Dave163 wrote:.
...why is it that with dslrs you put a teleconverter between body andlens, whereas with compact cameras you put the teleconverter on theend of the lens (OK there's nowhere else for it to go.).
Duh! .
What I mean is, given the choice between say a 2x Kenko teleconverterthat fits between body and lens, and a teleconverter lens with afilter size that will screw in to the end of whatever lens is alreadyon my dslr, which is better optically? Clearly the latter avoidsintroducing dust and is less fiddly to put on and off. Wouldautofocus still work (assuming it's on a lens that doesn't have arotating end element)? Would I lose the 2 f-stops I'd lose with aKenko? And would the weight damage my slr lens if I screwed a TC intothe end of it?.
Weight damage? No. Light loss? Definitely..
I'm sure there's an optical reason for it as well otherwise kenko wouldn't be so popular and raynox would sell a load more to SLR owners.Don't wait for the Nikon D-whatever, have fun now!http://www.flickr.com/photos/j_wijnands/..
Might be easier to design for through the rear element of the lens. As long as the lens mount is constant, the light path between the rear element of any compatible lens and the sensor should, hopefully, be somewhat easier to characterize...
Slightly different outlook. With a compact camera the only choice is to add converter lenses to the front..
With a DSLR, there is another possibility. For example, you have a 200mm lens, and want more reach, so take off the 200mm and attach a 400mm. Well, that is an expensive approach, but it is likely to be better than using a converter. The disadvantage of attaching a 2x converter to a DSLR lens is the aperture, say f/5.6 now becomes f/11..
As to why the converter is not attached to the front. Well, size, weight and cost. For example, the Sony R1, uses a sensor similar in size to a DSLR. There is a 1.7x converter, weighs 950g. Look near the bottom of this page for the tele and wideangle converters for the same camera:http://www.kenrockwell.com/sony/dsc-r1.htm.
The above examples are probably unique, the quality may be very good, but not very practical...
Dave163 wrote:.
...why is it that with dslrs you put a teleconverter between body andlens, whereas with compact cameras you put the teleconverter on theend of the lens (OK there's nowhere else for it to go.).
You can do both with DSLRs, but there are relatively few choices that willwork well screwed onto the front of a lens. As you said, with fixed lens cameras you have no choice..
What I mean is, given the choice between say a 2x Kenko teleconverterthat fits between body and lens, and a teleconverter lens with afilter size that will screw in to the end of whatever lens is alreadyon my dslr, which is better optically?.
Depends on the lens and the teleconverter. Generally, what's up front is most important. If you have a not-so-sharp lens, any rear-mounted teleconverter will expose the lens' faults even more. But stick an excellent front-mounted teleconverter on the same lens and it won't show the lens' faults as much. But an excellent front-mounted teleconverter costs a lot more, primarily because there is a lot more glass..
Wouldautofocus still work.
Assuming it can provide a clear aperture that fits within the AF spec, yes..
Would I lose the 2 f-stops I'd lose with aKenko?.
Assuming it can provide a clear aperture, no..
And would the weight damage my slr lens if I screwed a TC intothe end of it?.
Here's the main issue. If you have a 200/2.8 and want a 400/2.8, you'll need a TC with a front element that is at least 142mm diameter. And if you want good quality you'll need 4 or 5 elements of that size. That's a lot of weight, probably 5 pounds. Not something most lenses would be happy holding by the threads..
And it's also expensive. So much so that you'd probably be better off just getting a 400/2.8. Which you can do with an interchangeable lens camera, but not with a fixed lens camera..
So there isn't a lot of choice for front mounted TCs on DSLRs because there is a better solution available: a lens of the appropriate focal length and aperture..
Seen in a fortune cookie:Fear is the darkroom where negatives are developed..
The difference lies in the nature of standard TELE-photo lenses, and their use of an optical device called a 'Barlow lens' that was first employed in astronomy..
Http://en.wikipedia.org/wiki/Barlow_lensOkay. Here is a little background...
It happens that normal photographic lenses are mounted at a distance from the image plane EQUAL to the focal length of that lens. This focuses subjects at infinity, with extra extension being added mechanically for subjects that are nearer the camera..
So a lens of 50mm f-length, say, is mounted at distance of 50mm, two inches or so..
100mm lens is placed at 100mm, on the front of an empty spacer tube of four inches approx..
1000mm similarly, but the empty spacer tube is a whole 1 metre in length over a yard!.
That's right! A long focus lens really was very long... the same length as it's f-length...... most of the barrel of the lens contained nothing but fresh air!..
Mr Barlow's discovery transferred from astronomy meant if a *negative* element (made with concave glasses instead of convex ones) was placed BEHIND a standard f-length lens the whole thing behaved like a narrow angle (long focus) lens......
.... but WITHOUT the physically inconvenient length of the original kind of long lens, which was a bit handy, to put it mildly, and so it was the telephoto lens was born..
BTW, this *telescoping" of the length* is why these lenses are called "teles" ... it isn't because the view is like that "through a telescope"... despite the fact that it might seem so. .
Note for your further information:-.
The addition of a further movable group of lens elements BETWEEN the front assembly and the Barlow group was how the early zooms came about. .
Now it becomes clear that the Barlow type lens can be sold separately and mounted behind ANY suitable lens to increase it's focal length... That is what is done, and, as it happens, they work BEST behind lenses that already have a Barlow group in them.... that is, they work best behind TELE-photo lenses..
