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Old Mar 7, 2008, 10:56 PM   #1
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what on earth is ev exactly? i have looked it up on the internet and just get a bunch of confusing math and more terms i dont understand, or terms i thought i understood but must not because they are not making any sense to me in this context.

here are som basic questions

what will changing ev do to my pictures?

Does changing the ev actual change your aperature or change your shutter speed.

is it basically a way to make your pictures more bright or dim?

when people advocate stepping a lens down is this done with the ev while on auto or program mode?

i now that these must be silly questions to alot of you, but i have no idea about this.

by the way i use a KM maxxum7d if that makes any difference
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Old Mar 7, 2008, 11:44 PM   #2
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EV is Exposure Value. There is an absolute standard for it, but in the context you are asking, it is relative to what the camera metering chooses as 'correct '.

what will changing ev do to my pictures?

It will give you overall lighter or darker pictures. A change of +-1 EV changes the amount of light by a factor of two.


Does changing the ev actual change your aperature or change your shutter speed.
Yes, it changes one or the other depending on the mode you are using. If shooting in Aperture priority, it will change the SS, and if shooting in Shutter ppiority, it will change the aperture.(Except in the case of some cameras which change the ISO sensitivity in one or more shooting modes) For a given sensitivity. the only way to change exposure is to change shutter speed or aperture (or both)

when people advocate stepping a lens down is this done with the ev while on auto or program mode?
Stopping down is closing down the aperture to obtain greater depth of field or to get longer shutter opening for special effects. It is normally done in manual or Aperture priority mode, since the idea is to pick the aperture and let the camera choose the shutter speed.

These are basic, not silly questions. The concepts take a bit of time to really hit home, and confuse most of us when starting out, until that moment when it all comes together. That is what the forums are for - to ask questions and be answered.



brian
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Old Mar 8, 2008, 12:16 AM   #3
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"In photography, exposure value (EV) denotes all combinations of camera shutter speed and relative aperture that give the same exposure. The concept was developed in Germany in the 1950s (Ray 2000), in attempt to simplify choosing among combinations of equivalent camera settings. Exposure value also is used to indicate an interval on the photographic exposure scale, with 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a "stop."[1]
Exposure value was originally indicated by the quantity symbol E[sub]v[/sub]; this symbol continues to be used in ISO standards, but the acronym EV is now more common elsewhere."

That's what Wikipedia has to say before the technical gobbledygook starts.

As far as I'm concerned, EV has just replaced the old f-value, or f-stop from the days of yore when film was the only game in town.

A one f-stop difference in light intensity is more-or-less equal to a doubling or halving of the light available to reach the film (or sensor). For example, with a lens aperture setting, f/8 lets through half the light as f/5.6 and twice the light of f/11.

For shutter speed, 1/250 sec. lets through half as much light as 1/125 sec. and twice as much as 1/500.

Each one of these standard aperture and shutter speed steps was called (and still is by many) a "stop". You can just replace the word "stop" with "EV".

Dr.Theo asks: what will changing ev do to my pictures?

It'll make them darker or lighter. Hey! that answers another question further down!

Does changing the ev actual change your aperature or change your shutter speed.

Yes, but it's an interdependent relationship. If your camera determines a certain combination of lens aperture and shutter speed to give what it thinks is a correct exposure, you can change either variable and the other will change also to maintain the correct EV. Such as if the camera gives you an aperture of f/8 and a shutter speed of 1/500, if you manually change the aperture to f/16, the camera will change the shutter speed to 1/250 to maintain the same EV.

is it basically a way to make your pictures more bright or dim?

Yes.

when people advocate stepping a lens down is this done with the ev while on auto or program mode?

Following from a previous answer, when you step a lens down, you are changing the aperture to allow less light to come through the lens diaphragm. The act of doing so changes the EV by whatever amount you step down. Going from f/4 to f/11 is a difference of minus 3 EV (f/5.6 = -1 EV, f/8 = -2 EV and f/11 = -3 EV). It doesn't really matter what mode you are using on the camera. Purists will probably say that in this case it is more proper to use the word "stop", since EV is more properly a value arrived at by working the aperture and shutter speed in concert to ensure that a certain intensity of light makes it to the sensor, but I've seen the two terms used as practical synonyms.

Stepping the exposure up with the lens aperture would yield positive EV numbers -- going from f/11 to f/4 is a change of +3 EV.

