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A basic question about exposure....

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Torbak

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A little embarrassed to ask, but can somebody please explain how both the power of light and an exposure can be measured in f numbers?

I understand the "tripod" of exposure - i.e. how aperture, shutter speed and ISO interact to give an exposure.

For me, an F number relates to an aperture. What I therefore don't understand are terms like "this light is at f11 on the subjects face" or "Dial in an exposure of f11 to the camera".

If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
 
then everything else will fall into place.

You are aproaching things through the back door.

Wikipedia probably has a good explination

TEdolph
 
Torbak said:
If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
Click to expand...
Well, you make it match. So if you're dialing in an exposure of f/11 you're setting your aperture to f/11. Then you adjust your shutter speed and iso so your meter says that is a correct exposure. That's in manual mode; there's also the option of using aperture priority to just set the aperture.

As you may know, there are trade offs to all of these settings. A very wide aperture (f/1.8 or something) has a much shallower depth of field than f/11 but it has way more light. So if you're in a dark room, it is going to be a challenge to get enough light to shoot at f/11. ISO most people understand - when you crank it up to high levels, you get noisy images. For shutter speed, usually you want it higher than 1/focal length of the lens and I try to keep mine above 1/60th if I can.

On the other hand, if things were too bright your camera might be maxed out at 1/4000th of a second so you have to decrease your aperture (increase your f/stop). That reduces your light so the camera can increase the shutter time (drop to 1/2000th or so). It's all just a balancing act.

I think it was easier when I was learning with film. Your ISO was set on the role you had in the camera; you only had the options of changing aperture or shutter.
 
Torbak said:
A little embarrassed to ask, but can somebody please explain how both the power of light and an exposure can be measured in f numbers?

I understand the "tripod" of exposure - i.e. how aperture, shutter speed and ISO interact to give an exposure.

For me, an F number relates to an aperture. What I therefore don't understand are terms like "this light is at f11 on the subjects face" or "Dial in an exposure of f11 to the camera".

If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
Click to expand...
Are you reading an old film days book?

It almost sounds like they are talking about the situation where you are using a fixed film ISO and flash, where the shutter speed doesn't matter if the flash is the dominant light source (up to max shutter sync speed of course). Under those circumstances, someone with a flash meter can take readings of the flash on the bright side and dimmer sides of the face and then quote those f/stop numbers.

In all other circumstances with constant light then the shutter speed can be varied to get the aperture to a more desirable number, and of course with digital and changeable ISO then that can move the whole f/stop & shutter speed relationship to a maybe better area.

Summary.... It definitely sounds like "out of the ark" talk in regard to exposures.

Regards........... Guy
 
jquagga said:
Torbak said:
If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
Click to expand...
Click to expand...
Here's a few tutorials:
http://www.cambridgeincolour.com/tutorials/camera-exposure.htm
I find this helpful:

"As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren't otherwise obtainable." In other words, if you don't need to bump up ISO, don't. Keep it as low as necessary to use the aperture and shutter speed you want.

http://www.cambridgeincolour.com/tutorials/camera-metering.htm#exposure-compensation

I've been wondering about adjusting EV more; I'm new to this myself. Seems to me you're basically recalibrating the camera's metering to meet the needs of the subject.

Shhhh ... if we're lucky, the Equivalence Gang won't hear about this.

If you don't know about the EG, be very, very glad. Some of them don't like Cambridge in Colour even a little bit, but I do.
jquagga said:
Well, you make it match. So if you're dialing in an exposure of f/11 you're setting your aperture to f/11. Then you adjust your shutter speed and iso so your meter says that is a correct exposure. That's in manual mode; there's also the option of using aperture priority to just set the aperture.

As you may know, there are trade offs to all of these settings. A very wide aperture (f/1.8 or something) has a much shallower depth of field than f/11 but it has way more light. So if you're in a dark room, it is going to be a challenge to get enough light to shoot at f/11. ISO most people understand - when you crank it up to high levels, you get noisy images. For shutter speed, usually you want it higher than 1/focal length of the lens and I try to keep mine above 1/60th if I can.

