Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1 | 2 | (show all)
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
F-number vs "fast" and "slow"
      #5734071 - 03/15/13 12:46 PM

I was researching the concept of f-numbers and I now understand what it means (ratio, focal length over aperture).

I think I also understand why my Celestron 6SE has a relatively short tube, but a 1.5m focal length... because the focal length is the distance the light travels inside the telescope (corrector to primary to secondary and back to the focal plane in the case of this SCT)?

Now, is the reason why they call a lens with a low f-number "fast" and one with a high number "slow" is because a fast lens can gather more light in a shorter period of time and produce the same image faster than a lens with a high f-number?

Juan

Edited by FlyBD5 (03/15/13 03:23 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
dan_h
Carpal Tunnel


Reged: 12/10/07

Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5734103 - 03/15/13 01:03 PM

Basically that is it. It is a leftover from the days of film photography.

dan


Post Extras: Print Post   Remind Me!   Notify Moderator  
BillB9430
super member
*****

Reged: 12/02/06

Loc: Illinois
Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5734107 - 03/15/13 01:06 PM

Juan, I believe your reasoning is correct. Historically, this usage likely came from camera lenses, where a small f/number lens required a "fast" shutter speed and a large f/number lens needed a "slow" shutter speed. Hence "fast" lens and "slow" lens". That is just another way to state the same reasoning you gave, however. - Bill

Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: BillB9430]
      #5734140 - 03/15/13 01:22 PM

Ok, guys, thanks. Slowly but surely I am building up the info required to derive knowledge and some day, wisdom.

So, if that's the case, how in the wide, wide world of sports does a focal reducer/corrector turn an f/10 lens into an f/6.3 or f/3.0 lens?


Post Extras: Print Post   Remind Me!   Notify Moderator  
ed_turco
Pooh-Bah
*****

Reged: 08/29/09

Loc: Lincoln, RI
Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5734284 - 03/15/13 02:59 PM

This only works for photography of extended objects. The fast lens doesn't gather light more quickly, but packs the light into a smaller area with the concurrent increased intensity.

By the way, for star images alone, the light intensity is the same for an x-magnitude star in either a fast or slow telescope. But put a nebulosity, (an extended object), in the picture and you want the faster system to bring it out on film, digicam, or whatever.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: F-number vs "fast" and "slow" new [Re: ed_turco]
      #5734300 - 03/15/13 03:06 PM

A "positive" lens produces a real focal length, meaning it forms a real (erect or inverted) image, one that can be seen on a piece of paper or a camera focal plane.

Adding another positive lens in the path shortens the focal length of the first lens. Thus the "system" focal length is shortened by the additional positive lens.

A 6" f/10 telescope has a (6)(10)=60" focal length. Adding a f/6.3 focal reducer (also called telecompressor) into the path reduces the system focal length to (6)(6.3)=37.8" focal length.

Helpful?
Mike


Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: ed_turco]
      #5734341 - 03/15/13 03:28 PM

Quote:

This only works for photography of extended objects. The fast lens doesn't gather light more quickly, but packs the light into a smaller area with the concurrent increased intensity.

By the way, for star images alone, the light intensity is the same for an x-magnitude star in either a fast or slow telescope. But put a nebulosity, (an extended object), in the picture and you want the faster system to bring it out on film, digicam, or whatever.




Hmm. But in a camera, the focal plane has to remain the same, on the sensor or the film. If a fast lens means you can use a faster shutter, wouldn't that mean the image would be smaller?

And did you get someone to design this and this just for you? LOL!

Juan


Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: Mike I. Jones]
      #5734350 - 03/15/13 03:30 PM

Quote:

A "positive" lens produces a real focal length, meaning it forms a real (erect or inverted) image, one that can be seen on a piece of paper for a camera focal plane.




Ok, so far so good.

Quote:

Adding another positive lens in the path shortens the focal length of the first lens. Thus the "system" focal length is shortened by the additional positive lens.

