Exposure analogy for laypeople

darich · Dec 29, 2006

Hi all.

I've got a presentation coming up in mid February and with an open subject i've chosen photography.
i'm going to talk about when you want shallow DOF and how to achieve it, long exposures and general photography terms. Nothing too heavy duty - it's only to be around 15minutes.

I cam up with an analogy of exposure and wondered if it was too silly or right on the money or somewhere in between.

Imagine a bathtub full of water.
The water in the bathtub is the amount of light required to correctly expose any given photo.
With a small aperture, or small plughole you need more time for the bath to empty and properly expose your image.
If you have a large aperture, ie a large plughole then the bath empties quickly so the exposure time is reduced.

The talk will be to others on my course and while i don't know for certain I'm pretty sure that none of them are any more advanced than p&s cameras. i did have a chat with a girl on the course and she said that a talk on photography would be quite interesting.

is the bathtub analogy a bit too weird?

Big Mike · Dec 29, 2006

That sounds like a pretty good analogy to me. I've also heard a version of that...but you are filling a bucket with water. Bigger hose --> less time. Smaller hose --> more time.

markc · Dec 29, 2006

It's an imperfect and tricky analogy, but it might work for people. I'm been thinking of this for a bit myself. Here's what I came up with:

Rather than a bathtub, you are spraying water onto a big sheet of something with a spray nozzle. The aperture his how far open you have the nozzle; the shutter is how long you have it open for; the light in the scene is the pressure of the water; and the ISO is how absorbent the material is. High ISO is a paper towel. Low ISO is a big diaper. Correct exposure is damp, but not soaked, so a paper towel will take very little water, but a diaper will take quite a lot. Overexposure will make it dripping wet, while underexposure will barely wet it. The pressure (light in the scene) will determine how much water is coming through just as the opening and time does. Even if you have a paper towel and a wide nozzle, if you have very low pressure (a night shot), it's going to take a while for that little dribble to wet the paper towel.

It's a bit complex, but it actually may be less confusing for some people.

RacePhoto · Dec 29, 2006

I like the water pouring or draining, but I think a bathtub is a bit large. :lol:

You might think of a bottle of water poured through a funnel with a small opening and the same size bottle of water poured through a funnel with a larger opening.

The funnel is the lens, which by coincidence funnels in light, and the opening is the speed of the lens, or as you are explaining aperature setting. You could attach a straw and make it run even slower.

This is something you could carry in and demonstrate the difference in time if you wanted to do a visual?

Shutter speed is measuring how long you pour in water to get 12 ounces.

If you pour water for 3 seconds and the whole 12 oz bottle full doesn't get through the funnel, you have under-exposure.

If you pour for 3 seconds and all the water is gone, before the time is up... you have over exposure.

A perfect exposure is pouring water for the exact time it takes for one bottle of water to go through the funnel and into the measuring cup in X seconds.

(your baseline time is whatever number of seconds it takes to make your example work, so you can pour one bottle through the correct opening, and have it all gone, at exactly the right time.)

ksmattfish · Dec 29, 2006

I like the standard bucket and faucet analogy I heard in Photo 101. Water is light, and the bucket is the film/sensor. Correct exposure is filling the bucket to the rim. The size of the bucket is determined by ISO. An ISO 100 bucket holds twice as much as an ISO 200 bucket, and four times as much as an ISO 400 bucket. Aperture is how much you open the faucet. Shutter is how long you leave it open. You could fill the bucket by turning the faucet on full blast for a short time, or at a trickle for a long time, or something in between. If the water overflows that's overexposure. If it doesn't go up to the rim, that's under-exposure.

markc · Dec 29, 2006

Yeah, people probably get the bucket thing easier, but I think it can be misleading. I still think you need to account for how much light is in the scene, as that's what's determining the exposure in the first place. Otherwise ISO is what determines how much light you need, since it's the size of the bucket.

