About Flash: Part Two

Now what you know about the power& distance limitations of a flash, lets see some stuff you can do with a flash.
There are two ways to use a flash:
1. As the primary source of light
2. As a secondary source (fill flash)

As a secondary source, you would expose the image for ambient light and let the camera adjust the flash strength via TTL(through the lens) metering, while giving a bias if needed(+1 flash EV will double the power, -1 will half the power that the camera has otherwise calculated using TTL metering). Av(/A) and Tv(/S) modes expose for ambient light and use flash as a 'filler'. Also called slow sync when long shutter speeds are used (slower than about 1/30s)

As a primary source, u underexpose the background by a lot and use the flash's power to completely light your subject(or whatever part of it you want to light up). In a reasonably dark place, P(rogram) mode does this... it will not expose for ambient light, only for the flash via TTL.  Will usually use a shutter speed of 1/60-1/200s. When used in this manner, the flash freezes the subjects movement and leaves no noticeable blur, because the flash strobe has a very short time duration (~1/10,000s) and the camera is NOT exposing for ambient light.

When to use the flash?
Indoors: When there is insufficient light or to balance a subjects exposure with backlighting if a window is open in the background
Outdoors: for portraits/ group snaps, to lessen harsh shadows and to create catchlights (the twinkle in the eye that you usually see)

In this example, I used flash to balance the indoor and window exposure. It was about 3pm and had I not used flash, there would be two possible outcomes depending on how you expose the snap:
i) The subjects would be properly exposed, but the window in the background would be completely blown out(highlights clipped), ruining the snap
ii) The window would be exposed correctly but the subjects would be silhouettes!
The only way out was to correctly expose ambient light of the window with the lightmeter while letting the camera use TTL-metered flash to expose the subjects.

Flashes used directly can produce harsh shadows and ruin texture and it is advisable to diffuse them i.e. make the source seem bigger. There are many ways to do this, the most effective is bouncing it off the ceiling/wall. This changes the light source from being hard to soft.

I may have mentioned in the past about the X-sync speed of a flash, which is ~1/200s on most cams, and it wont allow you to shoot faster than this with the flash on. However you can use an external flash which supports high-speed sync to use flash at higher shutter speeds. This is especially useful for shooting portraits outdoors when you want to use a large aperture to blur the background as well as use the flash to prevent harsh shadows.

About Flash: Part One

A fundamental requirement for a photograph is LIGHT. This light can be of various kinds. Some important ones are:
Hard light- Perceived as a point source, gives well defined, contrasty shadows. e.g. the sun on a clear day
Soft light- A large source, gives soft shadows. e.g. An overcast sky(the sun is hidden behind clouds)
Transmitted- When the source of light is in the frame. e.g. A sunset
Each type of light has its uses (for portraits, soft light is generally preferred), and you cannot say one type of light is 'better' than another(in the absolute sense).

There are many instances when we need to artificially add light into the frame, for various reasons(some of which I will discuss in later parts of the article). One way of adding light is using a flash.

Flashes may be built into the camera (e.g. a pop-up flash) or may be an external unit. The power of a flash is expressed as a "guide number" in meters or feet(@ISO 100).

This formula will tell u how far a flash can expose a subject.

f-number x distance x sqrt(100/ISO)= Guide number

*sqrt= square root for those who dont know.

My canon 400D flash has a guide number of 12m... lets see what dat looks like in a practical situation:
Say I use a reasonable aperture of f/4 and ISO 400... then
4 x D x sqrt(100/400) = 12m
4 x D x 1/2= 12m
D=6m

So the flash can effectively expose a subject upto a distance of 6 meters, with the above settings.

If I keep the aperture constant and change the ISO to 100, the max distance the flash will expose is 3m.
for ISO 800, it will be 6 x 1.414= 8.5m

The reason for the square-root is because of the way light intensity varies with distance (inverse-square rule).
Say I have a constant source of light. Suppose, when I stand at a distance of 10 m from it, I receive 100 lux of light intensity, if I go another 10 meters away from the source (total of 20m from the light source now), I will receive 25 lux.(1/4th the original intensity when at double the distance)

#Remember that doubling the f-number (say from f/4 to f/8) reduces the light by a factor of the square of the ratio(in this case (8/4)^2 or by a factor of 4), that is why there is no square root function required for the f-number.

For 'ordinary lenses', the maximum aperture available varies with zoom. Take an 18-55 f/3.5-5.6 lens. When zooming in therefore, the maximum range of the flash is reduced. (With a constant aperture lens, this does not happen)

------------ More coming later in Part Two---------------------------

About Histograms

One major advantage of Digital over Film is that you can see the results instantaneously, and if needed change the settings and re-take the shot. We normally use the camera's LCD screen for that. However, LCD screens can be deceptive. A more accurate way of determining whether the exposure was correct or not is using the histogram. Almost all digital cameras have them, so lets learn how to read and interpret them.

First of all, a histogram is nothing but a bar chart. Lets take a look at a simple example of a bar chart:

This bar chart is of a hypothetical mark distribution in a class. On the Y(vertical) axis, we have number of students. On the X(horizontal) axis, we have categorized marks into 4 groups, 0-25, 25-50, 50-75 and 75-100. You can see that the height of each bar is proportional to the number of students getting marks that fall into that group.

