rmalarz Loc: Tempe, Arizona

While writing another article which contained reference to photographic exposure, a thought occurred to me, one that simplifies understanding the relationship between f-stop, shutter speed, and ISO.

For the purposes of this article, we are going to need some definitions. These are not made up definitions, they are scientific in nature.

Exposure -
Exposure = Intensity x Time

Intensity is how bright, time is how long.

That is pretty simple. We have the luminance of the scene that passes through the lens and the length of time the shutter is open. That’s it. We can regulate the intensity through the use of an iris, or f-stop. Shutter speed is regulated by the shutter speed settings on the camera. Because of the mathematical nature of exposure, these two quantities work together.

These two settings, f-stop and shutter speed, regulate the light passing through the lens. These are located on one side of the camera. On the other side of the camera, usually the back, we have a photo sensitive material.


Sensitivity -
Sensitivity is the degree of response to light, preferably for this discussion within the visible spectrum. This sensitivity is given a rating which is defined by the International Standards Organization (ISO). The greater the numerical value the more sensitive to light the material is. Again, this is pretty simple.


There are a few other related concepts that are important to keep in mind, but we won’t concern ourselves with those, in detail, at present. Those concepts are shutter speed determining how fast the image is captured. If one is trying to capture moving subjects a faster speed is advised. f-stop affects depth of field. The smaller the f-stop the larger the depth of field. Now that we’ve mentioned those, we can put them aside. We are concerned with the relationship of shutter speed and f-stop (exposure), and ISO (sensitivity).

Now, let’s see how the two subjects of our discussion interact with one another. I know we’ve all been exposed to the Exposure Triangle. So, now, let’s forget that we’ve ever seen it.

Again, we are concerned with two items on one side of the camera, and one item on the other. This is the system with which we are going to do some mental exercises.

We are going to imagine this system is a beam balance scale (see below for the illustration). The balance works such that when the weight on one side is equal to the weight on the other, the beam of the scale is horizontal, as shown below.

One the left side, just for a matter of choice, let’s put two weights. One weight is labelled f-stop, the other shutter speed. On the other side of the scale, we are going to put a weight labelled ISO.

The concept is that we have to keep the scale balanced. If we change the amount of weight labelled f-stop, we’re going to have to change the weight labelled shutter speed to maintain that balance. A smaller amount of f-stop requires a larger amount of shutter speed to maintain equilibrium. Conversely, a smaller amount of shutter speed is going to require a larger amount of f-stop.

Now, if we change the ISO side, we have to change either one or the other, or both f-stop and shutter speed to compensate. That is actually how simple the relationship is. It’s not complicated at all.

The additional matters that one needs to keep in mind are, as mentioned earlier, depth of field, stopping motion, etc. This is why taking a photograph becomes more of a system management exercise than just pointing a camera and clicking a shutter. There are trade offs to be made. You, as the photographer, manage these trade offs. And that is the essence of photographic exposure.
--Bob

RichardTaylor Loc: Sydney, Australia

From a tutorial (of mine) written a while ago on a now defunct forum.

Exposure

(Download)

Uuglypher Loc: South Dakota (East River)

Hi,Bob,
I'm glad to see your re-examination of the so-called , and wrongly-called "Exposure Triangle".
As normally drawn, it would rightly be called "The Image brightness Triangle"because, as you astutely point out, only two factors, light intensity (aperture) and light duration(shutter duration) actually control the exposure of film or the semiconductor photon collector (sensor) to light (photons).

The events that transpire before an image is produced are:

exposure of photosites to photons,
conversion of photons, on a one-to-one basis, into electrons,

accumulation of those electrons in the "electron well" of the photosite, and..

then...

under the effect of the sensor's "native ISO" release of the voltage of the current of the electrons in the"electron wells" to produce the image with appropriate brightness at each of the image's "pixels" corresponding to the positions of each photosite"s position on the sensor's surface.

If the ISO had been increased above "native ISO" the current released by every photosite after exposurewould have been amplified to the same degree resulting in an over-all brighter image.

OK...so what about noise?

without complicating matters by discussing the different kinds of noise, let's concentrate on "read noise (luminance noise).

Luminance noise also results from release of "noise electrons" from the Electronnwell. The noise electrons are different and separate from the image electrons, but their numbers are relate to the number of image electrons in this way:

The number of Luminance noise electrons are equal to the square root of the number of image electrons!

So, in the Dark areas of the image, where exposure has been least, let's say a very dark detail was exposed by nine photons, so there would be three noise electrons also released (3 is the square root of 9).