I hope some of this makes more practical sense than the math charts! :?

Grant


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Old Mar 8, 2008, 12:28 AM   #4
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great, that helps alot thank you.

now one more question and this one has me like this:?

what is changing the ev doing when i change my to say +1 in program mode i see a shutter speed of 3 and my f is 2.8

then when i set the ev to zero go into manual mode set my speed to 3 and my F to 2.8 and i get 2 very different pictures,

the one with the EV dialed up one gives alot brighter some what washed out picture in the bright areas with greater depth , and color contrast in the dark areas

and with the set up of the ev at zero i get a picture that is much clearer and sharper throughout the whole frame, but i lose the depth and color in the darker areas

================================================== ========

so what i have gathered is that changing your EV is like going to middle spots on the apperatures, so instead of jumping from 2.8 to 3.5 you can effectively set it up to go somewere in between there or past your maxximum or minimum aperature sizes?
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Old Mar 8, 2008, 1:37 AM   #5
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When you set a camera's exposure compensation function to anything other than zero, you are telling the camera to add or subtract light from what it thinks is the correct exposure:

At a setting of zero, the camera sets the aperture and shutter speed (in program mode) or just the shutter speed (in aperture priority mode), etc., to give what it judges to be the correct exposure.

At a setting of +1, you are telling the camera to expose at 1 EV more than what the camera judges to be the correct exposure.

At a setting of -1, you are telling the camera to expose at 1 EV less than what the camera judges to be the correct exposure.

Dr.Theo says: what is changing the ev doing when i change my to say +1 in program mode i see a shutter speed of 3 and my f is 2.8

In this case, the numbers you are seeing (shutter speed of 3 and my f is 2.8) are the settings for the correct exposure as judged by the camera. You have set the exposure compensation factor to +1, so, in this case the camera will expose at 1 EV higher (twice as much light) as it thinks it should -- resulting in a brighter exposure.

then when i set the ev to zero go into manual mode set my speed to 3 and my F to 2.8 and i get 2 very different pictures,

Right! Because you set the exposure compensation factor back to zero, the camera exposes the way it wants to -- which is 1 EV less than when you set the EC factor to +1! So, you end up with a darker exposure than in the first shot. Changing to manual mode had nothing to do with it. You would get the same darker exposure with the camera still in program mode. It was changing the exposure compensation factor back to zero that made the picture darker.

Exposure compensation is useful under high contrast lighting situations where the camera's light meter might not give a good exposure of the subject.

Shooting a person's face against a bright sky can lead to underexposure of the face because the light meter takes an average of all the light it sees and the average is skewed by all the bright light from the sky. To help avoid this, you can use spot metering (if your camera offers it) to read more of the face and less of the sky, giving a better exposure of the face. Or, you can use the exposure compensation function to tell the camera to expose at a higher EV than it thinks it should (+1, + 1.5, whatever).

The camera is really pretty dumb, which is why they give you ways to override the camera's judgment so that you get the results that you want.

Grant

Edit: Sorry, I missed that last bit where Dr.Theo says: so what i have gathered is that changing your EV is like going to middle spots on the apperatures, so instead of jumping from 2.8 to 3.5 you can effectively set it up to go somewere in between there or past your maxximum or minimum aperature sizes?

Not really...EV is pretty much just the total amount of light that the camera figures has to reach the sensor to produce a correct exposure. The light gets to the sensor through the cooperative efforts of the lens diaphragm opening (aperture) and the shutter curtains. So, I suppose you might say that, philosophically, :roll: there could be three types of EV: The real, or the over-all EV, and the individual settings that are made to the lens aperture and to the shutter speed, the standardized measuring units of which are called EV's. Each unit measure increase or decrease in either the lens aperture (the f/numbers) or the shutter speed (1/30, 1/60, 1/125) is commonly referred too as one EV. The combined individual EV settings of the lens aperture and shutter speed determine the over-all EV in effect when the exposure is made.

Oh, my god.... I think that I've gone into an unrecoverable tail-spin. I know what I mean but I don't know that I am putting it in such a way that anyone else could know what I mean! If you have to bring philosophy into it, it might be time to cut your losses and go home.

At least I left the ISO adjustment out of it.


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Old Mar 8, 2008, 5:10 AM   #6
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well i suppose for the time being that i will just have to be happy with the fact that uping my ev makes pictures brighter even with out being able to open the apperature further or as far as i can tell holding the shutter open longer. with out understanding the how.

