Photoshop Levels Adjustment Help, please

[Tim]

Hi - in order to post some photos, I have been instructed:
"Avoid highlight and shadow clipping by using set RGB values in order to retain maximum detail. Set Black point no less than 3 and White point no more than 252."

I am using Photoshop CS3. I would like to know:

a/ how I can TELL when the values exceed these figures?

b/ how to correct: am I right in thinking that I would use "Image > Adjustments > Levels" then move the Black 'Output Levels' slider to read 3, and/or the White 'Output Levels' slider to read 252, in the little boxes below?

Regards to all, and thanks for your time,
Tim

[Colin Southern]

Hi - in order to post some photos, I have been instructed:
"Avoid highlight and shadow clipping by using set RGB values in order to retain maximum detail. Set Black point no less than 3 and White point no more than 252."

I am using Photoshop CS3. I would like to know:

a/ how I can TELL when the values exceed these figures?

b/ how to correct: am I right in thinking that I would use "Image > Adjustments > Levels" then move the Black 'Output Levels' slider to read 3, and/or the White 'Output Levels' slider to read 252, in the little boxes below?

Regards to all, and thanks for your time,
Tim

Hi Tim,

Not sure where you got that advice from, but it's the exact opposite of what I'd advise.

Images that DON'T have (usually small) areas of pure black or pure white have a tendency to look flat ...

... the first thing to do to avoid that is to ensure that your photo is using the full range of levels available. To do that what I normally do is add a levels layer and pring the black and white clipping points in until the image looks good. For printing the black clipping point is normally in the region of 8 (up to about 16) and the white normall in the 230 to 240 region (the exact amount depends on what I've already done in the previous step during RAW conversion).

A picture paints a thousand words - throw up one of your shots here and I'll give it a levels adjustment for you and we can see how it looks afterward eh?

PS: Welcome to CiC - great to have you with us!

[Tim]

Hi Colin - thanks for your comments, however in this case, the directive came from an image library that I am sending a profile to, so has to be adhered to!

I would therefore be grateful if someone could answer my original questions, please.

Tim

[Colin Southern]

Hi Colin - thanks for your comments, however in this case, the directive came from an image library that I am sending a profile to, so has to be adhered to!

I would therefore be grateful if someone could answer my original questions, please.

Tim

Hi Tim,

I just read your original post again. I think that what they're meaning is "don't set the black CLIPPING POINT higher than 3, or the white CLIPPING POINT lower than 252 - that would make a lot more sense.

Are you able to post a link to the exact instructions so that we can read them in the exact context?

[dendrophile]

I would therefore be grateful if someone could answer my original questions, please.

Tim,

Well, step one is to know if you have an image that has blacks less than 3, or whites greater than 252. One way to tell this is to look at the histogram (Window->Histogram). When you use the expanded view of the histogram (small black arrow with 4 black lines top right of the histogram window) the histogram will tell you how many pixels have the colour you select. As you move the cursor around the numbers at the bottom of the window will update, telling you for each level how many pixels have that value. Looks like you can select multiple levels by dragging a selection over the histogram.

Now we have a tool we can use to decide if a photo violates the image library guidelines.

Assuming we have a photo that has blacks less than 3 or whites greater than 252, we need to adjust the colours. I think we only need to use Image->Adjustments->Levels for this. In the Levels window type 3 and 252 into the Output Levels text boxes. This will *compress* the colours into your photo into the range 3-252. This will have the affect of reducing the contrast in the image. You may want to put an s-curve into Image->Adjustments->Curves to compensate.

Does this help?

Graham

[dendrophile]

Does this help?

Tim,

Guess it helps to say:

a) Histogram, as per my reply above.

b) Yes.

:-)

Graham

[Tim]

Hi Colin - well actually, my original question contained within "" was a copy and paste from the library instructions. Searching on Google brought up an identical phrase from Getty Images:

http://contributors.gettyimages.com/...q_one_page.pdf

(not the library I am submitting to.), so it is obviously a common requirement

QUOTE
Hi-Res TIFF Image File Detailed Technical Requirements
• If you are shooting negative film or transparency, the original analog image should be
professionally scanned with zero USM applied and cropped to appear without any visible
or black borders at the edges.
• RGB values for scanned material should be limited with a black point of no less than 3
and white point of no more than 252. This should be performed at the end of Photoshop
editing. In doing this, extreme highlight and shadow detail will retain all the information
needed to print successfully. This also will limit the possibility of color banding and
clipping within the file.
• Digitally captured imagery should also have RGB values limited between 3 and 252. In
both cases this should be the last action before saving the file using Photoshop in the
output levels of the histogram.
ENDQUOTE

Looking at the last paragraph I now see that they must be suggesting setting the points using the Output Sliders in 'Levels' as I assumed.

