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Grey scale . . . color chart ?
Old 03-30-2005, 08:15 PM   #1 (permalink)
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Anybody done any testing of their camera to determine accuracy of your flash meter and what your camera records using a ten-step greyscale and a color chart? I tried this recently and I seem to now have come up with more questions than answers.

To explain my procedure: I used a homemade grey scale from a laser printer (found an example on the net and printed it) and a homemade color chart from an inkjet printer (similar procedure).

Using a Wein 1000 meter, I dialed the power supply of my light system to set the light at exactly f/4. Nikon D100 ISO was set at 200. Color balance in camera set to flash with no adjustment to color temp. (For the D100 owners, you know that there are 3 steps up and three steps down from the base white balance setting for flash in this camera.) I used an 45" umbrella to evenly light the charts, which were shot simultaneously. Since the Wein meter is set to give base readings at ISO 50, the equivalent at ISO 200 should be f/8. Shot in NEF (Nikon's raw mode). I started shooting at f/5.6 and adjusted in 1/3 stop increments to f/11. Recording medium was a 12x Lexar 256 mb Compact Flash card. Camera set to manual exposure and shutter speed at 1/125. I have not printed anything yet, only viewed on the LCD in the camera and on the computer monitor. Doing a quick check using autocontrast in the editing software, both black and white improved, while retaining each of the steps.

With regard to solid, saturated blacks and clear bright whites, am I wrong to expect that somewhere along the range of exposures there should have been soemthing very close to the test target with little or no adjustments in photo editing software? To my eye the black was short of black and similar for white. Additionally, there was strange color banding patterns in the grey surrounding the ten-step scale.

Have I been too frugal, err thrifty, okay cheap? Would some of the problems gone away if I had used a "real" grey scale like the one available from Kodak and a real color chart from . . . is it MacBeth? (Those are aweful spendy.)

I am trying to crank up the quality of my work and am awaiting the delivery of a book on image editing. However, my idea for the test is that if you start with the best possible image out of the camera, your task in editing will be less cumbersome and produce much more satisfying results.

Your input will be greatly appreciated.
Cheers,
Clint
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Re: Grey scale . . . color chart ?
Old 03-30-2005, 09:26 PM   #2 (permalink)
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Use the color sampler in PS and check the numbers; you won't be able to know for sure by looking at a monitor.

Your 18% grey should read R=127, G=127, B=127. Your white should read above 245 for R,G&B. You black should be below 25 for each channel.

You won't necessarily move the scale off of one end or the other as long as the whole chart is within the dynamic range for the exposure. With digital imaging you measure luminance on a scale from 0-256. 0 is black, 256 is white, so the mid point is 127. Shadow detail begins to appear somewhere around 25 or above and highlights begin to disappear somewhere above 240.

If the values for the RGB channels differ (like R=127, G=129, B=131) then you have a color cast, probably a white balance issue. This, by the way, is a great reason for having a trustworthy grey card; you can shoot a test with the card in the scene and always have a reference for adjusting any color cast that happens to creep in.


Hope that helps some.
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Valuable info
Old 03-30-2005, 09:34 PM   #3 (permalink)
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[ QUOTE ]
Hope that helps some.

[/ QUOTE ]

Indeed it does. Thanks a bunch. Gonna go to work on that.

If others have any thoughts, I would appreciate hearing from you as well.

Cheers
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Re: Grey scale . . . color chart ?
Old 03-30-2005, 09:49 PM   #4 (permalink)
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Gee....I seem to recollect a post last week where I mentioned that a light meter only gets you in the ball park...but several of the experts pointed out that I was an idiot....so we will have to wait and see what they have to say.

You are never going to get absolute black and 100 percent white using a flash meter because for starters it is based upon 18% grey, and number two, you have to consider the absorbtion/reflectance of your material.

You can have enough light that you aren't getting pure white, and yet enough reflectance off the darker parts of the chart that they won't go black.....but you used a light meter and you have a perfect exposure.

Meter all you want, but stick a black laquered car with chrome rims on a white cyc in your perfectly metered light and I'm guessing you are going to have to rethink your strict constructionist rationale and rules.

Mark


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You, sir . . .
Old 03-30-2005, 10:06 PM   #5 (permalink)
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. . . have a " 'tude" [attitude] !

