Compact digital cameras Vs. DSLR

This is the 6th and final part of a tutorial about photography basics. So far we’ve seen

• Some physics background related to photography
• The pinhole camera and the basics of camera building
• The lenses: Zoom, Focus and Aperture
• The ISO value: Films and CCD sensors
• Mastering the Shutter speed

Finally, we have seen all the necessary elements to build a camera. The black box, the lens, and the shutter.

Traditionally, there were two kinds of film cameras. Cameras with Direct Viewfinder and SLR (Sinlge Lens Reflex, or just Reflex) cameras.

Direct Viewfinder cameras have and eyepiece that allow to look at the scene to be photographed and the lens that captured the image to be projected on the film to make the picture.

This camera is very simple to make, but brings up a problem. Because the scene you see and the scene you shot are slightly displaced the picture might not be what you expect, heads sometimes are chopped off the picture, and things like that.

This problem is called Paralax Error, and becomes more of a trouble as the the object photographed is closer to the camera. For landscape photography might even be no problem at all, but for close ups is a huge deal.

SRL camera internal diagram

SLR cameras work in a very different way. The image you see on the viewfinder is the same that will be photographed. This is accomplished by reflecting the light inside the camera with a mirror and a pentaprism.

Do you remember refraction we talked about on the first post? When a light passes from a medium with higher refractive index to one with a lower one the refracted angle is wider than the incident angle.

Well, when the incident angle is wide enough the refraction angle will be 90 degrees, and what if the angle is even wider? Then an effect called Total Internal Refraction in which the surface behaves like a mirror. This is also how to Optical Fiber works.

Total Internal Refraction

A pentaprism is a pentagon extruded on the third dimension. Just like a cilinder but with a pentagonal base, instead of a circular one.

If a pentaprism is made with an adequate geometry and refractive index, it is possible to make light to enter it perpendicular to one face, reflect internally and leave it perpendicular to another face.

Then, the light enters the camera through the lens, reflects on a mirror put at 45 degrees, and enters the pentaprism perpendicularly (see the image for details).

When the shutter is pressed, the mirror is lifted, the curtain behind it is opened and the image is formed either on the film or the CCD.

Now a question, where is the image formed when the mirror is down? It can’t be directly on our eyes, mostly because the image must be formed at a certain distance that is the focal length, and would be very hard to make this to be our retina.

Actually the bottom face of the pentaprism is what is called Unpolished Glass. That means that is a rough, translucent surface, just like the translucent paper we used on the black box model. Because the surface is rough the image forms on it, but because it is translucent the light goes through, then looking from behind, through the pentaprism one can see the image formed on it.

All this is very nice, precise and sophisticated, but is not all rainbows. A camera like this is heavier, more expensive, more noisy and inherently more fragile because there are more things that can break.

Modern digital cameras are also categorized in compact cameras and Digital SLRs (known as DSLRs). While some compact digital cameras still have Direct Viewfinders, they can also use the sensor to capture the image and show it on a digital viewfinder that is not only more comfortable, but also don’t have paralax error. Because most people prefer use this digital viewfinder, more and more, digital compact cameras are coming without the direct viewfinder.

A DSLR camera works exactly like the traditional SLR film cameras, except they have a CCD instead on film. Because the sensor is exposed only at the very moment of taking the picture, there is no way for it to continuosly display the image on the LCD viewfinder, you have to use the eyepiece and.

A compact digital camera’s sensor is continuously exposed to the light that comes from the lens. When taking a picture, it just saves one of the many frames that it is continuously receiving. This is very handy, but by far is not as precise as the mechanical scheme of a SLR that can achieve speeds of 1/8000 of a second and faster.