Proper Exposure Analogy

[bsharpe411]

<p>Recently I went to a local fast food restaurant and as part of my order, requested a soft drink. The young girl placed a medium-sized cup under the drink dispenser; pressed the button for that size; and then walked away to complete my order. When she returned, the cup was filled to the top with no overspill. As I sat there eating my lunch, it dawned on me that the drink dispenser and a camera had a lot in common in achieving the same basic objective.</p>

<p>Filling a cup with liquid or ‘filling’ a piece of film with light is basically the same thing. The variables are:</p>

<p><strong>LIQUID ------------------------------------------------ FILM</strong><br>

<strong></strong><br>

<strong></strong><br>

Cup size -------------------------------------- ISO speed</p>

<p>Water pressure -------------------------------------- Light intensity</p>

<p>Pipe size ----------------------------------------- F-stop opening</p>

<p>Time valve is open ------------------------------------Shutter speed</p>

<p>The reason I’m using the automated drink dispenser analogy is because anybody can fill a cup properly while watching it fill. You can’t watch light ‘fill’ a piece of film to the proper level (proper exposure). If any one of the 4 variables in filling the cup changes, the one or more of the other 3 variables must change to compensate. In most instances, the only normal change is cup size and the normal compensation is time. But what if the water pressure suddenly dropped or the size of the pipe changed? Some compensation would have to be done.</p>

<p>With the typical SLR in program mode, the user chooses the film ISO (cup size). The meter measures the light intensity (water pressure) and then the program selects an appropriate f-stop (pipe size) and shutter speed (valve open time). Is there only one right combination to select? Of course not. In full program mode-the camera decides which combination it wants. If you want the cup filled faster or slower, then select the time and let the machine choose the appropriate pipe size (shutter priority automation). If you would rather control pipe size…perhaps to avoid over-fizzing or splashing, then select pipe size and let the machine choose the appropriate time (aperture priority automation).</p>

<p>For those of us who don’t want the camera making the final decision, we usually first consider the water pressure we will encounter (light intensity) and then choose the approriate cup size (film speed). OK…measure the pressure (meter) and look at the possible combinations of size and time (f-stop/shutter) shown on the meter dial. Fortunately, the system of f-stop/shutter speed combinations goes in steps of halves & doubles (e.g. twice the opening=half the time). Then use that really wonderful computer we all have (a brain) to consider if we need to alter what the meter says to do. And if we ever find that we made a mistake (I never do :-)), then we store that information for the next time the same situation arises.</p>

<p>I know I’m pretty much preaching to the choir here, but in one of my former lives, I taught photography classes in adult education schools back in the 70’s. and 80’s. I hope Rashed and others may find this helpful.</p>

[Alan Marcus]

<p >I was also a teacher at about the same era as you. I used and continue to use the water glass filling analogy, same as you.</p>

<p > I like to add that the magic number for all this is 2.</p>

<p >We change our exposure using an increment that either doubles or half's (I know full well that changes are also made in 1/2 and 1/3 stop increment).</p>

<p >What I want to point out is the f/number (number set) is based on the square root of 2 which is 1.414. We don't need this level of accuracy, we round to 1.4. </p>

<p >The sequence as you well know is 1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 22 etc. Note each number going right is its neighbor on the left times 1.4. Each number going left is its neighbor on its right times 0.707 (.07 is accurate enough; this is the inverse of 1.4.</p>

<p >Why 1.4? The lens appearance (aperture) is a circular geometric figure. Multiplying a circle's diameter by 1.4, you get a revised value. If you use this value to construct a larger circle, the revised circle has double the surface area thus a lens so opened up passes twice as much light. Conversely, multiply a circle's diameter by 0.7 constructs a circle with half the surface area. </p>

[stp]

<p>Good analogy and useful information. Please don't take this wrong, but I smiled to myself as I read this and pictured you in the fast food restaurant thinking about the photography analogy while watching a cup fill with a soda drink -- it reminded me of a popular posting on this same forum a few days ago titled "The Insane Photographer" -- things that people do or think that shows they've "gone over the top" regarding photography in their lives. This one would have been a great entry! Cheers. [bTW, I've bookmarked your posting in the event that I ever have the chance to teach a photography course. Alan's response is also very helpful.]</p>

[JDMvW]

<p>Well, speaking only for myself, I find the water-glass analogy to be just as complex as the original situation you-all are trying to model. I think most people ought to be able to grasp "more light/less light" <em>just as easily</em> as they can an analogy to "hydrodynamics."</p>

[jim_momary]

<p>What is the implication if you get a free refill?</p>

<p>:o)</p>

<p>Jim (the kid who always sat in the back and was a snot)</p>

[Alan Marcus]

<p >The more/less light fails to convey how the 4 factors of the camera exposure that are intertwined.</p>

<p >1. ISO the size of the container you are filling i.e. film speed - glass volume</p>

<p >2. Shutter speed the time allowed to accumulate light/water</p>

<p >3. Abundance light intensity / water pressure</p>

<p >4. Aperture lens f/number / pipe diameter </p>

<p >The water glass being filled is the best analogy I have ever confronted in my 50+ years of teaching the stuff.</p>

