Prime?

Ok, I'm an noob and I admit it. Will someone tell me, please, just what is a "prime"? The converter I understand and will probably purchase one shortly. I'm happy with the 55-300, but I want to reach out further and Bigma is out of the question just now.

Thanks.

A prime is a lens with a fixed focal length.

In the olden days it was called 'a lens'. Then they invented zooms and needed to stop people going back to the shop claiming their lenses were broken so they invented the new name.

If it's not a zoom, it's a prime. What lenses do you have now?

And my teachers taught me that people who admit they don't know something are actually smarter than those who always think and act like they know everything.

Which converter are you thinking about?

They do have issues of their own and may not necessarily solve your problems.

wots a telephoto lens
then
neil

Telephoto always used to refer to anything longer than the 50mm standard lens, and wide angle to anything shorter.

The normal, sane and accepted meeting but strictly speaking it's only a telephoto if it's physically shorter than it's focal length - otherwise it's a long-focus lens. It's the 'reverse' of retro-focus designs needed for wide-angle lenses for SLRs that have an unavoidably-large registration distance because of the mirror. (Sorry, it's been one of those days... - to the tune of Ride of the Valkyries: 'Mr Pedantic, Mr Pedantic...')

So my 100mm macro stops being a telephoto if I focus close-up?
The F 100-300 is only a telephoto when zoom ed to beyond 170mm, whilst several of my 50mm lenses are telephotos at least when focused on infinity...
I think you'll find it's one of those words here the definition has changed. In a similar way that many companies sell 'macro' zooms that can't even get near to 1/2 life size on the recording medium - true macro should be 1:1 or closer.
At least with the changed definition of telephoto it improves the usefulness of the term!

Wow! Dangermouse Writes something that is to the point, clear and accurate....and you manage to comprehensively confuse it....I am impressed

Well I did say I was in a pedantic mood and to be more pedantic it's a function of optical design, effectively including a teleconverter-like group of elements at the back (or Barlow lens in telescope). I've always assumed the 'length' defn. only applied when focussed at infinity. I doubt many long 'telephoto' lenses aren't telephoto designs these days, except those insanely long cheap and nasty ones with weird names that you only seem to get on eBay.

I know what the term means in practice - I use it that way myself and I'm not going to the stake about this. But no reason not to know the technical meaning as far as I can see? (And my 100mm macro may not be a telephoto even at infinity since it's always > 100mm - just - from front element to sensor plane)

I see your adding the rear flange distance now (Your original definition only mentioned the length of the lens) Mine is ~95mm with both caps on.

I guess the 500mm Newtonian telescope I have is actually only telephoto if used with an eyepiece (and the camera of course) - it must be about 1m long

[pedant mode off]

Time for bed.

The focal length of a long lens is measured from the film (sensor) plane to the optical center. That measurement is done with the lens focused at infinity.

For a telephoto lens, the optical length is measured with the magnification ratio when the lens is focused at infinity. Note that the focal length of telephoto quit often changes a lot when it's close focused, especially the "internal focusing" ones.

The physical length of a lens has nothing to do with the focal length since the optical center might be way behind the front lens element.

Enough already. It's like having Stefan back!

But to repeat my question to wuttwutt, what is it you are hoping your teleconverter will do for you? Don't want to see you making a purchase you might regret.

[quote:3496ace15f="greynolds999"]

Enough already. It's like having Stefan back!

quote]

Seemed like a clear concise explanation for the noob to me

John