F ratio

[Scooot]

My dob has an f ratio of 4.8. So when I attach my DSLR to its focuser am I taking images at f4.8? In other words the shutter speed would be the same as if I set the AV to 4.8 with just the camera. (That is if I could set it at 4.8).

I hadn't thought about it before until I took some pics of the moon in manual mode with my camera on the dob.

Thanks for any help.

[Alien 13]

Yes thats how it works the scope behaves the same as F4.8 lens would regarding exposure length but with a much longer focal length.

I do use a chipped T adapter on my camera that has focus confirm (the beep when focused) and allows the F ratio of the scope to be dialed into the camera in manual or AV mode its quite useful for daytime photography.

Alan

[Scooot]

Yes thats how it works the scope behaves the same as F4.8 lens would regarding exposure length but with a much longer focal length.

I do use a chipped T adapter on my camera that has focus confirm (the beep when focused) and allows the F ratio of the scope to be dialed into the camera in manual or AV mode its quite useful for daytime photography.

Alan

Thanks Alan,

I was thinking of taking some of the moon rising with some of the horizon in view and wondered what the depth of field would be like. So I guess at f4.8 I won't have much of the foreground in focus but it might not matter much as I suspect it will be silhouetted.

[Alien 13]

The depth of field thing is the limiting factor of scopes on foregrounds and daytime objects however with something as bright as the moon you can use an aperature mask on the front of a scope to increase the F ratio (some end caps have a hole for this purpose) always worth a try.

Alan

[JRWASTRO]

Thanks Alan,

I was thinking of taking some of the moon rising with some of the horizon in view and wondered what the depth of field would be like. So I guess at f4.8 I won't have much of the foreground in focus but it might not matter much as I suspect it will be silhouetted.

Greetings Richard,

For a 250mm F4.8 lens the hyperfocal distance ( based on 8.2 micro-metre pixels... My 5D) will be about 18.5 km. For CCD's with smaller pixels the distance will be proportionally closer. Eg. With 4.0 micro-metre pixels this distance will be about 9.3 km. So objects closer than 9 km ( smaller pixels) will start getting blurred. So most of the " foreground" will be blurred. but not to worry as the moon on the horizon is so affected by seeing and the atmosphere ( it looks yellow!) that matters pertaining to sharp focus are academic so much so that I would not get too "stressed out" on the matter of sharp focus.

Yes, Alans' suggestion is very good. Stop down the lens to f/16 and the dof will drop to about 2.5 km to infinity ( 4.1 micron-pixel). Focus at infinity.

Jeremy.

[Scooot]

Greetings Richard,

For a 250mm F4.8 lens the hyperfocal distance ( based on 8.2 micro-metre pixels... My 5D) will be about 18.5 km. For CCD's with smaller pixels the distance will be proportionally closer. Eg. With 4.0 micro-metre pixels this distance will be about 9.3 km. So objects closer than 9 km ( smaller pixels) will start getting blurred. So most of the " foreground" will be blurred. but not to worry as the moon on the horizon is so affected by seeing and the atmosphere ( it looks yellow!) that matters pertaining to sharp focus are academic so much so that I would not get too "stressed out" on the matter of sharp focus.

Yes, Alans' suggestion is very good. Stop down the lens to f/16 and the dof will drop to about 2.5 km to infinity ( 4.1 micron-pixel). Focus at infinity.

Jeremy.

Thank you Jeremy, really interesting reply.

As a result I did a search for hyperfocal calculator and found this depth of field calculator which I think is very good.

http://www.dofmaster.com/dofjs.html

If I stopped it down to f10 the distance of the near limit Depth of Field comes down to 7.52 km from 15.7km. I'm not going to bother but its good information, all part of the learning process. -

[JRWASTRO]

Glad to be of service Richard.

I do 99.9% of calculations on my trusty HP50G calculator and if the calculator objects ( takes more than 10sec to do a calculation then I use MATLAB on my Win 7 machine. I make small reference tables to suite my lenses. I can provide an example if you would like.

OK, I have a couple of references for you.

1)

I do not subscribe to the use of Hyperfocal distance to maximise the DoF but I am swayed by the philosophy and arguments of Mr. Harold Merklinger ( Photographer Par Excellence) :

http://www.trenholm.org/hmmerk/DOFR.html

2)

Mr. norman Korens' excellent tutorial:

http://www.normankoren.com/Tutorials/MTF6.html

As the yanks say " Enjoy",

Jeremy.

[Alien 13]

Glad to be of service Richard.

I do 99.9% of calculations on my trusty HP50G calculator and if the calculator objects ( takes more than 10sec to do a calculation then I use MATLAB on my Win 7 machine. I make small reference tables to suite my lenses. I can provide an example if you would like.

OK, I have a couple of references for you.

1)

I do not subscribe to the use of Hyperfocal distance to maximise the DoF but I am swayed by the philosophy and arguments of Mr. Harold Merklinger ( Photographer Par Excellence) :

http://www.trenholm.org/hmmerk/DOFR.html

2)

Mr. norman Korens' excellent tutorial:

http://www.normankoren.com/Tutorials/MTF6.html

As the yanks say " Enjoy",

Jeremy.

Great stuff thanks.

Alan

[Charic]

Oh no! more studying to do?

Iv'e never taken much concern in my telescopes focal ratio for observational use, and do understand that the f/ratio is the result of dividing the focal length by the aperture so f/5.91 rounded to f/6 in my case, and understand that this is a representation of the "speed" of my telescope. Now my thinking was, there is more 'visible' aperture available from my telescope at prime focal, than there would be just using a Nikon prime lense set to f/6 and the fact that I cant change or set any focal ratio settings on the DSLR with the lens off, I've never bothered to study this any further?