Night Photography: A Tip to Photograph Stars (and Other Point Light Sources)

Mastering night photography is not that difficult, nonetheless it has its own peculiarities you should be aware of. In this blog post we will see how one of the basic rules we learn about exposure is no longer valid when shooting stars.

One of the first things you certainly learnt when you started learning photography was how exposure is determined by three parameters: aperture, shutter speed and ISO sensitivity. Each one has multiple effects on the final result (most notably depth of field, motion blur and noise), but each one can be used to determine how much light enters your camera and reaches the sensor. Aperture, though, has a peculiarity: it's not an absolute measure, but a relative one. In fact, the f-number is not a measure strictly speaking: it's a pure number. The f-number N is the ratio between the focal length f of the lens and the diameter r of the entrance pupil:

Basically, the luminance (the "brightness" of the resulting image) depends only on the relative aperture and not on the absolute value of either lens parameters alone. In fact, when evaluating exposure, you just use the f-number: no matter the focal length or, more generally, no matter which lens you're using, if the f-number is the same, exposure is going to be the same. If you stop your aperture up or down, exposure will stop up or down accordingly.

This is true most of the time and is a consequence of the physical model of an optical system such as a single aperture camera (or the human eye). We've seen many time the equation

which summarizes this basic rule: luminance (in f-stops, hence the logarithm) is proportional to the square of the aperture N and inversely proportional to time t the shutter remains open.

What Happens When Shooting Point Light Sources?

When shooting stars, or more generally point light sources, however, the model changes and this result is no longer valid. A point light source, in this context, will be defined as a source of light whose size in the resulting image will smaller or equal to one pixel. Perfectly in focus, and depending on your sensor's resolution, some stars and planets may in fact appear bigger than one pixel, but not that much. Hence, this approximation can be considered good enough.

The reason why this happens is not complicated but requires some knowledge of Mathematics and Physics but since a photographer is usually only concerned with results and the rules to apply, I'll try to provide just a very summarized and intuitive explanation.

Let's start with a couple of analogies, although pretty "rough". It's absolutely intuitive that, the farther from a sound source, the fainter the sound you perceive. It's also intuitive that when shooting with a flash, the farther from the subject, the fainter the light that reaches it and, hence, the fainter the light reflected to your camera sensor. Now: why doesn't a similar effect exists when shooting any subject? A picture is produced by the light reflected on the subject: why isn't exposure affected by the distance from it?

It turns out it's a consequence of two competing phenomenons which, under certain circumstances, "balance" themselves and cancel out the contribution of the distance. It also turns out that the result is the general well known law we were talking about at the beginning of this article, hence the importance of the relative aperture, the f-number, in the field of photography.

On the other hand, when shooting point light sources (as the majority of stars in the night sky can be considered) the two competing phenomena don't balance themselves any more. In fact, one of the two practically disappears and the focal length f of the lens doesn't affect exposure any more. In this case, the result is similar to what we described in the analogies we above: luminance is inversely proportional to the distance from the subject but, much more importantly, it is proportional to the diameter of the entrance pupil. Having disappeared f from the equation, the result depends solely on r² (a quantity proportional to the area of the entrance pupil) and not on the relative aperture. This fact is somewhat intuitive, if you think about it: the larger the area of the entrance pupil, the more light it can gather. Seen from this point of view, in fact, the usual rule is probably less intuitive: lenses configuration with the same aperture N may have entrance pupils of different sizes. Why, then, they give the same effect? That's because of the two components we were talking about, but we won't enter into mathematical details.

Since

it turns out that focal length does affect the final exposure, given N.

How? This model predicts that an increase in the focal length of the lens keeping the aperture N fixed increases exposure, since it increases the area of the entrance pupil. Although you won't be usually shooting skies with long lenses, you could take advantage of this fact to reduce shutter speeds, especially taking into account that detected luminance varies with r² and, hence, with f², the square of the focal length.

Some estimations are quickly done: if you increase the focal length from, let's say, 18 to 35 (using the same aperture), you'll increase the quantity of light reaching the sensor of a factor

that is, 2 stops.

It's important to realize that this effect applies only to point light sources, that is, small stars whose size in the picture is comparable to, or smaller than, the size of a pixel. It doesn't apply to the moon, to bigger stars and planets and not even to the sky itself. Nevertheless, it's a good trick to know if you want to maximize the number of visible stars in your picture.

