Exposure values vs light values

[Compaq]

Our camera measure the light reflecting off our subject. Our subject has a luminance, a light value. When we set our camera's settings, shutter speed and aperture, we set an exposure value. Clearly, the LV and EV must somehow be related to form the final image, or exposure. We connect the exposure value to the light value through our film's speed. The exposure value decide the amount of light hitting the film, the light value describes how bright our subject is and the film speed decide how much light must hit the film in order to render the image more or less "as we saw it". That was poorly formulated. The LV and the EV are independent values, but we connect them through the film/sensor speed.

Is the point to match the exposure value to the light value by selecting the appropriate film/sensor sensitivity? Part of me don't believe that is correct, but I need to ask nonetheless.

On my lens, I set the shutter speed and the aperture. There's a value there that changes with these settings. I believe this is the EV. In the viewfinder, I read another value, a value that depends on the brightness of the scene (light value) and the film speed. What value is this, exactly?

I thank you all for help 🙂

 

[KenR]

I'm not sure what you're really asking. The light value and the exposure value are only loosely related - film speed can be varied (see Ansel Adams and the Zone System) by development. Remember the example of the person dressed in black standing in front of the black wall vs. the same person, now dressed in white standing in front of the white wall. Both should get the same exposure, but the meter in your camera will be fooled and overexpose the first and underexpose the second. That's why some people swear by incident light meters, and others use spot meters to get more precise exposure readings.

 

[Gumby]

I think you are struggling with the definition of EV. There is a known and consistent relationship between LV and EV. Part of the problem is that the terms are not universally used "correctly".

EV was a 1950s concept of Compur to make it easier to set cameras. The linked EV mechanism allowed the photographer to set one exposure value (the EV) and that locked so that when the shutter was adjusted the same exposure was assured.

Thus, EV is essentially an index number to define a set of exposure settings (film speed + aperture + shutter speed) for a LV. For example, for a LV of some number (fairly bright light) the EV might be 13... which would equate to f/11 at 1/60 (and every other equivelent) at ISO 100.

 

[Gumby]

Is the point to match the exposure value to the light value by selecting the appropriate film/sensor sensitivity? Part of me don't believe that is correct, but I need to ask nonetheless.

If I understand you correctly, then this is EXACTLY what the light meter does. The meter measures light and a conversion is made for that light value to an exposure.

For internal caemra meters that exposure is then expressed to us by the meter display as an aperture + shutter speed considering the film speed that was set.

For a hand-held meter things are a bit more mechanical and that "translation" from LV to expsure is done via the hand-held meters calculator dial.

 

[Compaq]

But what exactly does that number in the viewfinder represent?

The number on the lens would be the exposure value? Because the EV is just a set of camera settings defined from the zero point EV0 (f/1, 1 second)?

 

[sevo]

LV and EV are often used synonymously - indeed, LV does not have a standardized (or even reasonably common) definition. See http://en.wikipedia.org/wiki/Light_value for the whole mess.

 

[Gumby]

But what exactly does that number in the viewfinder represent?

The number on the lens would be the exposure value? Because the EV is just a set of camera settings defined from the zero point EV0 (f/1, 1 second)?

It depends on the viewfinder. Most viewfinder displays have 2 numbers displayed: shutter speed and aperture (f stop). I don't personally know a viewfinder that displays a single EV number. That is more common on hand-held meters, but even they generally show EV plus aperture + f stop.

The number on the lens itself is most often the aperture (AKA f stop). When a shutter is in the lens you'll see shutter speed and maybe EV also.

 

[Gumby]

Here's something to consider to help you determine which number you are referring to. Generally speaking and not accounting for archaic cameras:

EV will be a whole number between 1 and 17 (or so). EV will generally be associated with an interlock system that has the two numbers, below, next to each other.

Aperture will be a number like 1.4, 2, 4, 8, 16, 32

Shutter speed will be a number like 1, 2, 4, 8, 15, 30, 60, 100, 125, 250, 500... where the number is the denomimator so "15" really means 1/15 second.

 

[sevo]

I don't personally know a viewfinder that displays a single EV number.

Well I have or had the Olympus 35SP and several generations of Hasselblad meter prisms. There must be some more - but the concept was only popular for the few years between the introduction of TTL and fully coupled meters.

 

[Compaq]

Maybe I should have mentioned that I use a 35 SP 😉 This camera operates in a "old-fashioned" way with EV numbers in the viewfinder. The aperture ring show, obviously, aperture values, and the shutter ring show shutter speeds. But, there's a window on there that shows an EV number. Also, in the viewfinder, there's the match needle that goes along an axis of EV number things. I read a number, for example 15 on a bright day, and match that with the number on the lens.

Maybe not all are familiar with how this camera works, but my question isn't what aperture and shutter speeds are, no need for that 😉, but rather how the number in the viewfinder is determined... I thought that EV only was a set of camera settings (read: aperture/shutter speed). This number in the viewfinder varies with the scene's brightness. That leads me to believe that I'm reading the light value of the scene in the viewfinder.

Here's a theory: If I set the ASA dial to 100, and point the camera at a scene, THEN I would be reading the light value (based on the camera's calculations). If I turn ASA to 200, then the calculations in the camera compensates by showing the LV plus 1.

I hope you're understanding my confusion 😛

 

[sevo]

If I set the ASA dial to 100, and point the camera at a scene, THEN I would be reading the light value (based on the camera's calculations). If I turn ASA to 200, then the calculations in the camera compensates by showing the LV plus 1.

Right, that (i.e. LV = EV@100ISO) is the most common definition. But there are others.