A 6" f/10 telescope has a (6)(10)=60" focal length. Adding a f/6.3 focal reducer (also called telecompressor) into the path reduces the system focal length to (6)(6.3)=37.8" focal length.

Helpful?
Mike




That's where you lost me. Is it as simple as multiplying the f-number?

Juan

Edited by FlyBD5 (03/15/13 03:30 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5734367 - 03/15/13 03:38 PM

Ah, I just looked at a pic showing the workings of the reducer and now I think I understand.



I still don't get how you can lop off almost 23 inches off the focal length and still be able to focus. But that's probably because this image is not to true scale.

Juan

Edited by FlyBD5 (03/15/13 03:39 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5734435 - 03/15/13 04:20 PM

Quote:

I think I also understand why my Celestron 6SE has a relatively short tube, but a 1.5m focal length... because the focal length is the distance the light travels inside the telescope (corrector to primary to secondary and back to the focal plane in the case of this SCT)




Juan:

With a simple telescope like a basic refractor or a Newtonian, the focal length is the distance from the mirror or objective to the focal plane. The focal ratio also represents and angle.

But with a compound telescope like an SCT there are multiple elements and so it is probably better to think in terms of "effective focal length" or "effective focal ratio." Your 6 inch F/10 consists of a corrector plate, a primary mirror and a secondary mirror. The primary mirror is about F/2, the secondary mirror is convex and it magnifies the image about 5X so the result is an effective focal ratio of about F/10.

Unlike the Newtonian or refractor where the distance from objective to the focal plane is essentially the focal length, compound scopes are a combination of two curved mirrors that each make their own contribution. The actual magnification, the actual effective focal length of an SCT depends on the distance between the mirrors. Since SCTs generally focus by moving the primary mirror, the actual effective focal length varies depending on the relationship between those two mirrors.

In terms of the focal reducer, the focal ratio only changes after the reducer so if the reducer is say an 0.6 in an F/10 scope and is placed 100mm inside the focal plane at F/10, the focal plane will be 100mm *.6 = 60mm from the focal reducer and therefore 40mm inside the original focal plane.

With an SCT, this is easily managed because of the ease of moving the F/2 primary mirror. With refractors and Newtonians, this can be a problem.

I hope this helps.

Jon Isaacs


Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: Jon Isaacs]
      #5734622 - 03/15/13 06:15 PM

Quote:

Quote:

I think I also understand why my Celestron 6SE has a relatively short tube, but a 1.5m focal length... because the focal length is the distance the light travels inside the telescope (corrector to primary to secondary and back to the focal plane in the case of this SCT)




Juan:

With a simple telescope like a basic refractor or a Newtonian, the focal length is the distance from the mirror or objective to the focal plane. The focal ratio also represents and angle.

But with a compound telescope like an SCT there are multiple elements and so it is probably better to think in terms of "effective focal length" or "effective focal ratio." Your 6 inch F/10 consists of a corrector plate, a primary mirror and a secondary mirror. The primary mirror is about F/2, the secondary mirror is convex and it magnifies the image about 5X so the result is an effective focal ratio of about F/10.

Unlike the Newtonian or refractor where the distance from objective to the focal plane is essentially the focal length, compound scopes are a combination of two curved mirrors that each make their own contribution. The actual magnification, the actual effective focal length of an SCT depends on the distance between the mirrors. Since SCTs generally focus by moving the primary mirror, the actual effective focal length varies depending on the relationship between those two mirrors.

In terms of the focal reducer, the focal ratio only changes after the reducer so if the reducer is say an 0.6 in an F/10 scope and is placed 100mm inside the focal plane at F/10, the focal plane will be 100mm *.6 = 60mm from the focal reducer and therefore 40mm inside the original focal plane.

With an SCT, this is easily managed because of the ease of moving the F/2 primary mirror. With refractors and Newtonians, this can be a problem.

I hope this helps.

