August 30, 2014, 08:17:56 AM

Author Topic: Grey card and spot metering  (Read 3777 times)

bc33

  • Guest
Grey card and spot metering
« on: April 22, 2013, 12:04:01 PM »
Hi All,

I take lots of photos of dogs and I'm trying to get more accurate exposures of black dogs and white dogs and other dogs that tend to confuse me and my 50d's metering.

I thought I'd try to set my exposure using a 18% grey card, thus getting a correct exposure no matter what colour of dog I was photographing.
To do this I bought a grey card which, as I understand it, I hold in the same light as the dog and take a meter reading from the grey card. So far so good.
So I hold the card at arms length and take a meter reading using spot metering. From this reading I expect to see a histogram with a spike in the centre of the histogram and the rest should be blank.
My grey card fills at least 50% of my 50d's frame so a spot meter reading from just 3.8% of the frame should see nothing but my grey card

However this is never the case. My histograms have spikes all over the range, so my camera is getting metering information from much more than just the central 3.8% of the frame.

I thought I could get away with just a small pocket sized grey card but if the metering is not actually the central 3.8% of the frame I will have to get bigger pockets and a much bigger grey card that fills the entire frame and then it won't matter what metering I use.

Does anyone know if canon spot metering is  supposed to just take a reading from just the central portion of the frame and nothing else?

Please try this for yourselves, hold anything with a solid colour up at arms length and take a spot meter reading and let me know what result you get.

Thank you. Simon



canon rumors FORUM

Grey card and spot metering
« on: April 22, 2013, 12:04:01 PM »

TrumpetPower!

  • 1D Mark IV
  • ******
  • Posts: 951
    • View Profile
Re: Grey card and spot metering
« Reply #1 on: April 22, 2013, 12:23:24 PM »
The histogram is built from the entire image. Unless the gray card fills 100% of the screen, you should expect to see all sorts of other stuff represented there.

Indeed, if it's only a linear 50% of the frame that you're filling the gray card with, the area that covers is much less than 50%....

But, if you're using a gray card as you describe, the histogram is pretty irrelevant unless you're watching against clipping of especially bright highlights...in which case, the meter reading off the gray card becomes irrelevant.

You might also consider a handheld light meter instead of a gray card. Logically, they function the same way, but the gray card has disadvantages -- not the least of which is that even the low levels of gloss on a gray card can wreak havoc with your camera's meter readings. Try it -- step outside on a sunny day, stand in a single spot with the Sun at your back, hold the card at arm's length, and rotate the card. See how much its brightness changes, even to your eye, depending on whether or not it's pointing near or away from the Sun! Now, which of those angles is the "right" one for setting exposure?

A meter doesn't have those problems, due to its design. Stand in the place of your subject (or as close as you can get), aim the dome at the camera, press the button, and you've got your exposure.

Cheers,

b&

hsbn

  • PowerShot G1 X II
  • ***
  • Posts: 36
    • View Profile
Re: Grey card and spot metering
« Reply #2 on: April 22, 2013, 12:23:53 PM »
Hi All,

I take lots of photos of dogs and I'm trying to get more accurate exposures of black dogs and white dogs and other dogs that tend to confuse me and my 50d's metering.

I thought I'd try to set my exposure using a 18% grey card, thus getting a correct exposure no matter what colour of dog I was photographing.
To do this I bought a grey card which, as I understand it, I hold in the same light as the dog and take a meter reading from the grey card. So far so good.
So I hold the card at arms length and take a meter reading using spot metering. From this reading I expect to see a histogram with a spike in the centre of the histogram and the rest should be blank.
My grey card fills at least 50% of my 50d's frame so a spot meter reading from just 3.8% of the frame should see nothing but my grey card

However this is never the case. My histograms have spikes all over the range, so my camera is getting metering information from much more than just the central 3.8% of the frame.

I thought I could get away with just a small pocket sized grey card but if the metering is not actually the central 3.8% of the frame I will have to get bigger pockets and a much bigger grey card that fills the entire frame and then it won't matter what metering I use.

Does anyone know if canon spot metering is  supposed to just take a reading from just the central portion of the frame and nothing else?

Please try this for yourselves, hold anything with a solid colour up at arms length and take a spot meter reading and let me know what result you get.

Thank you. Simon
This spot metering is only measured inside the circle you see in your viewfinder. However, the histogram is for the whole image, not just the central spot. So if you take the photo of course you'll see the histogram spike in many places not just in the middle unless your greycard fill the entire frame. The important thing is do get the correct exposure? Don't worry too much about spike in histogram.

