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/tutorials/dynamic-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 :p) 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 ... :eek:

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

[arthurking83]

....

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


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 :D

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?

[Adrienne]

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

Oh my, I thought you were on to a winner there, but guess what?? 2,2,2!!!

A

[ricktas]

OK, your camera is compensating for the darkness. How do we overcome that? What settings are you using? Is the camera on full manual?

[Adrienne]

Thanks Rick...yep it's in manual, ISO is 100, Auto WB and on f8. I did submit all my RGB readings to my tutor and he said he would accept the 222, but I am a slight perfectionist and I want to know why the camera won't go to 0.
:)

[kiwi]

How are you processing the image?

[Adrienne]

thanks for replying! I am using PS as the Nikon software doesn't seem to give enough info regarding RGB numbers...just histograms.

BTW I hope this doesn't post twice, I thought I replied but I came back to the thread and my reply wasn't there!!

A

[kiwi]

I wonder whether on import the image is being auto adjusted, if you are loading a raw file maybe.

[Adrienne]

Yeh thanks for that but the module notes made sure the settings on PS were on default and the WB set "as shot" and I'm not shooting raw.
I just wish I could get the RGB numbers through other software so as I could compare! Do you know of any other programs with a densitometer?

[arthurking83]

thanks for replying! I am using PS as the Nikon software doesn't seem to give enough info regarding RGB numbers...just histograms.

....

A

Two thoughts on this.

1.(software) Which Nikon software are you using? if ViewNX2(it has to be version 2!!) then the mouse pointer location wil give you a readout of the RGB values in the upper info bar(it may be coloured black if you use the default grey skin)
So if you have the image in the viewer pane, and not just looking at thumnails, just above the image view is a dark info bar, where you toggle on/off the histogram/info box and the pixel view size scale.
So as you move your mouse around the image (in this example on the shadows) you'll see the first two numbers are the pixel location(in the image).. so that as an example values 1024,900 indicate the pixel location on the image starting from upper left corner. The values in brackets are the RGB values for that pixel. PROBLEMS!! (that annoy me). If you zoom in to any of the zoom ratios, the mouse pointer turns into a hand. Great if you want to move the image around for pixel peeping purposes, but makes it hard to 'pinpoint' an exact pixel, or find the darkest shadow pixels. Using the full screen zoom factor on the scale makes it hard to be precise with your mouse pointer.(bloody annoying programming that is!) The hand still works as the pixel locator, but not very precise. ie. it still does what you want it too... but you have to have a lot of patience with it(I never use it for that purpose)
Oh! if you want to view the darkest shadows, press the S key on your keyboard. This shows lost shadow detail. if there is no dark spots on your white background(once you press the S key) you have no lost shadow detail(pressing the H key works in opposition and shows you lost highlight detail)
For that purpose, I use CaptureNX. When you use the histogram tool, click the enlarge icon on the histogram tool(small boat anchor looking widget) makes the histogram larger and hence easier to read accurately.
On the histogram tool, press the small Double Threshold tick box to enable the tick. Entire image shows black and white points on your image, if you have them. If not, once again.. you have no lost shadow detail and no lost highlight detail, you won't see the accompanying black or white speckles in the image. The image should turn grey with lost shadow and highlight details. You can also view the RGB shadow.highlight details by clicking Shift+S(for shadows) and Shift+H for highlights). These are not black/white points, but the colour version of lost shadows/highlight.
To view the pixlel RGB (or black/white points) values, drag the small slider at the bottom edge of the histogram, or simply type in a value in the boxes marked shadows or highlight. So if you type in a value of 5 for shadow detail, and you have the Double Threshold indicator still active, it will then show you the pixels that pertain to a value of 5 or less in black(on the image).
If you click on the 'Watch Points' arrow, the histogram then gives you another feature where you see the RGB values as you scroll the mouse pointer over the image, and you can use a sample area size or a point sample.. eg, you can set it to show pixel values on the pixel your pointer is on, or on an area, 3x3 or 5x5 pixels in size around your pointer. For more accuracy in pixel location, zoom in to 100%, or more, view.

