Nice things to know about Digital SLR Photography

I'm currently the proud owner a Canon EOS 20D camera (just upgraded from a 10D), but most of this article should apply to all digital SLR cameras. I hope this will help ease the learning curve for newcomers to the digital SLR world, and hopefully avoid all the pitfalls I have fallen into. This article is no way near complete, but it is a start.

Sensor
Focal Length
Focal Length Comparison
Focal Length Multiplier
Camera Shake Rule
Freeze the Action
Play with Time
Procedure for taking Pictures
Exposure Compensation
Aperture
Light Sensitivity of Lenses
Depth Of Field
Flash
Flash and Av Mode
Panning
Trouble with auto-focus and Sigma Lenses
Dust on the Sensor
White Balance
Black and White
RAW shooting
Test your lenses
 

Sensor
This is the digital equivalent of film - this is what "sees" the picture. The actual chip is typically called CMOS or CCD, but this is not nearly as interesting as its size. The problem is that these sensors are very expensive to make in full frame 35mm film size (24 x 36 mm). This raises some issues when using lenses intended for 35mm film SLR cameras, which will be discussed later. For more details on sensors see here.

Focal Length
Determines how much magnification there is. Focal length is not to be confused with the "zoom" buzzword you see in adds for compact cameras e.g. "it has 4x zoom". It is easy to calculate the "zoom" for an SLR lens: If it says "70-300mm" on a lens it means it has 300/70 = 4.3 x zoom. However this is not really all that interesting to know, i.e. a fixed focal length lens of 1000mm would have a "zoom" of x1, but it can actually magnify enough to take nice pictures of craters on moon.

50mm (on a 35mm-format camera) is considered a "standard" focal length, because pictures taken at 50mm have approximately the same perspective and field of view as humans see things. Wide-angle focal lengths (much less than 50mm) will exaggerate the perspective making the foreground appear longer that it is, and make things look further apart from each other. Telephoto focal lengths (much higher than 50mm) will have to opposite effect squashing things together. For a more formal definition of focal length see here.

When people sell their houses the real-estate dealer always uses a wide angle lens, because it make the rooms look enormous. Here is a living room with a wide angle and a normal focal length. Look at the plant in the background - it looks miles away on the wide angle shot:

Had I been able to step back more and use more zoom, you would have seen the perspective lines in the floor becoming more and more parallel.

Focal Length Comparison
"Okay, so you say that 50mm is "normal", what is this mumbo-jumbo on my compact Canon G5 camera then?":

This means that the focal length can go from 7.2mm to 28.8mm (the "1:2.0-3.0" is about light sensitivity which will be covered later). These focal lengths have no meaning unless we know the sensor size, so we can compare it to focal lengths on a 35mm full frame (if this is all nonsense - don't worry it will be explained).

The sensors diagonal on a Canon G5 is 8.94mm, which compared to a 43.3mm diagonal of a 35mm full frame give us a ratio of: 43.3mm/8.94mm = 4.84. So the focal length range of a G5 would go from 7.2mm · 4.84 = 35mm  to 28.8mm · 4.84 = 140mm, so the Canon G5 is not a super extreme wide-angle camera as the numbers could have led one to believe.

Focal Length Multiplier
This is sometimes also referred to as the "crop factor". This is actually the ratio we calculated in the previous section. In comparison, my Canon 20D has a crop factor of 1.6.

Let us take two pictures of Pelle the cat with the same lens set at a focal length of 65mm. The first one is with a 35mm film SLR and the second one with my EOS 20D with a crop factor of 1.6:

	35mm film SLR	Canon EOS 20D
What gets to the film/sensor. Both see the same image, since the both have the same lens on with the same focal length.
The final picture. Because the 20D only registers the middle part, the final picture looks magnified.
	65mm focal length	65mm ·1.6 = 104mm perceived

When I use a lens intended for a 35mm film camera on my 20D I need to multiply the given focal length by 1.6 to get the 35mm equivalents, so when I use a 300mm lens I'm getting the magnification of a 480mm lens. The lens is still only a 300mm, but the image is just cropped giving perception of more magnification.

This effect is great when large magnification is wanted, but really bad when wide angle is wanted. To get the field of view from a 24mm lens on my 20D I need to buy a 24mm/1.6 = 15mm lens, which is more expensive and less available than a 24mm lens.