Jon Isaacs




That was a GREAT explanation. If I got this correctly, the focal reducer only acts on the final path of the light from the secondary to the focal plane, not on the other two (corrector to primary, or primary to secondary). Given the three segments and the relationship between them to come up with the f/10, all the reducer has to do is manipulate the final segment to affect the entire f-number of the scope as a whole and achieve its intended goal. Did I get this right?

Now, that said, what are the tradeoffs of using a focal reducer? There has to be a catch to all this, it can't all be good news.

Juan


Post Extras: Print Post   Remind Me!   Notify Moderator  
highfnum
Carpal Tunnel
*****

Reged: 09/06/06

Loc: NE USA
Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5734629 - 03/15/13 06:19 PM

I built 4 inch f/120
Hence my handle name


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: F-number vs "fast" and "slow" new [Re: highfnum]
      #5734759 - 03/15/13 07:30 PM

Juan,
Indeed, there is usually some trade-off when attaching a focal reducer. This can be one or more of; stronger vignetting, stronger field curvature, worse off-axis aberrations or smaller fully-illuminated field.

Note that the faster system has a more steeply converging light cone.

You can have two lenses of the same focal length, and hence identical image scale. But make one of larger diameter and it becomes faster. The larger aperture collects more light for the focal length and so delivers a brighter image.

Note also that when you add an eyepiece, the f/ratio itself does not control image surface brightness; the exit pupil does. That's a whole other topic..


Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: GlennLeDrew]
      #5735037 - 03/15/13 09:40 PM

Quote:

Juan,
Indeed, there is usually some trade-off when attaching a focal reducer. This can be one or more of; stronger vignetting, stronger field curvature, worse off-axis aberrations or smaller fully-illuminated field.




(dang, here we go again... google v-i-g-n-e-t-t-i-n-g... Oh. Softening around the edges. Ok. Off... axis... aberration... coma. Ok. There goes my english out the door...) Ok, got it.

Quote:

Note that the faster system has a more steeply converging light cone.

You can have two lenses of the same focal length, and hence identical image scale. But make one of larger diameter and becomes faster. The larger aperture collects more light fir the focal length and so delivers a brighter image.




Ok. I actually got it without having to look up anything in google.

Quote:

Note also that when you add an eyepiece, the f/ratio itself does not control image surface brightness; the exit pupil does. That's a whe other topic..




And I looked that up too! Thanks!

Juan


Post Extras: Print Post   Remind Me!   Notify Moderator  
leonard
scholastic sledgehammer
*****

Reged: 10/19/07

Loc: West Virginia
Re: F-number vs "fast" and "slow" new [Re: FlyBD5]
      #5735073 - 03/15/13 09:56 PM

Hello ,

Interesting subject .

>>>> Now, is the reason why they call a lens with a low f-number "fast" and one with a high number "slow" is because a fast lens can gather more light in a shorter period of time and produce the same image faster than a lens with a high f-number? <<<<<<<<<

Is this thinking correct ? :

If you have two refractors say both 4 inchs in dia. , one at f20 the other at f10 , the f20 is called slower because in photography the image scale will be larger than the f 10 because the f 10 has steeper curves to focus the light closer to it and thus has a smaller image scale .
Is this correct ?????????

Leonard


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: F-number vs "fast" and "slow" new [Re: leonard]
      #5735096 - 03/15/13 10:09 PM

Leonard,
Image scale has nothing to do with 'speed', only f/ratio. These terms come from the ancient days of film photography. A 'slow' lens is one which builds up an image in a film emulsion more slowly, and a 'fast' lens which builds up an image more quickly.

A 28mm f/2.8 and a 300mm f/2.8, even though they form images of greatly differing scale, are equally fast.


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: F-number vs "fast" and "slow" new [Re: GlennLeDrew]
      #5735124 - 03/15/13 10:19 PM

Juan, et al,
Here's another perspective on lens speed, and its function in image surface brightness.

Imagine you're a very tiny bug sitting an a camera's detector. Looking up, you see the aperture as an illuminated circle. The larger this circle, the more light illuminating you. And the size of this light depends *only* on the f/ratio.

