Greyscale Test

[airgirl]

I am trying to do a stepped greyscale test and am having some trouble. I have done the test numerous times (and I mean numerous), but I'm still not getting the correct results.

I am using 18% grey card.
I am doing the test in complete shade.
My camera is set to ISO 100 and f/8 manual exposure.

I have a couple of questions, so if you could clarify for me, that would be great!

1. I have to take a meter reading off the grey card and set the shutter speed to get a correct exposure leaving the aperture at f/8. Does this mean just to take a picture of the grey card and see if the picture matches the colour of the grey card?

2. When I take 6 exposures above and below this reading, then read the RGB levels, the amounts are different. I have tried to rectify this over and over again, but to no avail. What else should I be doing?

3. Even when I have all of my exposures, starting from white, the RGB levels for the first picture should be R 255 G 255 B 255. The levels for the next exposure from this is still R 255 G 255 B 255. The further along the scale I go, that's when the values differ at each exposure (ie) R140 G 150 G 145.

4. Are you able to explain to me in layman's terms what the purpose of this test is. I understand that it's to measure the dynamic range of a camera, but WHY? Sorry if this is a stupid question, but am just having a difficult time understanding this.

Do you have any other hints or tips that might help me to succeed in completing this test?

Thanks so much!

[ricktas]

I think you are worrying about something you have no control over (the dynamic range of your camera). If you are doing a test and have no idea what the purpose of the test is, you are spending way to much time on the technical and electronic aspects of your camera, and not enough enjoying taking photos. I assume that as you posted this in the New To Photography forum, you are new to photography. At this stage, learning to get good exposures means knowing the exposure triangle and getting an understanding of the histogram your LCD can display.

The best use for a grey card is for setting a custom white balance, and you will find that is the only use that most photographers use the grey card for. The dynamic range of a camera sensor is set, most DSLR cameras have a dynamic range of 9-10 stops. You cannot change that, just get out and take photos and enjoy the experience.

Here is a guide to dynamic range : http://www.dpreview.com/learn/?/key=dynamic_range

[airgirl]

Thanks for your response, Rick.

The only reason I ask this is that I have an assignment to complete and obviously the purpose of the assignment is to work out what the dynamic range of my camera is.

Otherwise, I really wouldn't be bothering to do this. I would much rather be enjoying taking pictures, believe me.

I shall keep trying until I get a good result.

Luisa

[stixstudios]

Perhaps what you need is a 21 step grayscale step wedge - and then photograph that.
I can't see how you could work out the dynamic range with an 18% grey card?

Steve.

[airgirl]

Steve

Sorry....perhaps the 18% grey card was a little misleading.

For the assignment I need a photographic grey card.

Where I get the 18% from is here....and I quote

'There are lots of different methods for arriving at an accurate mid-tone reading and one method is to use an 18% grey card. If we consider the grey scale in terms of steps, rather than a continuous increase in tone and each step is double the density of the previous step, or one stop, we find that the grey that falls midway between black and white is the same as that of a grey card that reflects 18% of the incident light.'

I'm not quite sure what a 21 step grayscale step wedge is.....

[ricktas]

I am guessing here, cause I have never attempted this sort of test. It is easier to just search the net and find out the dynamic range of a given camera brand/model sensor.

So what I am guessing is the point of this test is you start with your 18% grey card and take photos based on your settings that you have from the course. You then need to move up and down from that point, 1 stop at a time, till the grey card appears either pure black or pure white, then from those two points you can ascertain the dynamic range of your camera based on how many stops there are between the white one and the black one. You will also need to perform this under a constant light source. So for example, doing it outside will not work cause the light can change as clouds move over the sun etc.

[ameerat42]

Hi airgirl.
I'm trying to figure out how you're supposed to figure out the DR of your camera using so humble a method, considering that at places like DP Review go to great lengths to do so.
As Rick said, look on-line, and the DP Review site is
http://www.dpreview.com/reviews/specs.asp
Your camera would surely be listed. Look for its "In Depth Review" and if you find one, try to find a "Photographic Tests (DR)" from the pull down menu.
As an example, here's the specific test for the Canon 550D
http://www.dpreview.com/reviews/canoneos550d/page16.asp
If the DR is not specified, try the manufacturer's specs on their site (though I rather think DPR's info would be more comprehensive.
Am.
PS: You'll also see a photographic greyscale wedge on the page.

[Kym]

This might help... http://www.cambridgeincolour.com/tut...amic-range.htm

You get more DR from raw than JPEG.

What Camera brand/model? How many bits in raw?

[airgirl]

Hi Kym

My camera is a Nikon D5000.

Sorry, I couldn't tell you how many bits in raw. How can I find this out? From the manual?

[airgirl]

Hi airgirl.
I'm trying to figure out how you're supposed to figure out the DR of your camera using so humble a method, considering that at places like DP Review go to great lengths to do so.
As Rick said, look on-line, and the DP Review site is
http://www.dpreview.com/reviews/specs.asp
Your camera would surely be listed. Look for its "In Depth Review" and if you find one, try to find a "Photographic Tests (DR)" from the pull down menu.
As an example, here's the specific test for the Canon 550D
http://www.dpreview.com/reviews/canoneos550d/page16.asp
If the DR is not specified, try the manufacturer's specs on their site (though I rather think DPR's info would be more comprehensive.
Am.
PS: You'll also see a photographic greyscale wedge on the page.