Sometimes you may be tempted to stop down the aperture to have a better focus at infinity, especially when the lens you're using hasn't got a hard stop at infinity (many cheaper lenses, such as most Nikkor DX lenses, have not). In this case, instead of indiscriminately or heuristically stopping down the aperture, use the hyperfocal distance instead (which we talked about in a previous post) to get a good focus lock at infinity and determine exactly the depth of field you need. If you can, open your lens as much as you can.

Flash Exposure Tutorial: The Basics

This is an introductory tutorial, aimed at photographers at their first experiences with flash, to help them understand how flash photography differs from what they're used to and, above all, why.

Some amateurs photographers with some experience with exposure basics often have serious problems using their flashes and soon give up, sometimes blaming their gear, just because they're missing the subtle differences that make flash light special.

I want to apologize to whoever has problems accessing the attached PDF file. I tried it hard to make a regular blog post out of it, but I gave up because the results were awful. Sure, I could get rid of all the Maths formulas inside the tutorial and publish the plain text. That would surely improve the reading experience, but I'm not sure whether the understanding experience would improve as well. I bet it wouldn't. This is a tutorial for photographer and, even if it has some formulas in it, they're only used as a vehicle to help you get a deeper understanding of the subject. If you feel like skipping the all, do it. At least, try to spend some minutes understanding the concepts behind them, which I strived to describe as clearly as possible. If you feel I'm missing my goal, please drop me an email or post a comment: any feedback is welcome.

The document is hosted in Google Drive and when you access it you can download a copy of it.

Flash Exposure Tutorial: The Basics v. 1.0 (PDF)

Adobe Lightroom Tutorial - Part XX - Exposure vs. Brightness

Part I - Index and Introduction
Part XXI - Making Photo Books

One of the questions I'm most frequently asked for by both blog readers and friends is:

"What's the real difference between Exposure and Brightness."

The answer to this will soon be irrelevant: the Brightness slider has been removed from the new development process (2012) shipped with the upcoming release of Adobe Lightroom. I quickly described some of the changes that we'll see in Lightroom 4 on a previous blog post and I described the major differences between the current development process and the new one. Also, I stated I'm really glad that both the Exposure and the Brightness tools have been merged into a new tool, still called Exposure, whose behaviour I find more intuitive and predictable.

However, here's a quick summary of the major differences between Exposure and Brightness in Lightroom and how you could take advantage of both to improve your results.

First of all, let's have a look at the authoritative "answer", although not satisfactory, that comes directly from Adobe. The Lightroom 3 official documentation, that you can find here, states the following (this is an excerpt):

• Exposure: Sets the overall image brightness, with a greater effect in the high values.
• Brightness: Adjust image brightness, mainly affecting midtones.

Lightroom documentation also suggests you adjust the overall tonal scale before adjusting the brightness.

Information provided by the documentation is crystal clear, but clearly insufficient. Although hinting at a major difference in how they alter the tonal scale of a picture, it fails to explains the real difference between them, leaving Lightroom users experimenting and trying to learn by trial and error. Furthermore, but this is just a personal opinion, the documentation only tells an half-truth. Based on my experience, in fact, the description is both incomplete and deceiving. In the following sections we'll see why.

In this blog post I'll use the following photo (unedited) as a test bench:

Grey Gradient - From Zone IX to Zone 0

Exposure and Brightness Behaviour

For small adjustments, both exposure and brightness will produce very similar results: you will begin seeing some differences when you push these sliders farther, on either end.

Let's begin increasing either adjustment:

Exposure: +2

Brightness: +100

The effect of the exposure increase on the image is greater than the effect of the corresponding brightness increase throughout the range.

The first thing we notice is something we already knew: as I've often stated in this blog, exposure is much less forgiving than brightness and burns the highlights very quickly. In fact, both zone VIII and zone IX are burned when increasing exposure by 2. On the other hand, increasing brightness has a much gentler effect on the highlight: in fact, no zone is yet burned out.

If you push these adjustments further, this behaviour is even more evident:

Exposure: +3

Brightness: +150

After increasing exposure by 3 steps, Zone from VI to IX are completely blown out. On the other hand, when increasing brightness to 150, zone IX is burnt (and zone VIII is very close to being so).

