Digital Camera Guide Part 1

Following the rousing success that was my guide to HD televisions I now turn to the world of digital cameras. This was prompted by the frustrations of DaBoss who is looking to replace his ultra-slim cool camera for something that will take decent photos. Not wanting to say told you so, but I told him so. The first time he showed me the camera I told him he was going to have problems with it in the normal situations he found himself, but it was soooo cool. To be fair none of the family were operating it quite right, but even so problems existed. To prevent anyone suffering the same type of misbuy herein lies the guide.

Digital Cameras

I don't think it's an exaggeration to state that digital cameras have changed the entire nature of amateur photography. No longer having to mess with film and deal with developers who print and charge you for the entire reel including that photo you took of your thumb; amateur photographers can take pictures of what they want, where they want, and when they want without care or worry. Instantly able to preview shots and delete and retake in an instant means you have a greater chance of getting the shot you want. Add in the internet to share your photos, home photo printers to print them yourself, and the rise in computing power and photographic software to tweak the images just so the amateur has never been closer to the professional in the images they can take.

But still a gap remains and to an extent it's down to both the specialist jargon of that world and the new jargon that has been built around digital cameras in particular.

DSLR, Compact, Sub-compact.

Even before you start looking for a camera you have to make a decision of what type you want. At first glance this is simply a case of size - the Digital Single Lens Reflex cameras are the bulky ones you see professionals carrying, the compacts something a little more handy, and the ultra-compacts something you can slip into a shirt pocket. However each also has their own strengths and weaknesses. I'll go into details under each section but a quick summation is as follows:

DSLRs are big bulky and tend to be aimed at the professional and thus generally do little to hide the complexity of their use; they normally have a separate body and lens which allows greater versatility in your shots but that can bump up the price.

Compacts are the general workhorse, not as big as a DSLR but something that would still be noticeable if stuffed into a pocket. With a fixed lens you lose the versatility of the DSLR, but aimed at the general public the interface is normally toned down. Normally the cheapest of the lot.

Ultra-compacts. Great for sticking in your pocket for an outing. Like the compact the fixed lens lacks versatility and cramming the electronics in a smaller case can bump up the price.

Time to get into specifics.

Megapixels

This is pretty much the first thing that's thrust into your face if you're looking at cameras and like so much in marketing it is both misleading and almost useless without extra information generally not provided at the same time.

The megapixel count is simply the number of pixels, in millions, that exist on the sensor. Each pixel is normally made up of three subpixels (some cameras use layered subpixels) that each sense one of the three primary colours - red, green, and blue. The more pixels the greater the detail you can retain when you print out large images or crop and zoom in on a particular section.

So as a marketing tool it's great - the higher the number the more detailed your image, end of story; not quite. Because the factor that isn't mentioned is the sensor size.

Image Sensor

In a digital camera the film is replaced by an image sensor made up of pixels. For the old film the generally used standard was 35mm which had a ratio of 3:2; in a feat of abject stupidity image sensors use a ratio of 4:3. Some DSLRs use the old 3:2, but everything else 4:3.

This should be a bonus for digital cameras as they gain more height for the same width as the old film cameras. This would indeed be great if any photo developer or photo printer manfacturer produced 4:3 ratio prints; this means your photo will either be cropped top and bottom or be bordered by an unprinted area.

This alteration also proves a problem with the traditional method of measuring the sensor size. Keeping it simple back in ye old days of photography the image formed on a glass tube which was specified by its diameter. It was soon found that only a portion of the tube was suitable for forming a good image - approximately two-thirds of the diameter. Everyone knew what size tube produced what size image so the measuring system stuck. Now we hit the 21st Century and we're still stuck with it.

So digging up the specifications for a camera (and you will need to dig at least a little) may well tell you that the image sensor size is 1/1.6" in modern terms about 15.875mm; this represents the diameter. Except of course it doesn't, it represents the diameter of the old tube on which a smaller image is formed. Dig deeper and you will find the actual sensor size; in this instance 7.78mm by 5.83mm with a diagonal of about 9.7mm.

So why bother, what's so important about this? Well the size of the image sensor is, in reality, the key to the camera, but it's complicated and not so easy a point to sell and thus gets pushed aside.

Pixel Density

This is somewhere you need to combine the proudly stated Megapixel count with your dug up information on the sensor size. I'll compare two unnamed cameras a 7.1 MP and a 12 MP, as per above the 12 MP is obviously the better of the two; but let's add in sensor sizes.

7.1MP 7.18 x 5.32mm
12MP 7.78 x 5.83mm

Divide the pixel count by the area of the sensor and we get back two figures

7.1MP - 185, 876 pixels per mm²
12MP - 264, 565 pixels per mm²

In order to increase its megapixel count the 12MP camera has had to cram more pixels into the same space. So what? Well that leads to the next bit.

ISO sensitivity

Again back in the old days you had a choice of what type of film you loaded depending on what you were shooting. If you were shooting in bright light you needed a film that was less sensitive to light so you wouldn't get patches of burn where the light levels overloaded the film. In dim light you need a film that was sensitive to light so you could maintain a high shutter speed and eliminate blurring while still capturing enough of the light to create an image.