More specifically, the illumination received scales directly as the solid angle of the aperture. Solid angle is area, and is much like square degrees, except it's a spherical measure, in steradians. For example, an f/2.8 aperture has a solid angle twice that of an f/4 aperture, and so delivers twice as much light to any given image point.

There are finer points to raise regarding the divergence between geometric f/ratio and aperture solid angle when aperture ratios get *really* fast, but for the purposes of the usual range of lens speed, where faster than f/1 is very rare, we can neglect this.


Post Extras: Print Post   Remind Me!   Notify Moderator  
FlyBD5
sage
*****

Reged: 02/12/13

Loc: Boston MA
Re: F-number vs "fast" and "slow" new [Re: GlennLeDrew]
      #5735395 - 03/16/13 12:41 AM

Quote:

Juan, et al,
Here's another perspective on lens speed, and its function in image surface brightness.

Imagine you're a very tiny bug sitting an a camera's detector. Looking up, you see the aperture as an illuminated circle. The larger this circle, the more light illuminating you. And the size of this light depends *only* on the f/ratio.

More specifically, the illumination received scales directly as the solid angle of the aperture. Solid angle is area, and is much like square degrees, except it's a spherical measure, in steradians. For example, an f/2.8 aperture has a solid angle twice that of an f/4 aperture, and so delivers twice as much light to any given image point.

There are finer points to raise regarding the divergence between geometric f/ratio and aperture solid angle when aperture ratios get *really* fast, but for the purposes of the usual range of lens speed, where faster than f/1 is very rare, we can neglect this.




Well, considering that in the grand scale of things we -are- bugs, I guess this pretty much makes sense.

Oh, heck, it's time for some homebrew. 9.5 Belgian Ale or 9.2% IPA? Decisions, decisions...


Post Extras: Print Post   Remind Me!   Notify Moderator  
leonard
scholastic sledgehammer
*****

Reged: 10/19/07

Loc: West Virginia
Re: F-number vs "fast" and "slow" new [Re: GlennLeDrew]
      #5735482 - 03/16/13 03:06 AM

Hi Glen ,

If I used the 4 inch lenes I posted one F 10 the other F 20 to take a pic. of a tree I'm guessing the F 10 lens would take the image to the desired film exposer faster than the F 20 lens would . If this is true is it because :

1 - the 4 inch F 10 lens lets in more light than the 4 inch F 20 . I don't see how that would work.
2 - Or does the F 10 lens record more of the space in front of it much like if used with an eyepiece of the same FL lets say 20mm and the F 10 lens shows a wider field of view than the F 20 lens would .
So would this mean the F 10 lens lets in more light because it sees more space thus recording faster ?
And if so would the image scale at F 10 be smaller per unit area ( The tree would record smaller ) or would the image scales be the same on each film ?

I ask this because I know little about using a telescope for taking pics.

Thnaks for any clairification , Leonard


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: F-number vs "fast" and "slow" new [Re: leonard]
      #5735541 - 03/16/13 05:10 AM

Quote:

1 - the 4 inch F 10 lens lets in more light than the 4 inch F 20 . I don't see how that would work.




Leonard:

Astronomers think in terms of aperture (objective diameter) and focal length because from that we can compute the magnification and the exit pupil, basically all we need to know.

Photographers think in terms of focal length and focal ratio and with good reason.

To a photographer, 100mm F/10 is a lens with a 100mm focal length and 10mm diameter lens.

So, compare the exposure time of a 1000mm F/10 lens to a that of 1000mm F/5 lens. Both have the same image scale but the F/10 lens is 100mm in diameter and the F/5 lens is 200mm in diameter. It captures 4 times the light so it only takes 1/4 the time to expose the film.

The exposure is directly related to the focal ratio and is independent of the focal length. The image scale is directly related to the focal length and independent of the focal ratio.

I hope this helps

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | (show all)


Extra information
15 registered and 22 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1301

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics