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02-25-2009, 10:44 PM   #1
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How to pick the right ISO without making dark scene brighrt than it actually is?

While trying to figure out how far I could stretch the SR feature, I've managed to create an accidental dilemma, as I kept pushing the ISO higher and higher. It appears that I've managed to make the scene brighter in the captured image than it actually was.

For instance, note the attached pic (Av mode, ISO3200, F2.8, 1/5sec, SR on, flash off, auto WB, NR weakest, natural image-tone). This is not what I saw with the naked eye, however, and I see pretty well. In fact this was the darkest corner of the room, and the camera had a hard tome focusing at all. The light against the back wall is from a distant light fixture far overhead, yet it seems like sunlight flooding in, lighting up the area. The color of the wall is also skewed greatly towards white, though it's a lot greener. The floor was also darker to the naked eye.

While I'm impressed with the SR quality -- 8 times improvement over conventional 1/40sec(focal-length) -- I don't like the fact that I've managed to make scene brighter than normal (without the aid of a flash). How does one go about selecting sensitivity that doesn't overly accentuate the available light?

Wasim


Last edited by wasim_altaf; 03-16-2009 at 09:12 AM.
02-25-2009, 11:42 PM   #2
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I think the best rule of thumb in selecting ISO is to use the lowest one you possibly can while still getting the exposure you want. Try instead to focus on using the shutter speed and aperture to create your desired exposure. It sounds like you are focusing on using the ISO to obtain optimum exposure in your above discussion. Lots of practice will guide you to success. Best of luck.

You can also use the histogram to check for proper exposure.
02-25-2009, 11:48 PM   #3
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Shoot in Tav mode and use some minus EV.
02-25-2009, 11:57 PM   #4
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QuoteQuote:
Shoot in Tav mode and use some minus EV.
Agree with Gary. I really like this mode when the lighting is difficult or changing quickly. I too play a lot with the EV which can control noice done correctly. I think the Canon / Nikon folks wish theyt had this mode at times. JIM

02-26-2009, 12:03 AM   #5
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Wasim, this is likely a basic exposure issue, not specifically ISO-related or the particular shooting mode you're using. Essentially, any of the three variables (shutter speed, sensivity (ISO), and aperture affect the amount of light gathered.

The camera lightmeter does not know whether you are pointing it into a dark corner or out a light window. Put another way, it does not know whether your subject is light or dark. The camera tries to take all the tones it "sees" and expose them to an average tone of 18% gray.

If you want to see this in action, point the camera towards a mostly dark subject, and also towards a mostly bright subject. In both cases, you will see the subjects grayer than they should look--the white gets underexposed to gray, the black gets overexposed to gray. Some people might say that the meter "gets fooled", but in truth it is the photographer's responsibility to figure out how a particular scene should be treated. A classic example is shooting in the snow--if left to its own, the camera will want to turn all that white background to gray or blue--the photographer must deliberately increase exposure relative to the camera's recommended exposure--if using autoexposure, this means dialing in a healthy dose of positive EV comp.

If you're using any autoexposure modes (basically anything but M), raising and lowering the sensitivity (ISO) will not make your picture darker or lighter because an offsetting change will be made to the shutter and/or aperture.

An observation about your posted picture: you have the spotmeter engaged. What this means is that instead of looking at the whole frame, the meter is only evaluating a very small portion at the center of the frame. In this image, it was probably seeing that battery charger or the dark floor next the the battery charger. As I said above, the camera wants to try to make subjects look gray--and sure enough, that battery charger that probably should look almost black looks quite gray in this picture.

It is highly likely that this picture would have been darker had you been using the 16-segment matrix metering or the center-weighted metering...though it still might not have been dark enough. For that, you would need to dial in negative exposure compensation. It is up to you to recognize how your picture should be exposed--which are the highlights of the image, and which are the shadows, and how that will fit into the exposure range that your camera can capture--roughly +/- 3EV. At ~3EV, highlights will clip to white, and ~-3EV, shadows will appear black.

There are tomes and tomes written on exposure, and different techniques work for different people. I'm sure you'll find plenty of helpful people here.

-Andrew

Last edited by AndrewG NY; 02-26-2009 at 12:08 AM.
02-26-2009, 12:31 PM   #6
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QuoteOriginally posted by AndrewG NY Quote
Wasim, this is likely a basic exposure issue, not specifically ISO-related or the particular shooting mode you're using. Essentially, any of the three variables (shutter speed, sensivity (ISO), and aperture affect the amount of light gathered.

The camera lightmeter does not know whether you are pointing it into a dark corner or out a light window. Put another way, it does not know whether your subject is light or dark. The camera tries to take all the tones it "sees" and expose them to an average tone of 18% gray.

