Gray card metering with 20D. whats up?

[johneric8]

I'll make this quick and 2 the point. I make a habit sometimes of putting my 20D in av priority and metering on a gray card. Whatever the shutter speed reading I get I switch over to manual mode and use along with the aperture that I had chosen in AV priority. Now, here is my question? When taking the reading in AV priority, does it matter which metering mode the camera is set to? for example, I had it set to evaluative when I took the reading in AV mode and for some reason all the pictures I took were on the bright side. would it make a difference if I changed it to Partial or center weighted metering? I'm waiting to get a nice hand held meter but until then I enjoy trying to set my own exposure with the gray card. Any info would be apreciated. I was using an old meter that my grandpa gave me made by argus that worked great but I dropped it and broke it when I did a photo shoot for a girl from hooters!! LOL

[kraterz]

Yes, it matters what metering mode you are in. In evaluative, the camera does all sorts of "smart" calculations, and gives you the shutter speed it thinks will give you the right exposure. Spot/partial and to a lesser extent center weighted, are far more reliable with a gray card. At least you will understand what the camera is doing. I'd go with spot/partial.

[Dante King]

Are Grey cards kinda redundant when sooting raw?

[johneric8]

Yes, it matters what metering mode you are in. In evaluative, the camera does all sorts of "smart" calculations, and gives you the shutter speed it thinks will give you the right exposure. Spot/partial and to a lesser extent center weighted, are far more reliable with a gray card. At least you will understand what the camera is doing. I'd go with spot/partial.



Thanks for the feedback Kraterz thats what I thought. to answer the question if shooting a gray card is redundant in Raw I will say this. Yes, if you are shooting the gray card for white balance. But, if your shooting in raw and want use a gray card as a setting point for exposure then it really does make a difference. the exposure that you get can make or break the picture even when shooting in raw. This is why I like to take a reading off my kodak gray card because I normally find it to be pretty reliable. It's something I dont do for everything but if I'm going out exploring and looking for cool things to photograph then I really love the gray card.

[pierrot]

Incidentally, there is no spot metering on a 20d. ;)
Johneric, you say your pics look the bright way. What does the histogram says?

[johneric8]

Incidentally, there is no spot metering on a 20d. ;)
Johneric, you say your pics look the bright way. What does the histogram says?


It did say they were on the bright side for sure! but not overexposed. But to my eyes some of the things looked overexposed like a white plastic lawn chair for example, in fact there were a couple of placed where the picture was blinking showing a few small areas of overexposere. I know there is no spot metering on the 20D but, the parial metering works in a similar way. Even the people at canon said this to be true when I talked to their suport the other day..

[griff2]

Are Grey cards kinda redundant when sooting raw? Not at all. You can set white balance from a gray or white card, exposed in the shot, when using RAW, but your results may be different if you'd done a custom white balance from a white or gray card.

[etaf]

use the Partial metering mode - thats about 9% of viewfinder

[PacAce]

Yes, it matters what metering mode you are in. In evaluative, the camera does all sorts of "smart" calculations, and gives you the shutter speed it thinks will give you the right exposure. Spot/partial and to a lesser extent center weighted, are far more reliable with a gray card. At least you will understand what the camera is doing. I'd go with spot/partial.
No, it doesn't matter. It's a gray card reading so no matter how you dice and slice it, you'll always get the same meter readings because the tonality of the scene (the gray card) is consistant all over (assuming the gray card covers the entire viewfinder like it's supposed to unless you're using partial metering). Now, if you were metering a scene with a lot of different light readings all over the place, then, yes, it would matter.

[scottbergerphoto]

1. Getting the exposure right is always important whether you shoot Raw or jpeg.
2. If the Grey card fills the entire viewfinder, it shouldn't matter which exposure mode you use.
Scott

[SirvoN]

Dante,

While I agree shooting in Raw provides lattitude to make corrections to exposure I wouldn't call it a substitute for nailing the correct exposure from the start. I shoot 90% Raw and 90% full manual (partial metering), and knowing where to set your exposure to maximize dynamic range and minimize noise is a worthwhile skill to learn.

To respond to the original question - many have already stated this. If you fill the screen with the grey card it doesn't matter. Otherwise, use partial and be sure the entire partial meter area falls on the card before determining the exposure. And it might save you time if you simple use manual mode rather than using Ap and then switching over to Manual.

Are Grey cards kinda redundant when sooting raw?

[pyterps]

I just have one question about the white balance. In what situations would you use a gray/white card to set white balance.

[Adam Hicks]

Yeah try shooting a girl in a white wedding dress against an otherwise dark background. The camera will average the scene and most likely blow out the detail on the dress. No amount of photoshopping a RAW image is going to get that back!

