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Magnification?

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#1 Ettu

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Posted 30 August 2019 - 04:20 AM

What is the " magnification" of a scope considered to be, photographically, at prime focus?

Specifically an 11" Edge HD at 2800 focal length, using a DSLR

Is there such a thing/number commonly used?

Since photographically you can "zoom" to almost any number size 

I know how to calculate it when using an eye piece

My best guess is that it might be considered as the FL, 2800mm divided by 35mm, the standard 1x lens of SLR cameras

I.E. about 80x

is that right?



#2 einarin

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Posted 30 August 2019 - 04:30 AM

In AP magnification has no relevance.

Just focal length and field of view and pixel scale is meaningful.



#3 james7ca

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Posted 30 August 2019 - 05:50 AM

A 35mm lens might be "standard" for an APS-C format camera, but for the full-frame 35mm format it's normally considered to be a 50mm lens (very roughly the diagonal dimension of a 35mm frame).

 

But, I agree with einarin and note that image scale also depends upon the pixel size of the camera.



#4 sg6

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Posted 30 August 2019 - 06:00 AM

The term is not applicable at the prime focus, consider it easier that you need an eyepiece for the term Magnification to apply.

 

The relevant bit is the size of the image formed.

That is as a good approximation: S = Tan(Obj)*FL

Obj = size of object in degrees, FL = Focal Length of scope.

 

So consider the moon - nice, easy, round - that is 0.5 degrees, your scope is 2800mm so image at focal plane:

S = tan(0.5)*2800 = 24.4mm, say 25mm for ease.

 

So to image the moon (full) in one image you would need a sensor of at least 25mm and that would be very very tight, 30mm sensor a bit easier.

 

As M42, M45 are bigger you need somewhat larger sensors, OK, very large sensors. M57 is OK as that is small.


Edited by sg6, 30 August 2019 - 06:01 AM.


#5 rolo

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Posted 30 August 2019 - 07:13 AM

Devide your focal length by 50 and it will give you the magnification using a full frame chip. So, 2800mm = 56x magnification at prime focus.


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#6 TOMDEY

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Posted 30 August 2019 - 07:33 AM

Problem is that there are three rigorous, and then numerous additional, casual "magnifications".

 

The rigorous ones:

 

Lateral magnification is the paraxial amplification of the real image to object absolute lateral (side to side) extent... For example, if your naked telescope forms an image of the moon that is 8.7mm across --- that is a lateral magnification of -2.60 x 10-9, with the minus sign indicating that the image is upside down.

 

Longitudinal magnification is the paraxial amplification of the real image to object absolute longitudinal (front to back) extent... and is the square of the lateral mag. In the example above, it is 6.8 x 10-18, making that image of the moon a mere 23 picometers thick, aka half that for the visible hemisphere! This explains why the center and limb are intimately parfocal... because the image form is a profoundly thin pancake... way thinner than the "depth of focus" of the referenced 1000mm telescope!

 

Angular magnification is the paraxial amplification of the virtual image to object absolute angular extent... and applies to afocal imagery (where object and image are both far away) aka a telescope with eyepiece. It is given by the ratio of objective to eyepiece focal lengths. e.g. The referenced 1000mm telescope with a 10mm eyepiece has an angular magnification of -100; same comment regarding the minus sign.

 

But, then... (alas!) the imagery ~experts~ have jumped in and defined other "magnification standards" ... which have nothing to do with optics, per-se, and everything to do with what they happen to be hawking, for that era. Enter the realm of professional photography and movie-making! Kodak's original movie film was 70mm wide strips. Then Dickson sliced it down the middle and introduced the compact roll-film camera. Sprocket holes ate up some of the width, finally settling on the standard real image format of 36x24mm, fed by a 50mm focal length lens, and defining that as standard unity magnification. And that is what we still call "full-frame" today! Note that our "half-frame" is therefore 24x18mm. [Further note that the aspect ratios are different!] Then, to further add to the confusion... camera manufacturers started adding "equivalent focal lengths" to pretend that their cheap, shrunken cameras were as good as the bigger ones. As we optics guys would say, "It's all a blur!"

