Chris Wilcox
super member
Reged: 10/22/04
Posts: 173
Loc: New Jersey
|
|
Another quick questions for the experts....
What mm EP does the DR act like at prime focus.
Thanks,
--------------------
Chris Wilcox
Orion Atlas EQ6
8" Orion Newtonian
|
Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
|
|
The way you determine this is that it = the diagonal length of your chip. A 35mm film frame is 24mmx36mm, so the diagonal is 43mm, which is why they say that a 50mm lens is the "normal" lens (close enough, and easy to mulitply by). That means that a 50mm lens produces 1x on film, so to get the magnification produced by a lens, it's lens fl / 50mm. So a 35mm camera acts like a 50mm eyepiece (well, really a 43mm eyepiece).
So, how big is your Digital Rebel's chip? 
Jimbo
--------------------
Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
|
jrcrilly
Refractor wienie no more
Reged: 04/30/03
Posts: 30716
Loc: NE Ohio
|
|
Quote:
The way you determine this is that it = the diagonal length of your chip. A 35mm film frame is 24mmx36mm, so the diagonal is 43mm, which is why they say that a 50mm lens is the "normal" lens (close enough, and easy to mulitply by). That means that a 50mm lens produces 1x on film, so to get the magnification produced by a lens, it's lens fl / 50mm. So a 35mm camera acts like a 50mm eyepiece (well, really a 43mm eyepiece).
Often when a question is asked about comparing an eyepiece to an imager the intent is to determine the relative field of view. The Rebel will image a field of about 25 X 40 arcminutes at 2000mm. If we use the smaller axis we can call it 25 arcminutes.
In the same telescope, a Plossl with a 50 degree AFOV would show a true FOV of about 50 arcminutes at 60X (34mm EP); that'd be about 25 arcminutes at 120X (17mm EP) - so the field imaged with a Rebel will be comparable to that displayed in a 17mm Plossl.
--------------------
John C
Battle Cry of Reno
http://www.wadsworthobservatory.com
My Cloudy Nights gallery
AT12RC
AT65EDQ
QSI683WSG-8
Roper Scientific Quantix 6303E "project" camera
mystery EQ mount on the way
|
Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
|
|
The formula for field size (width, height, or diagonal) in degrees is: 57.3 divided by the lens focal length in mm times the film dimension in millimeters.
FOV = (57.3 / lens focal length) * film dimension
The 300D has a chip that is 22.7mm x 15.0mm (diagonal of 27.2mm).
The FOV at 2000mm is: 25'x39' Diagonal FOV = 46.75'
The FOV at 50mm is: 17°x26°
TFOV ~ AFOV/mag = (AFOV/scope focal length) * EP focal length.
EP focal length to produce desired FOV, then, is:
EP focal length = TFOV/AFOV * scope focal length
so,
46.75'/50° * 2000mm = equivalent diagonal FOV EP = 31mm
39'/50° * 2000mm = equivalent horizontal FOV EP = 26mm
25'/50° * 2000mm = equivalent vertical FOV EP = 17mm
The box describing the FOV of a 300D would have a circumscribed circle close to the FOV of a 31mm Plossl and an inscribed circle close to the FOV of a 17mm Plossl.
You can also use these to derive a "normal" lens, producing about 1x. Using the diagonal length, a 30mm focal length lens should produce about 1x on a 300D chip, to you can calculate the magnification of a given lens (or scope) in the normal way:
2000mm / 30mm = ~67x. The TFOV (if you compare to a 50° Plossl) comes out close, too.
A 50mm lens produces 1x on film. Compared to a 50mm 50° Plossl on a 2000mm lens that produces 40x and a 1.25° field, a film camera would produce 40x and a 1.03°x0.69° (1.24° diagonal) field. So I say that a 35mm film frame is more or less equivalent to a 50mm EP (50° Plossl).
A 30mm 50° Plossl, same lens, would produce ~67x and 45' field. A 300D would produce a 25'x39' (46.75' diagonal) field. I'm going to call that ~67x magnification, and say that a 300D frame is more or less equivalent to a 30mm EP (50° Plossl).
Input: size of chip.
Outputs: TFOV for any lens, magnification for any lens, 35mm film equivalent length lens, comparable focal length EP.
These formulae also work for other sized chips (Toucam, ST-7, ...).
Jimbo
--------------------
Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: