what lens for a canon 400d

so , just to be absolutely sure i understand this, the 400d comes with a standard kit lens thats has 18-55mm written on the side. This means that attaching this lens onto the 400d, with a FLM of 1.6 the lens becomes a 2.8 - 88mm lens. Is this correct?
How does this affect the apature size? I would imagine that a 35mm lens at 2.8f, would be a much larger apature than a digital camera lens at 2.8, as the digital camera has a smaller sensor. is there a formula here to work out the f value?

No - an 18-55mm lens becomes the equivalent of a 28.8 - 88mm, this then gives you the zoom effect. Aperature is a different story altogether. The C sensor in the 400D has no effect at all on Aperature.
Aperature (in F numbers) is a function of the size of the opening e.g. 58mm for a typical SLR lens and the focal length of the lens plus a couple of other bits and bobs that I haven't yet researched. Generally speaking the longer the focal length of the lens the bigger the F number will become (same as scopes). In order to get a F2.8 on a 500mm lens the bigger the business end will become, hence the price will increase dramatically.

Sigma 70-300mm APO DG (Macro) F4-5.6.

Yep I have one of these too and highly recommend it as being great value for money. Great for regular photography too.

You just can't beat this for the price.

Hi John,

Not quite correct. The 300D and 400D have the same C size sensor. The only difference is the number of pixels on the sensor - which has no effect on the image size - only the quality.

# pixels as well, but the physical size of the sensor is different.
EOS 300D 22.7 x 15.1mm CMOS 342.77 mm^2
EOS 350D/400D 22.2 x 14.8mm CMOS 328.56 mm^2
www.canon.co.uk/For_Home/Product_Finder/...igital_SLR/index.asp

There are some posts in the archive from when the 350D came out complaining about the smaller sensor size.

Cheers,
~Albert

so , just to be absolutely sure i understand this, the 400d comes with a standard kit lens thats has 18-55mm written on the side. This means that attaching this lens onto the 400d, with a FLM of 1.6 the lens becomes a 2.8 - 88mm lens. Is this correct?
How does this affect the apature size? I would imagine that a 35mm lens at 2.8f, would be a much larger apature than a digital camera lens at 2.8, as the digital camera has a smaller sensor. is there a formula here to work out the f value?

No - an 18-55mm lens becomes the equivalent of a 28.8 - 88mm, this then gives you the zoom effect. Aperature is a different story altogether. The C sensor in the 400D has no effect at all on Aperature.
Aperature (in F numbers) is a function of the size of the opening e.g. 58mm for a typical SLR lens and the focal length of the lens plus a couple of other bits and bobs that I haven't yet researched. Generally speaking the longer the focal length of the lens the bigger the F number will become (same as scopes). In order to get a F2.8 on a 500mm lens the bigger the business end will become, hence the price will increase dramatically.

sorry, that 2.8 was a mistype, should have been 28.8mm. i corrected it above.

Albert, thanks for picking me up on that detail.

I believe the 300D scale factor was 1.5 whereas the 350D and all modern C sensors are 1.6, is this correct? (I was speaking in general terms to try to explain the effect of sensor size on image scale and magnification).

Sigma 70-300mm APO DG (Macro) F4-5.6.

Yep I have one of these too and highly recommend it as being great value for money. Great for regular photography too.

You just can't beat this for the price.

Seriously though - go for one of these - you won't be dissapointed. Manual focussing is a little stiff (and you will be focussing manually), but the difference between it and the Canon equivalent 90-300mm EF is quite large, plus if your into Macro photography, there is no better lens in the price bracket.