Adam

[Jon]

I just have one question about the white balance. In what situations would you use a gray/white card to set white balance.
Any conditions where you have doubts about either using one of the canned white balances or relying on AWB to get you correct colour renditions. Extreme examples will be under gas-discharge (fluorescent, sodium-vapor, mercury-vapor) lighting, of a subject which is receiving reflected light from a strongly-coloured environment (heavy vegetation, open shade in blue sky conditions). Possibly more typical would be under general incandescent lighting, open sun in the hours near sunrise/sunset (where subject, not sky colour is important). It'll also help in mixed lighting conditions as you'll be better able to zero out the effect of one lighting type, and only have to deal with the other later.

[pyterps]

JON,

Thanks for the information. I think I stated my question wrong. I guess it doesn't matter if you use the gray or white card to set white balance? When would you use the gray card over the white care or vise versa?

Dave

[Jon]

I'd always use the grey card over the white. What CWB does is figurte out how much adjustment to apply to 2 of the 3 color channels, say R & B, so in the final exposure on a neutral grey R=G=B. If you shoot a correctly-exposed white card as your "calibration", you risk blowing out the highlights, and the correction will be thrown off. For instance, assume that your recordable color values are 0-255 for each color channel. You shoot a grey card, properly exposed, and the camera determines R=126, G=128, B=130. So It'll boost the red a tad and pull back n the blue a little. You shoot a white card, it's right at the edge of your sensor's recording ability and your values are R=252, G=255, B=260 (which is recorded as 255, since that's as high as it can measure). Now the camera's going to decide that red needs to be boosted a little, but the blue's just fine, since it already equals the green. But it was actually over, and the results were lost in the blown highlights, so your CWB will be showing too blue. So you always want to use something that will give you a mid-grey exposure rather than a white, and my preference is to use something that will not only give me a mid-grey, but an exposure as close as possible to what my "real" pictures will be at.

[pyterps]

Jon....really like your explination and will use the grey card from now on. I is like working with the zone system and where zone 5 is middle gray. It would also add more latitude to push or pull....Dave

[RDKirk]

I had it set to evaluative when I took the reading in AV mode and for some reason all the pictures I took were on the bright side. would it make a difference if I changed it to Partial or center weighted metering?

Yes. The camera has an internal database containing tens of thousands of correctly exposed image (stored as mathmatic expressions). When you take an evaluative meter reading, it takes the active focus point as the subject, measures all 35 or so segments of the screen, calculates that into a mathmatic expression using a proprietary algorithm, then compares that expression with the tens of thousands in its database. When it finds a match, it sets the exposure.

When you present it with a gray card, the match it will find will be some dull, overall gray day photograph...so it will increase the exposure somewhat to lighten up the day. The problem, of course, is that if you're going to play Captain Kirk and fool the computer, you have to compensate for it.

I'm waiting to get a nice hand held meter but until then I enjoy trying to set my own exposure with the gray card. Any info would be apreciated.

In-camera metering is actually quite mature these days. I do remember in my earliest days that those new-fangled in-camera meters were still looked upon with great distrust. But evaluative is actually very good on its own, although a spot meter (and even the "large-spot" Canon 9% partial meter) is potentially better if you really know how to use it.

[johneric8]

Yes. The camera has an internal database containing tens of thousands of correctly exposed image (stored as mathmatic expressions). When you take an evaluative meter reading, it takes the active focus point as the subject, measures all 35 or so segments of the screen, calculates that into a mathmatic expression using a proprietary algorithm, then compares that expression with the tens of thousands in its database. When it finds a match, it sets the exposure.

When you present it with a gray card, the match it will find will be some dull, overall gray day photograph...so it will increase the exposure somewhat to lighten up the day. The problem, of course, is that if you're going to play Captain Kirk and fool the computer, you have to compensate for it.



In-camera metering is actually quite mature these days. I do remember in my earliest days that those new-fangled in-camera meters were still looked upon with great distrust. But evaluative is actually very good on its own, although a spot meter (and even the "large-spot" Canon 9% partial meter) is potentially better if you really know how to use it.


Thanks for this insight, this was what I was looking for in regards to my original question. This is kind of what I was thinking but wasnt sure...

[mbze430]

Yes. The camera has an internal database containing tens of thousands of correctly exposed image (stored as mathmatic expressions). When you take an evaluative meter reading, it takes the active focus point as the subject, measures all 35 or so segments of the screen, calculates that into a mathmatic expression using a proprietary algorithm, then compares that expression with the tens of thousands in its database. When it finds a match, it sets the exposure.


What RDKirk describe is based on Nikon's 3D Color Matrix I and II system. I don't know if it applies to the Canon system. Anyone want to find technical details based on the Canon system?

[johneric8]

What RDKirk describe is based on Nikon's 3D Color Matrix I and II system. I don't know if it applies to the Canon system. Anyone want to find technical details based on the Canon system?


I dont know for sure but I have heard that to be true of the canons as well. I'm quite sure they use the database system in evalutive. If I'm wrong, please someone let me know.