1) The advantage of the Barlow behind-the-lens arrangement is that image quality is high, especially if the extender is as carefully matched to the assembly as are the elements within that original lens..
1.1) Another benefit of this arrangement is that the close focusing power of the original lens is maintained at the new longer f-length... This is a real bonus in use..
1.2) Primary disadvantage is the effective maximum aperture is reduced over the original. This is because the light hitting just the middle of the frame spread out over the whole frame which weakens it, of course..
1.4x magnification = one stop light loss.
2x magnification = two stops light loss... which is very significant when the original lens is not a wide aperture type...
What about the add-ons for screwing on the front of a {fixed} lens.. ??.
2) Mostly the quality is lower, but (and it is a big but) usually it's a whole lot better than severe cropping of the image would have produced. Also, and similarly to conventional tel-extenders, where the manufacture has been matched to the fixed lens' configuration, the optical performance can be more than adequate. I have a 1.5x tele-add-on for my KM A2 which is not bad at all. .
2.1) If the front elements are made large enough, there need be NO LOSS of light from the use of a front-of-lens extender. This adds to weight and therefore expense, of course. (Hey! You didn't get into this to *save* money.. did you?).
2.2) It is unlikely that any matched front-tele-add-on would be made available for lenses where the manufacturer produces a Barlow type for the same lens... which means the choice would only be from generic types from 3rd party sources..
There you go. I hope this was clear, and has been helpful.Regards,Baz..
So on a DSLR front lens threads maybe anything from 67mm on up to 100s of mm. A lot of high end tele lenses don't even have front threads..
So, as a manufacturer you need to produce a range of converters for all those thread sizes, but you also need to understand the distance to the next element in order to understand how to refract the light. The combinations would require a dedicated converter per lens converted..
Alternatively there are only a handful of mounts (Canon, Nikon, Minolta, Pentax etc.) where the size is uniform (the size of the mount in question) and the distances are uniform since you are intercepting light destined forthe focal plane. Much cheaper, simpler and scalable..
Dave163 wrote:.
...why is it that with dslrs you put a teleconverter between body andlens, whereas with compact cameras you put the teleconverter on theend of the lens (OK there's nowhere else for it to go.).
What I mean is, given the choice between say a 2x Kenko teleconverterthat fits between body and lens, and a teleconverter lens with afilter size that will screw in to the end of whatever lens is alreadyon my dslr, which is better optically? Clearly the latter avoidsintroducing dust and is less fiddly to put on and off. Wouldautofocus still work (assuming it's on a lens that doesn't have arotating end element)? Would I lose the 2 f-stops I'd lose with aKenko? And would the weight damage my slr lens if I screwed a TC intothe end of it?.
Thanks for clarifying!.
Http://public.fotki.com/wibble/public_display/.
Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window..
Thanks Baz, your contribution was clear and helpful..
I've found the Olympus TCON-17 or B-300 1.7x teleconverters (on the front) to be optically pretty good and inexpensive (B-300 around $60USD)..
CK Shene showed that two of them can be stacked to get 2.89X..
Dave..
Dave Martin wrote:.
I've found the Olympus TCON-17 or B-300 1.7x teleconverters (on thefront) to be optically pretty good and inexpensive (B-300 around$60USD)..
CK Shene showed that two of them can be stacked to get 2.89X..
A quick check shows these lenses each has approximately 70mm front elements. Even one of these on a typical 70-300mm f/5.6 will vignette at 300mm f/5.6. You'd need around f/8 to hope to have no vignetting. Stack two and you are around f/13 to get no vignetting..
Seen in a fortune cookie:Fear is the darkroom where negatives are developed..
Nickleback wrote:.
Dave Martin wrote:.
I've found the Olympus TCON-17 or B-300 1.7x teleconverters (on thefront) to be optically pretty good and inexpensive (B-300 around$60USD)..
CK Shene showed that two of them can be stacked to get 2.89X..
A quick check shows these lenses each has approximately 70mm frontelements. Even one of these on a typical 70-300mm f/5.6 willvignette at 300mm f/5.6. You'd need around f/8 to hope to have novignetting. Stack two and you are around f/13 to get no vignetting..
Thanks for pointing this out..
With a single Oly TCON:.
Vignetting/exposure change is not an issue on my FZ30 at above about f/3.6 150mm equivalent (the actual focal length is about 32mm because of the 4.7x crop factor)..
Vignetting/exposure change is not a problem at f/1.8 50mm actual for my Canon 300D (crop factor of 1.6). I have not tried with longer lenses..
Vignetting is a problem with stacked TCONs, but not too bad for long reaches..
Dave..
Dave Martin wrote:.
Vignetting/exposure change is not a problem at f/1.8 50mm actual formy Canon 300D (crop factor of 1.6). I have not tried with longerlenses..
If this is the mk II version of the lens (plastic mount), hanging half a pound of glass off the front threads is a problem. The lens can snap in half..
Seen in a fortune cookie:Fear is the darkroom where negatives are developed..
Nickleback wrote:.
Dave Martin wrote:.
Vignetting/exposure change is not a problem at f/1.8 50mm actual formy Canon 300D (crop factor of 1.6). I have not tried with longerlenses..
If this is the mk II version of the lens (plastic mount), hanginghalf a pound of glass off the front threads is a problem. The lenscan snap in half..
Good point!..