 

Note: I didn't go (much) into the quantitative stuff... which is readily found in the popular avocational hobby literature.

PS: Note that the diagonal is ~43mm, no coincidence that this just comfortably fits within our "standard" two-inch drawtube! You see, all of these "standards" are cross-correlated and dependent on history, not fundamentals of optics.

 

Somewhere, somehow... all of these historical standards are traceable to Buggy Whips!   Tom

Attached Thumbnails

  • 100.2 35mm Eastman Film Photographic Standard Format.jpg
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#7 Bowlerhat

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Posted 30 August 2019 - 08:51 AM

Devide your focal length by 50 and it will give you the magnification using a full frame chip. So, 2800mm = 56x magnification at prime focus.

 

How about non full frame?



#8 martinl

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Posted 30 August 2019 - 09:58 AM

As per above, photographically the concept of magnification makes no sense. Only FOV matters. The idea that 50mm equals a magnification of 1 is a photographer’s myth from the time when cameras came with a 50mm lens and the viewfinder eyepiece was around 50mm. 50mm/50mm=1x.  

 

Why did cameras come with 50mm lenses? Because 50mm is essentially the shortest focal length you can build easily without resorting to advanced optical designs to cover a 35mm frame.



#9 Richard Whalen

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Posted 30 August 2019 - 02:46 PM

I think it is relevant, as far as image scale. It all depends on your focal length and chip size. Smaller chips give the effect of  more image scale, smaller fov. So it is important to match your chip size (like an eyepiece visually) to the object you are imaging to get the desired effect. 



#10 TOMDEY

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Posted 30 August 2019 - 05:09 PM

The manufacturers were in the conundrum that customers wanted to feel like they understood optics and photography. So, alas... the manufacturers coopted terms like ~magnification~ to make customers feel knowledgeable and comfortable --- in the mood to buy stuff. Unfortunately... once we wind up on more serious avocational sites... like Cloudy Nights, here... the chinks in the armor start to reveal as gaping cracks! Need to re-learn what we thought we already knew.

 

The result is that we tend to way overthink topics that are actually pretty simple!

 

The only solution is (if one is so-inclined?!) --- maybe sit in on a local optics course or two. Our local university here (U or Rochester, NY) allows that for alumni... whether they attended medical, music or sciences!    Tom



#11 Cotts

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Posted 31 August 2019 - 08:13 AM

I can see the term applying when comparing images from two lenses of any focal length.  

 

e.g.  I take a picture of a duck on a pond with an 80mm lens.  This produces an image of the duck on my film plane that is, say, 2mm long.   Now I take out my 640mm telephoto lens.  The image of the same duck will now be 16mm long on my film plane.

 

I have 'magnified' it 8 times compared to the first lens, the ratio of the two focal lengths.

 

For some reason a 50mm lens on 35mm film was considered 'standard' with a magnification of 1.0x.  I think the reason is that 50mm lenses were by far the most common general use lenses.  It was what most folks were used to.  Everyone had an idea of what things near, far, big and small 'looked like' in such a lens.  Telephotos then were compared to that familiar 'standard'....

 

It is also interesting to use the "50mm lens is 1.0x" concept with wide angle lenses.  Would a 10mm lens have a "magnification" of 0.2x?  You never heard those sorts of numbers bandied about... W.A. lenses compared fields of view, not magnification..

 

Dave


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#12 Ettu

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Posted 04 September 2019 - 07:56 PM

Wow, Thank you to all who contributed to this thread!

I got more about the history, and a way more "confusing" ;-) answer than I ever expected!

Thank you.

Okay, Just for fun, Here's a twist...

