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Archive 2013 · Way to measure amount of light?
  
 
Paul.K
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p.1 #1 · p.1 #1 · Way to measure amount of light?


So say I am in a church shooting f/1.4, ISO 3200, 1/100. Is there a way that I can calculate how much light there is in the church? I've heard of EV (exposure value), which seems close to what I want but it appears to be independent of ISO setting. Since ISO is not included, it doesn't really capture what I want. Obviously this is assuming that whatever exposure settings (aperture, shutter speed, ISO) are the correct ones for a good exposure.

The idea would be I'd get a number like 16 EV or something and then I could say something like, "Well, with 13 EV you can choose ISO 200, f/8, and use 1/400 of a second."






Feb 06, 2013 at 12:49 AM
scottam10
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p.1 #2 · p.1 #2 · Way to measure amount of light?


Strictly speaking EV is independent of ISO, however it is common practice to express luminance in EV based on ISO 100 speed

(eg Sunny 16 rule)

http://en.wikipedia.org/wiki/Exposure_value

To convert between iso values, you could just add (or subtract) 1EV for every stop the iso has been shifted by.

eg. from the wikipedia table f/1.4 1/125 second @ iso 3200 would be EV8 (at iso 3200).

If you want to convert back to iso100 equivalent: iso100 is 5 stops less light sensitive than iso3200. Therefore this is equivalent to EV3 (at iso 100)
- you can easily convert between iso values this way

From the table, if you were at iso100 you just go 5 rows higher ie you would need a 1/4 sec exposure at f/1.4



Feb 06, 2013 at 01:18 AM
Kerry Pierce
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p.1 #3 · p.1 #3 · Way to measure amount of light?


I'm not sure that I understand what you're asking for, so please don't get insulted if I've not gotten it right.

Seems to me that you're asking for a handheld light meter. I used one a lot back in the film days, but with the advent of relatively accurate TTL metering, I quit using one. Regardless, light meters come in a variety of flavors and costs. Here's a link to a B&H page on light meters.

They can get quite expensive.

If you already know this stuff and are asking about something else, please accept my apology.

Kerry



Feb 06, 2013 at 01:55 AM
Todd
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p.1 #4 · p.1 #4 · Way to measure amount of light?


Are you asking for the amount of lumens in the room? Or, are you wanting to measure the light with a light meter to aid in exposure?


Feb 06, 2013 at 03:44 AM
James R
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p.1 #5 · p.1 #5 · Way to measure amount of light?


Your camera reads the amount of light entering the lens. The measuring light in a room is easy, you simply measured number of lux by counting the number of lumen per square meter. So figure out the number of square meters in the church and then measure....

Anyway, is it possible you are worrying about ambient light effecting your image by back lighting your subject?



Feb 06, 2013 at 05:22 AM
runamuck
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p.1 #6 · p.1 #6 · Way to measure amount of light?


The sunny f16 rule is based on a shutter speed that is the reciprocal of the ISO. ISO 100 would use a shutter speed of 1/100 or even 1/125. ISO 400 would be 1/400 shutter speed

You don't need to worry about lumens, lux, or anything else.



Feb 06, 2013 at 06:31 AM
 

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Paul.K
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p.1 #7 · p.1 #7 · Way to measure amount of light?


Thanks for all the replies. I appreciate you guys taking the time to sort me out.

I am not really worried about lumens, lux, external light meters, etc. scottam10's answer is most what I was looking for. I think that is explanation gives me what I need.

@Kerry, I appreciate the response but I guess I wasn't fully clear in my first post. Thanks for responding.

@Todd/runamuck, thanks. I am not specifically looking for lumens or any quantity that is applicable outside of photography necessarily.

@James, haha, I enjoyed your joke. The purpose of my question is more to compare the relative light available between photos already taken.




Feb 06, 2013 at 08:12 AM
HelenB
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p.1 #8 · p.1 #8 · Way to measure amount of light?


There is a 'brightness value' that combines EV and ISO, but it has got a little confused by the use of EV at ISO 100, which seems to have become LV (light value) by making the 'speed value' of ISO 100 equal to zero. Originally ISO 100 (ASA 100 at the time - 1960, when this was written in an ASA standard) had a speed value of 5. Therefore there are now two different sets of values to choose from, should you wish to use such a system.

Ev = Tv + Av = Bv + Sv
Exposure value equals time value plus aperture value, which also equals scene brightness value plus film or sensor speed value. (All values are logarithmic.)

Therefore
Bv = Tv + Av - Sv

So the brightness value is 5 less than the EV at ISO 100.

The usual zeroes:
Tv = 0 at 1 s
Av = 0 at f/1
Sv = 0 at ISO 3 (nominal)

With a change of one stop being a change of one in the value.