Camera Shake Rule
This is an old school film SLR rule of thumb, that is still of great value today:

Okay, so what on Earth does that mean? Well, if I zoom in at 300mm, then my shutter speed needs to be 1/300s or faster, but if I zoom out at 70mm, then my shutter speed only needs to be 1/70s or faster.

You can rest your camera against a table, fence or use a tripod if you want to "break" the rule and still get sharp images, but bare in mind that you should not go much slower than 1/30s if there are people moving in the scene (unless you want it blurred).

Okay, but what about that "Focal Length Multiplier"? Does that change this rule? No, you do not need to multiply with anything. Use the focal length written on the lens - regardless of how much we crop the image, the rule still applies. It it just a rule of thumb. If we needed to multiply with the crop factor then my Canon G5 should not be able to take sharp pictures at full zoom at shutter speeds slower than 1/140s, but it does it just fine at 1/50s.

Freeze the Action
The shutter speed given by the "Camera Shake Rule" may not be fast enough if what you are photographing is moving fast, and you want to freeze the action.

The faster the motion, the closer the subject, and the longer the focal length, the faster the shutter speed you’ll need to “freeze” the motion. I generally use 1/250 - 1/1000s when I want to freeze some action - experiment!

Set the top-dial on the camera to "Tv" and set the shutter speed. Let the camera decide the rest, but change the ISO setting if needed yourself. If you are getting shutter speeds of more than 1/1000, you may want to lower the ISO to get less sensor noise.

Play with Time
You may think of the human eye as having approximately a "freeze speed" of 1/30 second. The more you change the shutter speed away from 1/30s, the more you can capture images the eye can't.

A 4 second exposure of a water stream blurs the water motion making it look as smooth as silk:

Or 1/500s to freeze the water splashing up after an "accidental" apple impact in my kitchen.

Procedure for taking Pictures
Okay, I understand the camera shake rule, but how do I go about "obeying" it? Here is a simple procedure that can be followed to get the fairly good result when you are not in a creative mood or are doing things manually:

1. Set the camera at "P" on the top dial, so you are allowed to changes parameters like ISO and exposure compensation, and the built-in flash doesn't pop-up at random.
2. Select ISO 100 (the lowest ISO setting to get the least noise in the picture)
3. Compose the wanted image, and press the shutter release button half way down.
4. Notice the exposure time is shown in the view finder in the bottom left. It may say "60". If you're not zoomed in more than 60mm, then take the shot. Otherwise increase the ISO and try step 3 again. If you reach ISO 800 without success, then it is time to evaluate if you want to:
       a) Use a flash
       b) Go beyond ISO 800 (do you want the picture blurred or noisy?)
       c) Use a tripod (if there is movement you may not want to go much slower 1/30s to avoid blurring the motion)
       d) Take the picture anyway and accept that this may be blurred

Naturally the above is never followed to the letters, and the are many other ways of going about it, but it actually doesn't take more than a few seconds once you get the hang of it.

Get in the habit of checking the suggested shutter speed before shooting. People complain that there is no ISO indication in the view finder. This is true, but getting a suggested shutter speed of 1/800 is a hard-to-miss hint that you have your ISO setting too high.

Exposure Compensation
If the picture is over- or underexposed you can compensate with the jog dial (when  in "P", "Tv" or "Av" mode). The compensation is clearly displayed both in the view finder and on the top LCD display:

WARNING! You have just taken some special close-ups of something, and had to exposure compensate to get it right. You are happy with it, and shut the camera off, and go look for something else to do. After some time the Bride and Groom at the party do something goofy and the photo opportunity is hard to miss. What happens? You forget all about your compensation settings, that the camera remembers from last time (even if you did turn it off), and you get the most under- or overexposed pictures in history. Why, Canon Why? Make it a custom setting please! Trust me on this one:

"If you compensate for exposure, make it a habit to reset it when you are done"

If you don't, then there will be a time when you forget, and after taking 50 pictures it's a tragedy to discover such a simple mistake has ruined all the pictures, and the trouble is that the LCD screen on the back isn't really that good at showing it, because the contrast varies a lot dependant on viewing angle.

