Magnification ratio with a camera

[exmoorjim]

Hi All, I know the method for calculating the image magnification ratio when using eye pieces, but what is the calculation when using a camera instead of an eyepiece.

I use an Sky Watcher Evostar 150 (150mm objective lens) and an ZWO ASI485 colour camera.

Thanks for any help.

[ONIKKINEN]

Magnification when using a camera is difficult (or rather not possible) to determine. With an eyepiece the magnification you see is defined by how much larger the objects looks than with your naked eye, but when viewing an image taken with a camera the size can change depending on what device is used to look at it.

For example lets look at the image on a smartphone screen at arms length, and next lets look at the same image on a 27'' display at arms length, surely the bigger screen image has higher magnification as the image is several times larger? As you can see it gets tricky right away, which is why the term magnification is typically not used for imaging purposes. You can approximate the effect of magnification somewhat by using the field of view you get from the camera/scope combo and comparing that to a view with a certain eyepiece. It does not equate to a magnification, but at least you now have something to compare against.

Handy tool for figuring out what your scope/camera combo can fit in a single image: https://astronomy.tools/calculators/field_of_view/

[vlaiv]

Magnification is meaningful with eyepieces because eyepieces produce light beams at angles that are magnified for our eyes to see.

With image - it not only depends on device you are viewing it - but also on your distance. Look at any image and then step back few meters and look again - it will be smaller.

Since telescope + sensor can be thought of as a projection device rather than magnification device - appropriate quantity to examine is sampling rate - or pixel scale. How much of the sky is covered with single pixel and is expressed in arc seconds per pixel.

If you then want to calculate "magnification" - you further need - DPI/PPI rating of your display device and viewer distance. From pixel per inch number you can calculate pixel pitch or pixel size and use that to calculate angle that this pixel subtends given observer distance to the screen - magnification is then ratio of these two angles - one that pixels covers in the sky and one at which pixel is viewed given the pixel size and observer distance.

[michael8554]

Hi Jim

Focus a planet onto two cameras.

One with a third inch diagonal sensor.

One with a Full Frame 35mm diagonal sensor.

Both with the same size pixels.

The planet would be a dot on the Full Frame image.

But almost fill the sensor on the small chip's image - wow, that's some magnification ! 

Maybe   :-<

Crop the Full Frame image to the same pixel dimensions as the small chip, the results look the same.

So you can see how the concept of magnification when imaging is hard to quantify.

Michael

[vlaiv]

3 minutes ago, michael8554 said:

Hi Jim

Focus a planet onto two cameras.

One with a third inch diagonal sensor.

One with a Full Frame 35mm diagonal sensor.

Both with the same size pixels.

The planet would be a dot on the Full Frame image.

But almost fill the sensor on the small chip's image - wow, that's some magnification ! 

Maybe   :-<

Crop the Full Frame image to the same pixel dimensions as the small chip, the results look the same.

So you can see how the concept of magnification when imaging is hard to quantify.

Michael

Here you are talking about software "zoom" or resizing image to fit the display device and adding another (maybe unnecessary) layer of manipulation.

Size of the planet in both images, when viewed at 1:1 or 100% (when 1 pixel on display corresponds to 1 pixel of the image - or "native" display mode) will solely depend on pixel size if both sensors are used with same telescope.

Field of view / sensor size plays no part here.

[michael8554]

Isn't that what I said ?

Same pixel size ?

Michael

[vlaiv]

2 minutes ago, michael8554 said:

Isn't that what I said ?

Same pixel size ?

Michael

If the pixel size is the same - planet size will be the same in both images when viewed 1:1 or at 100% zoom

It won't be dot in one image and "normal" in second.

That only happens when you resize image for display purposes - when you attempt to put whole FOV on screen or similar. That is also "magnification" - but does not depend on telescope+camera so I don't think it is relevant for original question. You can perform that with any image not just astro images.

[Gfamily]

As a rule of thumb, I reckon it's better to consider the field of view rather than the 'magnification'.

The fov of a camera is similar to that of a Plossl eyepiece with the same focal length as the diagonal of the sensor. 

So an APS-C with a diagonal of ~28mm will give roughly the same fov as a 28-30mm eyepiece.

ETA: and your ASI seems to have a diagonal of about 12.5mm, so should give an idea of the fov to expect.

 

ETA2: of course I should have read all through the reply from @ONIKKINEN first.

Edited August 1, 2023 by Gfamily

[vlaiv]

Well, this raises interesting question.

I'm used to thinking about the image in terms of pixels and not in terms of FOV constraints.

If one forgets about the pixels - then same rules apply - except this time FOV is governing factor in any calculated "magnification" - it would be size of display screen and size of FOV in angles (say 1'20" x 50" or something like that).

However, I would advise all who think of doing astrophotography to think more in terms of individual pixels and pixel scale rather than FOV.

Same FOV can be covered by 1200x900px or 2400x1800px or even 4800x4600px. This will inevitably lead to over sampling in some cases and poor image quality due to lack of enough exposure on such high pixel scales (over sampling) - something people should really attempt to avoid in astrophotography.

[andrew s]

Of course if you want the linear magnification work out the size of Jupiter in the image (~pixel size x no pixels across diagonal) and divide by the size of Jupiter!

Regards Andrew 

[michael8554]

Perhaps we're thinking "Pixel size" eg 1200x 800.

And "size of pixel" eg 3.75um.

I'm saying a planet on a 6000 x 4000 pixel camera will be a dot.

On a 600 x 400 camera it will "fill the image"

But the 6000 x 4000 cropped to 600 x 400 will look the same,

Michael

[AstroMuni]

1 hour ago, michael8554 said:

I'm saying a planet on a 6000 x 4000 pixel camera will be a dot.

On a 600 x 400 camera it will "fill the image"

I think I understand what you are saying and my interpretation of what vlaiv is saying is ...If you have a monitor at 600x400 resolution,  the 6k x 4k image at 1:1 would overflow your computer screen and the image of planet would look like the image taken with 600x400 camera viewed at 1:1 as that image too would occupy your full screen. So in effect you are both saying the same thing 🙂

 

[Elp]

Isn't all this the same as crop factor when comparing DSLRs and mirrorless? They don't talk in magnification, it's a crop factor, usually compared to full frame 36 x 24mm sensor size.

Edited August 1, 2023 by Elp