What Graham says is confirmed in this page on the web:
http://www.zuberphotographics.com/co...els-output.htm
(Go to the section headed 'Table 2 Output Levels').

'Compress' is rather a frightening word, but only applies below the lower limit and above the upper one, so its not too life-threatening. The values below (in my case) 3 will all become 3, and any values above 252 will all become 252.

New Question: I think is is therefore safe to assume that if I made a Photoshop Action that applied those changes, I could apply that Action to every image: my reasoning being that if an image had a value less than 3, it would become 3, but if it didn't the image would stay as it is, because values of 3 and above are untouched. Do you think that's correct logic, Graham? Also, please can you explain exactly, with an example, what you mean by 'You may want to put an s-curve into Image->Adjustments->Curves to compensate.'

Thanks for the hint of the Expanded Histogram - I hadn't seen that before.

Best wishes to all,
Tim

[Colin Southern]

Hi Tim,

Thanks for that.

It's an interesting philosophy - Although what they're suggesting will reduce contrast slightly, I'm guessing that they want it this way so that they can then subsequently normalise it to what they want at their end. I keep forgetting that not everyone works from a calibrated and profiled monitor -- so it's probably a step that they've deemed necessary to overcome images damaged by people with incorretly setup PP equipment.

Essentially what they appear to be trying to achieve is a full tonal range, and then "pull it back a bit" - the minor problem you'll face though is that the absolute amounts on the INPUT sliders (or a levels layer) will depend on your exposure - so what I would do is set the INPUT black and white clipping points to the edge of where activity is seen on the histogram, and then set the OUTPUT sliders to whatever they ask for. Flatten / Save / Job Done.

Make sense?

PS: You are working from a profiled and calibrated monitor aren't you

[dendrophile]

Tim,

New Question: I think is is therefore safe to assume that if I made a Photoshop Action that applied those changes, I could apply that Action to every image: my reasoning being that if an image had a value less than 3, it would become 3, but if it didn't the image would stay as it is, because values of 3 and above are untouched. Do you think that's correct logic, Graham?

Yes, that is safe to assume. Note, that when I experimented with the output levels I had not noticed that the effect was to clamp at 3 and 252, I had assumed that the result was the full 0..255 range get squashed into 3..252. With clamping there would be no need to revisit the contrast.

Also, please can you explain exactly, with an example, what you mean by 'You may want to put an s-curve into Image->Adjustments->Curves to compensate.'

Note that I don't think you'll need to do this now, but, for the record, here is a screenshot that shows an s-curve. This particular one is on the lightness channel from an image in LAB space, but could be applied to RGB. Try doing this on one of your photos, and you'll quickly realise that this curve shape increases global contrast.



Best,
Graham

[Tim]

Thanks for all the points, guys - they have been a great help. Now I must sit down and process all the photos!!

Tim

[Tim]

PS: You are working from a profiled and calibrated monitor aren't you

Hi Colin - I meant to ask you for your comments about that. I have a problem when working from my inexpensive Lenovo laptop: as the lid/screen is tilted at different angles, so the image that the eye sees changes in brightness. This is very annoying, as I generally have to 'guess' where the correct angle of the lid should be. Can you recommend some way of always setting it to see a standard brightness?

Tim

[heather4279]

PS: You are working from a profiled and calibrated monitor aren't you

Colin, how do you calibrate the monitor?

Thanks in advance,
Heather

[Colin Southern]

Hi Colin - I meant to ask you for your comments about that. I have a problem when working from my inexpensive Lenovo laptop: as the lid/screen is tilted at different angles, so the image that the eye sees changes in brightness. This is very annoying, as I generally have to 'guess' where the correct angle of the lid should be. Can you recommend some way of always setting it to see a standard brightness?

Tim

Hi Tim,

It's a common issue with laptop screens, unfortunately.

Only two things that I can think of ...

1. Use an external monitor for post-processing, and ...

2. Tilt the screen so that your eyes are at right-angles to it ... best way I've found to do that is to put the reflection from your eyes right smack-bang in the middle (usually easiest to see if the screen is dark).

Not ideal, but that's the best I can think of.

Hope this helps

[Colin Southern]

Colin, how do you calibrate the monitor?

Thanks in advance,
Heather

Hi Heather,

The BEST way is to use a hardware calibration device such as a Spyder II or Spyder III (or equivalent) -- but they're not cheap.

Other than that, about all you can do is adjust brightness and contrast to (hopefully) get your black and white points correct, but it's pretty hit and miss.

[heather4279]

Hi Heather,

The BEST way is to use a hardware calibration device such as a Spyder II or Spyder III (or equivalent) -- but they're not cheap.

Other than that, about all you can do is adjust brightness and contrast to (hopefully) get your black and white points correct, but it's pretty hit and miss.

Looked at the prices. At my current standings, they are out of my skill level anyhow. Thanks for the info.

[Tim]

Thanks for the hints, Colin.

Tim