With the OP conditions of a full frame, DSLR, at prime focus of a 2800mm scope,  and adding here that it's a 21mp camera

For my "twist" I'm going to agree with the "Standard" of 1x for full frame camera is 50mm. Just because, as a norm, a 500mm lens bought for such a camera is almost always called a 10x lens.

So therefore my 2800mm C11Edge is a 56x lens

But any of us who've taken deep sky images with aforementioned setup, would agree that seems low for a "magnification" number.

So here's my twist to correct that....

Hi resolution images are often considered to be 300dpi, - right?

Or in our digital world, 300px/in (ppi)

Normally you print things out on standard 8.5x11 paper, with the image (with a border) say....11x7.4 (maintaining aspect ratio)

BUT the image of a 21mp camera printed on paper big enough to hold a 300ppi image would give you (roughly) an 18.7x12.5 image

Or, roughly 2.85x bigger than fitting the whole image on a standard sheet of paper.

SO FINALLY,  by my convoluted reasoning, I'm going to take my 56x "lens" and multiply it by 2.85 = 160x as my hi rez magnification

And that folks is what I'm going say is a reasonable number for this setup's magnification

And I'm not going to argue about it! ;-)

To each his own I say, eh?


Edited by Ettu, 04 September 2019 - 08:02 PM.


#13 james7ca

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Posted 04 September 2019 - 11:30 PM

Wow, Thank you to all who contributed to this thread!

...

Normally you print things out on standard 8.5x11 paper, with the image (with a border) say....11x7.4 (maintaining aspect ratio)

BUT the image of a 21mp camera printed on paper big enough to hold a 300ppi image would give you (roughly) an 18.7x12.5 image

Or, roughly 2.85x bigger than fitting the whole image on a standard sheet of paper...

Well, yes, you can use any "magnification" value that you want (to each his own...).

 

But, I don't understand why you are taking the areas of the two sheets of paper to calculate the difference in your version of magnification. That's about like squaring the focal length when you compare a 50mm lens to a 100mm lens. I think we'd agree that going from 50mm to 100mm is a 2X change, but it's NOT a 100^2 / 50^2 = 4X change.

 

So, if you want to go by the printed paper size the difference would only be 18.7 / 11 = 1.7X, not 2.85 (which is approximately the square of 1.7).

 

Thus, 56 x 1.7 ≈ 95.



#14 elwaine

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Posted 08 September 2019 - 09:56 PM

As per above, photographically the concept of magnification makes no sense. Only FOV matters. The idea that 50mm equals a magnification of 1 is a photographer’s myth from the time when cameras came with a 50mm lens and the viewfinder eyepiece was around 50mm. 50mm/50mm=1x.  

 

Why did cameras come with 50mm lenses? Because 50mm is essentially the shortest focal length you can build easily without resorting to advanced optical designs to cover a 35mm frame.

Maybe it's semantics, but I have to disagree in principle. Photographically, magnification makes perfect sense.

 

Using a 35mm format camera, film or digital (regardless of pixel size or array ) photograph a specific object 50 feet away using a 50mm focal lens. Repeat with a 100mm focal lens... and then with a 200mm focal lens. Using the photo taken with the 50mm lens as a reference, the targeted object will appear twice as big in the photo taken with the 100mm lens (2x mag), and 4 times as large in the photo taken with the 200mm lens (4x mag). 

 

What one does with the image afterwards has no bearing on the magnification acquired in the original photograph. I.e., cropping and blowing up the targeted image increases the apparent size of the object but does not increase the magnification scale of the original photograph.


Edited by elwaine, 08 September 2019 - 10:02 PM.


#15 Erik Bakker

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Posted 09 September 2019 - 02:01 AM

On an APS-C sensor, dividing the focal length by 35 would give you an indication, on a full frame sensor dividing the focal length by 50mm is a good starting point to estimate the calculation factor of a scope. But that is not hewn in stone.



#16 JimFR

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Posted 09 September 2019 - 10:11 PM

For maximum magnification, hold the picture right against your face.


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