Feb 06, 2013 at 09:28 AM
Graystar
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p.1 #9 · p.1 #9 · Way to measure amount of light?


Paul.K wrote:
So say I am in a church shooting f/1.4, ISO 3200, 1/100. Is there a way that I can calculate how much light there is in the church? I've heard of EV (exposure value), which seems close to what I want but it appears to be independent of ISO setting. Since ISO is not included, it doesn't really capture what I want. Obviously this is assuming that whatever exposure settings (aperture, shutter speed, ISO) are the correct ones for a good exposure.

The idea would be I'd get a number like 16 EV or something and then I could say
...Show more

You can do what you want to do implicitly with a gray card.

First, you meter a gray card. In manual mode, the center of the exposure meter display represents the current EV level of the scene. Meanwhile, the exposure indicator represents the EV of the aperture/shutter combo. By changing aperture/shutter, you move the exposure indicator along the exposure meter display. When the exposure indicator is in the center of the exposure meter display, the EV of the aperture/shutter combo matches the EV of the meter. This situation is called Standard Exposure and is represented by the APEX equation that was already mentioned...
EV=Av+Tv=Bv+Sv

Exposure Value has two meanings. First, it is a combination of aperture and shutter. Second, it is a combination of scene luminance and ISO. When the two EV values match, you have Standard Exposure. In your camera, the center of the exposure meter display always represents the EV of the scene luminance/ISO combo, and the moving indicator always represents the EV of the aperture/shutter combo.

Technically, you could look up your aperture/shutter combo and get the numeric EV value of the scene. However, as a stand-alone piece of information, it's not very useful. What's important is how EV relates...not what EV is. All that really matters is that the EV of aperture/shutter match the EV of scene/ISO...and you don't actually need to know the EV in order to do that. After metering a gray card, you simply center the exposure indicator in the exposure display, and you're done.

A long time ago...before cameras had built-in meters...knowing the EV value was important. Camera lenses had aperture, shutter, and EV scales on them. Using a meter or exposure calculator, and considering film sensitivity, you would determine the EV of the scene. You'd then set that EV on the lens. Now, when you changed the aperture, the shutter would automatically change to maintain the set EV. And when you changed shutter, the aperture would change. In a way, these lenses were the precursors to the modern Aperture priority and Shutter priority modes. After all...maintaining EV equivalence is what the "Auto" of auto modes is all about.

In manual mode this is very limiting. The only time you have the correct EV is when you're actually metering the gray card. When you remove the gray card, you're left with the correct aperture/shutter, but no good way to change them easily. This problem is addressed in different ways, depending on the camera maker. Pentax cameras have a function where pressing the AE-L button will allow you to change both aperture/shutter with one control dial. Nice, but leaves ISO out of the equation. The better solution is to use AE Lock. To do this on a Nikon means that you have to change your "Auto Meter-off Delay" (now called "Standby Timer" on the D600) from the default 6 seconds to 10 minutes or longer (I use 30 minutes.) Also, change your AE-L button to "AE Lock (hold)"

Now, you simply spot meter your gray card, press the AE-L button, and you've locked in the EV. It's just like setting the EV value on those old lenses. You can now set the camera to any mode (I prefer A mode) and turning the dial will change both aperture and shutter, maintaining your locked exposure. If you change ISO, then shutter (in A mode) will change to maintain the locked exposure.

So you see, by utilizing the functions of the camera you can achieve the same goal. You wanted to know the EV level so that you can select exposure values. By locking your exposure in the camera, you implicitly "know" the EV level. And when using auto modes, the only choices you get for aperture/shutter/ISO are those that provide your locked exposure. And don't forget that you can always tweak that exposure using the Exposure Compensation function. All-in-all, using an extended AE-Lock with a gray card represents a significant amount of EASY control over exposure.



Feb 07, 2013 at 05:02 AM
Paul.K
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p.1 #10 · p.1 #10 · Way to measure amount of light?


Wow, thanks Helen and Graystar for the additional information. Those equations are what I was looking for too. Thanks.

Graystar, thanks for typing up all of that and sharing your knowledge. Having never used a gray card, I've always just assumed they were just for white balance. Using them like that is a technique I've not heard of. Thanks for explaining it so thoroughly



Feb 07, 2013 at 05:35 AM
Two23
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p.1 #11 · p.1 #11 · Way to measure amount of light?


This is exactly the kind of stuff you learn when you shoot large format. You have to do it all--there are no magic buttons and the cameras have no batteries. For weddings inside a church, I normally still use a hand held incident lightmeter to get started. I've been trusting them for nearly 20 years.


Kent in SD



Feb 09, 2013 at 03:31 AM





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