Mt Spokane Photography

  • Canon EF 600mm f/4L IS II
  • ********
  • Posts: 8460
    • View Profile
Re: Grey card and spot metering
« Reply #3 on: April 22, 2013, 12:43:56 PM »
 
As noted, be sure to cover the entire frame with your gray card.  If you are using one of the tiny ones, they are generally best for color correction.
Be sure to check things out before actually using it, as Thom notes, camera manufacturers do not use 18% reflectance to calibrate your camera, so your exposure might be off.
http://www.bythom.com/graycards.htm
 

TrumpetPower!

  • 1D Mark IV
  • ******
  • Posts: 951
    • View Profile
Re: Grey card and spot metering
« Reply #4 on: April 22, 2013, 01:07:46 PM »

As noted, be sure to cover the entire frame with your gray card.  If you are using one of the tiny ones, they are generally best for color correction.
Be sure to check things out before actually using it, as Thom notes, camera manufacturers do not use 18% reflectance to calibrate your camera, so your exposure might be off.
http://www.bythom.com/graycards.htm

Thom's points are all correct and useful...but also somewhat dated. These days, if you're looking for perfect
exposure, there are much better methods than a gray card or even a handheld light meter. Specifically, you'll be wanting to get your hands quite dirty with ICC profiles....

But, of course, Thom's suggestions are an excellent starting point, as they're plenty "good enough" for the overwhelming majority of photographers. Unless you have problems doing as he says, there's no point in worrying even about the exact reflectance of your gray card.

Cheers,

b&

Mt Spokane Photography

  • Canon EF 600mm f/4L IS II
  • ********
  • Posts: 8460
    • View Profile
Re: Grey card and spot metering
« Reply #5 on: April 22, 2013, 05:13:38 PM »

As noted, be sure to cover the entire frame with your gray card.  If you are using one of the tiny ones, they are generally best for color correction.
Be sure to check things out before actually using it, as Thom notes, camera manufacturers do not use 18% reflectance to calibrate your camera, so your exposure might be off.
http://www.bythom.com/graycards.htm

Thom's points are all correct and useful...but also somewhat dated. These days, if you're looking for perfect
exposure, there are much better methods than a gray card or even a handheld light meter. Specifically, you'll be wanting to get your hands quite dirty with ICC profiles....

But, of course, Thom's suggestions are an excellent starting point, as they're plenty "good enough" for the overwhelming majority of photographers. Unless you have problems doing as he says, there's no point in worrying even about the exact reflectance of your gray card.

Cheers,

b&
How do you install a ICC profile in your camera to get a correct exposure?
 
ICC profiles are more about getting colors correct.

TrumpetPower!

  • 1D Mark IV
  • ******
  • Posts: 951
    • View Profile
Re: Grey card and spot metering
« Reply #6 on: April 22, 2013, 05:35:11 PM »
How do you install a ICC profile in your camera to get a correct exposure?

Sadly, of course, you don't....

Quote
ICC profiles are more about getting colors correct.

But exposure is the most fundamental part of getting colors correct! And, in a fully-ICC-managed workflow, the profile is going to correct any exposure errors at the same time it gets the colors in line. A hybrid workflow will at least correct exposure and white balance, which is all you can realistically do outside the studio, leaving it up to your supply of carefully-constructed generic profiles (one in daylight, one in shade, one in tungsten, etc.) to get you "close enough."

Obviously, you need to shoot a target in the same conditions at the same settings as the "real" shot. If you're shooting tethered in the studio, you can quickly build a profile on the fly and analyze it to tell you the exact exposure adjustments and white balance settings you need, and you can then decide if you want to make your exposure adjustments with the camera / lights or (if there're reflective or illuminated highlights that'll get clipped) in post-production. When I'm doing art reproduction, I do so and adjust the lighting to within 1/100 of a stop of perfect exposure in camera.

The same process also (of course) tells you how to adjust exposure and white balance in post-processing, at which point all you're left with is applying the ICC profile.

I use a similar workflow in the field when I'm doing landscape work, except that I generally don't shoot tethered and thus I'll bracket (usually with the auto-HDR because it's so convenient) and pick the best / least-worst exposure as the starting point, and then use the profile of the image of the chart to determine post-processing adjustment of exposure and white balance. Works awesome. Even if you significantly miss the exposure, you've got so much latitude with modern cameras that, so long as you didn't clip the highlights, you can perfectly salvage all but the worst mistrakes...so long as you got that shot of the target (I use a ColorChecker Passport in the field) in the light with the same settings.

