How do focal reducers/barlows work?

[pipnina]

I've been thinking about what a barlow lens/focal reducer actually does, I think it's meant to make the primary look closer to the secondary than it really is to the EP. Would this be correct? If so, I imagine the reducer has the opposite effect.

Although, the bigger question is... Does a barlow/focal reducer affect exit pupil? I'm not asking much from an observing point, but from a photography point of view. If you use a powerful reducer, won't vignetting become a problem? I mean, the sensor you're using is already like a 25-35mm EP. And if you use a 3/4/5x barlow... will vignetting become a problem due to the exit pupil shrinking?

These objects confuse me.

    ~pip

[ronin]

A barlow is said to multiply the focal length of the scope however I suspect the more accurate description is that it multiplies the size of what would have been the prime image.

Think that most these days use a negative lens to do it but the explanation is easier with a positive lens.

Place a 12mm focal length positive lens 18mm beyond the prime image and it will form another image of 2x the size at 36mm away.

So in 54mm you have doubled the image size and in effect the scope focal length.

A reducer will/could be the other way round, same 12mm focal length lens at 36mm from the prime image and the lens creates an image at 18mm that is half the size, so appearing to have halved the focal length.

Guess a reducer uses a positive lens and a barlow uses a negative lens and I'm too lazy to work out an example for the negative. And using one suitable positive means that the same item can do both. However curvature etc is a factor.

[pipnina]

A barlow is said to multiply the focal length of the scope however I suspect the more accurate description is that it multiplies the size of what would have been the prime image.

Think that most these days use a negative lens to do it but the explanation is easier with a positive lens.

Place a 12mm focal length positive lens 18mm beyond the prime image and it will form another image of 2x the size at 36mm away.

So in 54mm you have doubled the image size and in effect the scope focal length.

A reducer will/could be the other way round, same 12mm focal length lens at 36mm from the prime image and the lens creates an image at 18mm that is half the size, so appearing to have halved the focal length.

Guess a reducer uses a positive lens and a barlow uses a negative lens and I'm too lazy to work out an example for the negative. And using one suitable positive means that the same item can do both. However curvature etc is a factor.

That's very interesting. Thank you.

[Mr Spock]

The Barlow uses a negative lens to intercept the light cone from the primary. In doing so it increases the length of the light cone thereby giving an apparent increase in focal length. Using the same power negative lens and a longer tube, the focal length would increase further; a shorter tube would increase the focal length less so.

A focal reducer works the opposite way. It has a positive lens which reduces the length of the light cone from the primary to create a shorter focal length.