Quote:
Originally Posted by Tallstock
Hi,
I connected - prime focus - my new Canon 550D DSLR (Image Sensor size 22.3 x 14.9mm with multiplying factor 1.6x) to my dob recently to look at the Moon and showed my wife the result using Liveview on the laptop. The first question she asked "what's the magnification"?
I had no answer.
We are used to looking through various eyepieces and learning about magnification so we are interested in this discussion. She and I don't want to get into discussion about a "meaningful concept".
So if my 1200 focal length/50 old film camera = 24x magnification how do I correctly calculate magnification?
Peter
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OK, I'm from the days of film and have not done any astrophotography recently other than a few afocal shots with a compact digital. I did spend a bit of time with both prime focus and eyepiece projection SLR astrophotography many years ago.
In the days of film there was 35mm format also called 135. There was also 126 (compact instamatic) and 127 (4x4cm roll format) cameras and there was also a small pocket instamatic that used 110 format film. See
http://en.wikipedia.org/wiki/Film_format for a list of many others.
For the professional there was also 4.5x6cm medium format, 6x6cm medium format, 6x7cm medium format and 10x12.5cm large format film for use in medium and large format cameras.
Since 35mm film which has dimensions of 24x36mm and was the most popular and is still sold and used today; it is this format that digital DSLR CCD/CMOS sensors and equivalent lens focal lengths are compared to hence the multiplying factor. You can see that if you multiply your sensor size by 1.6x you get 14.9x1.6=24mm and 22.3x1.6=36mm,
Now forget the multiplying factor for now as it does not affect the size of the image, only focal length of the lens or telescope does. The formula for image size at the film plane is as follows:
Linear image size = angular diameter of object x focal length of telescope(or lens) / 57.3. eg. The Moon is 0.5deg in diameter. For your 1200mm focal length telescope you get an image size of 0.5x1200/57.3 = 10.5mm. If you get some plastic film transparency (or tracing paper) and hold it flat at the focal plane (film plane) of your telescope (no camera or eyepiece attached) you will get an image of the Moon that is 10.5mm in diameter (once focused). You'll even spot some detail!
Now if you have a film camera your 10.5mm Moon will be a small circle compared to the 24mm x 36mm film plane area, but still a good size to give plenty of detail. If you now remove your film camera and insert into the focuser your DSLR with 14.9 x 22.3mm 'film plane' your Moon image will still be 10.5mm but it will fill more of your smaller total area of the sensor. If you were to multiply the Moon image size by 1.6x it would equal in ratio to that of 35mm film. You would get the same result if you had a telescope of 1200x1.6=1920mm focal length and a full 24x36mm DSLR or SLR. eg,. 0.5x1920/57.3 = 16.8mm. (10.5x1.6=16.8mm).
Your magnification factor is always referred to a 50mm lens on 35mm film. Therefore 1200/50=24x and 1920/50=38x. Now you can print an image taken with a film camera on standard 10x15cm paper and get a nice picture. The image of the Moon taken with the DSLR can also be scaled up to 10x15cm paper by a factor of 1.6x and will look bigger OR printed at the same scale with the 10x15cm paper cropped to produce a smaller print (6.3x9.4cm paper) but same Moon size as on the SLR.
I personally don't like the use of the magnification factor as it is really a meaningless term since there are so many different film formats and yet we chose to use the 135 format only to apply a magnification factor to. However, since 135 format was the most popular size I can understand its use to allow the user to easily convert focal lengths to a format they are familiar with. Just look at the lenses on the compact digital cameras; they are usually less than 10mm in focal length and this means nothing to the average person. The CCD sensor size too is small compared to film but the combination gives an equivalent 35mm film format ratio.
On a typical SLR, 50mm was considered normal, 35mm wide, 28mm very wide, 70-80mm portrait, 135mm telephoto, 200mm longer telephoto. The same size lenses on a typical DSLR with either 1.5x or 1.6x smaller chips effectively operate as 1.5x or 1.6x longer lenses. This is why DSLRs's are offered with zoom lenses beginning at either 18mm or 24mm to match either 28mm or 35mm in 25mm format.
I hope I have cleared up and not clouded the magnification issue at prime focus. The main thing to remember is a given focal length telescope will always give the same image size no matter what the sensor (film) size is. How much you then want to magnify further by enlarging the image when printing or displaying is up to you.