BetterPhoto Q&A
Category: New Answers

aperture and shutter speed

Thanks,
Paula

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January 10, 2006

Bob

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January 10, 2006

The best way to determine exposure is to use a light meter of some kind, either one built into the camera or a hand held meter that can take what's called incident or reflected readings or both. Most hand held meters will do both.

As to the relationship between the two, I'll give you a couple of examples and if you still want to make your own chart, just extrapolate from the numbers.

Let's say you're at 100 ISO, and you get a meter reading of f8.0 at 1/125th of a second. Here's how that works out: If you want a smaller f-stop to get better depth of field, say f 11.0, then you need to DECREASE your shutter speed by 1 stop or down to 1/60th of a second. As your f-stop numbers (narrowing the opening of the lens and letting in less light) you need to compensate for that by giving a longer exposure time.

Think of shutter speeds like f-stops in
sense that smaller lens openings (higher values like f16, f22, f32, etc.) require longer exposure times. For each increase in f-stop number, to maintain the same exposure value, you need to DECREASE or slow down your shutter speed by one stop or time increment.

Conversely, if you need less depth of field, to say blur out the background, and your original meter reading was f8.0 at 1/125th, then to get down to f2.8 for example, or 3 stops wider than f8 [f5.6, f4.0, to f2.8] then as you open up your lens wider, you need to SPEED UP the shutter 3 stops from 1/125 to 1/250th, to 1/500th to 1/1000. Seewhatimeanhuh?

OK, now a train leaves Kansas City Mo for Chicago at 3:30 p.m. traveling at 76 mph. And a train at the same time leaves Minneapolis headed for Chicago traveling at 57 mph,....[OHHHHHHHHHH nooooooooooooooooooooo, not ANOTHER train problem !!!!!] Saay, you need some scratch paper for this? lol !!

BTW, if you want an excellent description on the relationship between the lens opening and shutter speed, go to the library, find the Time Life series on Photography and get volume 1 on the CAMERA. You'll find all the basics in there.

Hope this helps rather than confuses you. Take it light. ;>)
Mark
Mark

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January 10, 2006

Click here.

It's a bunch of EV values that can be used.

Basically Paula, let's say using this chart you get f/16 @ 1/60 it's shown on the chart as "value '14'". then just go diagonal on the chart and all the 14's are the same value. let us know if anyone of us helped.

p.s., if you're new to photrography, marks paragraph above is LOADED with great great info.

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January 10, 2006

Some theory:

It might help if you keep in mind that it is all about the amount of light that you allow through the lens. There is a direct relationship between aperture, shutter speed and ISO which is called f-stop. One f-stop is either exactly half or exactly double the amount of light depending on whether you stop down or up.

The awkward numbers on your aperture ring (eg. 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16) discribe the relationship between the diameter of the opening of the iris and the focal length. eg. an f-stop of 1 on a 50mm lens would mean that the iris opening has a diameter of 50mm and when stopped down to 2 the iris would close to 25mm etc. Note that every two steps the number doubles. ie. stopping down from 2.8 to 5.6 would mean stopping down by two f-stops. Some lenses are also labeled with half stops such as a 3.5 between 2.8 and 4.

So the difference between 1 and 1.4 is that a setting of 1.4 will allow exactly half the amount of light that 1 setting would allow - or one f-stop less; a 2 would be half the the light of a 1.4, a 2.8 half of 2 etc.

ISO refers to the sensitivity of film and here you will find a similarity in that if the ISO no. doubles the sensitivity of the film doubles - or increases by one f-stop. ie. a ISO50 film needs twice the amount of light than a ISO100 which needs twice than an ISO200 etc.

Shutter speed also behaves in a similar way in that if the time doubles the amount of light doubles or in other words increases by one f-stop. eg. if you're set at 1/125 and you want to stop down by one stop (reduce the amount of light by half) you would change to 1/250.

Here's an example of what this all means:
If you are metering your scene and your camera is loaded with an ISO100 film and is set to 1/125 shutter with a 5.6 aperture and you want to stop down by one stop (maybe because your meter suggests you do) you can do one of three things: 1. You can close the aperture to 8, or 2. you can set your shutter speed to 1/250 or 3. you can take out your ISO100 film and replace it with an ISO50 film - which I would not suggest, though :-)

Which combination of shutter speed and aperture to use depends on various factors. Two things to keep in mind though are that a slower speed will increase your chance of blur when you're hand holding your camera and that the more you close your aperture the greater your depth of field (DOF = the area that is in focus) will be.

The slowest speed at which one can still safely take a handheld shot without bluring it to much depends on the focal length. As a rule you can roughly divide 1 by your focal length to get to the longest shutter speed. eg. with a 50mm lens this would be a 1/60, with a 90mm lens a 1/125, with 250mm lens 1/250 etc.

If you have any question, in case you are heavily confused now, do not hesitate to ask. :-)

Regards,
Peter

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January 10, 2006

:-)

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January 10, 2006

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January 10, 2006

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January 10, 2006

It should be Tempus Fugit.

I don't think you meant to say "Storm Flies". ;-)

Chris
(Former Catholic Schoolboy)

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January 11, 2006

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January 11, 2006

Thanks to all for your insight. I guess what it really boils down to is, I don't like math. I need to embrace it!
Paula

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January 18, 2006