Thanks for this!

I will look into these as soon as I can.

[airgirl]

I am guessing here, cause I have never attempted this sort of test. It is easier to just search the net and find out the dynamic range of a given camera brand/model sensor.

So what I am guessing is the point of this test is you start with your 18% grey card and take photos based on your settings that you have from the course. You then need to move up and down from that point, 1 stop at a time, till the grey card appears either pure black or pure white, then from those two points you can ascertain the dynamic range of your camera based on how many stops there are between the white one and the black one. You will also need to perform this under a constant light source. So for example, doing it outside will not work cause the light can change as clouds move over the sun etc.

Rick

That is exactly it!!!

I have done this test numerous times and I cannot get pure black. I do however, get pure white.

The RGB levels on each of the exposure above and below the initial exposure do not have the same numbers, where I'm pretty sure they are supposed to.

I'm going to have a play with the functions on my camera to check that the white balance is set correctly. This may be causing the inconsistent results.

[arthurking83]

D5000 is 12bit raw.

[kiwi]

Rick

That is exactly it!!!

I have done this test numerous times and I cannot get pure black. I do however, get pure white.

The RGB levels on each of the exposure above and below the initial exposure do not have the same numbers, where I'm pretty sure they are supposed to.

I'm going to have a play with the functions on my camera to check that the white balance is set correctly. This may be causing the inconsistent results.

If you are not getting pure black, you have too much ambient light

[fillum]

The RGB levels on each of the exposure above and below the initial exposure do not have the same numbers, where I'm pretty sure they are supposed to

Not sure exactly what you mean here. For a neutral colour (white, grey, black) R=G=B, however the exact values will vary at each exposure setting as you move from black (R=G=B=0) to white (R=G=B=255). In reality I'd image it would be very difficult to get the RGB numbers the same due to variations in colour temperature.

Not sure if these will help but maybe worth a try...
If the D5000 has Active D-lighting check that it is not turned on. Check your other camera settings for the Picture Mode you are using (e.g. have you increased brightness, contrast, etc). If you are using matrix metering, maybe give spot metering a try and see if that makes a difference.



Cheers.

[arthurking83]

Phil makes some very good points about metering and especially Active D-Lighting!!

The exact metering may also depend on on what lens is being used too.

some lenses vignette badly, and the processing program you use may affect that.

eg. 18-105VR lens vignettes wildly at almost all focal lengths at about 1stop form wide open(reducing as you stop down), and both ViewNX and CaptureNX compensate for that.
Every image I've shot with that lens has 80 out of a scale of 100 listed in the Lens Correction-Vignetting tool enabled by default. So these programs are affecting the actual exposure of this lens.
I don't get that with any other lens I have. And this is on a D300(if that makes any difference).

I just took two images of my grey desk(under light).
I set the D300 to both +5Ev and to -5Ev and both images came out according to what I expected them too.
+5Ev shot came out over exposed at pure white(except for the lower LHS corner where the values dipped down into the 246 values, but the -5Ev exposure(if you'd call it that ) came out with no values higher than 3 and generally 1's and 2's all over the frame. But no 0's anywhere.

Is that what you mean when you say you can't get pure black?
values of 0,0,0?
Hard to get and I think it's not important.
Can't remember the exact figures where black is black enough, but don't worry about it too much.

From memory the camera range below the Dxxx's don't have the ability to compensate automagically by more or less than +/- 3Ev(that has to be done with manual exposure).

[bigdazzler]

Geez and I just use my cameras to take pictures ...

At least youre doing this for an assignment airgirl ... AK does this stuff for fun

[arthurking83]

....

At least youre doing this for an assignment airgirl ... AK does this stuff for fun

LOL! I also pull things apart for fun.(except for my ye olde worlde PDA, which got electrocuted).

Once I get my greedy little hands on the likes of a D700/D700s/D3/D3s, the D300 will not be immune to my greedy little hands's 'dissassemblymania' problem

airgirl's experimentation sounds to me like 'pulling things apart for a better understanding', but without the physical aspects of pulling the device apart!
(and that sounds like fun too!)

[TrishM]

I am doing the same assignment as Airgirl, and have just read through all of your very helpful posts, so thank you to airgirl for asking and everyone else for replying!

[Adrienne]

Hi everyone! I am doing this test also and I also have the Nikon d5000. I CANNOT get it to shoot pure black either. The lowest RGB numbers have been 222. The latest test has been today in Melbourne on an overcast day and I swear there cannot be too much ambient lighting! I am in auto WB, ISO 100. Any suggestions?

I want to submit this assignment before Christmas, and I am sick of annoying my tutor!

Thanks guys, what I did read off this thread has been very helpful

Adrienne

[ricktas]

Hi everyone! I am doing this test also and I also have the Nikon d5000. I CANNOT get it to shoot pure black either. The lowest RGB numbers have been 222. The latest test has been today in Melbourne on an overcast day and I swear there cannot be too much ambient lighting! I am in auto WB, ISO 100. Any suggestions?

I want to submit this assignment before Christmas, and I am sick of annoying my tutor!

Thanks guys, what I did read off this thread has been very helpful

Adrienne

What happens if you take a shot with the lens cap on?