In a bright region of the image, say in a highlight detail, where 10,000 photons exposed the photosite and 10,000 image electrons were released, there would also have been 100 luminance noise electrons released. 100/10,000 is a far smaller proportion of noise than 3/9. Us the brighter the exposure of photosites, the lower the proportion of luminance noise.

So, if one "exposes to the left" to avoid risk of clipped highlight detail, they pay the price of capturing a greater proportion of noise in all regions of the image...from shadows to highlights, than if they assured maximal exposure at a level just lower than that which would have cause clipping of highlight detail by "exposing to the right" where the capture of noise would have been very much reduced.

Note then, that the effect of using a high or low ISO setting has its effect independent of the effect of the level of exposure of the photosites.
The aperture and shutter duration determine "exposure".
The chosen ISO affects image brightness (independent of exposure)

Think of the histogram frame. if all JPEG image data (the histogram) were collected in the left-most (darker) region, the noise in the resulting image would be far greater than if all the image data had been captured in the right-most (brighter) region.

And here's the important part:

if one's camera had, say, one full stop of extra raw-accessible dynamic range (ERADR) off to the right of the JPEG-adjusted histogram frame, and one were careful to use as much of that extra dynamic range as possible, they would capture THE LEAST POSSIBLE NOISE! as well as, by using the maximal amount of the available dynamic range, they would also capture the highest possible tonal spectrum at each and every level of image brightness, from the darkest shadows to the brightest highlight, and thus provide for maximum definition of detail in ALL regions of the image, including even the deepest shadows.

Check out the improvement in shadow detail in Bob's images; the improvement by use of EBTR is striking.

The points made above also explain why one does not practice EBTR simply by fiddling with ISO settings; they have no effect on actual exposure.

The point is, for the best image data quality, strive for maximal exposure of the photosites with photons!

Dave Graham

rmalarz Loc: Tempe, Arizona

Interesting, Richard. Wonderful thing, the internet. We can all collaborate.
Cheers,
Bob

rmalarz Loc: Tempe, Arizona

Excellent points, Dave. Thanks,
Bob

Uuglypher Loc: South Dakota (East River)

Hi, Bob,
I am particularly interested in anything Scotty has to say, given his strange ideas about how ISO operates!

Dave

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

An interesting way to look at it, and any thing that gets me to think about things in a different way is very helpfull. In addition there is an indefinable something that makes a good picture maker better than an average one. I call it- think in terms of pictures. As in "That would make a good photograph" A good photographer will look at everyday, common, objects and think-- Hey that would make a good picture. I can't define it or measure it but it's there never the less.

wilsondl2 Loc: Lincoln, Nebraska

All very interesting but looks to me you are saying the same thing just in a different way. You still need to consider all three to get the exposure you want. - Dave

rmalarz Loc: Tempe, Arizona

boberic, that's why I posted this here. That article I was writing got me to thinking about this in a bit different manner. Thanks for taking the time to read and comment.
--Bob

rmalarz Loc: Tempe, Arizona

Dave, thanks for reading and commenting. Yup, we're dealing with three things here. It was just another way of looking at it.
--Bob

BebuLamar

There are only 2 things. Exposure and Sensitivity.
On the exposure side aperture isn't the only thing that can change the intensity. You can add light to the subject. Subtract light from it via filter.
The shutter speed isn't the only thing that controls the time. When doing flash exposure the flash controls the time. So all of that is one. The exposure. The other one is sensitivity.

rmalarz Loc: Tempe, Arizona

Exposure is composed of intensity and time, as the equation states. So, that's two. The third is Sensitivity.

Adding light affects intensity. Subtracting light affects intensity.

We go back to the scale. If you add intensity, you will need to subtract time to keep the balance. Diminish intensity and you'll need to increase time, again to maintain balance. Time and intensity are two things linked to create exposure. The third factor is the sensitivity of the photosensitive material.
--Bob

Rongnongno Loc: FL

You know for 'normal folks' you can simply set the ISO as fixed the use the A or S mode (Nikon) to achieve that as this is what all the manufacturers do in 1/3 auto mode.

(the 2/3 being ISO on auto mode with A or S is selected). Meaning that the balance is simply upside down...

This does not take anything away from your explanation but just indicates how to exploit it in the real world for those inclined to use an auto mode.

wilsondl2 Loc: Lincoln, Nebraska

If you want to forget the whole thing just do like "The Famous Weegee" did and use a Press #5 flashbulb at f/16 and camera focus at 10' and think only of composition. - Dave

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

Flash bulb? Where do you find them? I thinkn the last one was made in the 60's