focal lengh 6000mm?? (1 Viewer)

kakapupu

Member

• Oct 4, 2006

• #1

i'm using CP 990 with Optolyth 80 TBS scope 20-60x.

camera gives 3x magnification 8-24mm (~35-105mm)
and eyepiece at 60x

so maximum focal length is 105mm x60 = 6300 mm ??

is it correct?

 

Recurvirostra

Just call me Avocet

• Oct 5, 2006

• #2

Sounds right to me. And much easier than carrying around a full-frame SLR with a 21 foot lens!

 

H

Hottentot

Fig

• Oct 5, 2006

• #3

Technically it should be referred to as effective focal length (EFL) as you obviously aren't carrying a 21 foot long lens around. This distinguishes it from the focal length quoted on prime lenses which is measured differently.

 

delia todd

If I said the wrong thing it was a Senior Moment

Staff member

Opus Editor

Supporter

• Oct 5, 2006

• #4

Hi Kakapupu

I see this is your first post, so may I extend a warm welcome on behalf of the Staff and Moderators.

You appear to have had your question answered already I'm pleased to say 'cos I wouldn't have had a clue

D

 

J

johnruss

Well-known member

• Oct 5, 2006

• #5

Yes you have all those mms for viewing but remember that with all those mms you have almost no depth of field and focus for photos is very hard at long distances.

 

kakapupu

Member

• Oct 7, 2006

• #6

tnx to todd....

 

westi

Well-known member

• Nov 20, 2006

• #7

johnruss said:

Yes you have all those mms for viewing but remember that with all those mms you have almost no depth of field and focus for photos is very hard at long distances.

Click to expand...

Hi,

Is there a magic formula to calculate the DOF? I assume the longer the EFL, the worse it gets.

BR
Ian

 

rezMole

Well-known member

• Nov 20, 2006

• #8

Erm, yes there is (are) a formula, but it's all very complicated. There's some stuff on wikipedia, but it's all too much for me to understand!

Link to Wikipedia - Depth of field

 

westi

Well-known member

• Nov 20, 2006

• #9

Errr - Yes - thanks for that rezMole!! :h?: Anyone know a good stress counsellor? :-C

I see you have a Tamron 200-500mm in your kit list. Ever had any problems with it. Mine recently seemed to disconnect within and would only focus (manually or AF) right out at 500mm. Its being fixed right now, but I wondered if the problems had been seen elsewhere!

BR
Ian