2.(camera)Now! as to why you see no 0,0,0 black pixels.. the only reason I can think of is that Active-D Lighting(ADL) is still enabled in the camera, and possibly in Auto mode. This will try very hard to affect the exposure to the point where it'll try not to produce 0 valued blacks. Of course you still can, but it tries even harder to counter your efforts again.
Now even though PS can't effectively see(or read or understand) Nikon's in camera features, the camera still effectively adjusts exposure to suit it's ADL function, even in Manual mode. It works in degrees of ability.

you said that you used Auto WB. Use manual WB.. any value, but try daylight. Turn off ADL(if it's on) and aperture makes no difference(if you're doing the lens cap test). F/4 and 1 sec exposure with lens cap on will give you only black pixels across the entire image, especially at ISO100.
As a side note, if you use AWB, the camera attempts to balance out the pixel values so that you don't get many lost pixels due to colour balance.
Eg. if in late afternoon sunset conditions, the camera naturally tries to make a scene look more blue that it is, as red pixels blow out much easier than the blue channel does. So for sunset scenes, I adjust WB values(raw images of course) to something like cloudy(with a Temperature value of say 6000-7000K, depending on scene.. and possibly a tint adjustment as well) Doing this then blows the red channel out, and many red pixels are lost(easy to recover with contrast saturation lowering in PP).. But where the histogram initially looked good in exposure terms the processed image now has blown a red channel. Image in camera was too blue. Using AutoWB effective adjusts exposure but in the RGB channel level.. ie. blue pixels tend to underexpose more, so the camera sets a WB value to counter this. Luminance(green channel histogram) effectively stays the same(move a touch this way or that, but close enough to unchanged, even after WB processing adjustments.
Finally just to be 100% sure.. do you have AutoISO disabled as well? Sounds like you have because you can't use ISO100 with AutoISO enabled anyhow.. but it's better to ask than to assume. Default settings is that AutoISO is set to off anyhow.
But the first place to check is the ADL feature.

[arthurking83]

If shooting jpg then the camera is definitely processing the image, before you get it into PS.

Non Nikon software, generally don't recognize any of the camera settings if you shoot NEF mode. ie. thirdparty software allows you to see only the raw data your sensor captured(that's part of my dislike about it, and hence my preference for Nikon software).

shoot raw :th3:

[Adrienne]

Phew!! Thanks so much, I have just downloaded NX 2 (I didn't realise I didn't have it, just Nikon transfer.. duh) Just had a quick play with my images and now I am down to 1,1,1. Already an improvement! Tomorrow is another day where I will fiddle with the ADL and the white balance (although the assignment specs were to keep it in auto). AND set to shoot in raw.

Thanks Arthur, I really appreciate your in depth advice!:D

Merry Christmas to you all, it's been a consuming technical day!

Adrienne :xmas31:

[arthurking83]

..... (although the assignment specs were to keep it in auto). ....

I don't fully understand the Auto mode reasoning, but if you are using any Auto mode, are you allowed to use any user adjustable variables as well?

Auto mode is really no different to full manual mode(if you understand it properly) in that even in Auto mode, you can meter off any part of the scene(using any of the three metering modes available) and use exposure compensation to achieve the exposure you want.
So (I'm only assuming) that the assignment is trying to teach you to achieve a true black point using exposure compensation, as most Auto modes will try to prevent that(and hence give you 1,1,1 or 2,2,2 black points).

There are differences in the types of Auto modes available as well. There are the true Auto modes(Scene types) that set all variables such as shutter/aperture/ and so forth, and then there are the semi Auto modes like Program mode and Shutter and Aperture Priority modes. The semi auto modes only take into consideration the exposure of the scene(as set by you, the operator).

[Adrienne]

I couldn't tell you why they want the camera in auto white balance but the aim of this part of the assignment was to create a zone ruler and learn
the DR of the camera. To help the student become familiar with exposure and pre-visualisation. (?)

[arthurking83]

AH! sorry ... my mis interpretation... WB in Auto... not.... camera in Auto mode! :th3: (got it now :o)