F-Ratio and brightness

[Ags]

Olly the tragedy is that I am a 'professional' technical writer! My explanations should be more effective than they are, clearly...

[Bazzaar]

No Tim, only aperture determines how much light enters a scope.

The fact that a scope has a greater field of view makes no difference.

I can see how coming from a photography background this is what you might think so let my try and explain,

Terrestrial photography involves many radiant sources nearby scattering light at all angles. The whole field of view is full of light. So increasing field of view can capture more light.

In astronomy all objects are virtually at infinite distance, infinity for focus. Star can be considered point sources.This means all photons are travelling parallel to each other. Not diverging, not converging. So field of view makes no difference to the light gathered, only the aperture.

Also dont get exposure meter light levels, a total of all the light in a terrestrial picture, and brightness of a specific object in the night sky to be the same thing. They are certainly not.

Barry

[Bazzaar]

A - Focal length = 800mm, Diameter = 200mm, F-Number = f/4

B - Focal length = 1600mm, Diameter = 200mm, F-Number = f/8

Note that:

1) the diameters are the same, the f-numbers are different

2) the field of view of telescope A is twice as big as telescope B.

3) telescope A collects four times as much light as telescope B (it's 2 stops faster) <== This is wrong

With telescope B, the user only has to magnify by 50x (using a 32mm eyepiece) to see the image at the same size. <==This is wrong You are tring to say 50 times magnification = 100 times magnification???

And the same mistake made in Proof part 2

[Bazzaar]

Yes, but this lacks rigor. What do you mean by magnification?

If you mean the ratio of the projected image on your retina compared to the size as seen by the naked eye then this holds true (as I've now managed to prove to myself a couple of posts back).

If you mean the specific magnification of the chosen eyepiece, then this statement is false.

Tim

Eyepieces dont have a magnification, they have a focal length, magnification is the product of eyepiece f/l and objective f/l. The definition of magnification is the apparent increase in size of an object compared to that with the naked eye.

[GazOC]

I've read in several places that the only the aperture controls brightness for visual observing, and not once have I seen this qualified with "for a given magnification"

I take it you're not bothering to read my posts then?

(I've made the point in both threads you've started on this subject)

[Moonshane]

my take on this as a strict layman would be as follows:

photography (not astro) works by using a iris in the lens which 'stops down' aperture to a certain size so for arguments' sake f8 (say 4mm) = twice as small a hole as f6 (say 8mm). f6 will have a light gathering area of 4x4x3.142 = 50mm sq whereas f8 will have a light gathering area of 2x2x3.142 = 12.5mm sq. this is regardless of the lens you use. if you use a 50mm lens (say = x1 magnification) the image will be at maximum brightness. if you use a 300mm lens (= x6 mag) then the image is less bright at the same shutter speed as the available light has been spread over a wider area. perhaps though the apertures the irises in different focal length lenses are variable (between lenses) and not fixed like a telescope?

I think this logic also works with telescopes.

e.g. for an 8" scope (aperture) the light gathering area is 4x4x3.142 = 50 inches sq and for a 12" scope = 113 inches square.

if both scopes are f5 for astro photography, the focal length of the 8" is 1000mm and for the 12" it's 1500mm. if using prime focus photography the 8" covers a wider field of view than the 12" due to the focal lengths. to get the same field in the 12" you'd need to decrease the focal length which would decrease the focal ratio.

to calculate the field of each (assuming film width is the field - say 36mm which I believe is comparable to field stop size) :

FOV= (36mm / Focal length scope)*57.3

so 36 / 1000 x 57.3 = 2.06 degrees at prime focus

or 36 / 1500 x 57.3 = 1.38 degrees at prime focus

so to make the comparison valid, if we assume the image is taken during the day then the image in the 12" scope has been 'stretched' by the longer focal length (= more magnification) by 2.06 / 1.38 = a factor of 1.49. but as the 12" scope gathers (113/50) or 2.26x more light, the image will be brighter even though the focal length has created more magnification and albeit at the same focal ratio. this proves it is a character of aperture and not focal ratio. if you view the aperture of the iris in the camera as an aperture and not a focal ratio then this also works in that case.

for visual, it's simpler as you use magnification to govern image scale and brightness to a large extent, e.g to get 100x magnification in the 8" we need a 10mm eyepiece and in the 12" a 15mm. the image in the 12" will be brighter than in the 8" at the same magnification despite the focal ratio being the same.

I think all this works and explains the way that it really seems the same (to me anyway).

I am sure this will be shot down but it will be interesting to understand why.

