exposure control with grey card

andreacioci

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Hi all,
I'm new in the forum. I've a Nikon D5000 with a 18-105mm and I'm trying to understand the use of the grey card 18%. Using it for the white balance is pretty simple and I want to use it for a correct exposure. Here is what I did:

- I put the camera on my tripod with 105mm focal length and measure the exposure with the light meter of the camera on the grey card. I took a pic and,when I open it through photoshop, the grey card is properly exposed with RGB values around 127.

- I changed the focal length to 18mm but I kept the same ISO, shutterspeed, aperture and took a second pic. If I measure the grey card, now the RGB values are around 140.

How comes that, changing only the focal length, the same object is exposed differently?
Thanks for your help.

Andrea
 

Forkie

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Two questions:

1. What metering mode are you in? If you are on evaluative (Canon) or matrix (Nikon) and anything changes in the frame, this will affect the overall exposure reading.

2. When you zoom in, are you stepping back to ensure the same area is covered by the card in the frame? if you shoot with the card small and in the middle and then the second shot with it taking up most or all of the frame, the exposure will be different again.
 
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andreacioci

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Hi Forkie,

1. Spot metering. Consider that I did not evaluate the exposure in the second shot. I used the same settings of the first one.

2. The distance between the camera and the grey card is the same in both shots. Therefore the second pic covers a wider area. Again, why this should change the result on the grey card if I'm not measuring the exposure again?

Thanks for your help.
 

Ysarex

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Hi Forkie,

1. Spot metering. Consider that I did not evaluate the exposure in the second shot. I used the same settings of the first one.

2. The distance between the camera and the grey card is the same in both shots. Therefore the second pic covers a wider area. Again, why this should change the result on the grey card if I'm not measuring the exposure again?

Thanks for your help.

It wouldn't change the grey card exposure then if you used the same settings on the camera and did not move the card or camera -- the two exposures were the same then.

You got different results for the grey card between the two exposures and I'm going to guess that's due to software. Either the software in your camera that created the JPEG or the software in your converter that's interpreting the raw file.

Repeat your test and save a raw file for both photos and evaluate both photos in RawDigger and you should get the result you're anticipating.

Joe
 

Big Mike

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Are you certain that the exposure settings were exactly the same? Most lenses have a different maximum aperture at one end of the zoom vs the other end. On that lens, the max aperture is F3.5 when at 18mm, but only F5.6 when at 105mm.

Also, your camera and lens are consumer level...they are not highly calibrated scientific instruments. You may get a little more or less exposure simply by changing the zoom.
And further to that, with most still photography equipment, extreme accuracy isn't required. For example, if you look at film/movie camera lenses, they talk about T-stops, which are a more accurate way of describing the amount of exposure, than F-stops.
 

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Another consideration as the size of the gray card changes as the lens focal length is changed, is that the spot is about 2% the size of the image frame.
As the FoV (field of view) gets wider with a shorter focal length, the spot gets bigger as the gray card gets smaller in the image frame.

Plus, with Nikon cameras the spot metered is tied to the selected auto focus point, so the focus mode and focus area mode the camera is set to can influence where in the frame the spot is.

http://www.greyorgray.com/
 
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andreacioci

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Ok guys...I need help :)
This is what I have done:
- on the camera I set the color space to Adobe RGB, it was on sRGB that, as far as I know, is more compressed.
- I took a picture, saved in RAW (obviously) and opened with RawDigger.

What the hell?!??!
I have tried to understand how it works...I think the maximum values for my camera is 4095, but what I read on the grAy card is R=260, G1=335, B=170.
How can I understand if the exposure is correct?

And again, I took the same pic at 18mm and the rgb values of the gray card are higher...

I really don't understand what's going on...

Thanks in advance
 

Big Mike

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Firstly, the color space setting has nothing to do with your raw file, besides an 'as shot' parameter.

When you 'open' a raw file, all you really see is the embedded jpeg preview...it's not actually turned into an actual image until you process that raw file....and at that point, you can specifiy which color space you want to assign to the image. The raw processing software will have it's own working color space...ACR or LR use something that is much bigger than either AdobeRGB or sRGB.

I'm still trying to figure out why you are going to such lengths (depths) to figure out exposure. :scratch:
 

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Ok guys...I need help :)
This is what I have done:
- on the camera I set the color space to Adobe RGB, it was on sRGB that, as far as I know, is more compressed.
Adobe RGB and sRGB are colorspaces. sRGB has a smaller color gamut than Adobe RGB does, but not by much.
The web is strictly sRGB. Some print labs can print images that are in the Adobe RGB colorspace.
Most image editing experts recommend doing all your image editing in a wide gamut colorspace like ProPhoto RGB, and as a last step converting to a colorspace appropriate for the image use.

Compression is about file type- NEF, CR2, TIFF, JPEG, PNG, GIF, etc - and file size as it relates to bit depth.




Color space - Wikipedia, the free encyclopedia
Image file formats - Wikipedia, the free encyclopedia
Image compression - Wikipedia, the free encyclopedia
 
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andreacioci

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Exactly...my nikon camera has a setting for the color space so, as i understand, the raw file is affected by it.