<p >I am always looking for better ways to explain. Perhaps you would share with me your equivalence.</p>

<p > </p>

[JDMvW]

<p>When I've taught basic camera 101 to novices, they never seemed to have any problem understanding the concepts in terms of more/less light and <strong>of course </strong> (re Alan) the <em>other</em> direct variables such as film speed, ISO, aperture, and the like. I think concepts about these issues are common experience in the real world, and there is no need to confuse people with such a complex set of analogies to some other experiences not actually direct sensory events. I just don't see where memorizing/making a bunch of soda cup analogies makes anything more understandable or clearer.<br>

If you think it's better, that's your right, but the OP asked for commentary, after all.</p>

<p> </p>

[charles_stobbs3]

I think a better analogy would be a kitchen sink faucet because you can vary how much you open it whereas no one is going to replumb the drink dispenser and the container should be a quart measuring cup with a different line for each ISO speed and the top line well below th brim because it is possible to overexpose any film speed.

[Alan Marcus]

<p >I agree with Charles. Actually, I use the kitchen sink analogy and not the soda dispenser. The hardest concept to get across is the fact that both film and the digital chip accumulate light. Next hardest is the f/number system. Over the years, we have tried several methods to state aperture, which is a ratio. Noting works like focal ratio with a notation like f/11 or f/4. </p>

[arie_vandervelden1]

<p>Water pressure is some function of pipe size, cup size, and time? Not intuitive to me.</p>

<p>How about:<br /> Pipe diameter = aperture<br /> Time = shutter speed<br /> Pressure = iso<br /> amount of liquid dispensed = exposure</p>

<p>If the amount of liquid dispensed = cupsize then the exposure is correct.<br /> For overexposure, the cup overflows. Analogous to clipping highlights.<br /> Underexposure = underfill.</p>

[Alan Marcus]

<p >Hi Arie,</p>

<p >Pressure is not ISO</p>

<p >Pressure is the amount of light energy at the instant of exposure - flash - ambient or combination .</p>

<p >ISO is the cup size. 50 ISO big cup - 200 ISO medium - 800 ISO small cup</p>

<p >Any questions? </p>

[JDMvW]

<p>I will admit, I do think a B-cup is nice, all things considered.</p>

<p>From the discussion I am beginning to see just how much clearer the water/soda analogy makes the principles of light clearer (LOL).</p>

[bsharpe411]

<p>Thank you all for your comments. I tossed this into the ring simply to see how people reacted, changed, and/or modified the analogy. I saw a lot of interesting takes on it. </p>

<p> <strong>Alan M</strong>....I like that you mentioned about the source of those common f-stop numbers. For years it was a mystery to me. I suppose that if they labeled a lens with 'square root of 32' rather than f5.6, most people with a math phobia would have given up on photography. The few times I mentioned this connection in the adult photo classes, I saw many of the students begin to roll their eyes and I soon realized I've got to be careful not to scare them away from the camera. I taught photography in the evening because I loved it...but I taught high school math during the day so that I could afford a place to live and some food to eat.<br>

<strong>Stephen...</strong>thinking about photography is better than thiinking about what that bacon-cheese-double monster with super fries is doing to me.<br>

<strong>JMD...</strong>I'll send you a free certificate for a DD-sized Super Big Gulp....enjoy!</p>

<p>All of us here are teachers of photography because friends and relatives turn to us to explain the mysteries of the fancy new camera they just bought. Just remember not to scare them off with too much technical jargon right away. Let them grow into the complexities of the process slowly.</p>

[Ed_Ingold]

<p>Automatic fountain dispensers flow at a constant rate. The only variable to accommodate different cup sizes is the filling time. In photographic terms, it is the equivalent of keeping the aperture constant and varying the shutter speed (i.e., Aperture-Priority).</p>

<p>This talk about flow rates, pipe sizes and pressure is irrelevant - like those extra "facts" in story problems to confuse the students.</p>

[craig_gillette]

<p>A common and useful teaching technique is to activate prior knowledge, that is, to build off what the student already knows. Many people are used to the issues of filling or emptying water containers of various sorts and can relate to these analogies. From there, the camera and light may be just a very short step. Either way, it's best to keep it simple and not try to involve too many different steps in the process all at once. You can pour a lot of water for a short period of time or a little water for a longer period of time, it's still the same amount of water (or light). </p>

<p>The instructor should get a feel for the existing level of knowledge and also approach it from the point that different students will respond to different analogies or "aids." Some can hear and visualize the issues, others will do better seeing the water dumped from an open cup compared to slowly emptying from a small hole in the base and cup, pint, quart and gallon bottles/measuring cups, etc., to "see" the comparative quantity of water/light in an iso/sensitivity range. You can even engage students hands on in the water dumping. </p>

<p>Of course, the location of the class may not allow the actual water play and the ages/experience levels of the students may not require too much use of actual water and empty vessels may serve just fine.</p>