[GazOC]

for visual, it's simpler as you use magnification to govern image scale and brightness to a large extent, e.g to get 100x magnification in the 8" we need a 10mm eyepiece and in the 12" a 15mm. the image in the 12" will be brighter than in the 8" at the same magnification despite the focal ratio being the same.

Sorry to snip your post Shane but this is bang on the money.

[Moonshane]

no worries mate

I am interested in the views of others on my 'theory' though as I want to ensure I understand this myself

[GazOC]

http://stargazerslounge.com/beginners-help-advice/145038-focal-length-ratio-etc.html

Post number 14.....:)

[Ags]

Moonshane your post looks right to my muddled eyes...

Tim, if the issue of f ratio is too confusing, my advice would be to forget about it for now - there is nothing, aside from AP - that depends on f ratio.

All you need to know is the actual focal length and the actual aperture. These two measurements together give you the f ratio, but that is the least interesting figure.

The focal length determines the maximum true field of view. For example, using 1.25 eyepieces, a 1800mm focal length scope has a max field of view of 0.9 degrees. A 600mm focal length scope (1/3 the length) has a maximum FOV of 2.7 degrees - three times as much.

Focal length also determines magnification. So a 10mm EP in a 1000mm long scope gives 100x, and in a 2000mm focal length scope gives 200x.

Aperture determines the light available. So a 4 inch scope shows you stars down to 12th magnitude, a 10 inch scope shows 14th magnitude stars and a 25 inch monster dob would show 16th mag stars - these figures are all approximate.

The brightness of extended objects is determined by exit pupil - aperture / magnification. There is a maximum brightness imposed by your eye of around 5mm to 7mm exit pupil.

So keep the above in mind for now and forget about f ratio, it is just causing confusion. I'm pretty sure the penny will drop before long.

[Tim Armes]

A - Focal length = 800mm, Diameter = 200mm, F-Number = f/4

B - Focal length = 1600mm, Diameter = 200mm, F-Number = f/8

Note that:

1) the diameters are the same, the f-numbers are different

2) the field of view of telescope A is twice as big as telescope B.

3) telescope A collects four times as much light as telescope B (it's 2 stops faster) <== This is wrong

With telescope B, the user only has to magnify by 50x (using a 32mm eyepiece) to see the image at the same size. <==This is wrong You are tring to say 50 times magnification = 100 times magnification???

And the same mistake made in Proof part 2

Since the two telescopes have a different field of view, once will produce a smaller image of the object than the other one. That image will need to be magnified a different amount by the eyepiece to produce the same size image on the retina.

The end result is that they both have the same magnification compared to the naked eye.

[Tim Armes]

I take it you're not bothering to read my posts then?

(I've made the point in both threads you've started on this subject)

Sorry, I think I missed that bit.

What I really needed to get my head round it was a concrete demonstration using numbers. I've managed to do that myself now, as posted above.

In case case, between you all, you got my head round it. Thanks

[Tim Armes]

Tim, if the issue of f ratio is too confusing, my advice would be to forget about it for now - there is nothing, aside from AP - that depends on f ratio.

I think I got my head around it a few posts back (the long one I made), so all's well unless I fundamentally misunderstood something.

Tim

[Macavity]

I sense, one can over-analyse? Theory aside, to record "faint objects", it seems a "good idea" to have... certain scope parameters. (Deliberately vague!) <G> In "Deep Sky Video Astronomy", Massey and quirk tabulate experimental(?) real-time limiting magnitudes for the "Gstar" video camera with various scope combinations...

Once you've exceeded 6"/F5 the remaining (common) scopes have a limiting magnitude 15.6 - 16.2 (16.9). Of course, this may make a *big* difference re. the number of stars you see. Then, there's (optimal) sampling, "PSF"s etc. All presumably to be set against "sky background"... Mag 18 per square arc second(!) for my "5.6" skies, apparently? For better or worse, I'm messing about with (preliminary!) practical tests. It *will* get brighter - Honestly!

Re. video astronomy, I've had SOME notable success with my "unsuitable" MAK150 (ad-hoc [working!] focal reduction) etc. Never say die, Eh? In my "excitement", I bought a TS/GSO photo-newt... Who knows if this was wise? As ever financial and practical considerations dominate here. <sigh> Maybe the smart(er) money is now on Skywatcher "Quattro"? I can't resist a slight chuckle on that one though: "To the QUATTRO, Lads"?

[dph1nm]

As Karsten pointed out, point sources like stars are not subject to stretching by the eyepiece as they are infitintely small to start with - so stars stay at a brightness determined by aperture (not F ratio) regardless of magnification.

Unfortunately this is not true unless you are in space. Star images do have a fixed size, which depends upon the seeing, and is usually around 1 arcsec or so. So once your telescope/eyepiece can resolve this size, then any more magnification will stretch them just like extended sources.

