[andy5452]

Hi.. Only a memeber for 12 hours, already got lots of info from the site!

I`m new to photography and a little confused. I have an S2 pro with a 28 - 70 Sigma zoom and a fixed 100mm Sigma Macro. (bought as a job lot from a friend). What confuses me is the way lens are rated on websites 70-200 f4.5-5.6 for example. I don`t know what it means! I`ve asked in Jessops etc. and told this would be a slow lens and would be better with a quicker lens (a lot more money) what do you get for your money?? The two lense I have go from f2.8 - f32. Are they "quick lenses?"

I`m basically looking for a decent zoom lense suitable for a intermediate hobby photographer ... Can any one offer any guidence??

Andy

[silkstone]

Let's take the example you mentioned... 70-200 f4.5-5.6

70mm is the minimum focal length, and 200mm is the maximum. This is the 'zoom range'.

f/4.5 is the maximum aperture at 70mm focal length. f/5.6 is the maximum aperture at 200mm.

The f-number is actually the ratio of aperture diameter to focal length (hence to be strictly correct it is written with a / ). So... at 70mm and f/4.5 the actual diameter of the iris is 70/4.5 = 15.6mm. At 200mm and f/5.6 the iris diameter is 200/5.6 = 35.7mm.

That's why long lenses with large maximum apertures are large and expensive - for example, a 300mm f/2.8 lens would have a maximum iris diameter of 300/2.8 = 107mm.

'Fast' lenses are those with large maximum apertures (small f-numbers) - but that's all relative to focal length. The Canon 50mm f/1.8 is pretty fast (although there's also an f/1.4 version which costs a lot more), but as you go to longer focal lengths it isn't practicable to have huge apertures. A maximum of f/2.8 is fast for zooms and most prime lenses.

Remember that this figure is only the maximum, and that most lenses give better results at one or two stops down from wide open. Also the depth of field - the range of distances that appear to be in focus - varies with aperture and focal length. Large apertures and long lenses give a very small DOF, which is fine if you want the background to be blurred but not so good if you'd like everything to be in focus.

HTH.

[andy5452]

Thanks, So as a newboy to all this I was considering buting a "Nikon AF Nikkor 70-300mm f/4-5.6 G Lens" as an introduction into the world of Zooms (£80) (If I undestand you correctly) That being max app at 70mm of F4 and a max app at 300mm 5.6?? That now seems a very restrictive amount of control over the depth of field, but quite a large amount of zoom?

Andy

[silkstone]

The f/4-5.6 is just the *maximum* aperture at the extremes of focal length. The lens will probably stop down to f/22 (???) throughout the zoom range, which will give a much greater depth of field.

Usually these lenses perform best at around f/8 or f/11.

[orangepeel]

Nikon AF Nikkor 70-300mm f/4-5.6 G Lens: -
f/4-32 @ 70mm
f/5.6-45 @ 300mm

[andy5452]

Brilliant. Thanks.

Next question then (if I may) I here a lot about "no fungus" when looking at various used lenses. Fungus, well I know what that is. Can I assume if a lens has fungus on it, then it`s uselss??

Andy

[Dave]

Hi Andy

In vast majority of questions I would walk away from such a lens. Plenty more fish in the sea as they say . I have heard of people being offered a lens for a song in such a condition & having a go at repairing themselves......not for the faint hearted I would expect

Have you been offered a lens in such a state ?

BTW glad you are finding the masses of info on this site of use All down to the excellent member base

[andy5452]

Yes such a lens is on offer, though the fungus is on the inner lens and appears not to be noticible on the final image.

Andy

[silkstone]

I wouldn't touch it TBH. Anything like that will affect the final image, and fungus indicates that the lens has been stored in damp conditions.

[andy5452]

Ok then. Thanks for everybodies help on this one.
Andy