what have i done!!??

[neil_ross]

oh dear! i bought a a canon 50mm f1.8 a week ago and the quality is seems so much better than my other 2 lenses that i just dont want to use anything else!

the problem is i can now see why its worth buying some decent L lenses!!

[chris-p]

I know the feeling. I bought a 35mm f/1.8 which is probably my most used lens. Makes you go off zoom lenses rather...!

[neil_ross]

Originally Posted by chris-p

I know the feeling. I bought a 35mm f/1.8 which is probably my most used lens. Makes you go off zoom lenses rather...!

not half! was at a wedding yesterday and even from across the room i was using! i might have to buy a few other sizes if they are all the same sort of price!

[chris-p]

If only they were all similar prices...

Depends on what you shoot as to what lenses to look at next I suppose. I went for a longer one (85mm) but you've got 50mm which is short-tele on an APS-C camera anyway. Sadly, no-one makes a real "normal" lens for APS-C cameras (on a Canon that would be 26.7mm). There are some good wideangle ones and the proper telephoto ones are stunning (but very very expensive)...

[neil_ross]

dam it!

this might be a stupid question but why is the quality so much better than something like my canon 18-55?

even when its not all the way down to 1.8, if i have both lenses with the same settings the prime lens seems to much clearer...

[chris-p]

Any lens has to be optimised for sharpness, CA, falloff and other geometric aberrations. If you make a lens of a single focal length you can correct and optimise it properly. Zoom lenses cannot be optimised across the whole zoom range (you can get pretty good ones but they're very expensive).

Thats why primes are generally sharper, brighter, more contrasty and generally better.

The same applies with the aperture. The f number is the size of the aperture at a specific focal length (f/2 @ 50mm means 50 divided by 2 which is 25mm - this means the aperture is 25mm across). Obviously, the physical size of the aperture changes with focal length (f/2 at 50mm = 25mm across, f/2 at 100mm = 50mm across) and so, as the focal length gets longer on a zoom lens, the aperture has to actually get physically larger to maintain the same f/ number. Fixed focal length means it's really easy to make much larger apertures...

[neil_ross]

ah i see!

that makes alot more sense now! might have to wonder out and use it a bit more today!!!

cheers chris!

[donoreo]

I love my Canon 50 mm f/1.8. Takes great pics. Trying only using your 50 for a while, it makes you think more about composition and things because you have to move, you cannot zoom.

[Dagwood]

Originally Posted by chris-p

The same applies with the aperture. The f number is the size of the aperture at a specific focal length (f/2 @ 50mm means 50 divided by 2 which is 25mm - this means the aperture is 25mm across). Obviously, the physical size of the aperture changes with focal length (f/2 at 50mm = 25mm across, f/2 at 100mm = 50mm across).

Chris, help me here,as I'm only using a compact and bridge cameras (Lumix TZ3 and FZ28)I have nothing like as large lens as you guys with DSLR's . My point is is that it seems an awfully big aperture to have a 50mm (2") opening or am I misunderstanding what you are saying? If I extrapolated this to, say, an f/2 at 150mm we are talking 3"! I'm obviously getting the wrong end of what you are saying, aren't I?
Dagwood

[chris-p]

OK, you've not misunderstood what I said but you've not spotted the difference with compact cameras... This may be a relatively long answer so buckle in!!

First, a definition: Focal length is the distance between the focal point in the lens (the bit where the glass elements cause the light rays to converge) and the imaging plane (the film or the sensor). It is measured in millimetres.
When you look through a camera you don't see "focal length" you see the angle of view - how wide or narrow that view is depends on the focal length. As focal length increases the field of view gets narrower and magnification increases (zooming in).

The problem is that for decades we've been used to talking about what we see through the viewfinder or on the LCD screen in terms of the focal length of the lens, rather than the field of view (measured in degrees). This is because it's easier than standardising which AoV you use (you can calculate the AoV horizontally, vertically or diagonally - see my tutorial here for more info).

This leads to confusion. In fact, you're probably confused already. To add to this confusion, when you alter the size of the imaging device (for example APS-C sensors are smaller than 35mm film) you change the AoV. Simply, a smaller sensor means a narrower AoV. What this means is that if you look through a 50mm lens on a film (or full frame) camera the angle of view is wider than what you would see with a 50mm lens on a smaller sensor camera.
This leads to the "focal length multipliers" that people talk about. Actually the focal length is the same - 50mm is 50mm (unless the whole universe gets smaller and defies the laws of physics) but, on a smaller sensor, the angle of view is narrower and the image is magnified compared to 35mm film.
To put it another way round, to get the same AoV on an APS-C sized camera as a 50mm lens gives on a full frame camera, you need a shorter focal length.

All this confuses people so, to try to simplify things, people often convert focal lengths back to "35mm equivalents". With DSLRS this is less common now as people are more used to the 1.5x and 1.6x "crop factors".
However, with compact cameras like yours this still happens because the sensors are so much smaller than a full frame one. For example, my Nikon D90 has a sensor that is 15.6mm high and 23.7mm wide. The sensor in your TZ3 is 4.26mm high and 5.68mm wide. To put it another way, the sensor in my D90 is 15.3x bigger than that in the TZ3.

Lets re-cap... Longer focal length = narrower AoV and increased magnification (zoomed in). Smaller sensor = narrower AoV at a specific focal length (more zoomed in).
So, because compact cameras have such small sensors you have to use really short focal lengths to get wider AoVs. What they do is replicate a particular field of view (in full frame terms) and quote the 35mm equivalent focal lengths.
Your TZ3 says 28mm - 280mm but thats in full frame equivalent terms. The real focal length is written on the lens. Around the rim of the lens it says "DC Vario-Elmar 1:3.3-4.9 4.6-46 ASPH". Thats the real detail. The actual real focal length of the lens is 4.6mm to 46mm. Because of the size of the sensor in the TZ3, 4.6mm has the same angle of view as a 28mm lens on a full frame camera.

Now the aperture thing makes sense...
f/ numbers are fractions (hence f/). It means "focal length divided by..." so f/2 means focal length divided by 2. f/2.8 means focal length divided by 2.8. They are also sometimes expressed as ratios (1:2 or 1:2.8 in this example).

Finally, we'll work one out. The actual, physical focal length of your TZ3 lens at it's widest AoV is 4.6mm. It's widest aperture at that focal length is f/3.3 (1:3.3). 4.6 divided by 3.3 = 1.39mm. So at its "28mm" setting with it's aperture wide open, the physical hole in the iris is 1.39mm across.