 

[skibeerr]

Read this, our own head bartender's

http://www.cameraquest.com/olysp.htm

EV is a light value that refers to a aperture/speed combination, change aperture and speed follows automatically. See Hasselblad and Rolleiflex TLR's.
EV is also camera specific i.e. If your (older)camera does not have this feature it is of no concern and you're back to aperture/speed reading and setting.

EV was the mechanic predecessor of the AE system 🙂

 

[Gumby]

... but my question isn't what aperture and shutter speeds are, no need for that 😉, but rather how the number in the viewfinder is determined... I thought that EV only was a set of camera settings (read: aperture/shutter speed). This number in the viewfinder varies with the scene's brightness. That leads me to believe that I'm reading the light value of the scene in the viewfinder.

OK, I understand your question now. You want to know HOW the camera meter is determining exposure. The sensor, a CDS cell in your camrea's case, indeed measures the amount of light (LV) and then the meter circuit adjusts that for the ISO setting to give the displayed number -- the EV you need to set on the lens. The internal "logic" of how it does that basically follows the formula that Sevo gave. From that EV setting, you select your desired combination of speed + aperture. So EV will always be a function of LV and your observation is absolutely correct: more light (higher LV) results in a higher EV value than less light.

 

[Richard G]

Your camera has a light meter. The light meter is for metering the light. The light reflected from the scene is metered by the meter cell, CdS, which alters output commensurate with the reflected light input. Match needle systems generate an EV, arbitrarily defined, where an EV of 15 coincides with the amount of reflected light to properly expose ISO 100 film, loaded in the camera, with a setting of 1/125s shutter speed for the camera's shutter and f16 aperture of the camera's lens opening. There are hand held meters and camera meters that bypass the intermediate EV step altogether.

As mentioned above, EV values had greatest utility with compur shutters, where a setting of shutter speed and aperture on the lens barrel allowed one setting with an EV value and coupled rotation of the lens barrel apparatus allowed a convenient doubling of shutter speed with a doubling of aperture width to maintain the correct exposure for all available variables. Increase in shutter speed is coupled to a decrease in f number.

 

[Compaq]

Thanks guys.

One question that may be a little too detailed: the conversion of the LV to the EV needed to properly expose ASA-100 film is based on which function? I'd like to see the function, basically. But you say that the definition isn't absolute, it varies. Basically I'd like to know the empirical relationship between measured light and the film speed. 🙂

I study chemistry, and are therefore, perhaps too, interested in details 😛

 

[Gumby]

Thanks guys.

One question that may be a little too detailed: the conversion of the LV to the EV needed to properly expose ASA-100 film is based on which function? I'd like to see the function, basically. But you say that the definition isn't absolute, it varies. Basically I'd like to know the empirical relationship between measured light and the film speed. 🙂

I study chemistry, and are therefore, perhaps too, interested in details 😛

The meter works in accordance with the general formula sevo provided, and the method described by Richard. What sevo stated IS the formal definition of EV. Calling it arbitrary might get mistaken with "random" but EV is anything but random.

You might need to seek a primer on the design and calibration of circuitry involved in a CdS meter to get an answer more fitting to your needs.

I wish I could answer your question in more detail, but explaining curciutry is beyond my abilities. I'm generally happy to understand what exposure information I am getting and how to use it (of change it) to suit my phoitographic needs. I respect your detailed inquisitiveness. 🙂

 

[Gumby]

p.s. There are ISO standards for most of this topic -- ASA/ISO film speed definition and determination, and EV. The ISO standards are not particularly easy to access but perhaps your local or school library can get them.

 

[Richard G]

Returning to the title of your thread and your most recent question, it is better not to think of a function, in the mathematical sense, that links EV to LV, although this is often stated. The EV and the camera settings for aperture and shutter speed and ISO are linked in that way, but LV, being related to the reflected light depends on the properties of the subject. For instance, bright sunshine, "Sunny 16", correlates to an EV of 15, ISO 100 and 1/125s and f16. But in bright sunshine on a beach, an exposure compensation for the increased LV, the increased reflectivity of sand and glinting water might require further stopping down to f22 or shortening the exposure to 1/250s. With incident readings at the beach or in the city, the reading in direct sun will be the same, but the LV in those places will be different. Photographing Ghiberti's bronze baptistry doors in shade in Florence will require either a reflected reading governed by the LV which determined say f2.8 at 1/125, with an EV in camera of 10, or an incident reading of f4 at 1/125 based on the ambient light, but you open up one stop to get the detail. Conversely, your German girlfriend's face in front of those doors will give a different LV in the same light, and this might be f4.5 at 1/125 but you open up half a stop determining from experience a different EV for the exposure you want to have for that shot. So LV is related to subjects and their reflectivity and is different for the same ambient light depending on the nature of the subject, and the EV is the exposure you set, overriding the brute ignorance of a reflected light meter reading.

 

[geotrupede]

if you want to have fun with octave / matlab try the following: it is a function to generate tables with f,t,EV,lux per given ISO


copy below and save to 'exposure.m'
then run octave and just type 'exposure(100)' for 100 ISO

have fun

G

----------------------------------------------------------------------
function exposure(ISO)
S = ISO;
f = [1 1.4 2 2.8 4 5.6 8 11 16 22] ;
t = [1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 1/4000 1/8000];


for i = 1:size(f,2)
for j = 1:size(t,2)

EV100 = log2((f(i)^2)/t(j));
EVS = EV100 + log2(100/S);

L(i,j) = 2^(EVS-3);
E(i,j) = 2.5* 2^EVS;
EVS_out(i,j) = EVS;

end
end


save -ascii EV.txt EVS_out;
save -ascii L.txt L;
save -ascii E.txt E;
------------------------------------------------------------------

 

[Freakscene]

Here is a good explanation of why exposure and luminance are different, especially if you want a scene to look how you want it to look, not how the camera or meter thinks it should look.

Marty