when there's too much light for the selected aperture and the base ISO of your camera then having faster shutter speeds can afford you more room to avoid overexposing.

I have some issues with this article. First and foremost is the proposition that changing ISO changes exposure, it does not. Related to that is that ISO changes the sensitivity of the camera's sensor, and technically it does not. All this leads the the bad conclusion "...if you can shoot at a lower ISO, you should" offered at the end of the article, and that's wrong too. The exposure is the amount of light that hits the sensor, and the ISO is what is done with the exposure after it has been taken. The article offers examples of the "effect" of using higher ISO settings on a Canon 5D Mark IV camera, but what is ignored in those comparisons is that they are all taken using correspondingly different exposures. In point of fact, if you keep the exposure constant and raise the ISO as high as you can, or to put it the other way around if you don't do that and instead lower the ISO using the same exposure, then generally the higher ISO will offer a better result than the lower ISO (exactly the opposite of the advice offered in the article). To illustrate what is wrong with the conclusion offered in Sutton's article, take a look at this chart using the camera he used: Photographic Dynamic Range versus ISO Setting Now go to any ISO higher than 200 and hover your cursor offer it to see what the "PDR" is for that ISO setting. For example, for ISO 200 that is 10.36, then go one stop down and look at the next ISO setting, in this instance that would be ISO 400 which is 9.74; if you just underexpose by a stop then the difference in PDR should be one stop, which in this instance would be going from 10.36 to 9.36, whereas 9.74 is actually just over a third of a stop better than that. Ergo, if highlight retention allows it, you should use a higher ISO rather than a lower ISO. Why should a beginner care? It's a classic example of GIGO. Teaching a flawed premise at the beginning will result in flawed results later on when the beginner starts applying in practice the principle (in this instance, choosing the lowest ISO they can get away with because "...if you can shoot at a lower ISO, you should").

I believe you meant Manual Exposure mode because the Metering Mode would typically be Spot, Center-weighted, or Evaluative (or Matrix) with other options available on many cameras. This is important because the metering mode is generally a vital part of attaining a good exposure. Speaking of exposure, ISO is not a part of that equation, so you have a "good" exposure when you have optimally exposed the image while retaining a desired aperture and shutter speed (i.e., the only two parameters available to you in the camera to control exposure are those two). Since I practically always shoot Raw format, and I often use Auto-ISO on my Nikon cameras (currently a D800), I do have a few thoughts on my approach to how to best utilize that feature. Yes, I generally start out in Manual Exposure mode, and I use Center-weighted metering (set to "Average" on my Nikon cameras), and I set EC (exposure compensation) to zero or sometimes an appropriate negative value to preserve highlights. To put that last setting into context, Auto-ISO is my preferred approach when coming off of base ISO where optimal exposure is a given, and when I am using Center-weighted metering on my D800 (and most other cameras I have used) my default setting for EC is +.7 and sometimes that even goes to +2 though usually a stop from default is close to ideal. One last point about "...the lower the ISO the better rule." As I stated above, it's "the more the exposure the better rule" and ISO is the means we use to compensate for a less than ideal exposure.

To say a Raw file has a look is the same as saying a musical score has a sound.