These different types of film were differentiated by an ISO number. The less sensitive the film the lower the number, say ISO 50; the more sensitive the higher the number, again say ISO 200. The film itself was made more or less sensitive by adjusting it's chemical properties, to keep things simple think of them as grains in the film. Less sensitive ISO 50 had smaller grains then high sensitive ISO 200; this in turn meant that ISO 50 film captured smaller details then ISO 200 film and was thus less grainy.

All well and good but how does this apply to digital cameras, you can't change the size of the pixels. No you can't what you can change is their threshold value, that is the amount of light that has to hit them in order to be triggered.

This is where the pixel density comes into play. Electronics at this small level can be highly susceptible to interference (also known as noise). Now if we imagine a single pixel on its own with light coming at it, it'll trigger if the light level is above its threshold; for bright images we increase the threshold value; for dim light we lower it.

So if we have an arbitrary value of 50% light hitting a sensor and the threshold is set at 51% it won't trigger and that pixel stays dark. So far so good in isolation; but put another pixel next to it and feed it 52% light, it'll trigger and create a small electronic current, this in turn could induce current in the pixels surrounding it and push our original pixel's value above 50% and cause it to trigger.

Essentially the smaller the pixels and the closer they are the more likely you are to experience this 'noise'. Cameras do try to eliminate it sometimes by clumping pixels together and pretending to be larger 'grains', but as with film that results in a loss of detail.

It's important to note that in nice bright condition you're unlikely to ever see any problems at all, but if you're inside or at a party your pictures may well start getting blurry and/or noisy. This is where the ultra-compacts can fall down, sure they may be easy to slip into a pocket for a party, but unless you use a bright flash everything's going to be dim and noisy.

Sensor size also plays another part with yet another old measurement that played by the lens.

Camera lens

If you look at the lens of a camera you can be forgiven for being confused, you will often be presented with a bald summation of 5x f=6.4-32mm 1:3.3-5.1. The first bit is easy it's the optical zoom, more on that later, the second part I'll deal with here.

the part that begins "f=" represents the actual physical distance from the lens to the sensor; except sometimes it doesn't. Sometimes it represents a theoretical distance - if the sensor was in fact a 35mm piece of film, what distance would the lens be to represent the same field of view.

Yeah complicated so diagram time.


The lens is represented by the blue oval and the 35mm film by the green rectangle (36mm in width). At the distance of the white object with a simple straight through lens the full length will be captured on the 35mm film as represented by the red lines. With a smaller sensor (orange) it needs to be positioned closer to the lens in order to capture the same image. So in this instance the film is set 28mm away from the lens and the smaller sensor at 6.4mm, as they'd capture the same image for this camera the f=6.4 is the equivalent to a 35mm f=28.

Of course it's never that simple. For the camera specifications I'm using the sensor width is 7.78mm, as per the diagram above to be a 28mm equivalent at 6.4mm the sensor requires a width of 8.23mm or needs to be at a distance of 6.05mm. It might not sound much, but at this scale that's a huge difference ~5% out.

To reconcile this requires a choice - either the manufacturer is lying when they claim that 6.4mm is the same as 28mm or it really does capture the same width. Assuming they're not lying the only way to get the same image is by using a shaped lens, one that bends the light inwards like so.


Using a wireframe to show the difference, the rectangle on the left represents the 7.78mm lens at a distance of 6.4mm from the lens on the right. The lines that continue past the sensor represent the path of light for a straight-through lens, the lines connected to the sensor represent how much the light has to be bent in order to deliver the 'same' image as a 35mm/28mm combination.

Again it doesn't look much, but again at these scales it's significant and means that, as the lens is circular, the only item that appears at it's true height and width is anything in the centre. As you get closer to the edge most noticeably on each side things will get slightly squished.

Of course all of this is pointless if your photos are so dim you can't make anything out and that leads me to the final and shortest section of this part of my guide.

Lens Size

This is where DSLRs rule as they're both bigger and have interchangable lenses. In this case the maxim is simple - the bigger the better. A bigger lens means more light getting to the sensor, means a faster shutter speed and less blur, and a lower ISO and more detail and less noise.

As I mentioned before this is where the ultra-compacts can start to fall down; to make them that small the lens has to be smaller too. So to get an image where you're most likely to take an ultra-compact (a party) you need to use the flash a lot, which bleaches the scene and drains the batteries; use a low shutter speed and introduce blur; use a high ISO 'threshold' and introduce noise and grain; or a combination of all three.

Conclusion

If you're taking quick shots outdoors and don't mind the lousier qualities of indoor shots then an ultra-compact is a good bet

If you've got the money, the patience to learn what all the buttons mean, and the strength to lug it about you're looking at a DSLR.

For something the equivalent of your old bog-standard film camera a simple compact will do fine.

In all instances though don't get blinded by the big numbers go through the steps I've outlined here and if at all possible try the camera before you buy it.

In the next part I'll detail using the digital camera, the various settings and how to get the most out of it.