Fixed-lens digital camera question

[Moose.]

Probably going to sound like a fool here, but what exactly is a fixed-lens digital camera? Are they just a digital SLRs with a lens fixed at say 50mm or is it another word for point-and-shoot cameras?

The reason I ask is that in a book I'm reading, "Understanding Exposure" (as recommended by you guys here ), it mentions that the aperture values of fixed-lens digital cameras are 4 times smaller than those of the equivalent SLR. For example if you select f/4 on a fixed-lens digital camera, you are actually shooting using an equivalent aperture of f/16 on an SLR. And f/5.6 would be f/22 etc.

Is my Canon S90 a fixed-lens digital camera (it has a 28mm to 105mm zoom lens)? If so, what does that mean in the real world when taking photos?

Anyone able to clear this up for me a bit please

[//dean]

My understanding of it is that a fixed lens means that you can't change it, ie it's fixed to the body. The lens could be, and probably is a zoom.

[ivaylo]

It's a bit misleading as being 4 times smaller is only as far as depth of field is concerned, not exposure. F/4 on S90 will give you the same exposure as F/4 on a 1Ds but rather different depth of field.

[itsnotarace]

It's all down to sensor size, this article explains it better than I can

http://www.cambridgeincolour.com/tutorials/digital...

Edited by itsnotarace on Thursday 15th April 08:53

[K12beano]

As said above really.

It's a bit of a complicated way of saying things, so I don't think you're being a "fool" at all.

Yes, you appear to have what the text means - a "fixed lens".

Yes, the text is correct in surmising, through the fact that the actual sensor size in such cameras is significantly smaller than most slr, or "non-fixed lens", cameras, that the aperture is "smaller".

Physically the size of the lens and the aperture can be smaller for the smaller sensor, and yet still (because they are relative terms) the "f-number" and field of view can be the same.

And yes, the main point to drive out of this is that at f/4 when covering a BIG sensor, whilst the exposure will be the same as for a small sensor, two factors will come into play. the focal length of the lens, and then by implication the depth of field on offer. So, if you want the camera to do the shallow depth of field trick you are at a massive disadvantage on a small sensor. (So do it in post processing!)

[Moose.]

Ah! Makes a bit more sense now. Thanks folks

I went out earlier and experimented a little with different aperture settings on the same subject:

http://www.flickr.com/photos/indo_pilot/sets/72157...

I see what you mean about achieving those photos using the shallow depth of field trick. I can get photos which do have this look but I've seen plenty of others where it's much more pronounced. I guess SLRs still have a place at the moment

[K12beano]

Here you go - the power of post-processing......