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Old Feb 16, 2006, 12:41 PM   #31
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Yeah i know what ev means now. That wasnt my original question.

Im trying to get a similar image with the 2 cameras. One has an ISO of 160 and one has an ISO of 200. Now since i can use the EV to alter the exposure i wanted to try and get them to match.



So my answer is no, there is no formula or you guys DONT KNOW the formula or what ever the calculation would be to get a similar exposure based on the fixed setting of the camera.

I know the apature on both and these i cant change, i know the ISO on both and i cant change them. So the only thing i can change is the ev wich seems to control the shutter speed resulting in an image compinsation.


You told me and repeated i can get the same exposure using different settings, and THATS what im trying to do. I thought it would be easier to try and calculate this rather then shoot thousands of images adjusting each camera to get the same exposure.


ill leave now. Thanks for everyones help.

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Old Feb 16, 2006, 12:50 PM   #32
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AGAIN, Aperture X Shutter speed X Iso = exposure value. Exposure value compensation (lets not abreviate here, like that crummy manual), ±EVC, takes the exposure value already calculated (Aperture X Shutter speed X Iso = exposure value, you have no control over this, thats the "auto" part of auto exposure) and modifies it by the amount you specify. Your not directly entering an exposure value, your entering a COMPENSATION amount, to compensate for the inadequacy of the camera's meter.

If you use a fully manual mode where YOU specify (Aperture X Shutter speed X Iso), then EVC is not possible.

Using big round numbers as an example....
5 X 1 X 200 = 1000
4 X 1.25 X 200 = 1000
1.25 X 5 X 160 = 1000

Three different ways for a camera to make the same exposure (1000). The camera decided (based on its meter) that 1000 is the right exposure. If you think its wrong, you can tell it +200EVC and it will change to 6x1x200=1200 instead.

You can not change what it thinks is the right exposure (1000 in this example). You can only modify its choice.

If you MANUALLY input 10 X 2 X 200 you will get 4000. You can not apply compensation to this, because the camera is not in control.

In your recent example you manually input f/5.6 x 0.5s x iso200 which equals an exposure value of 5. There is no part of that equation that is under the control of the camera. On AUTO, if its meter said that EV4 was the right exposure it would choose f/5.6 x 1s x iso200 = 4. THEN you could say +1evc and it would change the equation so that the exposure value is one stop higher. ±EVC is simply an add-on tool that allows you to compensate slighty the cameras AUTO EXPOSURE settings.
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Old Feb 16, 2006, 12:57 PM   #33
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You may not be able to get the two cameras taking photos that look exactly the same, no matter what you do.

It's not all about aperture and shutter speed. Image processing algorithms come into the equation (amount of contrast, saturation, sharpness being applied to the image captured by the sensor).

Given the same metering algorithms, the cameras should be exposing them the same way.

No calculations or adjustments are needed to accomodate the difference in ISO speed. The camera is doing that already. That's how autoexposure works (and even if you're in your camera's so called "manual exposure" mode, it's still autoexposure, based on the lighting the camera's meter sees with the models you're using).

If you find that one tends to expose a bit darker than the other because of a difference in metering sensitivity, use a higher +EV setting under Exposure Compensation for the one exposing darker. That's all you can do for the exposure part (the part impacted by aperture or shutter speeds).

For other differences in the images (color accuracy, contrast, saturation, sharpenening), you may not be able to do a thing. WB settings can help with color accuracy. That has nothing to do with exposure.

Sometimes you'll find that images are more "contrasty" from some cameras. The image processing algorithms are just trying to give them more "punch" by making the bright areas a little brighter and the darker areas a little darker.

Again, that has nothing to do with exposure (ISO speed, aperture, shutter speed). It's just that a given camera model processes the images differently, just like different film types can give you a different look, even with identical camera settings.

If your camera has a setting for contrast, try it. Using lower contrast will retain more detail, so that there is less difference between bright and dark. That gives less punch, but can help retain detail when cameras try to process the images so that you have a greater difference between darker and brighter areas.