On the other hand, if things were too bright your camera might be maxed out at 1/4000th of a second so you have to decrease your aperture (increase your f/stop). That reduces your light so the camera can increase the shutter time (drop to 1/2000th or so). It's all just a balancing act.
Click to expand...
Makes sense to me.
jquagga said:
I think it was easier when I was learning with film. Your ISO was set on the role you had in the camera; you only had the options of changing aperture or shutter.
Click to expand...
Maybe it was easier in the film days because there were far fewer parameters to manipulate on a camera, but digital has the undeniable advantages of speed and cost. Instantaneous review of "free" images in-camera can definitely speed up the learning curve when it comes to technical experimentation. Too bad that doesn't speed up the art side as much.

--
http://453c.smugmug.com/
 
jquagga said:
Well, you make it match. So if you're dialing in an exposure of f/11 you're setting your aperture to f/11. Then you adjust your shutter speed and iso so your meter says that is a correct exposure. That's in manual mode; there's also the option of using aperture priority to just set the aperture.
Click to expand...
But if you're using your meter, you are balancing A/SS/ISO to get correct exposure - in which case the reading of F11 seems irrelevant as it could just as well be F8 with a faster shutter speed or F16 with a slower one - the exposure would be the same
jquagga said:
As you may know, there are trade offs to all of these settings. A very wide aperture (f/1.8 or something) has a much shallower depth of field than f/11 but it has way more light. So if you're in a dark room, it is going to be a challenge to get enough light to shoot at f/11. ISO most people understand - when you crank it up to high levels, you get noisy images. For shutter speed, usually you want it higher than 1/focal length of the lens and I try to keep mine above 1/60th if I can.

On the other hand, if things were too bright your camera might be maxed out at 1/4000th of a second so you have to decrease your aperture (increase your f/stop). That reduces your light so the camera can increase the shutter time (drop to 1/2000th or so). It's all just a balancing act.
Click to expand...
yes - this I understood already
jquagga said:
I think it was easier when I was learning with film. Your ISO was set on the role you had in the camera; you only had the options of changing aperture or shutter.
Click to expand...
I am sure the Eureka moment will come - it just hasn't yet...
 
Can we have the context?

Sounds like a very old photography book to me, and I vaguely remember this stuff assumed ISO100 1/125 of a second.

But it may be something more modern in which case I have no eye deer.

Anyway, it certainly isn't some obvious thing you are supposed to know and we are all sniggering.
Torbak said:
A little embarrassed to ask, but can somebody please explain how both the power of light and an exposure can be measured in f numbers?

I understand the "tripod" of exposure - i.e. how aperture, shutter speed and ISO interact to give an exposure.

For me, an F number relates to an aperture. What I therefore don't understand are terms like "this light is at f11 on the subjects face" or "Dial in an exposure of f11 to the camera".

If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
Click to expand...
--
http://www.flickr.com/photos/acam
http://thegentlemansnapper.blogspot.com
 
jquagga said:
Torbak said:
If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
Click to expand...
Well, you make it match. So if you're dialing in an exposure of f/11 you're setting your aperture to f/11. Then you adjust your shutter speed and iso so your meter says that is a correct exposure. That's in manual mode; there's also the option of using aperture priority to just set the aperture.
Click to expand...
that is really not what it means.

The use of phrases like, "dial in an exposure of f/11" happens in flash photography, such that the shutter speed is used to control the back ground exposure and the aperture is used to control the foreground subject exposure. If you use a slow shutter speed to get the background to show (e.g. where a slow sync. speed is used) then a small aperture is necessary to prevent the subject illuminated by the flash frfom being blown out.

This is less important today where TTL flashes can control thier own output, but is still relvant for studio lighting with constant output lights.
jquagga said:
As you may know, there are trade offs to all of these settings. A very wide aperture (f/1.8 or something) has a much shallower depth of field than f/11 but it has way more light. So if you're in a dark room, it is going to be a challenge to get enough light to shoot at f/11. ISO most people understand - when you crank it up to high levels, you get noisy images. For shutter speed, usually you want it higher than 1/focal length of the lens and I try to keep mine above 1/60th if I can.
Click to expand...
Agian, if the OP would start with an understaning of Exposure Values, all of this would make a lot more sense.
jquagga said:
On the other hand, if things were too bright your camera might be maxed out at 1/4000th of a second so you have to decrease your aperture (increase your f/stop). That reduces your light so the camera can increase the shutter time (drop to 1/2000th or so). It's all just a balancing act.
Click to expand...
See above. You are not telling him anything that he doesn't already know.
jquagga said:
I think it was easier when I was learning with film. Your ISO was set on the role you had in the camera; you only had the options of changing aperture or shutter.
Click to expand...
Still, when doing studio work (and underwater photography) you had to understand how to use aperture to control the expousre of the subject, and shutter speed to control the exposure of the background scene.

