what lens for a canon 400d

for astrophotography, do you want high f values ( smaller apature) or low f Values? I know higher f value means smaller apature and better dof, but less light. what is generally the rule of thumb for astronomy?

Smaller F number means bigger aperature - hence more photons. The same rules apply as for scopes. With lenses get a smaller F number, this will mean shorter exposure time is necessary hence reducing tracking errors.

but what is written on the lens is the 35mm equivalent focal length, so the FLM is taken into account? eg, i currently have a panasonic with a 36 -432mm lens. I know the ccd is only tiny so the actual focal length is 6-72mm, but as they write 36-432 on the side of the lens, the image is the same size as an image from a 35mm with a 36-432mm lens, therefore any crop(magnification) caused by a small sensor is taken into account in this number.

sorry for all the questions,but this is how ive understood this for a few years, so i want to be sure i understand this correctly

but what is written on the lens is the 35mm equivalent focal length, so the FLM is taken into account? eg, i currently have a panasonic with a 36 -432mm lens. I know the ccd is only tiny so the actual focal length is 6-72mm, but as they write 36-432 on the side of the lens, the image is the same size as an image from a 35mm with a 36-432mm lens, therefore any crop(magnification) caused by a small sensor is taken into account in this number.

sorry for all the questions,but this is how ive understood this for a few years, so i want to be sure i understand this correctly

The panasonic lens might be an exception to the rule. Any lenses I've come across have the focal length stated as they would be for a 35mm slide. You need to do the conversion.

It may seem strange but it makes sense. E.g. the 300D and 400D have different sensor sizes but the same lenses will work with both. So you can't take the sensor size into account in the focal length printed on the lens.

How large a focal length you want depends on what you want to do with it. Though bear in mind if you are going over 500mm you are heading into expensive lenses (especially for low f numbers and decent optics) and might be better off with a small scope.

I'm not familiar with the Panasonic you are using (presumably a compact?) but think of it this way, in the SLR world all lenses focal lenghts are written verbatim - i.e. 55mm is 55mm. What you need to take into account is the sensor size. If you are using a full frame sensor like a Canon 1D MkIII, then you will get a true 55mm focal length. If however your SLR has a C sensor (400D et al.) then you need to multiply the focal length by 1.6 to get the true image scale.

cool. thanks guys. my panasonic isnt a dslr, and more of a very fancy point and shoot, and from what i know, most point and shoot cameras give the 35mm equivalent so i had no idea SLR's used a different convention.

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 28.8 - 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?

It may seem strange but it makes sense. E.g. the 300D and 400D have different sensor sizes but the same lenses will work with both. So you can't take the sensor size into account in the focal length printed on the lens.

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.