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[TECH TALK] Digital: Highlight Recovery, Part One

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koaflashboy says:

OK - so I volunteered to head up our Tech Department here in .The Portfolio, and our first topic I grabbed is going to be recovering highlights. So to dive right in, I'll attempt to answer the question "how much highlight detail can I recover?"

This is a two-part answer.

Part 1 - if you shoot JPEG - none. There, that was simple.

Part 2 - if you shoot raw - it depends. There, not so simple.

To elaborate - if the pixels are completely blown out, meaning they are clipped in all three channels (appear white when you opt/alt-click the Exposure slider in Camera Raw) there is no highlight detail to recover. If only one or two channels are blown, Camera Raw will do its best to recover the detail by way of adjusting the Exposure slider (and, in Beta 4 of Adobe Lightroom available now and Camera Raw 4.0 to be released with PS CS3 - the Recovery slider).

So I know this is a tech thread, but I will try to keep the higher-level tech stuff out and concentrate on getting down to the basics, so some things to keep in mind while trying to recover highlight detail -

1) view the histogram on your camera if it offers it - if you see you've blown highlights, adjust the exposure or compensation, and/or add a neutral density filter and reshoot

2) shoot raw if your camera has that capability (see Part 1 above for why)

3) when attempting to recover a good bit of highlight detail, it is better to adjust the exposure before setting the white balance because it's likely to change as you stretch the highlights due to the fact that the white balance scales the clipped channel(s) to match the unclipped one(s)

4) most cameras will allow about a quarter-stop of highlight recovery (that's something but read not much)

5) because of 4), see 1)

One important aspect to keep in mind while shooting digital is that it is not film. OK, glad we got that out of the way. What I mean, of course - rather than just stating the obvious - is that digital capture is linear. Our eyes our nonlinear. This is what allows us to see in hugely varying light conditions - conditions both film and digital sensors have a tough time capturing.

This is important because the typical digital camera can capture roughly six stops of dynamic range - and of these six stops, half of the information captured (independent on the bit-depth of the camera's encoding) is in the brightest stop. This translates to (drumroll) correct exposure in the digital realm is key, and it means keeping the highlights as close to blowing out without actually doing so. To distill it even further - expose for the highlights (as opposed to the old film adage of "expose for the shadows and print for the highlights" because with film - if no detail is captured in the shadows, you'll only get a black blob when you go to print).

Hopefully that helps and provides a little insight. I think this is a really important topic and I've only just scratched the surface, so let me know if you'd like more or if this is too high-level and I'd be happy to translate some.

cheers
Originally posted at 9:35PM, 26 January 2007 PDT (permalink)
koaflashboy edited this topic 86 months ago.

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jmark media llc is a group administrator jmark media llc says:

@ Tom ~ This is VERY good stuff here my friend!! Can you tell us a bit more about reading the histogram? As you know, I shoot in color in the middle of the day with an eye towards converting to B&W in post. I for sure push to the very edge of blowout. What should we look for in our histogram to ensure we correct exposure issues?

p.s. Have you EVER shot digital in your whole life ever ever ever? Ever? I know you shoot different film formats but was unsure if you could even spell digital. ;-D
Originally posted 88 months ago. (permalink)
jmark media llc (a group admin) edited this topic 88 months ago.

Flawed Emmy [deleted] says:

give me more....
88 months ago (permalink)

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Gerla Brakkee says:

Great work!! Keep them coming!
88 months ago (permalink)

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william c hutton jr says:

Digital exposure is similar to transparency film exposure – As koaflashboy said, "expose for the highlights". A half-stop can be important. As stated above, You can't recover information you didn't record. When there's no data (a channel's physical ability to count photons is exceeded), there's nothing to recover.

I agree with his recommendation for the Recovery Slider in Lightroom.

I am fond of spot meters. If I used a digital camera: I'd compose, move the frame to manually spot meter for the brightest light level, mentally compensate (see below), set the exposure, and then recompose. This obviously won't work for action shots.

If the highlight in a frame has very different reflectance than a 18% gray card, then exposure compensation is useful. Snow, for instance. reflects more light than a gray card. So, the meter-sensor photon count appears high; the recommenced exposure is too low, and the result (in a B&W image) is gray snow. The solution is to manually expose for snow in a shadow region and re-compose, or set exposure compensation for +1 to +2 stops and let the software do its thing.

DSLRs have automated exposure features that may or may not be as reliable as manual exposure and compensation. In my view one can learn how the camera's software engineers think about exposure and use these automated tools. Or, one can learn how to evaluate the light at hand and manually set the exposure. If the design of the camera/lens is such that manual operation is frustrating and inefficient, then it's better to try and reverse engineer how the software engineers want you to set exposure and do it that way. What doesn't work is to try and make the automated exposure features work the way you think about light and camera/lens operation (unless you just happen to think exactly like your camera's software designers think). Most people will discover that their brain and eye will work better than the software. After all, frame composition is not automated and we enjoy composing our photographs. Setting exposure is just as enjoyable if our camera/lens will let us do it.