IOW, you're barking up the wrong tree, trying to find a formula to change aperture or shutter speed to get them looking the same way. That's not the problem.

Use an editor. Play with the settings during post processing and see what impact they have.

Ask for assistance in our retouching and editors forums if you have a particular problem that you're trying to solve with a given image (too much contrast, etc.).


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Old Feb 16, 2006, 12:58 PM   #34
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Bottom line:

The camera's meters should already, automatically, adjust the other two variables to equalize the ISO.

They should be the same.

f/5.6 x 0.5s x iso200 = EV 5
f/5.6 x 0.65s x iso160 = EV 5 (roughly)

The camera meter decided that EV5 was the right exposure, it dosent matter that one variable is fixed in the equation, because it can adjust the other two so that the answer comes out to 5. This should happen the same on both cameras, unless there is something wrong with them or they are inherently miscalibrated. The ONLY way that ±EVC should be neccasary to balance them, is if one is miscalibrated. (Or as Jim said, they process the images differently when writing the jpg file which can change the look some).

If you can use manual settings, then this is all very easy. Iso160 is a third of a stop less sensetive than iso200, so either your aperture or shutter must be set a third stop more open on the iso160 camera, and otherwise the same. If you truly want an apples to apples comparison you should not use the auto exposure.
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Old Feb 16, 2006, 1:27 PM   #35
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tmoreau wrote:
Quote:
Bottom line:

The camera's meters should already, automatically, adjust the other two variables to equalize the ISO.

They should be the same.

f/5.6 x 0.5s x iso200 = EV 5
f/5.6 x 0.65s x iso160 = EV 5 (roughly)

The camera meter decided that EV5 was the right exposure, it dosent matter that one variable is fixed in the equation, because it can adjust the other two so that the answer comes out to 5. This should happen the same on both cameras, unless there is something wrong with them or they are inherently miscalibrated. The ONLY way that ±EVC should be neccasary to balance them, is if one is miscalibrated. (Or as Jim said, they process the images differently when writing the jpg file which can change the look some).

If you can use manual settings, then this is all very easy. Iso160 is a third of a stop less sensetive than iso200, so either your aperture or shutter must be set a third stop more open on the iso160 camera, and otherwise the same. If you truly want an apples to apples comparison you should not use the auto exposure.

tmoreau's explanation of EV Exposure Value is the closest so far. Exposure VALUE and Exposure COMPENSATION are two entirely different things.

The only camera in my possession that makes any reference to Exposure VALUE is over 50 years old!!and is not automatic (it doesn't even have a built-in exposure meter)
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Old Feb 16, 2006, 1:31 PM   #36
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BoweR64:

You'll find the forum members here perfectly willing to help out. I think the main problem is that you're asking the wrong questions. ;-)

We hate to sound like broken records, but these two cameras should already be exposing the images exactly the same way, even though they have different base ISO speeds.

If you're seeing a difference, you can use Exposure Compensation to help (just use a higher +EV setting on the one exposing darker). That's all you can do with your cameras because they don't have true manual exposure.

Even if they did, you wouldn't be doing anything via manual settings of these paramters that you couldn't do with Exposure Compensation anyway, unless you need to expose brighter or darker than the amount of setting adjustments your cameras give you.

There is no magic formula to make them expose the same way, because the camera's autoexposure should already be exposing the images the same way with both cameras, given the same amount of Exposure Compensation in the same lighting.

Sometimes you will see a difference in metering senstiivity. That has nothing to do with the difference in base ISO speed you see between these two models.

Your main issue is probably a difference between contrast between the two cameras, even when you try to use Exposure Compensation to get them looking close. WB differernces also come into the equation.

That happens. Different camera models process images differently, even from the same manufacturer. You have different sensors that react differently (dynamic range is an example), with different tone and contrast curves being used by the camera's image processing algorithms, better or worse auto white balance between models, etc.

Many factors go into how an image looks after processing by a camera.


P.S.