[ QUOTE ]
Gee....I seem to recollect a post last week where I mentioned that a light meter only gets you in the ball park...but several of the experts pointed out that I was an idiot....so we will have to wait and see what they have to say.

[/ QUOTE ] etc.

Help me. Help me. I like the tude. [img]/ubbthreads/images/graemlins/cool.gif[/img]

Thanks for the post. [img]/ubbthreads/images/graemlins/wink.gif[/img]

I will also be waiting to see what the experts have to say. [img]/ubbthreads/images/graemlins/grin.gif[/img]

Cheers.
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Black/gray/white...
Old 03-30-2005, 10:29 PM   #6 (permalink)
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I think the you may have done a bit of an overkill.
If you just take a Black/gray (18%)/white card and photograph it full frame with your camera, and then examine the histogram you will see three spikes when you have the proper exposure. If one of the spikes is clipped or missing, then you need to adjust the exposure till you have three good spikes. Also, always try to move the three spikes as far to the right of the histogram as you can without clipping the highlight spike. This will give you the most information in your shadows. Also be sure to include the target in one of the shots of each lighting set and it will prove of great value in post processing.
Cheers,
rfs
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Re: Grey scale . . . color chart ?
Old 03-30-2005, 10:38 PM   #7 (permalink)
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I'm with you on the meter. I use a meter, I like my meter... but sometimes it just ain't right... it always tells the truth, but it ain't always right.

I remember a re-shoot with a guy who had light grey hair, a pink complexion and liked to wear all black... I should've known better but I was checking ratios with an incident meter and completely spaced on the black clothing... almost no detail in the clothing at all and we had to make a lifesize cutout of the dude. He was not happy about re-shooting.
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18% grey...
Old 03-30-2005, 10:59 PM   #8 (permalink)
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A bit of a further explanation (I used to tech this shyt). If you are using a reflective meter, ie an in camera meter and metering from your subject, I agree completely with you. Is the subject light or dark, reflective or absorbant can make a huge dffference, it the old black cat in a coal bin or a fair blonde on a white sandy beach scenarios can cause massive problems with reflective meter. That reflective meter believes it is always reading 18% grey and when it isn't there are problems, that's where the shooter's head and knowledge enter into the picture by knowing when and how much to adjust the exposure from the meter reading.

An indicent meter removes that problem. An incident meter doesn't care how reflective the subjet is, it measures the amount light falling on the subject before it reaches the subject. Its frosted white dome reflects away 82% of the light falling on it, allowing only 18% (exactly 18%) to pass through and reach the meter. If exposure is corect for 18% grey, then it's correct across the scale. The rest of it is contol of the light present, avoiding glare, fill light etc. the only remining questions are wher to point the reflective meter, toward the light source, toward the subject or in beteen. That's where the photogs judegement enters the picture.

A reflective reading from a grey card gives an approximation of an incident reding. It can be exactly the same or sometimes way off, depending on how it is done. Is the grey card in the same slght as the subject? Is the grey card tilted or angled? The degree of angle in relaton to the light can make a difference in the accuracy of the results.

Note, there is another technique which is less known. Take a clean, white styrofoam coffee cup and place it over the lens so that it covers the lens completely, then use the in camera meter as though it were an incident meter. White styrofoam also pases through only 18% of light striking it.

For what it's worth, this is one of the topics I was grilled on many years ago in a previous life before being qualified as an 'expert witness' in a criminal trial in Federal Court. Try it, it works!
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Re: 18% grey...
Old 03-31-2005, 01:45 AM   #9 (permalink)
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[ QUOTE ]
Note, there is another technique which is less known. Take a clean, white styrofoam coffee cup and place it over the lens so that it covers the lens completely, then use the in camera meter as though it were an incident meter. White styrofoam also pases through only 18% of light striking it.


[/ QUOTE ]

I've tried the white styrofoam cup a long time ago and did not have good luck with it. What I did find that worked was 1 or 2 white coffee filters used in the same manner.

Cheers,
rfs
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Re: 18% grey...
Old 03-31-2005, 11:38 AM   #10 (permalink)
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O.K.

Let me run this up the flagpole and see if anyone salutes.

