Curves & Vital Statistics by Philip Lawson FRPS DPAGB el-imâge

Curves & Vital Statisticsby

Philip LawsonFRPS DPAGB

Curves & Vital Statisticsby

Philip LawsonFRPS DPAGB

http://www.el-image.co.ukel-imâgeel-imâge

All pictures are composed of pixels

All pictures are composed of pixels

All pictures are composed of pixels

Binning and Histograms

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First count the number of black pixels

Binning and Histograms

Next count the number of dark grey pixels

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Binning and Histograms

Repeat for all shades of grey

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Binning and Histograms

Note the fact that, as there are equal numbers of shades 1 and 2, this produces a flat top “mountain”.

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Histograms

In Photoshop there are 256 bins per colour (R,G and B) represented over 256 pixels.

In a typical picture there may be 3M pixels and this is represented by the area of the histogram.

0 255

3M

Histograms

Bin heights are therefore about 10000

Represented in only 100 pixels, so 10000 and 9800 will look the same

10000

0

Some Typical histograms

NormalCentre weightedClearly drops to zero at both ends.

Some Typical Histograms

High KeyLight end weightedBut still clearly drops to zero at both ends.

Some Typical Histograms

Low KeyDark end weightedBut still clearly drops to zero at both ends.

Perfect Exposure

Data in the histogram is totally contained within the box at light and dark ends.

Perfect Exposure

Whilst it might appear to be so, the histogram does NOT show that the image is “off scale” in the dark grey region or in anyway wrong or correctable.

This “topping out” of the histogram is just an indication that there are simply, approximately the same large number of pixels in the picture of shades 20 through to about 50, the dark grey shades.

Perfect Exposure - Highlights

The acid test of perfect exposure is that fine detail is revealed in the highlights when the light end of the curve is pulled dramatically down.

The bias of the histogram shifts to the dark end.

Perfect Exposure - Shadows

The acid test of perfect exposure is that fine detail is revealed in the shadows when the dark end of the curve is pulled dramatically up.

The bias of the histogram shifts to the light end.

Over Exposure

Data in the histogram is clearly off scale at light end.

Over Exposure - Highlights

The acid test of OVER exposure is that fine detail is LOST in the highlights when the light end of the curve is pulled dramatically down.

Under Exposure

Data in the histogram is clearly off scale at dark end.

Under Exposure - Shadows

The acid test of UNDER exposure is that fine detail is LOST in the shadows when the dark end of the curve is pulled dramatically up.

Working with Photoshop 1

Important to work in 16-bit mode for as long as possible[converting to 8-bit just before printing]

Only use CURVES and beware of ALL other adjustments[unless for a quick preview]

Convert the image to 360 dpi in Photoshop[if printing on a 1440 dpi dot-matrix (inkjet) printer]

Working in 8-bit mode

8-bit mode is a scientific description for the fact that the shades of grey are described by an 8-bit binary number.

Mathematically, 8-bit numbers can represent anything from 0 to 28-1, 0 to 255 to you ‘n’ me.

That is 256 shades of grey0 128 255

Whilst you cannot see the individual shades, this does have serious repercussions.

Working in 8-bit mode

The histogram shows the number of pixels in the image (or selection) of each shade of grey.

In the original 8-bit grey scale, every single shade of grey exists in approximately equal proportions.

256 step wedge in Photoshop

Working in 8-bit mode

When you modify the curve, you redistribute the histogram.Where it is stretched, you leave gaps and this can create visible posterisation in the image, (although this may not be immediately obvious except on bland skies).

Working in 8-bit mode

Where it is compressed, you double the number of pixels with certain shades of grey – but cannot create any intermediate, half tones. There is no such thing as grey shade 128.5

Working in 16-bit mode

In an 8-bit image (24-bit colour) luminosity and each colour channel is represented by 256 shades (28).

In a 16-bit image (48-bit colour) luminosity and each colour channel is represented by 65536 shades (216).

This derives from the fact that each shade is represented by a 16-bit number in the computer and, mathematically, and a 16-bit binary number has 65536 permutations. These range from 0 to 216-1 or 0 to 65535 to you and me.

Working in 16-bit mode

Performing the same operation in 16-bit mode has the same effect, but here you are dealing with 65,536 shades of grey.NB. On screen histograms only show 256 levels.

Working in 16-bit mode

Extreme manipulation of the curve, to reveal shadow detail, severely redistributes the histogram and appears to leave many missing shades.

Working in 16-bit mode

However, when the 16-bit mode is finally converted to 8-bit, 65,536 shades are “squeezed” down to 256 and the histogram becomes smooth again – no missing shades.

Example

Original histogram of eye.

Example in 8-bit mode

Original histogram of eye.

Before

After

Resultant histogram of eye.

Example in 16-bit mode

Original histogram of eye.

Before

After

Resultant histogram of eye.

Example

Although it may not be overly apparent, the 8-bit transformation has reduced the number of shades to almost a half of the original dynamic range.

The 16-bit transformation has protected the full dynamic range.

8-bit histogram of eye.

16-bit histogram of eye.

Dynamic Range too high

Quite often the dynamic range of the subject is too great for the Digital sensor.