[mbze430]

This is the closest thing I found on the net from Canon regarding their evaluative metering system

http://www.canon.com/technology/detail/35mm/21_exposure_con/

[PacAce]

Yes. The camera has an internal database containing tens of thousands of correctly exposed image (stored as mathmatic expressions). When you take an evaluative meter reading, it takes the active focus point as the subject, measures all 35 or so segments of the screen, calculates that into a mathmatic expression using a proprietary algorithm, then compares that expression with the tens of thousands in its database. When it finds a match, it sets the exposure.

When you present it with a gray card, the match it will find will be some dull, overall gray day photograph...so it will increase the exposure somewhat to lighten up the day. The problem, of course, is that if you're going to play Captain Kirk and fool the computer, you have to compensate for it.

Although you a good understanding of the evaluative metering system works, your assumption on how it would apply to a gray card seems to be totally off. As I (and ScottBergerPhoto) have stated previously, when you meter a gray card that completely fills the viewfinder, it does NOT matter which metering system you use. And simple math will prove this. A gray card will give you the same meter reading no matter which of the 35 (or however many there are) sensors is used. Let's assume that the reading is 5 in a scale of 1 to 10 (and it really doesn't matter what kind of scale is used either).

If you average out this reading, you will get

35 sensors * 5 / 35 total sensors = 5.

Now, assume we're using CW-averaged metering. In this mode let's arbitrarily say that the 9 center sensors are weighted 3 time the others (the exact weight doesn't matter here either). So the total reading from center will be

9 center sensors * 5 * 3 (for the weighting) = 135

And the reading from the rest of the sensors is

(35 sensors - 9 center sensors) * 5 = 130

So, to get the CW Avg,

(135 + 130)/((9 * 3) + 26) = 5

We can go on and do the same type of math for Partial metering and Evaluative metering but at this point you should get the idea already. No matter how your slice and dice the gray card reading and apply all sorts of algorithm to it, the fact remains that all the reading from the different sensors started out being equal so your end result will also be the same. No matter how smart the Evaluative system is, it will not alter the original sensor reading in any way. It only uses it for doing the complex math to arrive at a single reading to use for the exposure. And, no, it will not take a gray card reading and think it's a dull, gray day because it will be smart enough to know that, hey, all the sensor readings are the same, it must be a solid colored card of some type.

And to prove my point, I did an experiment. I shot a grayish card outside in all 4 metering modes and this is what I got:

http://www.tanseikai.com/pacace/images/test2.jpg


Then I went indoors and got this:

http://www.tanseikai.com/pacace/images/test1.jpg


And if you still don't believe me, you can easily try it at home yourself.

[RDKirk]

What RDKirk describe is based on Nikon's 3D Color Matrix I and II system. I don't know if it applies to the Canon system. Anyone want to find technical details based on the Canon system?

Pull out your copy of the July 1987 Modern Photography magazine (you still have it in your closet, right?). That edition has the stripdown test report of the new Canon 650 and its new Evalutative Metering system. Maybe your local library has it on microfiche.

Yes, the Nikon Matrix metering does work the same way. Canon has never revealed exactly how many image expressions are in their database (Nikon actually quoted 90,000+).

[RDKirk]

We can go on and do the same type of math for Partial metering and Evaluative metering but at this point you should get the idea already. No matter how your slice and dice the gray card reading and apply all sorts of algorithm to it, the fact remains that all the reading from the different sensors started out being equal so your end result will also be the same. No matter how smart the Evaluative system is, it will not alter the original sensor reading in any way. It only uses it for doing the complex math to arrive at a single reading to use for the exposure. And, no, it will not take a gray card reading and think it's a dull, gray day because it will be smart enough to know that, hey, all the sensor readings are the same, it must be a solid colored card of some type.

And to prove my point, I did an experiment. I shot a grayish card outside in all 4 metering modes and this is what I got:

http://www.tanseikai.com/pacace/images/test2.jpg


Then I went indoors and got this:

http://www.tanseikai.com/pacace/images/test1.jpg


And if you still don't believe me, you can easily try it at home yourself.

Did you actually print any of your pictures?

Actually, the last time I ran the exercise, I did it on Kodachrome on a brand new Elan (mark I)--it didn't have a histogram. But it did produce images of gray cards about half a stop too light.

I do like the fact that your histograms put your gray card just to the left of center...good.

But did you actually print out your pictures?

[PacAce]

Did you actually print any of your pictures?

Actually, the last time I ran the exercise, I did it on Kodachrome on a brand new Elan (mark I)--it didn't have a histogram. But it did produce images of gray cards about half a stop too light.

I do like the fact that your histograms put your gray card just to the left of center...good.

But did you actually print out your pictures?
What's there to print. They all look the same. And it's a "gray" card. The aperture and shutter speed is what's important and they're all the same.