A camera has a useable range of tones it can capture. Depending upon whether you are using film, digital, or chromes, the latitude varies, but any tone within that latitude is useable and is neither overexposed or underexposed.

You can use a light meter and theoretically, it gives you a base exposure, ie the theoretical proper exposure for capturing the widest ranges of tones in an image.

So lets say I meter my lights at f8 and plop a model down in front of my camera. Theoretically, if I set my camera to F8 and the meter says f8, I have set the "optimum" exposure.

But f8 isn't the only tone that camera will record. It will record highlights up to the point of overexposure, and it will record shadow detail down to the point of underexposure.

The "range" between a highlight burning out and a shadow going completely black is your "latitude".

My argument with "meter freaks" is they act like (some not all) that you then don't take your subject into account at all.

If I want to, I can have the lights set at f8, and assuming I have three stops of latitude, I can set the camera at f5.6. This will have an effect on the subject. The subject will be "brighter" than if I shot her at f8. However, it will still be within the latitude of the camera to capture. So long as I don't burn out any highlights, there is nothing "wrong" with shooting the photo at f5.6.

If I want, I can stop down to f11 (leaving the lights at f8) The subject will now be darker than when shot at f8. So long as I don't block up my shadows, I am within latitude of the camera to capture the image.

You might say "well, why would you want to do that". Well, because, in most cases, I am not just metering a single light on a single subject at a single spot.

I might want a highlight at 5.6 on the left side of the face, middle tones acroos the center of the subject, and dark shadows on the right. If the camera has 3 stops of lattitude, all three areas will capture, and yet only one of the three areas will meter at f8.

I have neither overexposed/underexposed any part of the image.

I might want to shoot a dramatic shot with a grid on a light and put the light right on the models face. I might meter at f8 but set the camera to 5.6 so that the face is "very bright" in relation to all the other tones in the image and I might want my shadows and fall off to be very dark.

In a studio, where you can control the light, any light from any direction that is within latitude, is neither overexposed or underexposed, if it is within the capture latitude of my device.

I can control the lights to any degree I want so long as I stay within the latitude of the capture device.

Outside, in many cases, the available light is many stops outside the latitude of my capture device.

I can therefore place my subject in a shadow, and sacrifice the highlights in the background for the amount of light I want on the subject.

Metering gets you close, and then from there you have the right and ability to control highlights, shadows and the subject within the latitude.

The subject then effects my choices. If I shoot a grey mouse on a grey blanket under flat lit conditions, I may be able to shoot the shot with several different f stops as I will still be within the range and latitude and am neither sacraficing higlights or shadows because the light falling on the subject is a very narrow latitude.

On the other hand, if I put a black shirt and a white sweater on a model who is fair skinned, I may find that I have to make the choice of sacraficing the detail in the black shirt to get the light I want on the model's skin where I want it. I might end up sacraficing the highligts on the white sweater for detail in the black shirt.

I understand and I am not arguing the point that the meter gives you the base theoretical exposure that may be "optimum" but YOU have control over tones in an image as long as they are within the latitude of the film and and you control the light to avoid highlight burnout and blocking up the shadow areas.

There are two styles of black and white figure photography that are posted here on a regular basis. One is dark, moody and somber and the other is the more blown out style.

In fact, it is a well known fact that you can "overexpose" black and white....which is actually a misnomer....you are recording light within the latitude of the film, but above middle grey, for the effect it has on the subjects skin tones.

When you do this, you are exposing the film above the meter reading, but within the latitude of the film.

It's a choice you have available.

Being a slave to one and only one meter reading is what I object to.

Perhaps we are saying the same thing.

I might agree that metering the lights at F8 gives you the theorectical reading to provide the greatest and most accurate range of tones in an image. But I might want to skew the balance (but within the latitude) by either opening the aperature or closing it down....in which case I am not shooting what the meter tells me.

In very low light (available light conditions) where the light is very flat, I can set the the camera one to two stops away from the meter reading and get better shots than using the meter setting.

I understand the difference between using a reflected reading and an ambient reading, but once you have your ambient reading, you can still "deviate" as the case may be.

Theorectically, if you meter at f8 on a white background, you background should be white, but we all pump more light on the background. Why do we do that?

Mark

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