Aperture
The aperture is a variable size opening in the lens, that the light passes through before reaching the sensor, much like the iris in our eyes. Aperture size is usually given in f-numbers. It is the source to some confusion, because the larger the number, the smaller the opening.

Aperture size together with shutter speed determine the amount of light falling on the sensor (the exposure). The aperture is sometimes called the "stop".

If you set a 50mm lens to f/1.4, then the diameter of the aperture opening is defined as 50mm/1.4 = 36mm. You can actually tell this is the definition just by looking at the notation "f/1.4" = "focal length / 1.4". Other notations like "F1.4" are also used.

The following table show combinations of shutter speeds and f/stop's that give the same exposure.

This gives you great freedom in choosing what it is you want to achieve, e.g. freezing the motion or changing the depth of field, which we will look at in a moment.

Most lenses take the best quality of pictures in the middle of its f-range. Film photographers have been saying "f/8 and be there" for years, and it bares some truth. Having a lens fully opened isn't going to get you the very best quality out of the lens, but it will get you the most light.

It is quite easy to verify the table above, if one is so inclined, by simply finding the area of light that reaches the sensor, e.g. the area of light is 1000mm² at f/1.4 and 250mm² at f/2.8 (for 50mm). So only 1/4 of the light get through when we choose f/2.8 compared to f/1.4. This is why the table has a factor 4 change in shutter speed for these apertures to achieve the same exposure - simple, but boring.

If you take a lens apart you may find the lens manufactures use all kinds of optical tricks, so the actual aperture is smaller than you would think, just like you can find a lens is shorter than the focal length is gives. This really doesn't matter, the lens manufacturers can use all the tricks they want, as long as the lens performs as specified.

Light Sensitivity of Lenses

Looking at lenses you will see things like "F3.5-4.5", or "F2.8". So, what does that mean? "F3.5-4.5" means that the maximum opening of the aperture is f/3.4 at the most wide angle the lens offers, and f/4.5 is the maximum opening at maximum focal length. The maximum opening is interesting, because it tells us how must light we can possible get through the lens.

"F2.8" means that is has a maximum opening of f/2.8 in the entire zoom range. This is an extremely nice feature if you are taking pictures with manual exposure calculations, because it means you can zoom in or out without changing the exposure setting. However, it has very little impact if you are doing all the exposure automatically.

An 50mm F1.4 lens is considered a very "fast" lens (meaning it has high light sensitivity). It has a very large aperture opening of  50mm/1.4 = 36mm. I can highly recommend Canons 50mm F1.8 - it's cheap and looks like at toy, but it performs beautifully. By the very nature of F-stop definition you will not find a 500mm F1.4 lens - it is just not realistic, so don't be so quick to discard tele zoom lenses as being slow if it says F6.3 at 500mm.

Depth Of Field
The distance between the nearest and furthest objects in a photograph that are considered to be acceptably sharp. Abbreviated DOF. It is dependant on aperture, focal length and focused distance.

There are all kinds of exact calculators that will let you figure out what DOF is, e.g. this one, but you rarely need them. But you do need to know what influences DOF and how: DOF is increased by the following

1. The smaller the aperture (bigger F-number)
2. The wider the lens (smaller focal lengths)
3. The further the focused distance

Okay, let's make a thin DOF line by zooming in, settings the smallest F-number, and focus on something close by:

You can se on the paint that the DOF is just a thin line, and the top of the rusty nail is actually blurred, outside DOF.

Let's say I use the same lens on a full frame camera like the EOS 5D and my 20D. I take the same picture with the same focal length settings on the lens with both cameras. Then I can crop the 5D picture and get EXACTLY the same picture as I got with my 20D - same DOF, magnification and everything (the 5D has 1.5 times as many pixels as the 20D, so it is as close to the 1.6 as one could ask).

Flash
Buy an external flash! I can't stress this enough. Built-in flashes produce "hard" ugly flash shadows and cause red-eyes.

Red eyes are caused by flash light reflecting from blood vessels in the eye. The annoying "red eye reduction" that most cameras have, it just a series of short flash bursts designed to get your iris to close more before the final shot. This delays the picture, ruins the surprise element, and people are not smiling very much after being blinded. The best pictures of people are usually pictures where they didn't know a picture was taken. Turn off red-eye reduction!