A similar behaviour can be observed when reducing a negative adjustment with these tools:

Exposure: -3

Brightness: -150

When decreasing their value, the effect of a brightness reduction is now greater than the effect of the corresponding exposure reduction. Once more, the overall effect of a brightness adjustment throughout the range is gentler and more uniform than the effect of an exposure adjustment. Also, a brightness reduction has a stronger effect on the shadows than an exposure one although, in this case, no zone has gone into zone 0 (pure black).

We can draw the following conclusions:

• The overall effect of a brightness adjustment is gentler and more uniform throughout the dynamic range in both directions.
• Increasing the value of either tool has a stronger effect on the shadows.
• Decreasing the value of either tool has a stronger effect on the highlights.
• Increasing exposure quickly burns the highlights and has a stronger effect throughout the range.
• Decreasing brightness has a stronger effect throughout the range.

This basically mean, also, that the effect of either tool is not linear throughout the range, peaking at a specific value: the shadows, when increasing their value, or the highlights, when decreasing it. However, the overall effect of a brightness increase is more gentle and uniform throughout the dynamic range.

This asymmetric behaviour has interesting consequences that, once understood, can be incorporated into your workflow to improve your results:

• If you need to reduce the highlights of a picture (relatively to other ranges), you should consider reducing the brightness. 
• If you need to increase the shadows of a picture (relatively to other ranges), you should consider increasing the exposure.

Using this asymmetry at your advantage may signify improving the tone distribution of your image, especially when you want to push back the highlights towards the middle tones while retaining all of your shadow details and, on at the same time, preserving the natural look of your image.

Some Example Pictures

I took this picture on the Roman Bridge in Salamanca using matrix metering and no exposure compensation:

Original Image - Matrix Metering - Exposure Compensation: 0 EV

Clearly, the sky and the colour of the rock deceived the camera and the resulting image is pretty overexposed. If we try to fix this picture using only the exposure slider, using the church as a reference, an acceptable result could be the following:

Exposure: -1

Not bad, but I'm not satisfied with the tone of the shadows nor with the overall tone distribution. This is a consequence of how exposure works: decreasing exposure has a stronger effect on the highlights, but the effect throughout the range is less uniform than brightness' (as seen in the previous zones samples).

Let's try to reduce the highlights bringing brightness down:

Brightness: -10

Please beware that, at least when processing Nikon RAW files (NEF), the default Brightness value is 50, not 0. As a result, the brightness value of the previous picture has been decrease by -60. The result is much more satisfactory, as far as it concerns overall tone distribution.

Starting from this picture, we can further tweak it until we're happy with the result. In this case, I'd just apply a small exposure adjustment to bring some light back into the shadow range:

Exposure: +0.1 - Brightness: -10

The following picture suffers from the same problem:

Original Image - Overexposed

Trying to bring the rocky cross and the church into a satisfactory range using exposure gives this result:

Exposure: -1

Once again, the result is somewhat flat and the tone distribution doesn't feel natural. Using both brightness (to reduce the highlights) and exposure, we obtain:

Exposure: -0.2 - Brightness: 0

Look at the much more natural color of the rock: this is a much better starting point for further processing.

Clipping and Color Shift

One of the the issues a photographer is always struggling with is clipping and all its incarnations. This is true both on camera and in post production. The exposure sliders in the current Lightroom development process (2010, in Lightroom 3), is the risk of clipping some channel when increasing its value.

When working in the digital domain, tools may be less forgiving than film and paper are, since clipping is a phenomenon that occurs abruptly. Hopefully, this problem will be partially mitigated in Lightroom 4 but, in the meanwhile, we've got to live with it.

One of the problem that may arise when clipping a channel is color shift: when a channel fills up, the relative proportions among the channels cannot be respected and the hue of some colors may begin to drift.

That's the reason why I never push any tool any further when a channel start to clip. If I need further tonal adjustment, I'd rather use the curves instead, leaving the white point fixed.

Unexpectedly, color shift is another reason why you should consider brightness over exposure. This is a reference image I'll use to explain this problem:

Color Reference Chart

This color reference chart is made up of three rows, each of which contain:

• A pure RGB color in the first square.
• Two differently saturated versions of the same color on the remaining two squares.

