Help me please, im a newbie

Started May 3, 2012 | Discussions thread
Forum ProPosts: 26,131
Re: heliocentric heresy!
In reply to jrtrent, May 6, 2012

jrtrent wrote:

Best to use the term "brightness" to refer to the brightness of the photo, and the term "exposure" to refer to the exposure.

They hardly seem separable. When a picture is too dark, it has been underexposed, and when it is too light, it has been overexposed.

Just change the ISO and abracadabra it is no longer 'underexposed'. What you are describing is the behaviour of a film emulation system built into are digital cameras, it is nothing fundamental to exposure.

It's all well and good to say that exposure is strictly a function of scene luminance, t-stop, and shutter speed, but when we want to take a picture of something, scene luminance isn't something we ordinarily control, it is simply presented to us as a working condition (e.g., bright and sunny, dark and overcast, etc.). Sensitivity, however, is something we can control, either through choice of film or ISO setting on a digital camera, hence the oft-referred to "exposure triangle."

You cannot control sensitivity on a digital camera, and the illusion that you can leads people to think they get something for nothing. Of course you are right that you generally cannot control the scene luminance, so if you want to take a photo as good as possible, you need to get as much exposure with that scene luminance which simply means setting the smallest f-number and longest shutter speed you can work with. ISO doesn't help you make that decision at all. Technique based on ISO generally leads to one choosing less exposure, and thus a worse result in limited light, than simply choosing the largest exposure you can.

"Photography is built on the three pillars of exposure: shutter speed, aperture and sensitivity. Shutter and aperture are controls for adjusting how much light comes into the camera. How much light is needed is determined by the sensitivity of the medium used. That was as true for glass plates as it is for film and now digital sensors. Over the years that sensitivity has been expressed in various ways, most recently as ASA and now ISO."

Yup, Nikon's technical writers get it wrong too. Of course, they refer back to film, and with film the speed rating was in some way a sensitivity rating.

Or, as another site put it, the three elements of an image's exposure are ISO, aperture, and shutter speed, "Most importantly – a change in one of the elements will impact the others. This means that you can never really isolate just one of the elements alone but always need to have the others in the back of your mind. . . In many ways it’s a juggling act . . . Keep in mind that changing each element not only impacts the exposure of the image but each one also has an impact upon other aspects of it (ie changing aperture changes depth of field, changing ISO changes the graininess of a shot and changing shutter speed impacts how motion is captured)."

Fairly typical of the confusion you get if you get the basics wrong. It isn't a 'juggling act' - you simply choose the largest exposure that you can subject to your own constraints (or your camera will handle)

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