[beaumont]

Pardon my ignorance, but with regards to the newly announced Pentax Rear Teleconverter...what exactly does this do? What about extension tubes, what is thier purpose?
Thanks in advance...

[Marc Sabatella]

Pardon my ignorance, but with regards to the newly announced Pentax Rear Teleconverter...what exactly does this do? What about extension tubes, what is thier purpose?
Thanks in advance...

Both teleconverters and extension tubes go between the lens and the body. A teleconverter increases the effective focal length of the lens. So, for example, a 1.4 TC turns a 90mm lens into a 135mm lens (90*1.4 = 135).

Extension tubes have no glass in them at all - they are just hollow tubes that move the lens further from the body. In doing so, the lens loses the ability to focus on fistance subjects, but gains the ability to focus more closely than it would have otherwise.

Both have the effect of making things bigger in a way. A TC changes the focal length of the lens, so standing the same distance from the subject, it looks bigger. Thus instead of a distant animal taking up only half the frame, it might take up most of the frame. Whereas a TC lets you take bigger pictures of objects by letting you get closer to them than the lens otherwise would have allowed. Doesn't help at all with a distant deer, but helps a lot with an insect right in front of you.

[beaumont]

Thanks for the explanation. I assume that there must be some sort of a compromise with the Teleconverter? Are the pictures going to be as sharp? CA, or AF issues?
Thx again...

Teleconverter has lens within, imho every extra lens will loose minor sharpness but within teleconverters, the difference is not easily noticeable.

What you have to remember is that teleconverter will "eat" your light with same rate you gain focal length. So that 90mm lets say F3,5 lens with 1,4 TC as mentioned above will become 135mm F4,9 lens (F3,5*1,4=F4,9).

[beaumont]

Teleconverter has lens within, imho every extra lens will loose minor sharpness but within teleconverters, the difference is not easily noticeable.

What you have to remember is that teleconverter will "eat" your light with same rate you gain focal length. So that 90mm lets say F3,5 lens with 1,4 TC as mentioned above will become 135mm F4,9 lens (F3,5*1,4=F4,9).

Aaaah, thanks for the response...always a trade off I guess. With the newly announced Pentax unit, am I correct to say the AF will work on my DA lenses?

[MikePerham]

Teleconverter has lens within, imho every extra lens will loose minor sharpness but within teleconverters, the difference is not easily noticeable.

Actually, the better quality TC's have more elements, not less. And even with the very best you do lose a bit of IQ wide open, but just stop down a bit and you will be OK. Of course, as mentioned, your f2.8 becomes an f4 (with a 1.4X TC) and your f4 becomes an f5.6 ..so you are losing a stop right off the bat and when you stop down to 5.6 on your lens, you are actually at f8.

Mike.

Actually, the better quality TC's have more elements, not less. And even with the very best you do lose a bit of IQ wide open, but just stop down a bit and you will be OK. Of course, as mentioned, your f2.8 becomes an f4 (with a 1.4X TC) and your f4 becomes an f5.6 ..so you are losing a stop right off the bat and when you stop down to 5.6 on your lens, you are actually at f8.

Mike.

Yeah, true, let me rephrase what i meant

Adding more optics between lens and camera body will not improve image sharpness.

It would be nice tough, add some "eye-classes" to a bad lens to get sharper image



(pardon my grammar im not native english writer)

[beaumont]

Yeah, true, let me rephrase what i meant

Adding more optics between lens and camera body will not improve image sharpness.

It would be nice tough, add some "eye-classes" to a bad lens to get sharper image



(pardon my grammar im not native english writer)

So on a faster lens, the loss of speed may not be as big an issue, but my faster lenses are also a short focal range so the improvement isn't that big. (1.4 X 31 = 43ish) However on my 18-250 the improvement would be larger, however they would have to be slower tripod shots as it's already a slow lens wide open. (Might be an interesting partner for a 77ltd at 1.8?)