This is what happens when increasing either exposure and brightness over a certain level:

Exposure: +1.5

Brightness: +150

It's evident that exposure and brightness treat distinct RGB channels in a different way. With a 1.5 exposure adjustment, you can clearly see a color shift. Indeed, if you take a measurement of the darkest squares, corresponding to pure RGB colors, you get:

• Red: (220, 61, 68)
• Green: (221, 255, 160)
• Blue: (125, 76, 255)

On the other hand, measuring the color of the same squares when a brightness adjustment is applied, you get:

• Red: (223, 91, 96)
• Green: (223, 255, 211)
• Blue: (122, 122, 255)

I'm not sure about the rationale behind this, but I suspect the exposure behavior is related to the characteristics of the human eye: since we're not equally sensible to light waves of different lengths, "lightening" a color is not a linear process amongst the RGB channels. That's pretty evident if you look at the relative luminance equations in colorimetric spaces.

If, on one hand, the exposure behavior may be justified, on the other hand you should be warned that strong exposure adjustment may lead to undesired color shifts.

Conclusion

Although similar, exposure and brightness have got completely different behaviour throughout the tonal range. Learning to use them is critical to master Lightroom and get the best out of your pictures. Also, every picture is unique and there's no rule but your personal taste when deciding which adjustments to apply in post production.

As we've seen, using one or the other may lead to very different results and even to unexpected (and undocumented) problems such as color shift.


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Adobe Photoshop Lightroom Tutorial - Part XI - The Tonal Scale

Part I - Index and Introduction
Part XII - Presence Controls

Adjusting the Tonal Scale

The Basic panel of the Develop module contains the editing tools to control the tonal scale of the image. Finer tonal scale adjustments can be performed in a variety of ways:

• Using the basic tonal scale sliders.
• Using the Tonal Curve tool.
• Using the histogram.

The effect of using the tonal scale sliders in the Basic panel and the histogram is equivalent: in fact, the histogram just provides an easier (and more "visual" way) of adjusting the tonal sliders. The Tonal Curve tool (or an external program as well) provide a more flexible and powerful way to adjust the tonal scale, but my advice is: always start editing your images fixing the tonal scale using the tonal scale sliders. Only then, perform additional corrections using more advanced tools.

Lightroom provides the following tonal scale sliders:

• Exposure.
• Recovery.
• Fill light.
• Blacks.
• Brightness.
• Contrast.

The first four sliders let you adjust the tonal scale of the image with specific emphasis on a specific region of the histogram. Lightroom visually divides the histogram zones into the regions shown in the following images:

If you want to adjust tones in a specific region, you can click and drag the region to be modified in the histogram: depending on the direction you drag the histogram to, the tonal scale will be adjusted accordingly. Each region corresponds to one of the tonal scale sliders and the corresponding adjustment will mainly target a specific zone, although the entire histogram will be modified as a result (shifting it, expanding it or compressing it).

In the following sections we will describe each tonal scale adjustment.

Exposure

The Exposure adjustment is usually the first adjustment you will use since it fixes the overall exposure of an image, with particular emphasis on the high values of the histogram. The adjustments is measured in f-stops (in the [-4, 4] range) and its effect is very similar to modifying the image exposure by the corresponding number of stops (for example, stopping the aperture up or down).

The effect on the histogram can be seen in the following screenshots:

Raising the Exposure level shifts the histogram to the right while lowering the exposure level shifts the histogram to the left.

The exposure slider is used to correct the overall exposure of the image and ensure that the white is at right level. Sometimes, it's not that easy to spot overexposed pixels in an image: if you click the Alt/Option button while adjusting the Exposure slider, the canvas will turn black and Lightroom will only reveal the pixels a color channel of which is going to be clipped.

If there remain overexposed pixels after correcting the image exposure, you can try to fix them using the Recovery slider.

Recovery

After setting the overall image exposure to the right value, the Recovery slider can be used to correct the tones of overexposed highlights. If the image being corrected is RAW, Lightroom can used additional sensor data and succeed in recovering details of pixel overexposed even more than 1 f-stop.

Adjustments of the Recovery slider are made in the [0, 100] range. The effect of adjusting this slider can be seen in the following pictures:

Raising the Recovery value compresses and shifts left the highest zone of the histogram, thus revealing "lost" details that may be present in the RAW file. On the other hand, if the image being adjusted is not a RAW file, clipped channels will result in information being effectively lost: Lightroom will still compress and shift down the histogram, but it won't be able to recover missing details.

