Go Back   Steve's Digicams Forums > Digicam Help > General Discussion

Reply
 
Thread Tools Search this Thread
Old Feb 14, 2006, 2:14 AM   #1
Senior Member
 
Join Date: Jan 2006
Posts: 142
Default

Say 2 cameras have the same settings for EV but one is fixed at ISO 160 and the other at 200. Could i set the 2 to the same setting?



Like say the 200 needs +0.9 where the 160 needs +1.2 or somthing? can you calculate that some how and get the same exposure?
BowerR64 is offline   Reply With Quote
Sponsored Links
Old Feb 14, 2006, 5:36 AM   #2
Moderator
 
Nagasaki's Avatar
 
Join Date: Jan 2004
Posts: 897
Default

Not really clear what you are asking. For a standard exposure with no EV adjustment the difference in ISO would give a different shutter speed or aperture. The differeence between 160 and 200 is very small about half a stop. The difference between 100 and 200 is 1 stop so if I use that as an example if you used ISO 100 and the correct exposure was 1/125 at F8 the 200 ISO setting would require half the light so would use 1/250 at F8 or 1/125 at F11.A change of +1EV would mean a change in exposure of 1 stop regardless of the ISO used the difference would be in the starting point.
Nagasaki is offline   Reply With Quote
Old Feb 14, 2006, 9:35 AM   #3
Senior Member
 
Join Date: Oct 2003
Location: Indian Rocks Beach, FL
Posts: 4,036
Default

EV is a correction after a shot is metered for the existing light, aperture, shutter speed and ISO. It doesn't matter whether the camera is properly metered at f2.8, 1/500 and ISO 50 or at f5.6, 1/1000 and ISO 400 - a +1.0 EV increases your exposure by one f-stop.

The camera has to do some computing to coordinate the settings and it is fully capable of calculating f-stops.

slipe is offline   Reply With Quote
Old Feb 14, 2006, 10:22 AM   #4
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

What you're asking about is Exposure Compensation. As others have already mentioned, you're just altering the exposure from what the camera's autoexposure would normally use.

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

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

Unless there are metering differences between models, you should see identical exposure (brightness of the image), regardless of the ISO speed used. Ditto for changes to Exposure Compensation. The same settings (+ or - EV setting for Exposure Compensation), should have the same impact on exposure.

Correct Exposure comes down to the amount of light, the ISO speed, and the aperture. A variety of combinations will produce identical exposure. You only need to use Exposure Compensation if you want a brighter or darker image compared to what the camera's autoexposure would normally give you in the same conditions.

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 (as the expense of the rest of the image). If you brighten the exposure for one part, the rest may be overexposed.

If your subject is much brighter than the rest of the image, 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.



JimC is offline   Reply With Quote
Old Feb 14, 2006, 12:59 PM   #5
Senior Member
 
Join Date: Jan 2004
Posts: 824
Default

EV or Exposure Value is actually a standard for measuring the absolute amount of light reaching a certain point (namely, the light meter). The baseline is considered to be measured at ISO 100, and consists of a continuous string of integers from 1 to 23 or so (although my old Sekonic zoom meter only has a range from 3-17; each light meter's range will vary depending on its design and so on). Each change in integer represents a doubling (or halving) in the absolute amount of light on the scene, which just happens to correspond to an single step in f-stop or shutter speed (cause that's the way they're designed).

Now assume I'm using ISO 100 B/W film, and I want to take a picture of a gray card and reproduce perfectly its middle gray color. I use my hand-held light meter (the same Sekonic zoom) that reads out directly in EV numbers.

Since I'm using ISO 100 film, I first rotate the center dial on my light meter's calculator so that it's at ISO 100.


I point the light meter at my gray card and let's assume the light level is 10 EVs. That's about the amount of light from a 60W lightbulb at 1 meter distance (roughly). I rotate the outer dial on the side of the light meter so that the pointer is at the 10 EV mark; when I do that, around the dial all the possible combinations of shutter-speed/f-stop are aligned.



