# How do you explain exposure?

Discussion in 'Exposure Discussion' started by JBrunner, Oct 29, 2010.

1. ### JBrunnerModeratorStaff MemberModerator

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Tried to explain exposure reciprocity to someone today, and didn't feel like I made much headway. For instance, they had trouble understanding the difference between film speed and shutter speed, and the whole half and double relationship between shutter speed, film speed and aperture.

What is your best method for explaining exposure? Water, funnel, and a bucket?

2. ### MattKingSubscriber

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For f/stop, I like using a door.

If a door is open all the way, it lets in X amount of light

If the door is open 1/2 way, it lets in less light

If a door is open 1/4 way, it lets in even less light

If a door is open 1/8 way, it lets in even less light.

The ratios are related to the area that is open.

3. ### 2F/2FMember

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If all else fails, I do it with Polaroids!

4. ### Q.G.Inactive

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The "classic" way is using the water tap and bucket analogy.

Good exposure is the bucket filled to the rim.
Underexposure a not yet full bucket.
Overexposure a bucket overflowing.

Film speed is the size of the bucket. A faster film is a smaller bucket, a slower film a larger one.

Shutterspeed the time it takes to fill the bucket.

Aperture how far the tap is opened, i.e. the rate of flow.

Turn the tap open more, and it will take less time to fill the bucket.
Close the tap a bit, and it will take longer.
Change the size of the bucket leaving the tap as it is, and it will take longer or shorter, depending on whether the new bucket is larger or smaller.

Something everyone will understand, even 3 year olds.

5. ### AgXMember

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film speed and shutter speed

I assume one problem lies in designating two different things with the same term "speed" in common English.

In German the terms "sensitivity" and "time" are used respectively.

6. ### RalphLambrechtSubscriber

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I agree. This is the most popular way to explain it, and it is used in several text books. Just google for 'exposure bucket' and you'll find many examples. Here is one:

7. ### Q.G.Inactive

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Yet, using the bucket and tap analogy, the word "speed" doesn't create confusion: no matter the 'speed', the rate of the flow of water, it takes less time (i.s, is speedier) to fill a small bucket than it takes a large one.

8. ### annSubscriber

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when it comes to dealing with halfing and doubling i use the relationship to 1inch to 2 inch which seems to help.

I can still remember many years ago (and i mean many as i have been doing at this for over 60 years) that relationship made no sense to me; and all of a sudden one day the light bulb went on and in fact i have seen a diagram using lightbulbs .

9. ### hpulleyMember

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Aperture is simply the size of the hole that lets the light into the camera, onto the film. Aperture is a ratio of lens diameter divided by lens focal length, always reducing the diameter to 1 for easier comparison. Since 1/2 is bigger than 1/4 that explains why 1/2 lets more light in. Understanding that the area of 1/2 is 4 times that of 1/4 tells you why it isn't 2x the light. Area is always squared (volume is worse, no one gets volume being cubed).

Shutter speed is a poor term, if it actually WAS a shutter speed number it would be alright but it is not. It is shutter duration. Again it is a fraction so 1/2 is longer than 1/4 second. Long shutter means blur, short shutter means frozen action. Shutter speeds are not areas so they are not squared, 1/2 is twice as long as 1/4 so it lets in twice as much light.

Film sensitivity isn't actually included in the exposure but we still need to consider it to choose an appropriate exposure. Film sensitivity is a much better term. More sensitive to light means you can use a shorter shutter duration to get enough light or a smaller aperture. 200 is twice as sensitive as 100 which is twice as sensitive as 50 so 200 is 4x as sensitive as 50.

Reciprocity failure is explained by a few chemical properties. Activation energies mean that at very low light levels there may not be enough energy to trigger the chemical reaction caused by normal levels of light. Over time reacting there is less chemical left to react is another factor. Some very sensitive films for short exposures are very poor in long exposures; some fairly slow films are actually better at long exposures. Only the data sheet can tell you about the failure of one particular film.

10. ### ic-racerMember

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With pre-teenage children in the darkroom I totally avoid the need for any understanding of reciprocity. All the negatives are printed for 60 seconds and we lith print. The kids can easily understand the longer the print is in the developer, the darker it gets; and exposure is out of the discussion.