Cheers,

b&

canon rumors FORUM

Re: Grey card and spot metering
« Reply #6 on: April 22, 2013, 05:35:11 PM »

eml58

  • 1D X
  • *******
  • Posts: 1427
  • 1Dx
    • View Profile
Re: Grey card and spot metering
« Reply #7 on: April 22, 2013, 09:11:59 PM »
I use the X-Rite Colour Checker Passport, you can find info/videos here.

http://xritephoto.com/ph_product_overview.aspx?id=1257

I basically set up in LR4 a "Profile" for each Camera Body I have (1Dx x2, 5DMK III x 2, 1DMK IV), it's important to set up a different "Profile" for each separate Body, and I go to the trouble of setting up again a separate "Profile" for the most used ISO's, i.e., 100/200/400/800/1250/5000, again for each Body.

Once that's done you can pretty well shoot in RAW, then adjust in Post in LR4 before bringing into PS CS6/Whatever you use, or stay in LR4.

It sounds a bit of mucking about, but once the profiles are set up in LR4 it's as simple as a single click, your done, mostly.

I also use a Sekonic Light meter in the field, not a lot, but for the difficult lighting shots I find it invaluable, I use the L-758DR & Now the L-478DR, basically similar units except the 758 does 1 degree spot & the 478 does 5 degree spot.

Good Luck.
The only thing necessary for the triumph of evil is for good men to do nothing

TrumpetPower!

  • 1D Mark IV
  • ******
  • Posts: 951
    • View Profile
Re: Grey card and spot metering
« Reply #8 on: April 22, 2013, 10:08:33 PM »
I use the X-Rite Colour Checker Passport, you can find info/videos here.

The Passport is a very good portable target and what I use out in the field. The software that ships with it leaves a great deal to be desired, even if it's significantly better than using a gray card or any of the other popular tools.

A big part of the problem is DNG profiles, which really leave a lot to be desired, on so many levels. But it's what you're stuck with if you're processing with Adobe products...and it, in turn, is so much superior to Canon's Picture Styles that it's not even funny....

In the studio, I use my own target that includes a replica of a ColorChecker and a couple dozen other paints and a dozen wood chips and a few hundred patches printed on an iPF8100 and a light trap and some Teflon and probably some other bits and pieces I'm forgetting at the moment....

Cheers,

b&

gbchriste

  • EOS M2
  • ****
  • Posts: 173
    • View Profile
Re: Grey card and spot metering
« Reply #9 on: April 22, 2013, 10:29:18 PM »
I absolutely love the Lastolite EZBalance.  The 12-inch model pops open to a 12-inch wide square with 18% grey on one side and white on the other. It is a non-reflective surface so doesn't get tripped up by reflections.  It is also completely neutral for dead on white balance calibration - either in camera or in post processing.  And with a couple of flicks of the wrist it folds down to about the size of a drink coaster and slides easily in the back pocket.

The 12-inch size is also very handy to easily fill up the frame in the view finder.

I find that centering my exposure needle when spot metering on it still tends to underexpose a bit so I center the needle, and then bump the exposure up 1/3 to 1/2 stop.  I also always do a custom in camera white balance using the grey target as the reference image and get spot on accurate white balance every time.

http://www.amazon.com/Lastolite-LL-LR1250-12-Inch-Ezybalance/dp/B0009QZDL6/ref=sr_1_1?ie=UTF8&qid=1366683856&sr=8-1&keywords=lastolite+ezbalance

Mt Spokane Photography

  • Canon EF 600mm f/4L IS II
  • ********
  • Posts: 8460
    • View Profile
Re: Grey card and spot metering
« Reply #10 on: April 22, 2013, 10:59:53 PM »
We appear to be addressing two different issues.
 
The OP is trying to get a correct exposure, while others are talking color correction in post.
 
I think we might be confusing the OP's question of how to get a correct exposure so that extreme measures are not required in a attempt to save a image.

eml58

  • 1D X
  • *******
  • Posts: 1427
  • 1Dx
    • View Profile
Re: Grey card and spot metering
« Reply #11 on: April 22, 2013, 11:47:39 PM »
We appear to be addressing two different issues.
 
The OP is trying to get a correct exposure, while others are talking color correction in post.
 
I think we might be confusing the OP's question of how to get a correct exposure so that extreme measures are not required in a attempt to save a image.
Maybe, but I wasn't, the Colour Checker Passport does operate for Colour Balance that's true, but it works just as well for Exposure balance also, and I feel it does it much better than a simple Grey Card, I also mentioned in my Post that the use of a Light meter in my own work is extremely helpful, at the end it's just an opinion, the Op may find it useful, he may find it total nonsense, I'm Ok either way. In the attached lifts from X-Rite web site, the Grey Card, is a Grey Card, for assist on Exposure, the bottom row of Grey blocks on the other lift, are, Grey Cards, to assist in Exposure.
The only thing necessary for the triumph of evil is for good men to do nothing

TrumpetPower!

