Exposure Basics, lesson three?

Started Mar 18, 2013 | Discussions thread
Najinsky Veteran Member • Posts: 5,739
Re: All at once or nothing at all?

Najinsky wrote:

Exposure has many definitions.

They believe Exposure has a fixed definition.

I agree, it has different meanings in different contexts. But when talking about the effect of exposure on image quality, the definition of exposure is pretty clear and undisputable

Photographers have always known that the amount of light hitting the recording surface is also a definition of exposure and is controlled by the relationship of the Aperture and Shutter speed. But since the 1950's the term used to describe this is 'Exposure Value' or Ev.

Well, that is not fully correct. Exposure value (Ev) is only the combination of shutter speed and f-stop. The amount of light hitting the sensor is the combination of exposure value and scene illumination (and one way to measure that is the Ev(100), the scene brightness that for a given Ev results in ISO-conform exposure of an ISO 100 film).

For film/JPEG/TIFF shooters, the term 'Exposure Triangle' has to include the three components of Aperture, Shutter and ASA/ISO in order that the photographer can control the appearance of their recordings to their liking. This is the correct foundation for beginners where the overwhelming majority of cameras and photographers shoot JPEG.

Yes, the term 'exposure triangle' has a well-defined and undisputed meaning. But when one asks people (eg, you) what is the precise definition of the word exposure that is consistent with the use of the exposure triangle one usually gets a blank stare or some ill-defined waffling.

For raw shooters and for some sensors, they may indeed be no reason to shoot at anything other than the cameras optimum (base?) ISO, save for the fact that the camera JPEG used to review the image will not look correctly exposed.

Which is only a problem for framing/focus/timing/blur evaluation since evaluating noise on the camera display is not that reliable, and which could easily be solved by using Auto-ISO for only the jpeg generation.

But it absolutely should not be the foundation of beginning photography. It is way too technical and removed from the act of picture making for the introductory section of a beginners course.

Yesterday, I proposed a very simple alternative to the world of ISO (and triangles) for jpeg shooters that doesn't require talking about sensitivity but just about image quality and the amount of light.

High Key, Low Key, Under exposed, over exposed are all terms that contrast with a normal (or by convention correct) exposure. That is why the exposure triangle is the foundation, to provide the context to relate these too; closely followed by DOF and motion blur as a consequence of the selected values.

Maybe I am one of the smart people you criticised at the beginning but I always found the exposure triangle to be a way of explaining something dead simple in a way too complicated manner. I mean, what is so hard about learning that one f-stop doubles or halves the light and that doubling (or halving) the ISO value, allows for a shutter speed to be set one stop shorter (or one stop longer)? Nobody needs a triangle to understand or apply that, at most you need your ten fingers to count the number of stops.

Perhaps you are too smart to understand the simplicity of the use of the exposure triangle. It happens.

My course started with introductions; why they were here, where they are in their photographic journey, what camera they are using and how they shoot it, and what they hoped to gain from the experience.

The words 'my photos are really bad' invariably come up.

So then I show some slides of bad photos, heads chopped off, tilted horizons, under-exposed, over-exposed, out of focus, grainy night scenes, various blurs, washed out flash, and so on. Then I show them some nice photos. Then I explain that to reduce the number of bad photos and get more good photos, we need to understand what happens in the camera when we press the shutter to capture a photo. By understanding what the camera is trying to do and why, we can start to take control of process to ensure the errors don't happen, and we can start getting the shots we really want. I also explain that there's a lot more to it than the camera, like seeing and choosing nice compositions, seeing the right moment and the role of the lighting, but they come later and we'll start with just getting the operation of the camera under control and build from there.

I explain that there are two fundamental controls we have understand, the Aperture and the shutter speed, and also a camera setting that it helps to have an understanding of, especially if they like taking photos at night but find the shots turn out like crãp.

Then I show the exposure triangle. I explain that Aperture is the size of the hole that restricts how much light travels through the lens. I explain that the shutter goes back to the days of film, where the film is sat in the camera, waiting to capture the light that will hit it when we open the shutter. When we're ready to take the shot, we hit the shutter release, the shutter opens, exposes the film to the light and then closes again. I explain that the shutter speed controls how long the shutter opens for, and therefore also controls how much light the film got exposed to. I use an analogy of a water tap (faucet to our cousins). If we turn the tap fully on we create a wider opening (aperture) and more water flows through. If we run the tap for 10 seconds, we get more water than if we run it for 5 seconds, like the shutter speed being open for longer lets in more light. Later, I extend this analogy to include the water pressure as an analogy for the strength of the light source. Everyone is familiar with opening a tap and the water coming gushing out and splashing everything, as well as opening the tap to be greeted by a slow trickle. I relate these to a bright sunny day with loads of light, and sunset and night time where the light fades and it becomes dark. But before I do that, I complete the shutter speed value by explaining that with digital cameras, there is no film. Instead it uses an electronic imaging sensor that can measure and record the light hitting it, and the camera uses this recording to make the image. I explain that some cameras, especially camera-phones and some compacts, don't have a mechanical shutter at all, the recording can simply be turned on and then off to create the recording duration.

