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06-06-2014, 03:12 PM   #1
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EV value a confusing concept to me

I always thought the exposure was determined by the combined setting of the shutter speed and the aperature value (and perhaps ISO as a third variable).

As I read my Pentax manual, some times I come across where the EV can be changed in increments. That's what confuses me. Will changing the EV force the shutter or aperature to change. Or is EV a completely different independent setting ?

06-06-2014, 03:16 PM   #2
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The EV is not a setting. The EV is the result of the aperture/shutter speed setting. When you change the aperture or shutter speed you are changing the EV.
06-06-2014, 03:20 PM   #3
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Changing EV is just changing the overall exposure, overriding what camera thinks the settings should be for 0 EV. You can force the camera to overexpose or underexpose based on its current metering result. The way it achieve it is of course by adjusting aperture/shutter/iso in order to do what you tell it to do.
06-06-2014, 03:23 PM   #4
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EV compensation is changing the exposure (either by shutter speed, aperture or ISO) - to a different value than recommended by the meter.

If you shoot white snow at the meter setting it will come out gray. If you add +1 or +2 EV, it will overexpose (according to the meter) and produce white snow. You go the opposite direction (-EV) for large dark subjects.

06-06-2014, 03:32 PM   #5
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QuoteOriginally posted by SpecialK Quote
EV compensation is changing the exposure (either by shutter speed, aperture or ISO) - to a different value than recommended by the meter.

If you shoot white snow at the meter setting it will come out gray. If you add +1 or +2 EV, it will overexpose (according to the meter) and produce white snow. You go the opposite direction (-EV) for large dark subjects.

So basically if one adjusts the EV in the camera, the camera will then go adjust the shutter speed and aperature to achieve where you adjusted it to ?
06-06-2014, 03:47 PM   #6
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Depends on your mode, in Av mode it will adjust shutter, in Tv mode it will adjust aperture, P mode will adjust based on program line, etc.
06-06-2014, 03:50 PM   #7
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QuoteOriginally posted by Michael Piziak Quote
So basically if one adjusts the EV in the camera, the camera will then go adjust the shutter speed and aperature to achieve where you adjusted it to ?
Depending what mode your in. If you are in Av, it will adjust the aperture only, Tv - the shutter speed only; P: - may adjust both; Sv: might adjust the ASA also.

Photography is littered with old terminology like EV, ASA... bit confusing.
06-06-2014, 04:00 PM   #8
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ok it's all making sense now.

One last question, when you're in Program mode, what is the "program line"

06-06-2014, 04:02 PM   #9
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Essentially this is whats going on.


Correct exposure is the combination of shutter speed and aperture that at the iso chosen will render all parts of the scene so that they look like they did when you viewed it with your eyes.


The scene should look like it did in real life.


That's the intention.


To achieve that, a meter is installed in the camera and by sophisticated electronics it samples the scene and determines the correct exposure according to rules built in to the camera. Normal scenes have an overall brightness in the mid range.


The meter in the camera doesn't know what the scene contains, so it assumes that all scenes are supposed to be the same mid tone brightness. But this is not the case.


If you take a snow scene, to render it correctly requires it to look very bright, snow is after all white.


If you take a dark and foreboding storm scene, in twilight, that scene is supposed to be very dark.


The camera doesn't know that you are taking an unusually lit scene, so the manufacturer has added ev compensation so you as a photographer can adjust the overall brightness the camera will use for its metering to brighten up bright scenes and darken dark scenes. If that doesn't happen every picture will turn out the same mid tone grey and many will look unrealistic.


Choose the iso to give you a reasonable shutter speed and aperture, and use the ev adjustment if you encounter an unusually lit scene.
06-06-2014, 04:12 PM   #10
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QuoteOriginally posted by Imageman Quote
Essentially this is whats going on.


Correct exposure is the combination of shutter speed and aperture that at the iso chosen will render all parts of the scene so that they look like they did when you viewed it with your eyes.


The scene should look like it did in real life.


That's the intention.


To achieve that, a meter is installed in the camera and by sophisticated electronics it samples the scene and determines the correct exposure according to rules built in to the camera. Normal scenes have an overall brightness in the mid range.


The meter in the camera doesn't know what the scene contains, so it assumes that all scenes are supposed to be the same mid tone brightness. But this is not the case.


If you take a snow scene, to render it correctly requires it to look very bright, snow is after all white.


If you take a dark and foreboding storm scene, in twilight, that scene is supposed to be very dark.


The camera doesn't know that you are taking an unusually lit scene, so the manufacturer has added ev compensation so you as a photographer can adjust the overall brightness the camera will use for its metering to brighten up bright scenes and darken dark scenes. If that doesn't happen every picture will turn out the same mid tone grey and many will look unrealistic.


Choose the iso to give you a reasonable shutter speed and aperture, and use the ev adjustment if you encounter an unusually lit scene.
Thanks for the in depth explanation Imageman.