Having an external flash help the red-eye problem automatically, since the flash is positioned further away from the lens.

When using a flash the best results are usually obtained by "bouncing" the flash light of e.g. a ceiling or wall. This requires the flash head to be tunable (both up/down and left/right). Bounced flash light  is usually much softer and more natural looking than direct flash light, and ugly flash shadows that show up when people stand close to a wall disappear.

If the ceiling is high, colored or dark, then bouncing light off it is not an option. It depends on your flash what you want to do. White walls are seldom available for bouncing (there is always something in the way). Some use a diffuser like this one:

This makes the shadows less hard. Others use a "soft box" that typically attaches to the flash with Velcro. The Canon EX 580 offers two built-in options: White reflection card that the light bounces off from (which I prefer, but it doesn't give a lot of light), and a retractable cover plate:

Top of the line flash units are currently "Sigma EF 500 DG Super" and "Canon EX 580", which both are E-TTL2 compatible. The Sigma is half the price of the Canon!

Most of the time I have set the flash firing strength to "-1", because I don't need that much light, it keeps the recharge times down (time between each flash), and saves the batteries. I can usually fire a few quick flash shots of before the flash needs any noticeable recharge time.

Turn the flash shoe tightening screws on tight to ensure proper contact with all connectors, and don't lift the camera by the flash. It is a very delicate small piece of plastic that holds the flash on to the camera. I have only dropped my camera once from a ~10cm height. Unfortunately the Canon EX 580 flash that was attached, broke due to its own weight and the uneven impact angle. The flash itself didn't hit the wood floor, but the impact broke the shoe plastic beyond repair. The stress on that plastic shoe is too much for any super glue to hold. I got a replacement part and changed it myself, but ever since I'm really careful when the flash is mounted. The Sigma seems more robust and lighter of the two, so that would be my choice if I was to buy today (and the fact that it is half the price).

Flash and Av Mode
If you alternate between taking pictures with flash and without, then you may have accidentally set the camera to "Av" mode when you switch back to using flash. This is trouble waiting to happen, because Canon have decided that the flash is to be used as fill-in flash in "Av" mode, and does not use the flash to get the exposure right. In low light conditions (like at a party) it means that the shutter speed is set way slower than it should. The fact that the flash does fire may help to freeze the action, but unless it is really dark, the ambient light is enough to create blurred trails that you discover the day after on your PC. Flash in "Av" mode is great if the girlfriend is posing in front of the Eiffel Tower at night and you are using a tripod, but don't use flash and "Av" mode at parties!

Panning
This a way to get fast moving objects in focus by following the object while taking the picture. This can produce great looking pictures with a dramatically sense of speed (since the background is blurred). The shutter speed must not be to fast for this to succeed, since it would otherwise freeze everything. This can take some practice but it is fun when you pull it of.

Trouble with auto-focus and Sigma Lenses
Oh boy! Don't get me started on this one. Let me first say that I love Sigma lenses in general, and they offer great performance for the money. Having said that, after hundreds of focus test shots I have come to the conclusion that some Sigma lenses (specially those that don't have the DC mark), can't auto-focus properly at shorter focal lengths than 50mm on some camera bodies! Can you even imagine how hard that error was to track down? I recommend you to test Sigma lenses before you buy (or make sure you can return it). Sigma can adjust the lenses so they will focus, or so they claim. They have been unable to help me with my "Sigma 24-70mm F2.8 DG". What I focus on is never in focus, but what is in front of it, is in focus:

Please note that the light source angle in this image caused a refection in the ink, which makes the left side fuzzy. This has nothing to do with the lens or focus.

If you do perform focus tests yourself, please note that the focus points are not necessarily exactly located where the view finder indicates, and that the points are larger than indicated. Focus may be found anywhere within the focus point, so you must be careful when DOF is very thin, since you may otherwise wrongfully conclude you have focus problems.

Dust on the Sensor
Actually it is not dust on the sensor, but dust on the glass or filter in front of the sensor. It comes with dust directly from the Canon factory. It is easy to test if you need to clean the glass, but if you haven't seen this issue in any of your pictures, please don't be alarmed by this test - It is always possible to provoke a picture of dust. I needed to have my 20D cleaned because I can see vague spots in normal photos which annoy me more than I can describe, coming from a brand new camera.