  • 1D Mark IV
  • ******
  • Posts: 951
    • View Profile
Re: Grey card and spot metering
« Reply #12 on: April 23, 2013, 10:00:27 AM »
We appear to be addressing two different issues.
 
The OP is trying to get a correct exposure, while others are talking color correction in post.
 
I think we might be confusing the OP's question of how to get a correct exposure so that extreme measures are not required in a attempt to save a image.

I believe I addressed all that in my first post, the first response on the thread: the histogram shows the whole image, not just the metered part; using a gray card can be tricky due to gloss but works with practice and experience; and a hand-held meter is much superior to a gray card.

But in the ensuing discussion, I've also made note that I use my ICC-based workflow first to determine the proper exposure and then to correct color. Make no mistrake: the first (and, I'd suggest, by far the most important) part of my ICC-based workflow is determining proper exposure settings on the camera and then the proper exposure adjustment settings in development -- and that's precisely the topic under discussion (even if what I do is a bit overboard for what Simon is trying to accomplish).

Cheers,

b&

canon rumors FORUM

Re: Grey card and spot metering
« Reply #12 on: April 23, 2013, 10:00:27 AM »

gbchriste

  • EOS M2
  • ****
  • Posts: 173
    • View Profile
Re: Grey card and spot metering
« Reply #13 on: April 24, 2013, 07:54:47 AM »
Actually, I find exposure and color balance to be closely entwined.  I shoot mainly portraits and a overly cool skin tones can appear to be underexposed and vice versa - underexposed skin tones can appear to be overly cool.  Also, when adjusting exposure, the color tones in the skin don't scale linearly.  As exposure is increased, some underlying tones in the skin will increase in brightness at a greater or lesser rate than others, causing all types of wonky skin tone problems.  Therefore, in my work flow, I deal with exposure and color balance as a single, integrated problem at the point of image capture.

1. I have my subject hold the EZBalance right up to their face, literally with their nose touching it.  The 12-inch size approximates the size of the human head and face very nicely.

2. I step in to pretty much fill the view finder with the EZBalance, take a meter reading, and increase exposure usually about 1/3 above center.  As stated above, I find a centered reading using the EZBalance to still be very slightly underexposed. 

3. I take a shot of the EZBalance as it is filling the frame. 

4. I custom white balance using that image.

5. I take the EZBalance form the subject, fold it back up in to my pocket, step back, and take a quick, casual test shot to evaluate the histogram and look for obvious problems like undesirable highlight clipping.

6. Assuming all looks well, I go to work.

Having a properly exposed and color balanced image in camera has reduced my post processing time by about 80%.

TrumpetPower!

  • 1D Mark IV
  • ******
  • Posts: 951
    • View Profile
Re: Grey card and spot metering
« Reply #14 on: April 24, 2013, 09:37:19 AM »
Actually, I find exposure and color balance to be closely entwined.  I shoot mainly portraits and a overly cool skin tones can appear to be underexposed and vice versa - underexposed skin tones can appear to be overly cool.

There's an excellent chance that that's because the raw processing software is applying adjustments out of order. Properly, exposure and white balance should be combined into three linear multipliers for each channel; then the gamma curve should be applied; then an ICC profile conversion from the camera's native space to the working space; and then, finally, any aesthetic tweaks, often including at least an S-curve for midtone contrast enhancement and whatever picture style (etc.) pre-canned tweaks.

User-supplied adjustments need to get applied at the right point in the pipeline. If you change exposure or white balance, that needs to be added in to that very first step and not done as an added adjustment after all else has been done. If you tweak the curve, it should be done after conversion to the working space.

And there are other places for things to go worng...some of the working spaces themselves have fundamental perceptual problems; see Bruce Lindbloom on "Blue Turns Purple," for example.

Of course, this is all perfectly opaque to the user; you just move the sliders around. If the programmer screwed up and isn't doing things in the right order, there's nothing you can do to fix it except see if you get better results with some other raw processor. And there's really no way you're going to even be aware of the nature of the problem except in vague ways such as the color shifts you just described when adjusting exposure.

Cheers,

b&

canon rumors FORUM

Re: Grey card and spot metering
« Reply #14 on: April 24, 2013, 09:37:19 AM »