Basically, one can change the exposure by adjusting the shutter speed or aperature. Furthermore, if one wants the camera to over expose or under expose an image, one can adjust the EV value and then the camera will adjust the shutter speed and aperature to achieve the exposure you desire - that is if you're in the correct mode that allows you to change the EV number.
06-06-2014, 04:17 PM   #11
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QuoteOriginally posted by Michael Piziak Quote
ok it's all making sense now.

One last question, when you're in Program mode, what is the "program line"
Here's a pretty good discussion about it:

How do DSLRs figure out what aperture to select in P mode? - Photography Stack Exchange
06-06-2014, 04:28 PM   #12
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I highly recommend the book "Understanding Exposure" by Peterson. It goes into depth about the exposure triangle and terminology. My library has a copy, I didn't even have to pay to read it!
06-06-2014, 04:45 PM   #13
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EV (exposure value) is just a standard way to express light level, just as ISO is a standard way to express light sensitivity.

You don't ever "change" EV. It is the amount of light in front of you. Don't confuse EV with exposure compensation as others have done. Adding or subtracting 1 EV is equivalent to adding or subtracting 1 stop of light, and is simply a handy shortcut - but the EV level of the scene in front of you does not change.

If a given scene is, for instance, an EV level of 10, and your meter suggests 1/100, f/4, ISO 100, then:

1/100 f/4 100
1/50 f/5.6 100
1/25 f/8 100
1/100 f/5.6 200
1/100 f/8 400

will ALL give the same exposure for an EV level of 10.
06-06-2014, 04:53 PM   #14
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Re: Program Line options
QuoteQuote:
Normal Pentax calls this "Basic Program Automatic Exposure", and it tends towards middling values for everything.
Hi-speed Priority The documentation says "prioritizes high shutter speeds" (helpful, guys). Based on experience, the basic program for shutter speed takes the focal length into account (faster speeds for longer lenses), and I'm not sure if this acts as a weighting of that factor or if it has a pre-set idea of "high" in this mode.
DOF Priority (shallow) Pretty self-explanatory: the program attempts to hold aperture as wide as possible, unless it can't to make the exposure work. Shutter speed is informed by the focal length, and how that and ISO are adjusted depend on the auto-ISO setting (see below).
DOF Priority (deep) The opposite, of course.
MTF Priority When used with a Pentax lens with electronic coupling, prioritizes the lens's technically-best "sweet spot"
The K-01 nor the K-50's 'P' mode has no Program Line settings as explained above, which seems rather pointless to me because there's already an AUTO setting which does essentially the same thing (unless I've missed something which is possible). Sure you can adjust Av and Tv in P mode, but why not just use Av or Tv in the first place?
06-06-2014, 05:04 PM - 1 Like   #15
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The Wikipedia article on EV is pretty good, particularly because it includes a table with the EV of common subjects:

Exposure value - Wikipedia, the free encyclopedia

The way I prefer to think of EV is as the convergence of shutter speed, aperture and ISO. EV is most appropriately expressed as n(ISO) where n is an integer and ISO is, well, the ISO number. At its basis, EV is a unit-less numeric representation of subject luminance. Here are the bullet points:
  • Each number on the EV scale represents twice as much luminance as the number directly below it
  • 0 (zero) is always 1 second at f/1.0 regardless of ISO
  • If the ISO qualifier is missing, ISO 100 is assumed
  • A difference of 1 EV luminance corresponds to a 1 stop difference in exposure
Now for a real world example. The exposure meter on the K-50 has a listed sensitivity range of EV 0-22. Since the ISO is not explicitly stated it is assumed to be ISO 100 yielding EV(100) 0-22. To put the sensitivity in terms of exposure settings, the meter is linear and will give valid results from this minimum:
EV(100) 0 ==> the amount of light where an exposure of 1s @ f/1.0 @ ISO 100 is appropriate
Note that this is VERY dim. Below this amount of light the meter readout will blink. On the high end the meter is linear and will give valid results up to:
EV(100) 22 ==> the amount of light where an exposure of 1/4000s @ f/32 @ ISO 100 is appropriate
Note that this is VERY bright. Above this point the meter readout will blink.

Are we confused yet? If so, there is no reason for concern since it is unlikely that you will ever need to be concerned about any of the above in normal shooting, particularly if you use auto-ISO. The time it is used is when you are attempting precise determination of exposure for particular parts of the scene.*

Summary:

You can express subject luminance in terms of a single EV number which may correspond to any of an infinite combination of exposure settings. That EV number represents twice as much light as the preceding step and half as much light as the next step in the integer series of EV values.


Steve

* A common situation for difficult lighting conditions where you might use a hand-held meter and intend to "place" exposure on a specific part of the subject. Both of my hand-held meters provide output as EV as an option.

Last edited by stevebrot; 06-06-2014 at 05:24 PM.
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