1000d focal length

[Mav359]

Hi guys

Can anyone tell me what the focal length of a Canon 1000d with a 18-55mm Lens on is.

The Camera is mounted on the OTA for me to do widefield shots with but i like to have all my sensor details in stellarium so i can use the reticules to plan shots ahead. I have actually had the details before but i re-formatted my PC & i can't find them now.

The dimeter of the Lens assembly is 35mm but i don't know how to work out the focal length with the lens thats on the camera.

Is it just 55mm (the lens zoomed out)????

Cheers

[RikM]

Yes. The focal length is 18mm at the wide setting and 55mm when zoomed in. The aperture in mm can be calculated from the f-stop setting and the focal length you zoom to. E.g. 55mm and f4 the aperture would be 13.75mm.

[Mav359]

So in this case the F ratio zzomed out to 18mm in bulb mode is 5.6

so 18mm / F5.6 = 3.21

sound right?

[RikM]

Yes, that's right

[ronin]

Not necessarily 3.21, they will restrict the light internally with field stops to achieve a sharper image.

Also there will be several lens and if you assume 2% loss at each air/glass interface the lens themselves will cause the f number to be slower.

You are most likely looking at f/4 to f/5 as a best speed.

What does the lens say on it?

[Psychobilly]

I have always used  Aperture = Focal Length/ Focal Ratio for generating the Reticules and the results are pretty close if you put the resulting images  though a plate solver...

I tend to make sure there some room around the target anyway as the edges of the image can be a bit ropey and can then be cropped off.. 

Peter...