Thread: Focal length, does it matter?

1. Focal length, does it matter?

As an amateur photographer I am used to focal length translating directly to field of view. I bought a 10 inch Meade Schmidt Newtonian reflector over the weekend that is reasonably fast at f/4. That translates into a 1016mm focal length. I've seen other 10 inch telescopes that are f/10 and have a focal length of 2540mm. In photography that would give a much more narrow field of view over the same area, giving greater magnification (not the actual term for it since magnification is more of a macro photography thing, but it will work for this question) of the object.

So, my question is, is this true for telescopes and astronomy?

2. Re: Focal length, does it matter?

It was hurting my head trying to figure out the question, so I just decided to give you the answer.

The True Field of View (TFOV) with any scope is equal to the Apparent Field of View (AFOV) of the eyepiece divided by the magnification of the eyepiece. Or, TFOV = AFOV / magnification. Obviously, the same eyepiece in a longer focal length scope will give you a higher magnification (the denominator in that equation).

(Remember, that magnification = Focal Lengthscope / Focal Lengtheyepiece.)

Hope this helps.

Clear Skies

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jyanna (01-14-2015)

4. Re: Focal length, does it matter?

The most important thing having to do with focal ratio in amateur astronomy--when we are talking imaging--is that the faster your system is, the shorter your exposures can be. Just like in terrestrial photography.

5. Re: Focal length, does it matter?

To expand on John's answer, you would use the diagonal measurement of your camera sensor in place of the eyepiece focal length in the equation given above. That would translate into a longer focal length scope giving you more magnification (or a smaller field of view) than a scope with a shorter focal length.

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ImNewHere (01-13-2015)

7. Re: Focal length, does it matter?

I am definitely going to be doing some imaging with cameras that have an APSC size sensor, 1/2.3 sensor size, and whatever is in the Tucsen IS500 that came with my telescope purchase over the weekend.

8. Re: Focal length, does it matter?

From the point of view of a camera, a telescope is just a big lens. So everything you know about camera lenses applies equally to telescopes. A scope with a focal length of 1016 mm is just a 1016 mm lens. A scope with a focal length of 2540 mm is a 2540 mm lens. F/4 is f/4, no matter who makes the optics.

For visual observing, the focal length and focal ratio do not matter a great deal. What matters for visual use is aperture (the linear aperture in millimetres, not the photographer's relative aperture, which astronomers call focal ratio).

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ImNewHere (01-13-2015)

10. Re: Focal length, does it matter?

So for astrophotography a 10 inch with a 2540 fl will give me more magnification than a 10 inch with a 1016 fl. That's what I wanted to find out. Thanks!

11. Re: Focal length, does it matter?

One more question, does a barlow then act like a teleconverter for astrophotography when using one and going for prime focus? For example, using a 2x barlow would make my 10 inch f/4 OTA with a 1016 mm FL give the same FOV as a 10 inch OTA with an f/8 aperture and 2032mm of FL? When using a teleconverter with a lens you get a FOV that equals whatever the multiplier of the teleconverter is times the focal length of the lens, with the loss of however many stops of aperture which is again based on the multiplier of the teleconverter.

12. Re: Focal length, does it matter?

Yes a Barlow has the same effect as a teleconverter.