:G

I am sure i am just not fully understanding yet. thanks for all your help guys. it was alot more informative then the math charts online


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Old Mar 8, 2008, 6:01 AM   #7
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Dr.Theo wrote:
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well i suppose for the time being that i will just have to be happy with the fact that uping my ev makes pictures brighter even with out being able to open the apperature further or as far as i can tell holding the shutter open longer. with out understanding the how.
When you use Exposure Compensation, that's exactly what it does (changes either the aperture or the shutter speed to give you a darker or lighter exposure).

The only other variables are ISO speed (how sensitive the sensor is to light), and how much light you have. If you saw different exposures with the same shutter speed and aperture, then either the ISO speed changed (and it will not stay set when changing modes in some cameras) or the amount of light changed.

If you try to use a shutter speed that's too fast for the lighting, ISO speed and aperture, you'll get underexposed images (too dark).

If you try to use a shutter speed that's too slow for the ISO speed, lighting and aperture, you'll get overexposed images (too bright). ;-)

So, you can't just use anything you want to. Use the camera's meter to let you know if your settings will result in under or overexposure if you want to use manual exposure.

The camera has to keep the shutter open long enough to expose the film or sensor and the same concepts apply to film or digital.

The meter in your viewfinder is designed to let you know if your settings are going to result in a darker or brighter exposure, compared to the way the camera's metering thinks it should be exposed (and the metering mode and what you meter on will impact what the camera thinks is needed).

When you are shooting in non-manual modes, this meter is used for a feature called Exposure Compensation. If you set the camera so that the pointer is higher than zero, it will take a brighter exposure than the camera would have used by default. If it's set to a -EV value, it will expose it darker than the metering thinks is needed.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both of these variables, although most cameras have a meter that shows you how your settings are impacting exposure).

Exposure Compensation lets you alter the way a camera's autoexposure/metering algorithms expose an image (brighten or darken it compared to the way the camera metered the scene). It's one of my most frequently used settings on most cameras.

A +EV value gives you a brighter exposure. The camera uses a slower shutter speed and/or larger aperture (smaller f/stop number) to get a brighter exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

A -EV value gives you a darker exposure. The camera uses a faster shutter speed and/or smaller aperture (higher f/stop number) to get a darker exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

If you're in Av Mode (Aperture Priority) and use Exposure Compensation, the camera will vary the shutter speed (since you're setting the aperture). If you're using Tv (Shutter Priority) mode and use Exposure Compensation, the camera will vary the Aperture (since you're controlling the shutter speed).

If you're in Auto (or other similar modes), the camera may vary aperture or shutter speed when you use Exposure Compensation. In low light, since your aperture is already wide open, it varies shutter speed if you use a -EV setting.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both).

This is the concept you need to become familar with, so I'll make it bold:

Correct Exposure comes down to the amount of light, the ISO speed, the shutter speed, and the aperture. A variety of combinations will produce identical exposure.

These exposure calculators and simulators may help you understand it better, too:

http://www.robert-barrett.com/photo/...alculator.html

http://www.photonhead.com/simcam/shutteraperture.php

You use Exposure Compensation if you want a brighter or darker image compared to what the camera's metering would normally give you in the same conditions if you're not using manual exposure.

An example of when you may want to use a +EV setting is for a backlit subject, where the subject would normally be much darker than the rest of the image. Since the camera has a limited dynamic range, it doesn't know that you want the dark subject exposed properly (at the expense of the rest of the image). So, you can make the darker subject brighter for correct exposure (which might cause the rest of the scene to be overexposed some).

If your subject is much brighter than the rest of the image (for example, direct sunlight hitting your subject, even though most of the photo is in shadows), you may want to use a -EV setting for Exposure Compensation so that your subject is not overexposed (making the rest of the image darker, too).

The camera has a limited range of bright to dark that it can capture. So, it makes choices so that most of the iimage is correctly exposed, depending on your metering mode. Sometimes that may not be what you want. That's where exposure compensation comes in.

In manual mode, you're controlling the Exposure with your settings for aperture and shutter speed (and your meter will show you if the camera thinks they will result in under or overexposure, much in the same way it will show you if your Exposure Compensation settings in non-manual modes will expose brighter or darker).

