More on Aperture and Shutter

Showing posts for query exposure. Show all posts

Showing posts for query exposure. Show all posts

F-stops are the hole in the lens that lets the light in. The number is just the bottom (ok, the denominator) of a fraction.

• When the lens is set to f2, the hole in the lens is 1/2 as big as the lens is long.
• When the lens is set to f8, the hole in the lens is 1/8th as big as the lens is long.
• Which hole is bigger, 1/2 or 1/8?
• Which hose would let more water through, one that's a half inch in diameter or one that's one-eighth in diameter? (See, there's the first plumbing parallel.)
• Which f-stop lets in more light, f2 or f8?

Shutter speeds control how long the lens lets the light in. The number is just the bottom (ok, the denominator) of a fraction.

• When the shutter speed is set to 125, light comes in for 1/125th of a second.
• When the shutter speed is set to 8, light comes in for 1/8th of a second.
• Which is longer, 1/125th  or 1/8th?
• If you leave the water turned on for 1/8th of an hour (7.5 minutes), does more water flow than if you leave it turned on for 1/125th of an hour (less than 30 seconds)? (Another plumbing parallel.)
• Which shutter speed lets in more light, 1/8th or 1/125th?

Shutter speeds also control the action-stopping ability of the camera.

• If you aim a garden hose at your girlfriend and she is running, how much of her gets wet if you only open the nozzle for 1/1000th of a second?
• How much gets wet if you open the nozzle for 1/4 of a second?
• How much will you like it when she hits you with the bucket because you turned the hose on her?
• Here's the plumbing parallel - moving subjects move farther the longer the shutter is open (the longer the hose is turned on.) The more a subject moves during the exposure time (the length of time the shutter is open), the more the image of the subject moves across the film. The more the image moves across the film, the more blurred the image becomes. Short exposure times "freeze" action.
• Which shutter speed freezes action better, 1/8th or 1/500th?

A sunny day is like a high pressure hose

• If the light is bright, it's like strong water pressure.
• It takes less time for just enough light to get to the film.
• High shutter speeds are more suitable for bright light.
• Longer shutter speeds are suitable for dim light, just as you'd leave the faucet turned on longer to fill up a glass if the water pressure is low.

Film speeds can be compared to measuring cups

• Let's think of the film speed rating (ISO or ASA or DIN) as a standardized measuring cup.
• Fast films are like small measures. Let's call ISO 800 1 pint.
• We'll call ISO 100 8 pints (that's a gallon.)
• Which takes longer to fill up, a gallon or a pint?
• Which takes longer to properly expose, ISO 100 film or ISO 800?
• Plumbing and photography: Slow films (large containers) require bigger f-stops (larger diameter hoses) and/or longer exposure times (leaving the hose turned on longer) for proper exposure.

iso, aperture, and shutter

To be able to truly control your digital SLR to take pictures the way your mind envisioned them, you need to master at least the 2 basic mechanisms : Exposure and Focusing. I will cover Exposure in this article.
Assuming the ISO setting is constant, correct exposure is controlled by a combination of aperture and shutter speed. The camera has a built-in exposure meter, which appears as a dotted scale that runs from positive to negative, with a 0 point in the center. The job of the meter is to tell you if you are overexposing the shot (indicated by the pointer appearing on the positive side of the scale) or underexposing the shot (pointer would be on the negative side).
There is no absolute right or wrong exposure, so use the camera’s built-in meter only as a guide. Exposure is measured using a system of F-stops. These stops are controlled by moving the control dials on your DSLR body.
On most higher-end DSLRs there is one dial for aperture, and a second dial for shutter speed. Budget DSLRs tend to have only one dial, which by default controls the shutter speed, while the aperture is controlled by holding down the EV button (indicated usually by a +/- sign) and then moving the dial.
1 stop on your camera could mean either 3 clicks (moves) on the dial if you have set your camera at 1/3 increments, or 2 clicks if you’ve chosen 1/2 increments. I personally use 1/2 for speedy changing of exposure settings.
In Program/Auto mode
The camera will choose this combination for you.
In Aperture Priority mode
You set the aperture and the camera will set the shutter speed based on your choice of aperture.
In Shutter Priority mode
You set the shutter speed and the camera will set the aperture based on your choice of shutter speed.
In Manual mode
You set both the aperture and shutter speed. The camera will let you know if it’s underexposed or overexposed through the camera’s built-in exposure meter, but you have the ultimate control.
Shifting the Exposure
Assuming your camera meter has advised 1/500 (shutter speed) and F4 at ISO 100, for a proper exposure of a scene. In order to use back the same exposure but increase the depth-of-field (by using a higher F-stop) we can shift the exposure by counting clicks. Moving the aperture from F4 to F11 will take 8 clicks (if you have set it at 1/2 increments). Without even looking at the meter again, we can confidently move the shutter speed down by 8 clicks to 1/60 and get exactly the same exposure. This is all assuming the ISO does not change.
The Exposure Triangle: ISO, Aperture and Shutter Speed

		Aperture
		f8	f11	f16
Shutter Speed