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camera lens focal length ?


Gonzo
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I'm trying to get my head around this CCD pixel matching to your equipment and used the following tool:

http://astronomy.tools/calculators/ccd_suitability

I've picked the Canon 200mm f2.8 and to my surprised the focal length comes up as 348mm.

While for example my Megrez 72 is 432mm, f6.0 and aperture of 72mm "6x72=432", I honestly can't understand how this 348mm comes from for the Canon lens.

Anyone would like to school me on what is probably something basic.... lol

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The Canon 200mm lens has a focal length of 200mm but remember the aperture is variable when used with a DSLR.  No calculation required?  Are you trying to calculate arc seconds per pixel for your Atik 314L?

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Thanks for replying.

Yes, I' trying to calculate the the arc second per pixel if I was going to use a 200mm or 135mm lens in front the Atik. I'm planning to downsize my observatory (read: compact).

I get it that a 200mm lens has a focal length of 200mm. So I guess that 348mm in the tool comes from someone setting the aperture ring to 1.74 right?

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1 minute ago, Gonzo said:

Thanks for replying.

Yes, I' trying to calculate the the arc second per pixel if I was going to use a 200mm or 135mm lens in front the Atik. I'm planning to downsize my observatory (read: compact).

I get it that a 200mm lens has a focal length of 200mm. So I guess that 348mm in the tool comes from someone setting the aperture ring to 1.74 right?

but you cannot set that lens to f1.74, only f2.8 is the largest aperture.

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Just now, RichM63 said:

This hopefully does not look like a daft/thick reply, but you can't set the Canon 200mm f2.8 lens to more than f2.8 as this is fully open.

No it does'nt sound daft as it's correct.

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Cheers Mike i didn't think I had lost the plot. ;)

11 minutes ago, Gonzo said:

Thanks for replying.

Yes, I' trying to calculate the the arc second per pixel if I was going to use a 200mm or 135mm lens in front the Atik. I'm planning to downsize my observatory (read: compact).

I get it that a 200mm lens has a focal length of 200mm. So I guess that 348mm in the tool comes from someone setting the aperture ring to 1.74 right?

The 1.74 I guess is what I would term as the crop factor if we were talking DSLRs to obtain your quoted 348mm.

ie 200 x 1.74=348, which in normal photography gives an indication of focal length equivalents (somewhat false though).

eg with my 7D + 600mm (crop factor 1.6)  = 960mm ie the bird would look similar with a lens of 960mm on a full frame camera.

This doesn't really help you calculate arc seconds/pixel though.

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If you input 200mm focal length manually, it comes up as 6.65"/pixel for the 314L.

 

It looks to me like the Canon 200mm lens data was input incorrectly.

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So if I use 200mm lens, the only CCD that would be a "good" fit is the ZWO-ASI178MM/MC while the 314L wouldn't be that good.

I know this is all theory stuff, but how important is it to match the CCD as close as possible?

I have seen quite a few pictures (on this forum) taken with a 314L and a Takumar 200mm lens for example and they are amazing.

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41 minutes ago, RichM63 said:

Cheers Mike i didn't think I had lost the plot. ;)

The 1.74 I guess is what I would term as the crop factor if we were talking DSLRs to obtain your quoted 348mm.

ie 200 x 1.74=348, which in normal photography gives an indication of focal length equivalents (somewhat false though).

eg with my 7D + 600mm (crop factor 1.6)  = 960mm ie the bird would look similar with a lens of 960mm on a full frame camera.

This doesn't really help you calculate arc seconds/pixel though.

We must remember a 200mm lens is a 200mm lens no matter what camera it is connected to.

The only difference with each sensor is the FOV it records.

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3 hours ago, Gonzo said:

I've picked the Canon 200mm f2.8 and to my surprised the focal length comes up as 348mm.

Oops! Sorry :blush:

Cameras/scopes/lens details are submitted by users of astronomy.tools, we then check them and make them live. In this instance the data provided was wrong and we didn't notice. 

We will update the Canon 200mm f2.8 soon, probably tomorrow. 

HTH, 

Steve 

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