Although we have a lot of automation with newer camera models (both film and digital), the concepts of exposure still work the same way as they do with old manual only cameras.

You still have only 4 main variables to take into consideration for exposure (and I use the term "main" since there are a lot of nuances to how you measure the light (for example, your metering mode), as well as different film characteristics if shooting film, and camera settings if shooting digital for the desired tone/contrast curve within an image and more). Once you have a better idea of how these 4 variables work together to give you a properly exposed image, the other fancy features will make more sense.

1. Light (typically measured as EV for Exposure Value in Photography).

2. Aperture (which works similar to the pupils in your eyes, where you can open up the aperture iris wider to let in more light, or close it down to let in less light). If you let in more light with a wider aperture, you can expose the film or sensor faster. If you let in less light with a smaller opening, it takes longer to expose the film or sensor. Note that aperture is normally expressed as f/stop, which is a ratio between the focal length of the lens and the diameter of the aperture iris. So, smaller values represent a larger iris diameter.

3. ISO speed. This is how sensitive the film or sensor is to light and is the same thing as the older ASA rating for film. The higher the ISO speed, the faster you can expose it.

4. Shutter Speed (this is how long the camera's shutter stays open to expose the film or sensor).

IOW, it all boils down to how senstive the film or sensor is to light (which you control via the ISO or ASA speed of the film you use with film, or the ISO speed settings you use with digital), and how much light you need to let it see to "expose" the iimage (which you control via the aperture opening size and shutter speed).

So, you've got lots of fancy features on newer cameras to automate what settings it uses, and let you vary it's behavior to expose an image darker or brighter than the camera's metering would normally expose it. But, it really boils down to the camera changing the same things you had to worry about with a strictly manual camera without a fancy metering system, Automatic Exposure modes, etc.

So, Exposure Compensation is just another tool you have to work with, allowing you to expose an image brighter or darker than the camera's metering would normally expose it when using non-manual modes, and it varies shutter speed and/or aperture to accomplish that.

With manual exposure, you're controlling those variables (and your viewfinder scale for the metering will let you know if your setttings are exposing brighter or darker than the camera's metering thinks is needed).

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Old Mar 8, 2008, 6:08 AM   #8
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well that explains it, the iso must have changed when i changed modes. that was the only varyable i did not check.



thanks
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Old Mar 8, 2008, 6:56 AM   #9
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Quote:
As far as I'm concerned, EV has just replaced the old f-value, or f-stop from the days of yore when film was the only game in town.
Film bodies had EV compensation as well, and EV doesn't really replace f-stops. EV basically allows you to compensate for situations where your meter may be less than reliable. Metering isn't a perfect science and there are situations where values must be adjusted, such as snow or beach scenes. EV allows you to do so quickly and easily
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Old Mar 8, 2008, 6:20 PM   #10
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Dr.Theo wrote:
Quote:
great, that helps alot thank you.

now one more question and this one has me like this:?

what is changing the ev doing when i change my to say +1 in program mode i see a shutter speed of 3 and my f is 2.8

then when i set the ev to zero go into manual mode set my speed to 3 and my F to 2.8 and i get 2 very different pictures,

the one with the EV dialed up one gives alot brighter some what washed out picture in the bright areas with greater depth , and color contrast in the dark areas

and with the set up of the ev at zero i get a picture that is much clearer and sharper throughout the whole frame, but i lose the depth and color in the darker areas

================================================== ========

so what i have gathered is that changing your EV is like going to middle spots on the apperatures, so instead of jumping from 2.8 to 3.5 you can effectively set it up to go somewere in between there or past your maxximum or minimum aperature sizes?
You can't exceed the minimum and maximum apertures, which are built into the lens itself.

When you look at the camera settings, and see the SS and A values, and the EV setting, you are not seeing what the camera is actually doing. For that, you need to look at the photo with an Exif reader, which will tell you the camera settings, and also, the actual values. I the case of a shutter speed of 3 (which would be 1/3 second, I believe, on your camera), the actual shutter speed, after exposure compensation (which is what the EV value is) could be substantially different. For the EV value you give, it would normally be thought to be 2/3 s. The camera metering is only going to give you the equivalent exposure in preset values, though, so the actual speed could vary somewhat, and the compensated speed could also be off.

Even if you set compensation to '0', and then make the same exposure in manual, there may be a difference due to the 'rounding off' of the camera metering.

It gets less confusing after a while.:-)

brian
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