Trying to understand histogram/cache

[Rex]

Hi there,

I know that trying to understand the histogram and how to get the best from it is a big subject, but this is my question.

I open a RAW file in ACR, adjust so the the histogram is not clipping a either end and then open in CS3.

My histogram is docked to the right of the screen and shows Level Cache 3. Why, when I have not done any CS3 adjustments to the image yet? But the histogram looks like the one from ACR. So I double click on it to refresh, now it is level 1 but always shows clipping in the blue shadow.

Now I make an adjustment layer and the histogram is showing level 3 cache again; before I have done any adjustments. Why?

I have tried level 1 cache in Preferences but cannot work with the slow refresh rate.

And whatever adjustment I make to the image, I always have blue channel shadow clipping. I really don't understand and would be very grateful for some guidance.

Many thanks,

Rex

[Medley]

Hi Rex,

Sorry to leave you hanging before, but you actually seemed to have a better grasp of getting the histogram to perform than I did. I was hoping someone with more knowledge would step in for me.

The problem isn't with the histogram in your case. It's doing its job correctly- it's showing you what's comming out of the camera. So if you want to fix the blue channel shadow clipping, you'll have to adjust the camera settings.

Most digital software, as well as hardware, is biased toward clipping shadows rather than highlights. The reason for this has to do with the way a digital camera "sees" light. If you and I look at a dim light bulb in a dark room, we see a gradual progression from the lightest area into total darkness. Film sees, and captures, light in the same way. But a digital camera sees it as a series of half-lives, with each successive step being only half as bright as the step before it.

What this means to you and me is that in any given image (as shot), half of the digital information is in the brightest stop. 25% of the total information is in the next brightest, 12.5 % in the third brightest, etc. So in a typical camera with a six stop dynamic range, the deepest shadows contain only 1.5625% (one and 9/16ths) of the total information in the image. If you clip the shadows down to 0%, few people will actually notice the difference in information.

This is born out in a simple experiment. Take any image with relatively dark shadows, and brighten the shadows to mid-range level. See all the "noise"? That's a lack of detail, a lack of digital information. That's why HDR is popular with digital equipment- you can put all the information back into the shadows.

But, I'm off course again. If you want to solve the shadow-clipping problem, then adjust your exposure. If you're shooting in manual mode, then simply adjust the the shutter and aperture to be one notch to the right of center. If you're using a program or priority mode, then use the camera's exposure compensation feature to adjust the exposure 1/3 stop overexposed. However, both of these methods will adjust the entire histogram to the right, so be sure to meter for the brightest part of the image to avoid clipping the highlights. Once you have the shot, check the in-camera histogram to make sure the highlights weren't clipped.

When I shoot Raw, my goal is to shoot as overexposed as possible, without clipping the highlights. Darkening them in pp allows me to push some of that information back down into the shadow areas, giving them more detail.

Sorry for the long post Rex. I hope you found a bit of help somewhere in it.

- Joe U.

[Rex]

Joe,

Thanks very much for the great explanation. I have been submitting to photolibraries for too many years, but it was always with film. Then I started scanning my film, and now, I the last six months, have a D300.

Essentially, pressing the shutter is the same as it has always been, but certainly digital sees the subject differently to film, and I know which I prefer.

In my recent experience with digital, I am always checking the camera histogram and usually giving three stop bracketing, +1, 0, -1 to make sure that I have the exposure covered.

However, I would state that my first travel experience with the D300 was only two months ago, to Japan. Blue skies, sunny days, lots of red/yellow maple and dark eves to the temples. The contrast range would defeat film but then, shoot for the highlights and let the shadows take care of themselves. Some picture buyer in the future could decide what to do.

But with digital, the library will check the histogram (or assume that it is OK) and any clipping, ...... which the above scenario certainly gives in the shadows, .....

With my bracketed exposures, I have tried HDR but have little idea what I am looking at. Frankly, they look c..p! Or may be I am not doing the right thing. Most HDR images that I have seen have a slightly surreal quality that I don't think a picture buyer would go for.

Just have to persevere with adjustments the RAW file.

Rex

Rex

[Medley]

OK, just a few things that I thought of. Most of it is pretty straightforward, so I apologize in advance if it seems like I'm underestimating your ability.

With Photoshop's ACR (Adobe Camera Raw) plug-in, the default setting for the "Shadows" slider is 5. Solving your shadow-clipping problem could be as simple as running this slider back down to 0. With the default setting, I too generally find it clips the shadows a bit. If you still see clipping in the shadows with the slider set to 0, then the problem will have to be corrected in the camera settings.

I take mine one step further, however. I've created a preference with every slider set to zero, called (you guessed it) zero. I then set that preference as the "default", so clicking the default tab automatically sets all tabs to zero. Combined with turning off all in-camera adjustments, this gives me the unaltered view of what the camera captured.

I've learned a lot by doing this. In the beginning, my exposures were WAY underexposed, and the Auto tab in ACR always corrected them. Nowadays, my images need must less adjustment- always a good thing.

One more tip. Instead of simply opening a Raw file, Create a new document with the same pixel dimensions the image has. Make sure you choose the 16-bit option in the dialog box. Once you have the new document open, go to File> Place, and choose your Raw file.

The advantages outweigh the tediousness. Once opened, you can double-click on the placed layer to re-open the ACR settings and change them. You can also place the Raw file more than once, say to adjust for highlights and shadows separately, then blend the two layers with the Blend If: sliders under the Layer> Blending Options menu. This is, in fact, the first step in hand-merging HDR images, an incredibly tedious, yet wholly fulfilling task. You eliminate a LOT of the overprocessed surreal look.

- Joe U.

[Rex]

Many thanks for your reply and the useful information.

Must admit that I assumed the PS ACR default was the camera setting default rather than something Adobe had set. Think I will do as you suggest, set them all to zero.

I think the only 'in-camera' adjustment I have is to set the saturation up by one notch, to attempt to get the 'Velvia look.' That certainly could be a contributory factor.

I have just tried your 'place' example. Did not know about that; although I cannot get 'double clicking the PS placed image to open in ACR. I have been doing the old fashioned way; opening two versions of the same ACR corrected file and either dragging one on top of the other or copying. But this works better.

Many thanks,

Rex

[Medley]

Oops, I just double-checked, and you have to double-click the thumbnail image in the Layers palette to get the Raw adjustments back up. Double-clicking outside the thumbnail brings up the blending options. My fault.

- Joe U.

[Rex]

Yep, that works.

Looks like I have a new toy to play with!

It's no wonder that PhotoShop books run to thousands of pages; there are so many bells and whistles.

Rex