Now...doesn't anyone know how to read the rgb values shown by rawdigger? They are not referred as 8 bit depth as Photoshop does.
.
 

Big Mike

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Exactly...my nikon camera has a setting for the color space so, as i understand, the raw file is affected by it.
That's not really how it works.

A Raw file (as I understand it) doesn't really have a color space...it just has information from the sensor, that will eventually become colors in the image.
 

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It sounds to me like you are trying to do advanced calculus before understanding basic math.
 

Ysarex

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Exactly...my nikon camera has a setting for the color space so, as i understand, the raw file is affected by it.

Now...doesn't anyone know how to read the rgb values shown by rawdigger? They are not referred as 8 bit depth as Photoshop does.
.

Hey, I'm guilty here for setting you onto RawDigger in the first place, but it's after midnight and I'm going to bed. I'll check back in with you in the morning and help clear this up.

Joe
 

Ysarex

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Exactly...my nikon camera has a setting for the color space so, as i understand, the raw file is affected by it.

Now...doesn't anyone know how to read the rgb values shown by rawdigger? They are not referred as 8 bit depth as Photoshop does.
.

Good Morning Andrea,

First, Mike is correct; a raw file has no color space and the color space setting on your camera is meaningless relative to the raw file.

Given what you're trying to do, I suggested RawDigger because it will let you examine a raw file without interpretation/interference.

I think you're trying to photograph a grey card as a way to test the accuracy of your camera's meter. And you correctly assumed that if the camera was not moved and you used the exact same shutter speed f/stop combination the grey card should be exposed the same regardless of lens focal length -- good assumption.

Reading the grey card values is where it can get tricky. Back in the day (we old timers always have to refer to back in the day) when we shot film we had a device called a densitometer. We'd put a film negative on the densitometer stage and read transmission values of light passing through the negative. With a properly calibrated densitometer we got hard values that allowed us to do what you're trying to do now. It was nice back then to be able to hold and physically examine the film and then test the film; it gave us a feeling of certainty.

The data recorded by your digital camera is a lot harder to examine so directly. By the time you're trying to read RGB values, assuming middle grey should be near 127 in all three channels, the data from your sensor has passed through a lot of software changes. Just about all the software you use is interpreting that data so that the values you're getting aren't nearly as hard as what we used to get from film and a densitometer. You'd like to think for example that the raw conversion software you're using would have some kind of default standard setting where you can expect to be able to examine your data and that all raw conversion software includes this same default -- unfortunately that's not the case and they all process the data differently. For example, I just shot a grey card and processed the raw file through three different raw converters. I used a Canon camera so I first processed the file through Canon's own DPP, then Photoshop's ACR and PhotoNinja. I set each raw converter to it's default settings (linear curve) and then reading the center of the grey card DPP = 121, ACR = 115 and PhotoNinja = 104. Different software is spinning the data differently.

So we go to RawDigger as a way to get a harder look at just what's in our raw file; data without the spin applied.

First problem: Your two grey card exposures should give you pretty much the same values in RawDigger. Alt click to set a sample point in RawDigger and you'll get values for all four channels. Examine the green channel values and expect to see close to the same values from the grey card for both your exposures. I just did the same test.

I set a grey card in a window and set my camera to full manual with spot metering.
I used a zoom lens and started with the long end.
I metered the card at ISO 100: 1/30 at f/5.6 and took the photo.
I zoomed out to the wide end of the lens and took a second photo at ISO 100: 1/30 at f/5.6.

I didn't bother to use a tripod and so we can expect some variance from my hand-holding the camera.

In the first photo I placed a sample in the center of the grey card and I get a green average value of 1149.9
In the second photo with a sample placed as close as I can to the same center of the grey card I get a green average value of 1171.8

That's a slight difference and I'm happy to call the discrepancy due to my hand holding and the design of the zoom lens. If you're expecting a zoom lens to hold a constant aperture across it's zoom range you're going to be disappointed.

Interpreting the RawDigger values for useful exposure info is another task. The numbers and scales you see in Raw Digger will change from one camera to another depending on the sensor type in the camera so I can't for example tell you to expect a grey card to give you values similar to the one's I got. Here, the color of the light does play a role and RawDigger is showing you values for four color channels. When you're trying to evaluate your camera's metering system without running detailed tests, look at the green channel values. From the Window menu display RawDigger's histogram.

Across the top of the Histogram window make sure Show EV grid is checked. In that same box make sure Auto is checked and then look at the value for 0EV. With my Canon camera the 0EV value is 1024 and in these grey card photos I got values just a little above that around 1160. Now think in terms of EVs and see how close the green channel grey card value places relative to the 0EV position. If your meter is working well reading from the grey card you want to see the sampled green channel value fall within 1/3 EV of the 0EV position. Different camera manufacturers adjust their meters differently and frankly I wouldn't start worrying unless I saw that green channel value approaching 1EV different from the the 0EV value. Light color will play a role and you want to take your grey card photos in daylight conditions.

Hope this helps.

Joe
 

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