Nikon D50 SLR Digital Camera w/ 28-80mm

[Franglais]

Can you explain this just a little bit more?

Thanks Charles.

Well - I just checked out the price of Nikor lenses - GULP! I'm going to have to be satisfied with my current zoom!

This one comes up all the time.

The digital sensor in almost all Digital SLR's is the same shape as a 24x36mm film negative but it's smaller - about the same size as an APS negative. The lens draws the same 24x36mm image but the sensor only captures a part of it. It's like the sensor cropped the edges off the image, giving you a view which is more telephoto than what you would expect with the lens.

People talk about the "crop factor" which allows you to imagine what the view you get with a given lens is going to be in 24x36 terms. You multiply the real focal length of the lens by the crop factor to get the 24x36 equivalent.

For instance, the 18-70mm kit lens that you get with the D70 - multiply by 1.5 and you get 27-105mm. In other words the classic 18-70 on a Nikon DSLR is the equivalent of the 28-105mm lens that is a classic on film SLR's. And your 70-300 becomes the equivalent of a 105-450mm.

The true focal length doesn't change, and neither does the image. A DSLR just leaves out the edges of the image.

So why don't DSLR manufacturers just make a 24x36mm sensor so that the image looks the same on digital and film? Several reasons:

1. It cost several times more to make a 24x36 sensor compared with an APS-sized sensor
2. Light falloff. Unfortunately current sensor designs with microlenses are less efficient at capturing light arriving at an angle than head-on, and this is just what happens at the edges of a 24x36 sensor
3. Chromatic aberration. If the lens splits white light into a rainbow of colours then this results in coloured fringes and loss of definition on a digital sensor. This is just what tends to happen at the edges of a lens. If you just use the central part of the image you tend to avoid it.

The 24x36 sensor does have several advantages. It's bigger than an APS sensor so either it can have more photosensitive sites (pixels) or else the photosensitive sites are bigger and collect more light. This means that at a given ISO setting the electronic grain (noise) is lower. The transition in the highlights from exposed to overexposed is also softer and more natural. Canon seem to be absolutely convinced that it's the future...

Charles

[OBie]

So I'm not really gaining any zoom, it just looks that way ;)

Great explanation Charles. The camera store had explained it somewhat, but being the first time I was hearing it, it went into my brain and right back out. Thank you for being so clear.