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# Long and short focal lengths.

7 replies to this topic

### #1 Princess Leah

Princess Leah

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Posted 22 May 2024 - 08:49 AM

I thought this was a clever drawing by SteveNH on Cloudynights.

It is always helpful for me to have a diagram to help visualise the workings of a telescope.

I was wondering however why the arrows for short and long focal length don't extend right to the tree.

Perhaps Steve ran out of room? Or perhaps I don't quite understand this as well as I should.

### #2 WillR

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Posted 22 May 2024 - 09:12 AM

I don’t know but I have the same question. The diagram doesn’t really make sense to me.

I can see the vertical blue line to the right of the objective is where rays A and F cross in the short FL and where G and L cross in the long FL, but how are those angles determined? ( And what change letters from one to the next?)

Why are J and I coming out parallel in the second diagram. Coincidence?

It’s all a mystery to me.

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### #3 Overtime

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Posted 22 May 2024 - 09:39 AM

One question I had was the blue in the center a lens? I took it as such. Other parts of the pictured why mot it? This picture really gets me thinking.

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### #4 Princess Leah

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Posted 22 May 2024 - 09:47 AM

I should have included this. Hope that helps.

### #5 krishnak

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Posted 22 May 2024 - 10:35 AM

Hi, my understanding is that if the object is at infinity then the image is formed at the focal length (short focal length and long focal length). Since the tree is not at infinity its image is formed slightly behind the focal length. (Don't recollect exactly, so I could be wrong). Thats why short focal length and long focal length dont extend right to the tree image as the image is formed after the focal length distance. Thanks.

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### #6 Princess Leah

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Posted 22 May 2024 - 11:39 AM

Ah that's brilliant. Thanks for your help.

Apollo

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Posted 22 May 2024 - 12:26 PM

Wow! How is this the beginner forum?!?!?

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### #8 ChristianG

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Posted 23 May 2024 - 02:46 PM

Ha Ha!

But the formula is quite simple. If S (as in the drawing) is the distance between object and lens, and is larger than the focal length F (arrow on top of right portion of drawing), we can call the distance between the lens and the image S' and it needs to satisfy the lens formula:

1/S + 1/S' = 1/F   which can be rewritten 1/S' = 1/F - 1/S.

Adding and substracting fractions is not that difficult, in my humble opinion! But intuitively, one can understand how S' will be larger than F. One is subtracting something from 1/F in the above formula. So 1/S' will be smaller than 1/F. This means that S' will be larger than F. Cheers!

--Christian

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