Looking at reviews and specifications for these two cameras, you could run into another limitation in low light. They are both limited to a slowest shutter speed of 1/2 second. So, if light gets too low, Exposure Compensation can only do so much to make them brighter (once your camera reaches it's slowest shutter speed limits), if you think they're not being exposed bright enough using default settings.

Neither has manual exposure (not that it would help anything anyway, since a difference in the way the images are being metered and processed seems to be the main malfunction here).

But, don't worry about the difference in base ISO speed, or what aperture/shutter speed combination the camera is choosing. You can't directly control these parameters anyway with these models.

You can only make them look brighter or darker than the camera would normally expose them using exposure compensation, and you can help with color accuracy by using the White Balance Settings.

Other than that, you can only influence the images by using different lighting (more even lighting sources can help with contrast and dynamic range differences) and using an editor later to get them looking better.

There is no formula to accomodate for exposure differences, because there shouldn't be any with these models (both are autoexposure only, with your manual control limited to Exposure Compensation to brigthen or darken the images compared to how the cameras would normally expose them), or setting White Balance to help with color accuracy in the lighting you're shooting in.


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Old Feb 16, 2006, 2:11 PM   #37
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ok i figured out a little more, but i think there are some figures to get the same exposure because the ISO and the apature are different between the 2 that is what will need a calculation.

Ive figured so far that on these 2 cameras EV = the shutter speed. How do i know? i turned the lights off and the shutter speed stays the same. So reguardless of light the ev settings = shutter speed. On these 2 cameras anyway.

see images.

http://i5.photobucket.com/albums/y19...0EVchanges.jpg

http://i5.photobucket.com/albums/y19...0EVchanges.jpg

So the 2 common spots on these 2 cameras are +0.6= 1/2 sec (MX1200) and 0 step = 1/2 sec (A120) so since the MX1200 is an ISO of 160 it needs +0.6 to get the same exposure as the A120 at 0 with the ISO 200.

BUT... the exposure arnt the same because the apatures are not the same. right?
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Old Feb 16, 2006, 2:19 PM   #38
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BowerR64 wrote:
Quote:
ok i figured out a little more, but i think there are some figures to get the same exposure because the ISO and the apature are different between the 2 that is what will need a calculation.

Ive figured so far that on these 2 cameras EV = the shutter speed. How do i know? i turned the lights off and the shutter speed stays the same. So reguardless of light the ev settings = shutter speed. On these 2 cameras anyway.
No, that's not correct, it's a coincidence. If light was brighter, shutter speeds would be faster. The reason it was 1/2 second was because that's as slow as it goes with your cameras. ;-) See the P.S. section I added to my last post. If it could go slower, it would to keep from underexposing as much with the lights out.

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Old Feb 16, 2006, 3:08 PM   #39
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(MX1200) f/4.5 iso160 X sec
(A120) f/5.6 iso200 X*.8333 sec

Since you cant enter that shutter speed, you'll have to guess and check to coax the camera into it using EVC.

Set the MX1200 to ±0EVC and take a picture. If the shutter is 1/5 second, multiply by .8333 and get .16666 (1/6 second). Guess and check using ±EVC on the A120 until it gives you a 1/6 second shutter.

Since you are not in direct control of the camera, this is as close as you can get.

(Additional example, MX1200=1/100 A120=1/120) You should be able to do this with a calculator. If you can, then you have your answer. It also has very little to do with the ISO differences in your cameras, or exposure compensation. You're STILL barking up the wrong tree.
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Old Feb 16, 2006, 8:38 PM   #40
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Well i just wanted to compare the detail of the 2 with the same exposure but i didnt think it would be this complicated. I have decided they both oops. It was fun trying. I think ive shot 100 pictures on the A120 since i got it monday and i just had to recharge the batteries today. The A120 has a really cool feature called "battery discharge" i let the batteries do that to run em down a little further then they were.

The MX1200 has a little sensor on the front that meters the light the A120 doesnt have anything so i think it does it a little differnt.



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