With regard to using histograms, this link to Luminous Landscape's Exposure Tutorial is a nice one. The histogram should lean to the right but not clip the brightest tones. A tri-color histogram should be used. Some cameras only use the green channel for the monochrome histogram display, so the blue and red channels may be mis-exposed. The tutorial also explains the details behind koaflashboy's excellent synopsis.
Originally posted 88 months ago. (permalink)
koaflashboy edited this topic 88 months ago.

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serac says:

@Tom: Most excellent tip. Using "highlight exposure" and RAW capture is very powerful.

I'd like to add one point. In scenes with a high dynamic range (such as outdoor scenes with shadows and bright clouds), exposing so that the highlights are nearly blown out means that the overall image is underexposed.

Thus, though more information is captured at the highlights (i.e. you want to overexpose), the loss of information due to blowing out the highlights means that you'll probably be underexposing most high dynamic range shots (if you want to keep highlight detail). This is a bit counterintuitive.

Though no expert myself, I cannot stess the use of the histogram enough.
88 months ago (permalink)

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koaflashboy says:

>Daniel - correct, thanks for the clarification. To put more simply - you want to err on the side of slight overexposure when shooting raw because underexposure causes a significant increase in noise in the shadow pixels and so long as all three channels are not blown out, some detail can be recovered in the raw processor.

>Willie_901 - thanks for the additional comments (I edited your post to create a hyperlink from the URL you provided).

Another tip for highlight pixels that are only clipped in one or two channels -

• after converting into PS, inspect all three channels for the one with the most detail in the clipped area
• create a Channel Mixer adjustment layer, set to Monochrome and the value of the chosen channel to '100' - bring the other two channel values to '0'
• set the layer blending mode to 'Luminosity' and adjust opacity as desired; add a layer mask if needed

>Tiny - LOL:) - true, I don't think digital capture has matured to the point film has in some important areas and I have long ago swore to never shoot it for my landscape work so long as there is a scrap of film to be had somewhere in the world, but I do actually own a Canon G2 (!) and shoot raw (I consider those just snapshots).
Originally posted 88 months ago. (permalink)
koaflashboy edited this topic 88 months ago.

My Symbiosis [deleted] says:

Although I had to reread the technical things said a couple times to understand, I very much appreciate this thread, and threads of this sort. I learned quite a bit from above already.

Thank you very much for taking the time to post this, and thanks to everyone for their follow up comments.
88 months ago (permalink)

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koaflashboy says:

>Tiny - did your question get answered above? Basically, when reading a histogram (and there really is no absolute need to having a 3-channel histogram on camera - if your camera offers that, great - but any ol' histogram will do), since you now know that half of your image data is captured in the brightest stop, you want a to see a bell curve that starts at the shadows and rises to peak about 3/5 of the way over and then fall down in the highlights without spiking (i.e. so no clipping) - generally speaking. One such as this -




Since this only captures 5-6 stops, I'll discuss combining exposures to increase dynamic range in an upcoming segment.
88 months ago (permalink)

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Right Eye is a group moderator Right Eye says:

Hi. Great topic. I am trying to learn to take black and white digital. The following is my current workflow:


1. Take picture in RAW
2. Convert RAW to 8 bit TIFF in Canon DPP
3. Convert colour to bw in PS Elements (via gradient map)and any neccesary digital dodging and burning

I've got a few questions:

1. I understand that in digital photography I must expose for the highlights and make sure there are no clippings done (and maybe get a histogram like th eone above). Does this mean that the art and technique of the zone system may no longer be neccesary as I just have to keep exposing until I get an optimal histogram. To put in another way, its no longer neccesary to place objects within cetain zones because all i need to do is keep on adjusting the exposure until I get the histogram shape I want.

2. How reliable is the blown highlights indicator on camera (the blinking on the image preview).

3, If anyone uses Canon DPP, can you tell me whats the difference between checking and unchecking 'linear'.

Hope my question is clear. Many thanks in advace to anyone who respond.
88 months ago (permalink)

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Mike Stacey says:

Two things.

1. Use a 1 degree spotmeter
2. Use film.

Result: no blown highlights unless you want them.

Willie's correct re. the similarity of digital exposure lattitude to film's exposure range although most slide films are quite a bit less forgiving - but better in terms of highlights. They have roughly a 5 stop range with zone 5 (18% grey) right in the middle. Colour negative film on the other hand can have up to around 8 stops, B&W film developed correctly; 10 stops.

One of the biggest problems with fully automatic cameras is the fact that they have negated the necessity to learn the basics of light and photography. Don't be fooled by buttons which promise to do it all for you, buy some books, learn it yourself and take control of your camera! Better still, try shooting with a fully manual camera for a week, quite enlightening.

Mike
Originally posted 88 months ago. (permalink)
Mike Stacey edited this topic 88 months ago.

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koaflashboy says:

>Mike - isn't non-linear film just a beautiful thing? Particularly panchromatic b/w.
88 months ago (permalink)

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