How to calculaate the magnification using an SLR camera with a 1250 fl telescope

[David Stone]

Grateful if any could help

Dave Stone

[moondog]

Don't know a formula, but I worked out the approximate magnification for my 1125mm FL scope by comparing the magnification with some eyepieces I have. For the Canon 350D mine worked out at about 42x mag. Same as my 25mm TAL Plossl EP (50 deg FOV).

It would obviously vary a fair bit depending on the FOV of the EP you were using.

MD

[GordonCopestake]

For a 35mm camera simply divide the focal length by 50 to give an APPROX magnification, so 1250 / 50 = 25

For DSLR's with an APS sensor, you first multiply by 1.6 (or whatever your sensor multiplier is) so (1250 * 1.6) / 50 = 40

I think.... Someone will come along and correct me if i'm wrong!

[moondog]

Gordon your formula pretty much bears out my visual estimation. I guessed mine at about 42x whereas your formula would give 36x.

MD

[FLO]

Spot on. Most Nikons (D50/D70 etc) have a factor of 1.5x whereas for most Canons (D30/D60/10D/20D/300D/350D/400D) use 1.6x

[David Stone]

Many thanks everyone for your help.

Dave Stone

[moondog]

Dave

Glad you found your answer

Gordon

My DSLR's focus position is 100mm longer than the 'normal'. This is measured from the object lens to the focal plane of the DSLR. This would give my scope a FL of 1225. Using your formula this would give a magnification of 39.2x and not the 36x using the normal FL. This puts it closer to my visual estimation of 42x.

I suppose that, optically, the FL should be measured to the focal plane of the DSLR. What do you think?

MD

[GordonCopestake]

Yes, the focal length should be to the focal plane, anything that intercepts it (like an eyepiece) then changes the light path and "upsets" the focal length. The 50 rule is only approximate though