NigelM

[Ags]

Fair enough - inifinitely small was an exageration in any case. Nevertheless in practical terms star brightness is pretty independent of magnification.

[KaStern_Former_Member]

Hello Nigel,

seeing varies strogly. I can be very good to very poor.

If it is good I see a tiny disc of ligth surrounded by 1 or two faint rings,

at 300x in my 8" ATM Newt.

And I would like to point out:

How faint stars you ccan see depends a bit on mgnification.

I seeing is good enough you can see zhe faintest stars at high magnification.

For example 240x in an 200mm scope.

This is because the sky background gets darker and darker with hiher magnification.

Sky backgraund is somethink like an an extended light source,

and so exit pupil size plays a role there.

The star is a point-like light source. It stays pointlike until the magnification is so high

that you begin to see the star as a tiny dics (point spread function).

Then the star too begins to act as an extended light source.

In fact:

Bad seeing can make a star look like a fuzzy dics at under 70x in my 200mm scope.

In that case the star acts like an extended light source too.

Then it is time for me to pack my things and drive home.

Cheers, Karsten

[spanky]

Wow, I'm confused!

So what would happen if...

you took an exposure of say, 1 minute with a 6" f/5 and then with the same camera took a 1 minute exposure of the same target with an 8" f/5?

Would the camera suck up more light through the 8" ???

[LDUNN1]

.....I've read this thread from top to bottom.

I also come into astronomy (recently) with a life long footing in photography & have struggled with this sort of issue......it is reassuring that I am not alone.

In the early days I struggled with why astro folk talked about diameter & focal ratio to describe their kit ( & seldom focal length), while in the photo world it is focal length & min (smallest number, biggest hole) f-ratio & never the diameter of the front of the lens.

Using Spanky's example:-

The 8" scope will capture more light into the front of the scope than the 6"

However, as the focal ratios are the same at f5, the 6" has a focal length of 762mm vs 1016mm for the 8".

Therefore the 6" has a wider field of view & the objects in it appear smaller (than in the 8" longer focal length shot).

....& the 8" has a narrower field of view & the objects in it appear bigger (than in the 6" shorter focal length shot).

.......to make the images comparable (in the same fashion Olly promotes), I have to make the objects the same size in the final photo, so I would have to crop the image down taken with the 6" scope & enlarge what remains so that the object size in both photos matches.

For the same camera, I would lose resolution as I am cutting pixels away from the image (& that is another story!!!) ......but, that cropping action is akin to throwing 'light' away.

So, to photo that distant galaxy, I think I am better off with the 8" f5 image for two reasons:-

1) more light is entering the front of the scope in the 8"

2) I don't have to crop & blow up the image as much.

......the above assumes that you are focal length limited in the first place (ie the objects you want in the end photo easily fit in the photo with space around them to spare) - maybe if you wanted to fit a whole constellation in the image, the widerfield of view from the 6" would be more convenient to start with (no messing with stitching multiple images together).

Where things become more debatable in my mind, is if you had say a 6" f5 vs a slower 6" or 8" ....at some point there is a trade off between exposure time vs how much cropping due to less focal length I think.

Maybe my confusion in this explains why I still image with my DSLR & camera lenses ;-)

[JoLo]

I have always had trouble with these concepts as well, but in one of the astro imaging books I read (I think it was Robert Reeves) I found an explanation that keeps it straight in my head. The basic concept is that comparing visual and photographic light gathering is not an apples to apples comparison....you eye and brain recieves, processes and moves on....the camera chip gathers data linearly and continues to do so as long as the shutter remains open.

Keeping this in mind, and Olly's correct statement above that aperature always focuses the same amount of light on a retina or on a CCD chip regardless of focal length, Reeves (or whoever it was) gave the example of two scopes with the same aperature but different focal lengths and the light they forward to the chip.

8" aperture, f/4

8" aperture, f/8

For the f/4, the incident light is the same as with the f/8, but the optics concentrate the light on a smaller number of pixels in the center of the chip...so, the amount of time the scope needs to gather that light is shorter, and the FOV is larger.

For the f/8, the incident light is again the same, but it is spread over a larger area of the chip. Therefore, the optical system requires more time to gather the same amount of light, and the resultant FOV is smaller. It is, for lack of a better phrase, a dilutional effect.

I don't know if this helps, confuses the matter, or misses the point completely, but it has always worked for me.

Joe

[spanky]

I'm still confused lol

How about I put it another way...

Regardless of the difference in field of view, let's assume I'm going to crop the wider one so it matches the narrower one.