If you want to see this in action, point the camera towards a mostly dark subject, and also towards a mostly bright subject. In both cases, you will see the subjects grayer than they should look--the white gets underexposed to gray, the black gets overexposed to gray. Some people might say that the meter "gets fooled", but in truth it is the photographer's responsibility to figure out how a particular scene should be treated. A classic example is shooting in the snow--if left to its own, the camera will want to turn all that white background to gray or blue--the photographer must deliberately increase exposure relative to the camera's recommended exposure--if using autoexposure, this means dialing in a healthy dose of positive EV comp.

If you're using any autoexposure modes (basically anything but M), raising and lowering the sensitivity (ISO) will not make your picture darker or lighter because an offsetting change will be made to the shutter and/or aperture.

An observation about your posted picture: you have the spotmeter engaged. What this means is that instead of looking at the whole frame, the meter is only evaluating a very small portion at the center of the frame. In this image, it was probably seeing that battery charger or the dark floor next the the battery charger. As I said above, the camera wants to try to make subjects look gray--and sure enough, that battery charger that probably should look almost black looks quite gray in this picture.

It is highly likely that this picture would have been darker had you been using the 16-segment matrix metering or the center-weighted metering...though it still might not have been dark enough. For that, you would need to dial in negative exposure compensation. It is up to you to recognize how your picture should be exposed--which are the highlights of the image, and which are the shadows, and how that will fit into the exposure range that your camera can capture--roughly +/- 3EV. At ~3EV, highlights will clip to white, and ~-3EV, shadows will appear black.

There are tomes and tomes written on exposure, and different techniques work for different people. I'm sure you'll find plenty of helpful people here.

-Andrew
Andrew,

You are right, I did use spot-metering. That was the only way I could get the focus to lock. The over-exposed wall is probably due to spot-metering. I did focus on the charger, the darkest object present. I wasn't aware of the 18%-gray target. I noticed the grayness of the charger, though I thought that was due to noise! I hate bright-snow/gray-sky combo. I just can't get that exposure right.

I couldn't imagine using -EV comp in this pic, given how dark it was. (I bumped up the ISO instead, but it may not be equivalent). I used Av mode, dialed in an ISO value, and the camera returned a certain shutter-speed, presumably with the correct exposure, to capture the ambient light (variation) as is. This is where I'm a bit confused. The amount of light in the pic is greatly amplified, everywhere, compared to ambient, may be more than +3EV -- as if night had turned into day. Does higher ISO amplify the signal in an attempt to flesh out the brightness? I wonder if the pic will look the same had I used ISO800/~1sec shutter-speed exposure instead. May be I'm not suppose to treat ISO directly analogous to wider aperture beyond some sensor limit?

Wasim
02-26-2009, 12:46 PM   #7
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QuoteOriginally posted by Jewelltrail Quote
I think the best rule of thumb in selecting ISO is to use the lowest one you possibly can while still getting the exposure you want. Try instead to focus on using the shutter speed and aperture to create your desired exposure. It sounds like you are focusing on using the ISO to obtain optimum exposure in your above discussion. Lots of practice will guide you to success. Best of luck.

You can also use the histogram to check for proper exposure.
You are right. I'm not used to having variable ISO. I don't mind it as long as it doesn't somehow light up the scene.

Honestly, I don't know how to use the histogram yet!
02-26-2009, 02:13 PM   #8
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QuoteOriginally posted by wasim_altaf Quote
Andrew,

You are right, I did use spot-metering. That was the only way I could get the focus to lock. The over-exposed wall is probably due to spot-metering. I did focus on the charger, the darkest object present. I wasn't aware of the 18%-gray target. I noticed the grayness of the charger, though I thought that was due to noise! I hate bright-snow/gray-sky combo. I just can't get that exposure right.
OK, you should know that metering mode (spot/segmented/center has nothing to do with ability to autofocus--period. Metering is only about detecting light. The trick with focusing in low light is to find something that isn't too dark that has some contrast/texture. You can also use an AF illuminator--either with an accessory flash, or by raising the popup flash and letting it strobe.

QuoteOriginally posted by wasim_altaf Quote
Andrew,

I couldn't imagine using -EV comp in this pic, given how dark it was. (I bumped up the ISO instead, but it may not be equivalent). I used Av mode, dialed in an ISO value, and the camera returned a certain shutter-speed, presumably with the correct exposure, to capture the ambient light (variation) as is. This is where I'm a bit confused. The amount of light in the pic is greatly amplified, everywhere, compared to ambient, may be more than +3EV -- as if night had turned into day. Does higher ISO amplify the signal in an attempt to flesh out the brightness? I wonder if the pic will look the same had I used ISO800/~1sec shutter-speed exposure instead. May be I'm not suppose to treat ISO directly analogous to wider aperture beyond some sensor limit?

