This is two-fold. Firstly, it's an invitation for everyone to bend rules. The formulaic can become mundane, and judicious breaking of the rules can bring some excitement or novelty to your images. Secondly it's a comment that some of the rules themselves may well themselves be bent. Some might be so ill-defined they are meaningless, some might have so many exceptions that they're not worth considering a rule at all, some might be just plain wrong from a technical perspective.
There are plenty of expositions on the net why it's good. In summary - don't stick the thing you're interested in right in the centre of the frame, as it looks ugly, and also don't stick things too far off to one side.
Because its not a rule of thirds, it's a rule of typically avoiding the halves, zeroes and ones. There's nothing magical about thirds. Anyone who tells you there is is probably likely to tell you that the golden ratio is the most pleasing aspect ratio for a picture. Both are simply not true. The Mona Lisa's face is a quarter of the way down, not one third. The moon in Ansel Adams' Moonrise, Hernandez, New Mexico, 1941 is pretty much bang in the centre of the image - and the white crosses, deemed so important to the image that he never took a second shot as they'd fallen into shade, are at only about one eighth of the way up the image.
When you're Michelangelo, or when you're Ansel Adams. Or when the symmetry made by what surrounds the object you're putting in the middle has an intrinsic visual aspect that you wish to capture. Symmetry often leads to unease, we don't like things coming straight towards us, or staring straight at us, face centre. Use that tension or unease to enhance your photos; a perfect example would be Almost Closed by Kimhwan Seoulist (another copy, 500px doesn't work for me presenty).
Because it's a good rule of thumb for typical use of a camera. And in fact, many of the situations where it doesn't apply are things you shouldn't be doing.
Several reasons. It assumes that you have a stable stance or pose. Which you should have. However, if you have no choice but to literally shoot from the hip, such as when doing street photography where you don't want people to notice that photographs are being taken, then you will need a much higher shutter speed.
It also is a blanket statement which makes no reference to the resolution of the sensor. Assuming identical focal length and sensor size, If your movements are enough to smear a point light source around 3 pixels at 6000x4000 resolution, then they will only smear that same light source by 1 pixel at 1920x1280 resolution. And if you're taking quick 800x600 snapshots for a webpage, then the smear will be less than half a pixel. So, was the 1/30s exposure with a 50mm lens fast enough or not? It's impossible to say, unless you tell us what resolution you're shooting at.
Finally, it also ignores in-camera or in-lens image stabilisation.
Don't, it's bent enough already. Just learn what it implies, and how it applies. Find your personal, and your camera's, limit - take 5 photos, and see how many you consider devoid of shake. Only call it good if you can get 4 from 5.
In brief, but not necessarily simple, terms, noise is proportional to the square root of the signal. If you can quadruple the signal (exposing at +2 stops), you'll only double the noise, and so upon dialing the exposure back down, the overall relative noise is halved.
There are only two ways of exposing to the right - increase the aperture, or increase the shutter time. Changing the ISO is not exposing to the right, it's just taking the same signal and scaling it up, only for you to later scale it back down again. And when you scale up the signal linearly, you are scaling up the noise linearly too. You've gained nothing apart from having to click a few more sliders in Lightroom. So if you're not increasing the aperture or the exposure time, you're not exposing to the right, you're deluding yourself. However, if you are doing one of those things, then is your ISO as low as it can possibly be? If it isn't, then reduce the ISO, notice you're no longer ETTR, and increase your ISO again. Now notice that you've effectively just achieved ETTR by increasing the ISO from a lower setting, and therefore are just deluding yourself.
And of course, there's always a chance that you're blowing out a few specular centres of the highlights, as the on-screen histogram only tells you so much.
Let's not forget that if you're increasing the size of the aperture, you're reducing your depth of field, so will have less in focus, and if you're increasing the exposure time, then you are more likely to have motion blur or camera shake. So your desire to have less noise will cause less to actually be sharp in the final image. So you might be shooting yourself in the foot.
And of course, it presumes that the sensors are absolutely linear.
A better question is "when to follow it?". To which the answer is when you can do it at the lowest ISO, with a still life, indoors, and probably at no other time. However as a guideline, as long as your histogram is safe, and you can capture the shot you actually want, which might mean not changing the f-stop or the exposure time. Good luck with that.
Another hastily constructed page by Phil Carmody
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