So at this point, I have related the two controls of exposure.

The class are requested to bring their cameras with them, so I now get them to look at the numbers on the lens. I explain one of the numbers is a measure that describes how wide the Aperture can open and therefore constrains how much of the available light can flow through their lens. This brings in the concept of available light (the water pressure analogy) and the question how much light do we actually need to capture?

Here I give a very brief introduction to the concept of a meter (very brief and very basic) just to introduce the idea that there is a specific amount of light that we want to capture to enable a nice clear recording. I explain that when more light is coming through the lens, we only need to open the shutter for a shorter period, but when less light comes in, we need to open the shutter for longer. This introduces the effect of subject motion blur, and the idea that as well as controlling the exposure time, the shutter speed also plays a role in freezing the scene to prevent motion blur. This plants the seed that the controls for exposure also impact other characteristics of the captured image.

I then pose the question, if we don't have a lot of light to begin with, like at sunset or night, and the maximum lens Aperture further constrains that light, leaving us with a really slow shutter speed that makes the images all shaky and blurry, what can we do?

The common answers that come back at this stage are; use a tripod, use a flash, use the camera's night scene mode and occasionally, use a lens/camera with a bigger Aperture (that last one almost brings a rear to my eye). I explain how they all have their place, but some limits too. Sometimes, flash photography is prohibited and can give harsh ugly light. A tripod helps combat camera motion, but not the subjects who are moving or dancing. I explain that cameras/lenses with a bigger aperture are indeed bought for just this purpose but even then may still encounter the limits, but hopefully less frequently. Then I turn my attention to the answer 'use the camera's night scene mode'.

I talk about the 'Action' scene mode (get them to take a photo with various scene modes and ask them to listen to the sound the shutter makes. I also have my own camera so I can then let them hear a fast and a slow shutter speed). I explain the action mode gives priority to a fast shutter speed to help freeze the action. But for night scene, where we don't have enough light for a fast shutter speed, how does the camera compensate?

Then I refer back to the exposure triangle and start to talk about the ISO setting. I explain that while Aperture and Shutter Speed control the amount of available light to hit the recording surface (and the term exposure may get used) the ISO is a setting that helps the camera compensate for when we just can't capture enough light using the available aperture and required shutter speed. I talk about the days of film, and show some old Olympic photos capturing action in grainy black and white. I mention that they used 'high speed' film which is more sensitive to light and therefore allowed them to get by with the available light/aperture and required shutter speed. I also explain how the picture is very grainy and this a consequence of having used less light.

Then I come back to digital and talk about the sensor. I explain that it's sensitivity to light doesn't change as was the case with different film speeds, but it tries to simulate this by boosting the recording to make it look like there was more light captured than there really was, to prevent the image from looking under-exposed. However, like with fast films, there is a consequence to using this. When boosting the recording, random artefacts, called noise, which occurs naturally with nearly all types of recording, also gets boosted and this becomes visible in your images. The more the recording is boosted, the more visible noise you will see in the image. The ISO setting controls how much of a boost the recording gets. It can be useful when there is no other option and you only have a small amount of light to play with, but under most circumstances, you'll want to try to keep the ISO at its optimal setting to produce the clearest recording. I then explain that there is a little more going on here than given in the simple explanation, and that there's a few later sections of the course where some of this will be re-visited (e.g.: nose reduction).

So then I recap, Apeture and Shutter Speed control the amount of light falling on the recording surface, ISO is a setting that controls how the camera responds to that light. And that's the introduction done with (well, there's more but less relevant to this discussion).

So why have I done this. Taken time to write all this down like this?

It's three fold, first; I don't like the accusation that the teaching is damaging, based on peoples false assumptions about how the triangle is used. Hopefully it's clear from the above that the interests of Exposure are being fairly represented, in the context of a beginners course.

Second; all these areas get referred to at various stages of the 'discussion' and it makes more sense to write them once, in context, in the same place.

Third, I'm hoping to help those more technical minded people understand the intentions and the role of the exposure triangle.

The exposure triangle is no more an attempt at a precise technical definition of exposure than the Bermuda Triangle is an attempt to be a precise nautical chart/map of Bermuda. It is a graphical communication aid showing how three things have a connection in some context.

As hopefully demonstrated above, it provides plenty of scope for presenting the light gathering of Aperture/Shutter speed relationship in tact, while then relating it to a setting that is still (perhaps unfortunately) relevant to 99.99% of beginners (who are attending a course to learn both about their camera and photography). It is down to the instructor to ensure the correct meanings are conveyed. Presenting a single graphic with only three elements is a comfort to a beginner because it helps convey confidence that it's not as complex as they may have feared.

Graphical communication aids are used in all walks of life; from team tactics discussion for football games, to news broadcasts explaining the context of the latest conflict. The graphics seek to convey a message, not to be a definitive and authoritative guide.


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