I have 6" f/5 and an 8" f/5 side by side with the same camera on each and both pointing at the same target.

I do one sub on each for, say 10 mins.

Which one gives me a brighter image? Or are they both the same?

[ollypenrice]

I've just tried again to write an explanation of this for my website! Like your man, Joe, I have tried to address focal length but have come at it from the opposite end, keeping focal length the same in my version. I think the same thought process lies behind both attempts. Here are two exerpts from mine. Feedback welcome.

Focal Ratio. This is defined as the focal length divided by the aperture. The issue is clouded by something called 'the F ratio myth' so, to avoid getting involved in this bit of confusion, I am going to discuss f ratio only in the context of telescopes which have the same focal length. Let's say a focal length of 1000mm.

If the aperture of this 1000mm FL scope is 100mm it is an F10 scope.

If the aperture of this 1000mm FL scope is 200mm it is an F5 scope.

The area of aperture in the 200mm F5 scope is four times the area of aperture in the 100mm F10 scope. It therefore pours four times as much light onto the chip and needs an exposure a quarter as long as as the F10 scope. This is why the F5 scope is called 'fast' and the F10 'slow.'

(There is a caveat concerning point sources like stars not following this rule but I see it as having nothing to add to the discussion in amateur imaging terms).

and...

The F ratio myth identifies the following fallacy;

Take two telescopes of the same aperture, say 100mm. One is F10 so has a focal length of 1000mm. The other is F5 so has a focal length of 500mm.

You wish to image the same object in both, let's say M51. Will you gt the same result four times faster in the F5 scope? No. This is the myth!

Let's say that in the slow scope, with the longer focal length, M51 fills the chip. All the light from the scope's aperture is devoted to M51.

In the slow scope M51 only covers, by area, a quarter of the chip. So three quarters of the scope's aperture is devoted to collecting light from the background sky, not M51.

You have aquired no extra light from M51 in the fast scope and you have lost resolution because of the small scale of the image at the shorter focal length.

Only when the focal length is unaltered does a scope of twice the aperture require a quarter the exposure for the same result. (In fact a slightly better result because there is a gain in resolution arising from increased aperture as well.)

Olly

[DoctorD]

Hi Olly

I think what you are missing here is that in both cases the same amount of light from M51 hits the chip as the aperture is the same (assuming that M51 falls entirely within the FOV) however on the shorter focal length (faster scope) this is shared between fewer image receptors (pixels) and therefore a given exposure (number of photons hitting a receptor) is achieved in a shorter time.

(Larger pixel sizes results in more sensitive CCDs for a given technology but also in larger chip sizes for a given number of pixels)

I think that the original statement is flawed as has been said before - it needs qualifying "for a given image size - on the retina or the CCD" and assumes that visually we are only interested in point sources.

As much as I like double stars, visual astronomy would not be as interesting if this were the case

Clear skies

Paul

Sent from my iPhone so please forgive the spelling

[JoLo]

I think part of the confusion here is a difference in the approach and nomenclature between photography and astronomy. In photography, FOV / image scale / brightness are affected by altering the aperture of the imaging lens...when you zoom on a 70-200 lens, the aperture is being altered, but the focal length is constant.

This is not possible in a telescope based optical system..the aperture is constant, and the only way to alter the FOV / image scale / brightness is to alter the focal length...either through eyepieces of different FLs, or with the use of a barlow. The term "f-stop", so important in photography, is meaningless in astronomy.

Ultimately, both achieve the same thing, and equivilent exposures must be thought of as a function of aperture in photography, and of focal length in astronomy (assuming a constant exposure length).

[LDUNN1]

Olly - I totally follow your last explanation & agree 100% - that is probably the best description I've read & understood so far!

Joe - did you really mean to say "when you zoom on a 70-200 lens, the aperture is being altered, but the focal length is constant" ?????

.......When I zoom a 70-200 I think I change the focal length!

.......maybe you hit upon another way of explaining this to folkes with photo background ......

Consider a 70-200 with a constant f2.8 throughout its zoom range - it's front objective will be approx 71.5mm diameter (200/2.8)

Leaving the aperture ring set to the widest possible at f2.8 (so thats fixed) when you zoom in & out, you change the focal length......but look what happens to the 'front objective diameter':-

@ 200mm 200/2.8 = 71.5mm

@ 70mm 70/2.8=25mm

.......I have not changed the physical size of the front objective - it is the same lens & I have not moved the aperture ring from the f2.8 setting, & the diaphram that the aperture ring controls has not changed, & yet something else in the zoom lens is controlling the size of hole letting light in......a bit like a field stop in an EP.......I bet most photographers have not thought about it in this way before.

.....so what have I got wrong / missed????