This can occur in a camera when the scene is too contrasty.

This can occur in a film scanner when the transparency is too contrasty and the DMAX of the scanner too low.

Dynamic Range too high

Under these circumstances nothing can be done to restore detail to the burnt out sky and blocked in shadow (without trickery).

Dynamic Range too low

Equally common is when the dynamic range of the subject is too small for the Digital sensor.

This can occur in a camera when the scene is flat and lacks contrast.

This can occur in a film scanner when the negative is too thin and the DMAX of the scanner too high.

Dynamic Range too low

Here the recorded dynamic range is less than half the maximum.

Dynamic Range Stretching

Using the Levels tool, the dynamic range can be stretched.

Dynamic Range 8-bit

However, in 8-bit mode, the original (top) limited number of shades persists and visible posterisation occurs (it was already inherent in the picture).The final image contains no more than the original 100 or so shades.

Dynamic Range 16-bit

In 16-bit mode, the original (top) limited number of shades, whilst only being 50% of the maximum, were still about 30,000. Stretching these out has no effect on the final squeezing back down to 256 shades for printing.

8 bit vs. 16-bit

Posterisation is just visible, especially in thesmoother areas of the eye.

Photoshop CS

In developing Photoshop CS , Adobe came to the rescue of thousands of photographers who were used to many photographic concepts yet did not know how to achieve them through the Levels and Curves tools that had been present since the earliest versions.

An added bonus was the live Histogram palette, which makes seeing the informational content of changes highly visible next to the actual visible picture.

Photographic Filters

(Image->Adjustments-> Photo Filter… ) For many new-age digital photographers,

attaching a filter in front of the lens may not actually be a viable proposition.

Achieving the same effect as a filter on the digital image was not something that most people knew how to do.

TRY: New Layer, Paint Bucket Fill with Filter Colour. Set Layer Blend to Multiply, Opacity about 20%, Add Adjustment Levels Layer and trim Levels back to maximum.

Shadow/Highlight

(Image->Adjustments-> Shadow/Highlight…) The new tool makes it relatively easy to bring

out shadow and highlight detail. Unlike Brightness, Contrast and Levels, this new tool IS intrinsically safe.

Achieving the same effect using the CURVES was not something that most people knew how to do.

This is a shame, because it offers much more control and versatility.

Photoshop CS

What is more, the Shadow/Highlight adjustment can NOT be added into the adjustment layer stack.

As such, it cannot be retro-edited or combined with a layer mask. Its effect, blend and opacity cannot be controlled.

Fundamentally the Highlights control lowers the light end of the curve (leaving the dark end untouched), the amount determined by the AMOUNT and the dynamic range determined by the TONAL WIDTH % from the top (light end).

Shadows works in a similar fashion.

Shadows and Highlights

Curves offer all of this and more.

The original image is well exposed, highlight AND shadow detail being well contained within the dynamic range. However, the shadows appear dark and the highlights too bright. The histogram shows this in the dark and light peaks at each end.

Highlights

Placing the eye-dropper sampler over the dress reveals it to be in the upper 1/8th of the luminosity range.

Dragging this point on the curve down reveals the detail in the dress but severely darkens the shadows.

Highlights

Anchoring the mid-point does not completely resolve the situation.

The curve is like a piece of sprung metal. Anchoring the middle and pulling down on the right then lifts the left, lightening the shadows.

Highlights

To ensure that the shadow area is unaffected, it needs anchoring at a number of points.

Now, even extreme changes to the highlight end will have little discernable effect on the shadow end.

Beware to NEVER let the curve reverse (head down) as this will cause unwanted, strange derivative effects in the image.

Highlights

Now it is possible to experiment only with the highlights.

This curve introduces a classic S into the highlight end to increase contrast and reveal detail.

However, the very brightest highlights are lifted by this S curve and made unacceptably bright.

Highlights

In this curve, the S is lowered so that no part of the new curve is ABOVE its original point. It is still an S and thus increases the contrast and detail.

Before After

In this curve, the S is applied to the shadow region along with an overall lift to lighten and increase contrast and detail.

Before After

Shadows

Shadows and Highlights

Finally it is possible to construct the whole curve.

The highlights are subdued but with increased contrast.

The mid range has a subtle increase in contrast.

The shadows are lightened with increased contrast.

Shadows and Highlights

Before After

Highlights and Shadows

Remember All curves can and should be applied through

adjustment layers. Adjustment layers can be attached to masks

to operate only on selective areas. Adjustment layers have Blending Options.

This enables extremely fine control to be applied to very specific areas of the image without necessarily applying it to the whole image.

Summary

Use the histograms to optimise the capture of the image in the first place (in camera or in scanner).

Use the Levels to optimise a “thin” image. Only use the Levels and Curve tools whilst in 16-

bit mode. Use the Curve tools to affect ALL changes in

brightness, contrast and colour balance as it is intrinsically safe.

Use Curve Adjustment Layers because these are – retro-editable, have attachable region Masks and have adjustable Blending Options.

and Finally . . . .

Enjoy The rewards can be worth it.

Philip and Janet www.el-image.co.uk