As you can see from the above picture, to take a picture of that gray card and reproduce the gray properly I can use any of the following combinations on the outer two dials above:
f8 @ 1/15 s
f5.6 @ 1/30 s
f4 @ 1/60s
f 2.8 @ 1/125 s

And so on. All those are equivalent to reproducing a middle gray tone at a lighting level of 10EVs with ISO 100 film.

Now, suppose I am using ISO 400 film (I'll ignore the 160 value you mentioned just to keep us in even numbers here).
Setting the center wheel of my light meter's dial at ISO 400, I now find that at the same EV 10 value of ambient light, I can use the following combinations:


f8 @ 1/60
f5.6 @ 1/125
f4 @ 1/250
f2.8 @ 1/500
and so on

The thing to remember is that the "EV" represents an absolute amount of light; how to use that light to produce the same exposure differs depending on the ISO of the film (sensor) you are using.


Norm in Fujino is offline   Reply With Quote
Old Feb 14, 2006, 1:56 PM   #6
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Here is a handy online calculator to help you see the relationship between light in EV, shutter speed, aperture, and ISO speed (film speed in the calculator):

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

But, the EV settings you're referring to (the OP) are not the same thing as measured EV for exposure calculation purposes. You're only varying the exposure by the amount in the settings using a camera's exposure compensation settings, which is different than the lighting's measured EV.

From your description and question, that's what it sounds like you're asking about to me (exposure compensation, using +or- EV settings to brighten or darken your exposure).

What cameras are you comparing where one has a fixed ISO speed of 160 and the other 200? Added.. or was that just an example? ;-)

There are probably more important things to worry about compared to the differences in ISO speeds (lens quality, sensor quality, image processing algorithms, speed and reliability of focus, cycle times, ergronomics, etc.). ;-)

JimC is offline   Reply With Quote
Old Feb 14, 2006, 3:17 PM   #7
Senior Member
 
Join Date: Jan 2006
Posts: 142
Default

They are 2 cheap P&S cameras that i have, one is a fuji MX1200 1.3mp it has a fixed iso of 160 but has the same ev settings as the finepix A120 -2.1 to +1.5 in 0.3 increments.

I wanted to compare the images with the same settings but since one has a 200 fixed ISO (A120) and the other is 160 they dont come out the same. If i set the ev on them both to +0.9 the one with the 200 iso is a little over exposed compaired to the 160 if this makes any sence?

I thought maybe there is a formula i could st them both at the same to get a similar exposure that would trip the shutter (since i cant control that either) to the same speed. They will both be using the same lighting and same subject but like i said one has a fixed ISO a little higher then the other.


MX1200 says F=6mm and F4.5, the A120 says f=6mm F5.6
BowerR64 is offline   Reply With Quote
Old Feb 14, 2006, 3:25 PM   #8
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

They should be overexposed by the same amounts if you set Exposure Compensation to the same +0.9 EV setting on both.

Given identical metering and autoexposure processing, one camera's image shouldn't be any brighter than the others given the same settings.

You're just seeing a slight difference in exposure or metering between the two models.

The Exposure Compensation settings are not designed to compensate for ISO speed differences, only in how bright an image is exposed compared to the way the camera's autoexposure algorithms would expose them.

You could use Exposure Compensation to get one camera's shutter speed the same as the others by under or overexposing. But, that's not comparing properly exposed images from both. You'd need something other than autoexposure to control shutter speed if you really wanted them to be the same (open the aperture on one more to get it's shutter speed to match).
JimC is offline   Reply With Quote
Old Feb 14, 2006, 3:51 PM   #9
Senior Member
 
Join Date: Jan 2006
Posts: 142
Default

Thats what happens, the same settings have different shutter speeds.



maybe this will help some?


BowerR64 is offline   Reply With Quote
Old Feb 14, 2006, 4:03 PM   #10
Senior Member
 
Join Date: Jan 2006
Posts: 142
Default

Norm in Fujino that is cool, that is on your flash? so when you set that up on yoru flash it will adjust your flash?



Is there a little calculator on the web somewhere i can figure for say 75watt bulbs at 1 meter? would that be like 12EV or lower? like 8EV since they are higher watt then your figure 60watt?
BowerR64 is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 12:08 AM.