Wasim
You need to remember that what the camera tells you appropriate exposure is not necessarily correct, it is only a recommendation. Correct exposure is the exposure that results in the picture you want. Negative EV comp is exactly what you need here to avoid overexposure. That corner really is darker than average--if you want the picture to convey that darkness, you'll need to tell the camera to darken it--the camera wants to make everything average. That being said, it was probably the uncompensated spotmeter that caused this picture to be so badly overexposed. To use spot on that battery charger you probably should have used -1.7 or -2EV (a guess).

If you want blacks to look black, whites to look white, and everything else in between, you will need to take an active role in providing the necessary camera inputs. Each metering mode has trade-offs in this respect.

Matrix metering tries its hardest to do a good job with minimal input from the user--tries to guess what the photographer wants to do, using algorithms that we aren't really privy to. For example, if it thinks it sees a backlit subject, it will probably try to 'correct' the image by allowing the background to overexpose and brightening up the subject...to a certain extent. Matrix divides the scene into 16 areas and uses mysterious logic to try to average this out, but it still might not be enough for your tastes.

Center-weighted metering is in a sense less sophisticated but more predictable. Center-weighted is the classic metering mode that most classic film cameras of the 70's and 80's employed. It relies on you to be a little smarter about how much correction for a given subject is required--possibly requiring a greater degree of correction--but the advantage is that it is more predictable in its behavior...in this sense it rewards the photographer who is willing to work at it, because it can be "learned."

Spot metering is the precision instrument here. It looks only at a very small region in the center of the scene. The recommended exposure will be an attempt to turn this small region 18% gray (the other metering modes also use a 18% gray target as well). If you point that spot at a white area, it will try to darken it, if you point at black, it will try to lighten it. To get an understanding of this, enable the spotmeter, and survey the scene you want to photograph. Watch how as you aim the camera at objects around you how the meter responds. Bear in mind that the meter only sees luminance (brightness) and doesn't see colors. Try this exercise in manual mode. Point towards a medium-luminance subject and press the green button to get an initial automatic exposure. Now survey the scene and watch how as you aim at brighter objects the meter indicates positive steps, and as you point the camera towards the shadows the meter shows a negative offset. Your camera can handle roughly +/- 3EV, for a roughly 6-stop range (it may be a little more than this but you'll figure that out). It is up to you to set the exposure so that each part of the image falls into the right part of that range. Really bright stuff should be >2EV. Almost black shadows should be <-2EV. Other stuff should fall in between...typical caucasian skin should be approximately +1EV (try this--spotmeter your palm, setting it to +1EV and see how the rest of the scene tends to fall into place!)

I'm going to say this again--if you're using AE (autoexposure) modes like Av, changes to ISO will NOT normally affect the brightness of your picture because as you raise and lower it there will be a corresponding offsetting change to shutter speed (in Av mode, where aperture is fixed). If you raise the ISO from 800 to 3200 (+2EV) the shutter speed will be simultaneously increased, for example, 1/15 to 1/60 (-2EV). The exception to this case is when you reach the minimum or maximum shutter speed but when this happens the numbers in the viewfinder will blink.

Cheers,
Andrew

02-26-2009, 10:05 PM   #9
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QuoteOriginally posted by AndrewG NY Quote
OK, you should know that metering mode (spot/segmented/center has nothing to do with ability to autofocus--period. Metering is only about detecting light. The trick with focusing in low light is to find something that isn't too dark that has some contrast/texture. You can also use an AF illuminator--either with an accessory flash, or by raising the popup flash and letting it strobe.



You need to remember that what the camera tells you appropriate exposure is not necessarily correct, it is only a recommendation. Correct exposure is the exposure that results in the picture you want. Negative EV comp is exactly what you need here to avoid overexposure. That corner really is darker than average--if you want the picture to convey that darkness, you'll need to tell the camera to darken it--the camera wants to make everything average. That being said, it was probably the uncompensated spotmeter that caused this picture to be so badly overexposed. To use spot on that battery charger you probably should have used -1.7 or -2EV (a guess).

If you want blacks to look black, whites to look white, and everything else in between, you will need to take an active role in providing the necessary camera inputs. Each metering mode has trade-offs in this respect.

Matrix metering tries its hardest to do a good job with minimal input from the user--tries to guess what the photographer wants to do, using algorithms that we aren't really privy to. For example, if it thinks it sees a backlit subject, it will probably try to 'correct' the image by allowing the background to overexpose and brightening up the subject...to a certain extent. Matrix divides the scene into 16 areas and uses mysterious logic to try to average this out, but it still might not be enough for your tastes.

Center-weighted metering is in a sense less sophisticated but more predictable. Center-weighted is the classic metering mode that most classic film cameras of the 70's and 80's employed. It relies on you to be a little smarter about how much correction for a given subject is required--possibly requiring a greater degree of correction--but the advantage is that it is more predictable in its behavior...in this sense it rewards the photographer who is willing to work at it, because it can be "learned."