Auto TTL fill flash made most of that unecessary.

The jargon still lives on though.

TEdolph
 
Eureka!
Guy Parsons said:
Are you reading an old film days book?
Click to expand...
No but...
Guy Parsons said:
It almost sounds like they are talking about the situation where you are using a fixed film ISO and flash, where the shutter speed doesn't matter if the flash is the dominant light source (up to max shutter sync speed of course). Under those circumstances, someone with a flash meter can take readings of the flash on the bright side and dimmer sides of the face and then quote those f/stop numbers.
Click to expand...
They were talking about flash though - and of course shutter speed therefore doesn't matter. Oddly enough I just figured that out properly yesterday when reading an article on strobist about balancing ambient and flash together - but hadn't put 2 and 2 together and therefore considered that an exposure could be expressed as an aperture in that context.

Still not quite sure how ISO is covered by that though - even if SS is taken out of the equation it would still have to be an exposure of F11 at a given ISO if the ISO changed, the A would have to change too.

And does this mean a light meter reading can't be expressed as an aperture (F number) when it is continuos?
Guy Parsons said:
In all other circumstances with constant light then the shutter speed can be varied to get the aperture to a more desirable number, and of course with digital and changeable ISO then that can move the whole f/stop & shutter speed relationship to a maybe better area.
Click to expand...
yep - got that.
Guy Parsons said:
Summary.... It definitely sounds like "out of the ark" talk in regard to exposures.
Click to expand...
yes - i think that was what was throwing me.
Guy Parsons said:
Regards........... Guy
Click to expand...
 
Guy Parsons said:
Torbak said:
A little embarrassed to ask, but can somebody please explain how both the power of light and an exposure can be measured in f numbers?

I understand the "tripod" of exposure - i.e. how aperture, shutter speed and ISO interact to give an exposure.

For me, an F number relates to an aperture. What I therefore don't understand are terms like "this light is at f11 on the subjects face" or "Dial in an exposure of f11 to the camera".

If an F number is an aperture, and an an exposure is a combination of A/SS/ISO, how can an exposure be expressed as an aperture without any reference to shutter speed and ISO?
Click to expand...
Are you reading an old film days book?
Click to expand...
Sounds like it doesn't it?
Guy Parsons said:
It almost sounds like they are talking about the situation where you are using a fixed film ISO and flash, where the shutter speed doesn't matter if the flash is the dominant light source (up to max shutter sync speed of course). Under those circumstances, someone with a flash meter can take readings of the flash on the bright side and dimmer sides of the face and then quote those f/stop numbers.
Click to expand...
Yes, the meters just used to tell you what F stop to set after you had dialed in everything else (still do!).
Guy Parsons said:
In all other circumstances with constant light then the shutter speed can be varied to get the aperture to a more desirable number, and of course with digital and changeable ISO then that can move the whole f/stop & shutter speed relationship to a maybe better area.

Summary.... It definitely sounds like "out of the ark" talk in regard to exposures.

Regards........... Guy
Click to expand...
TEdolph,

a/k/a Noah
 
http://en.wikipedia.org/wiki/Exposure_value

Lots of scary log equations there...but I already understood the basic premise that "exposure value (EV) denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure"

What was confusing me was how aperture could be expressed as an exposure WITHOUT reference to SS (and ISO)
tedolf said:
then everything else will fall into place.

You are aproaching things through the back door.

Wikipedia probably has a good explination

TEdolph
Click to expand...
 
Torbak said:
Still not quite sure how ISO is covered by that though - even if SS is taken out of the equation it would still have to be an exposure of F11 at a given ISO if the ISO changed, the A would have to change too.
Click to expand...
Yes.
Torbak said:
And does this mean a light meter reading can't be expressed as an aperture (F number) when it is continuos?
Click to expand...
For continuous lighting an exposure would be quoted at, say, "f/8 at 1/250 at ISO 200". All three need to be quoted. The same exposure would work for any camera, any sensor size, any lens, digital or film.

Regards....... Guy, Noah's navigator.
 
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