In the following pictures you can see how Recovery helps recover missing details in a picture that was intentionally overexposed by more than 1 f-stop:

You can see as adjusting the Recovery slider to +70 helps recovering details of the overexposed cloud that would have been lost otherwise. The histogram of the pictures before and after the Recovery adjustment are the following:

As you can see, the Recovery adjustment has shifted and compressed the histogram slightly to the left, leaving space to reintroduce the information recorded in the clipped highlights. The following is the histogram of a correctly exposed picture of the same cloud:

As you can see comparing the histogram of the corrected picture above with the histogram of the correctly exposed one, the slope and the profile of the two histogram in the highlights zone is very similar.

Fill Light

The Fill Light adjustment allows you the lighten the shadows of your image without affecting the black point. Adjustments of the Fill Light slider are made in the [0, 100] range, and its effect on the histogram can be seen in the following pictures:

Raising the Fill Light value has the following effect:

• It slightly compresses and shifts the histogram to the right of the fill light region, without affecting the white point.
• It slightly expands the histogram to the left of the fill light region, without affecting the black point.

Blacks

The Blacks adjustment is used to set the black point of the image. The adjustment is made in the [0, 100] range. Raising the Blacks value moves the black point to the right, resulting in a higher luminance level to be mapped to black. The effect of this slider mostly affects the darkest tones of the image, leaving midtones and brighter tones almost unaffected.

The effect of the Blacks adjustment can be seen in the following pictures:

Raising the Blacks value has the following effect:

• It slightly expands and shifts the histogram left, without affecting the white point.
• Blacks are increasingly clipped, according to the part of the histogram that's shifted beyond the black point.

The Typical Workflow

The typical workflow when adjusting the tonal scale of your image is reflected on the order of the basic tonal scale editing tools in the Lightroom user interface:

• Overall exposure and the white point is corrected using the Exposure adjustment.
• White point is optionally fixed using the Recovery adjustment.
• The brightness of the shadows is corrected using the Fill Lights adjustment.
• The black point is corrected using the Blacks adjustment.

After correcting the tonal scale of your image with the basic tools, you can fine-tune it using more advanced tool such as the Tone Curve (described in another part of this series).

Difference Between Brightness and Exposure

The Brightness adjustment allows you to correct the overall image brightness, primarily affecting the midtones. Small brightness adjustments may be necessary to correct midtones brightness after correcting the tonal scale of the image with the basic tools. Many Lightroom users wonder what's the difference between Brightness and Exposure. The main difference should already be clear:

• Exposure affects the image brightness throughout the histogram and moves the white point (where highlights are clipped) shifting channel data beyond the end of the histogram.
• Brightness mainly affect midtones and affects black and highlights clipping points (black and white points) to a lesser extent, try to not shift channel data beyond the end of the histogram.

To clearly understand the difference between the two adjustments, then, let's look at their effect on the histogram. The effect of Exposure has already been shown in the pictures in the Exposure section; the effect of the Brightness adjustment can be seen in the following pictures:

Brightness - Original (+50)

Brightness - +20

Brightness - +80

Brightness - +100

As you can see, modifying the Brightness level shifts the midtones region of the histogram to the right or to the left. Despite what happens when adjusting the Exposure value, however, Lightroom compresses the histogram towards the clipping points in an attempt to not modify them.

However, huge brightness adjustments end up affecting both the black and the white point: that's why you should only need slight brightness adjustment after tuning the tonal scale with the tonal scale adjustments.

Contrast

The Contrast adjustment allows you to correct the image contrast, mainly affecting the midtones. The effect of adjusting the Contrast value on the histogram can be seen in the following pictures:

Contrast - Original Histogram

Contrast - +100

Contrast - -50

The effect is the following:

• The right half of the histogram is expanded or compressed to the right, depending on whether contrast is raised or lowered.
• The left half of the histogram is expanded or compressed to the left, depending on whether contrast is raised or lowered.

As in the case of the Brightness control, it's important to point out that this adjustment affects midtones and can modify both the black and the white points.

Also, if you need to modify the local contrast in another histogram region, Contrast might not be the tool for you: you should have a look to Tone Curves instead.

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