Spot metering is the precision instrument here. It looks only at a very small region in the center of the scene. The recommended exposure will be an attempt to turn this small region 18% gray (the other metering modes also use a 18% gray target as well). If you point that spot at a white area, it will try to darken it, if you point at black, it will try to lighten it. To get an understanding of this, enable the spotmeter, and survey the scene you want to photograph. Watch how as you aim the camera at objects around you how the meter responds. Bear in mind that the meter only sees luminance (brightness) and doesn't see colors. Try this exercise in manual mode. Point towards a medium-luminance subject and press the green button to get an initial automatic exposure. Now survey the scene and watch how as you aim at brighter objects the meter indicates positive steps, and as you point the camera towards the shadows the meter shows a negative offset. Your camera can handle roughly +/- 3EV, for a roughly 6-stop range (it may be a little more than this but you'll figure that out). It is up to you to set the exposure so that each part of the image falls into the right part of that range. Really bright stuff should be >2EV. Almost black shadows should be <-2EV. Other stuff should fall in between...typical caucasian skin should be approximately +1EV (try this--spotmeter your palm, setting it to +1EV and see how the rest of the scene tends to fall into place!)

I'm going to say this again--if you're using AE (autoexposure) modes like Av, changes to ISO will NOT normally affect the brightness of your picture because as you raise and lower it there will be a corresponding offsetting change to shutter speed (in Av mode, where aperture is fixed). If you raise the ISO from 800 to 3200 (+2EV) the shutter speed will be simultaneously increased, for example, 1/15 to 1/60 (-2EV). The exception to this case is when you reach the minimum or maximum shutter speed but when this happens the numbers in the viewfinder will blink.

Cheers,
Andrew
Andrew, thanks for the detailed explanation. This is turning into a highly informative tutorial for me. I took another stab it, following your advice. This time I switched to matrix-metering. I took a pic by focusing on the brighter wall first, just above the white baseboard. This is close to the real thing. Then I went back to focusing on the charger, this time with -1EV in M mode. Very close! The white baseboard must be fooling the meter a bit still. Another -0.3EV just might do it.

I also switched to cool-white-fluorescent for WB, which seems to add a yellow tone to white, but the rest look natural. WB is another gray area for me, unfortunately. As for using the flash as focus-assist, it is somewhat cumbersome having the focus in lock while closing the shutter. Do you turn off AE-lock-with-AL-lock for when doing this?

Wasim

Last edited by wasim_altaf; 03-16-2009 at 09:12 AM.
02-27-2009, 12:14 PM   #10
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QuoteOriginally posted by wasim_altaf Quote
Andrew, thanks for the detailed explanation. This is turning into a highly informative tutorial for me. I took another stab it, following your advice. This time I switched to matrix-metering. I took a pic by focusing on the brighter wall first, just above the white baseboard. This is close to the real thing. Then I went back to focusing on the charger, this time with -1EV in M mode. Very close! The white baseboard must be fooling the meter a bit still. Another -0.3EV just might do it.

I also switched to cool-white-fluorescent for WB, which seems to add a yellow tone to white, but the rest look natural. WB is another gray area for me, unfortunately. As for using the flash as focus-assist, it is somewhat cumbersome having the focus in lock while closing the shutter. Do you turn off AE-lock-with-AL-lock for when doing this?

Wasim
I think you're on your way--the most important part of the camera remains the lumpy grey part 6 inches behind the viewfinder.

Spotmeter is powerful but it can be hard to know what it's really reading unless you move it around a bit and watch it change.

I don't use AE-lock with AF-lock, I keep them separate. Another thing you might want to consider is decoupling the half-press for autofocus and rely on the AF button instead though I admit it can be problematic if you hand the camera to someone else!

I agree that the pop-up flash-as-AF-illuminator solution is clunky. Decoupling the half-press as above can help. You could also switch to manual focus as soon as you get the lock, then close the flash, but be careful not to move much afterwards. Another thing to be careful with is that when the flash is raised, the camera will consider its output in in the autoexposure logic so if you're not going to fire it you need to do your metering with it closed.

If you have an accessory hotshoe flash it will have a spotbeam mode that won't fire the flash, only the red illuminator is enabled...so even without flash output, an accessory flash is nice to have in lower light.

WB for artificial lighting can be a little tough, and flourescents tend to vary. If you have the time, you can use manual white balance. The best thing to do for WB is to shoot RAW because you can correct it later with no penalty though I usually try to do it in camera too so that the preview looks right. I think for me I usually use AWB or Tungsten, with the custom setting to automatically use flash WB with flash enabled. I will occasionally use manual WB or the bottommost of the flourescent settings. RAW also of course reduces the penalty for adjusting exposure and color during post-processing.

-Andrew
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