Big Mike · Dec 29, 2006

I agree with Mark, the bucket/tub analogy is imperfect....and your paper towel is more accurate...but if the audience knows very little...the bucket is probably easier to understand at first. I guess it depends on who you are talking to.

markc · Dec 29, 2006

Agreed. Either way, it's a pretty complex thing to try and get a simple analogy for. It might be best to just talk about opening and time and leave ISO and the rest out of it.

Torus34 · Dec 29, 2006

I took my own crack at it in

http://www.thephotoforum.com/node/36

To be quite honest, the only way I've been comfortable with describing DOF vs. aperture has been through diagrams showing how a smaller aperture narrows the cone of light rays and how this affects the circle of confusion [yet another concept!] The use of the simile of 'squinting' is a real cop-out, but it's all I could come up with barring a long, drawn-out, boring full-bore explanation.

It's one heck of a concept to try and get across in 15 minutes! I wish you the best!

Don Simon · Dec 29, 2006

Personally I found it easier to understand when it was explained to me using the analogy of, well, exposing film to light :lol:

I do understand some people find it easier to learn in different ways, but I think any analogy is going to be imperfect. Though I agree the paper towel / diaper thing at least covers the issue of sensitivity better than most.

ksmattfish · Dec 30, 2006

Here's the perfect analogy:

You have this light proof box with a closed hole at one end. Inside is a light sensitive material. Lets call the hole aperture, and the cover of the hole the shutter. How sensitive the material is to light will be described by ISO. You are trying to expose the material to enough light so that it turns middle gray. If you don't expose it to enough light, it'll be lighter than middle gray. If you expose it to too much light, it'll be darker than middle gray....

craig · Jan 1, 2007

I think it is about light control. Not water control. If I was giving a 15 minute lecture on photography I would speak of how it is a marriage between art and science. Exposure is only a small part of the deal. Get them interested in photography. Not bored by it.

markc · Jan 1, 2007

Good idea, craig. I'm going to be doing a class in a couple of weeks for beginners, and I'm not even going to touch on most of the technical stuff. It's definitely info that's important to know if you want to improve, but I'd rather come at it from a different angle. Rather than say aperture does this and focal length does that, I'm going to talk about the different choices in how a photo can look, and then explain how to get there. Hopefully that will get people excited about creating images.

darich · Jan 2, 2007

markc said:
Good idea, craig. I'm going to be doing a class in a couple of weeks for beginners, and I'm not even going to touch on most of the technical stuff. It's definitely info that's important to know if you want to improve, but I'd rather come at it from a different angle. Rather than say aperture does this and focal length does that, I'm going to talk about the different choices in how a photo can look, and then explain how to get there. Hopefully that will get people excited about creating images.

That's exactly what I'm going to do. When you want a small aperture or long exposure and when you don't. So the photo has a different look eg portrait with small aperture doesn't look as good as large aperture. I know exposure is only a small part of the whole subject but it's a fairly important part!!

I'll show examples of shots with large and small aprture and show why one is better than the other.

The analogy will only be used if someone asks me a question relating to aperture,light and shutter speed.

I'm deliberately steering away from being too technical. Aperture, exposure ISO and composition will all be mentioned but only briefly so the audience can see there are many variables that have to be set correctly to achieve a correctly exposed photograph.

blork · Jan 2, 2007

The water analogy is one that I've always used in my head, and even out loud when speaking to neophytes. I like the bucket example better than the tub one, expecially with the refinement that bucket size corresponds to ISO speed.

In my experience, beginners really don't get the idea that there is only one "correct" exposure -- that a change on one input (e.g. aperture) has to correspond to a change in the other input (shutter speed). Also, that even if the scene changes (e.g., gets darker) then you still have to let the same amount of light in, so you either have to open one thing (aperture) or the other (shutter speed).

Once you get it it's dead simple, but this analogy works very well for people whose cognition is more geared towards images and analogy than technology or science. It's not best for everyone (the science-oriented people will get it right away based on the facts), but it works for some.