A histogram is basically the same thing. Instead of number of students on the Y axis, we have number of pixels. On the X-axis, instead of categorizing students by their marks, we categorize pixels by their brightness values, into 256 categories, ranging from 0-255 (for jpeg, 8-bits=>2^8=256 values). 0 represents pure black, while 255 represents pure white. A typical histogram looks like this:

You will not see all this, of-course. The camera will only display the distribution, to save space:

So now, the taller the bars in a specific region, the more the number of pixels having those brightness values. So how do you use this for exposure?
A normal image will have a full spectrum of tones ranging from black to white, well distributed. If the histogram has too many pixels on the left side, it means your image is underexposed, while if there are too many on the right side, the image is over exposed. If you have a very tall bar just near 0 or 255, it means the shadows or highlights respectively are clipped(lost all detail). You may have many times experienced highlight clipping... the sky becomes completely white, losing all detail of clouds, etc.

See this example of histograms of differently exposed images:

Courtesy: http://www.thecolorblindphotographer.com/

So next time you take a picture, don't rely on the LCD screen. Take a look at the histogram too!(and underexpose/overexpose if necessary)

PS: Sometimes the histogram SHOULD have more values on one side, for example snow. It should appear white-ish in the image, so you want most of the pixels to be on the right side. However using the histogram, make sure the highlights DO NOT get clipped.

The Zoom Perspective

You may have heard people say that a Wide Angle lens expands space while a Tele Photo lens compresses it... What exactly does that mean? 

Have a look at these 2 pictures:

focal length=17mm

focal length= 55mm

I've tried to keep the shoe(say subject) size the same in both photos by moving away when shooting on the tele end. But what is the big difference between these two?

Its the BACKGROUND, or rather the size of it.

When shooting with a wide angle lens, the background becomes a lot smaller than the foreground(as if when viewed with the naked eyes, they were separated by a large distance- hence the term expansion of space).

On the other hand, when using a tele lens, the relative difference between the size of the background and foreground is much less( as if when viewed with the naked eye, they are close by-> hence compression of space).

This also means that a wide angle lens gives more background than a tele lens.

I think the following diagram is self-explainatory as to why it happens:

So when composing an image, if you want to show a subject in its environment, use a wide angle lens. However, if your purpose is only the subject, a telephoto is preferable, so as to reduce the amount of clutter in the background.

Note: the 'normal' perspective of human vision is said to be equivalent to a  50mm lens on a 35mm film SLR/ full frame DSLR (sensor size same as 35mm film), which is ~30mm on a Canon crop body and ~35mm on a Nikon crop body DSLR.

Why I still love my nifty-fifty

left: Canon 17-55mm f/2.8 USM IS (f/2.8 is the fastest they make zoom lenses till date)

right: Canon 50mm f/1.8 (popularly called the nifty-fifty)

Although I recently got an AWESOME Canon 17-55 f/2.8 constant aperture zoom lens (which allows f/2.8 regardless of how much I zoom in, unlike other typical zooms [eg canon 18-55 f/3.5-5.6]), I still love my 50mm lens. Here's why:

1. Size: See the comparison picture above!

2. Weight: the nifty is just 130g in weight, while the 17-55 weights 645g, which is heavier than my 400D body!(and can get quite tiring if used over long periods of time)

3. Speed(aperture): The nifty can take in more than double the amount of light than the 17-55, when both apertures are set wide open( f/1.8 vs f/2.8), effectively allowing higher shutter speeds to be used with the same ISO thereby reducing motion blur, or alternatively lower ISO to be used with the same shutter speed thereby reducing noise.

4. Price: Primes are cheaper than zooms(especially large aperture zooms). Period!

5. No Zoom!- Zooms tend to make me lazy. The nifty makes me walk around searching for the best vantage point to shoot from.

6. DoF: The shallowness of the depth of field at f/1.8 is enchanting!

The nifty is also more flair resistant and suffers from less chromatic aberrations (like purple fringing at high contrast edges)

I've tested the sharpness of both lenses @ f/2.8 and found them very similar. I surprisingly found the 17-55 slightly sharper, but just by a hair, probably because of the USM accuracy. Both these lenses blow away normal zoom lenses in terms of sharpness.


Depth of field example @ f/1.8

I highly recommend buying the 50mm f/1.8 (or the f/1.4 if you can afford it- 3 times the price of the f/1.8). It should be the 2nd lens you own (the first being the kit lens that comes with your DSLR). I guess the 35mm [f/2(canon) or f/1.8(nikon)] is also great for non full frame DSLRs.

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Composition Examples

Example 1:

The first thing you will notice is it follows the rule of the thirds.... There are 2 important points of interest here... The first one is the eyes, which have been placed on the upper 1/3rd horizontal line.

The second point of interest is what the eyes are looking at- the figure being drawn, which has been placed on the lower 1/3rd horizontal line

The frame has also been almost filled by the subject. A mistake many beginners tend to make is including the whole body. This creates unnecessary waste of space and also brings in distractions. Don't be afraid to cut out the top of the head, etc by the frame edges.

Shot with my beloved 50mm f/1.8 lens @ f/2.8, ISO 400, 1/40s on aperture priority mode. I used an aperture value of f/2.8 to blur the background while keeping most of the subject in reasonable focus.

I focussed on the pencil tip, here's why- when there is a human in the frame, the first thing we look at is their eyes. If their eyes are not looking at the camera, but instead gazing at some other object, our eyes immediately look at that object next, within a fraction of a second.

If a person is looking into the camera you should in 95% of the cases FOCUS ON THEIR EYES.

Example 2:

The charm of this shot is its simplicity. Subject placed on the lower right intersection of the thirds. No distracting elements in the frame.

Shot @ ISO400, f/2, 1/125s on 50mm f/1.8 lens

Example 3:

Follows rule of thirds(fingers). Uses converging lines(strings) to draw the viewers eyes into the photograph. Directional lighting reveals texture, form and depth. Background with no distractions.

I took it using my iphone 3Gs's cam near a window which provided directional lighting. Adjusted black point using levels to make background pitch black.

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