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Why the aversion of front mounted TCs? They are better.

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You seem to not understand the concept of f-stop at all. Hint: format size doesn't affect entrance pupil size.

--
Lee Jay
(see profile for equipment)
 
Jurgen Larssson said:
With a front mounted TC you gather and focus more light
so you won't lose any stops of light when exlarging 2x-3x.

Easy to remove/flip up

In theory you could make a 70-200 2.8 IS into a 70-200 1.8 IS
using a front mounted TC just gathering more light.

Or a 140-400 2.8 IS

Or a 210-600 2.8 IS

Are there really no applications for this?

---

Look at Tcon 17 and Tcon 300.
You can mount them on regular SLR lenses with step rings.
At 200 mm you only use the center of the lens anyhow.
It'll work, and won't weigh much more.

Any tests?

F
Click to expand...
--



http://www.pbase.com/zylen

'Never argue with an idiot, they drag you down to their level and beat you with experience'
 
Not a TC.

I don't agree with what Jorge Larson says oh-so-eloquently most of the time, but with this I do agree somewhat .

A high-quality adapter that would decrease focal length should be able to decrease f-stop as well. Although this would still be better applied between the camera and the lens.

I don't see this violating any physical laws, but I do see complications, like maybe focusing distances and aberrations. And, economical reasons for a business like Canon not to do it (EF-S lenses).

But imagine putting this on the 20D and the 24-105/4. It could become a 15-65 f/2.5. Which makes me think why Canon didn't do such a lens instead of the 17-55. Maybe the aberrations are much harder to control at those focal lengths.
mdg said:
I can't decide whether you're trolling or just seriously
mistundertand how camera lenses operate (and have not listened to
the people trying to explain it to you above). I suggest you
re-read lfigers posts.
Click to expand...
 
Every single pixel "sees" the entire aperture (well, except for that portion of the sensor that may vignette a bit at the edges).

A lot of people seem to think that a smaller sensor only looks through the center of the lens elements. That is just flat wrong.

--
Lee Jay
(see profile for equipment)
 
saynomore said:
A high-quality adapter that would decrease focal length should be
able to decrease f-stop as well. Although this would still be
better applied between the camera and the lens.

I don't see this violating any physical laws, but I do see
complications, like maybe focusing distances and aberrations. And,
economical reasons for a business like Canon not to do it (EF-S
lenses).
Click to expand...
Yeah, focusing distance is a huge problem--the main lens would need to be closer to the sensor than normal, and yet have more glass put between it and the sensor. Only practical if the lens has a much greater registration distance than the body it's being used on. It also needs a proportionally larger image circle.

It has been done--the 1996 Nikon E2 had the converter built into the body. But several fast lenses vignetted wide open, some wide lenses vignetted at any aperture, and image quality was so marginal people griped about it even on a 1.3mp camera. Nobody has ever tried it again.
saynomore said:
But imagine putting this on the 20D and the 24-105/4. It could
become a 15-65 f/2.5. Which makes me think why Canon didn't do such
a lens instead of the 17-55. Maybe the aberrations are much harder
to control at those focal lengths.
mdg said:
I can't decide whether you're trolling or just seriously
mistundertand how camera lenses operate (and have not listened to
the people trying to explain it to you above). I suggest you
re-read lfigers posts.
Click to expand...
Click to expand...
 
ljfinger said:
Every single pixel "sees" the entire aperture (well, except for
that portion of the sensor that may vignette a bit at the edges).

A lot of people seem to think that a smaller sensor only looks
through the center of the lens elements. That is just flat wrong.

--
Lee Jay
(see profile for equipment)
Click to expand...
 
mdg said:
I can't decide whether you're trolling or just seriously
mistundertand how camera lenses operate (and have not listened to
the people trying to explain it to you above). I suggest you
re-read lfigers posts.
Click to expand...
Not just possible, it's been done, in the Nikon E2. It didn't work terribly well and got ditched, but it did function as advertised with at least some lenses.
 
Roger Krueger said:
mdg said:
I can't decide whether you're trolling or just seriously
mistundertand how camera lenses operate (and have not listened to
the people trying to explain it to you above). I suggest you
re-read lfigers posts.
Click to expand...
Not just possible, it's been done, in the Nikon E2. It didn't work
terribly well and got ditched, but it did function as advertised
with at least some lenses.
Click to expand...
 
ljfinger said:
Every single pixel "sees" the entire aperture (well, except for
that portion of the sensor that may vignette a bit at the edges).
Click to expand...
Which is not the same as using every bit of the front element. A given pixel only uses that portion of front element it can "see" past the aperture.

For a small format change it may still use all of the front element because there probably wasn't enough front element for full illumination wide open at its intended format. But put a 4x5 lens on a 2/3rds sensor and you definitely won't use all of the front element.
Lee Jay said:
A lot of people seem to think that a smaller sensor only looks
through the center of the lens elements. That is just flat wrong.
Click to expand...
But it wouldn't be wrong if lenses actually had big enough front elements to not vignette wide open on their intended format. It's just that making the front element considerably more expensive for a half-stop in the corners wide open--that 95% of people will never care about--is economically silly.

A simple thought experiment: Put a step-down ring on a lens so it definitely vignettes wide open. For slower, wider lenses at least there will be a patch in the center that won't be darkened by the vignetting (though of course there'll still be regular falloff.). The size of that patch is the format that only uses the chunk of glass you've left usable with the step-down ring.

A tangent on this thought I'd like your opinion on... it would seem that the width on the image plane of the ring from "no vignetting" to "dead black" should be the width of the aperture--the theoretical width, not the reduced-by-optical-trickery width. It would also seem to follow that a lens that covers without vignetting stopped down enough could have no more than 1 stop of light lost to vignetting wide open. Or am I being totally clueless?
 
Roger Krueger said:
ljfinger said:
Every single pixel "sees" the entire aperture (well, except for
that portion of the sensor that may vignette a bit at the edges).
Click to expand...
Which is not the same as using every bit of the front element. A
given pixel only uses that portion of front element it can "see"
past the aperture.

For a small format change it may still use all of the front element
because there probably wasn't enough front element for full
illumination wide open at its intended format. But put a 4x5 lens
on a 2/3rds sensor and you definitely won't use all of the front
element.
Roger Krueger said:
A lot of people seem to think that a smaller sensor only looks
through the center of the lens elements. That is just flat wrong.
Click to expand...
But it wouldn't be wrong if lenses actually had big enough front
elements to not vignette wide open on their intended format. It's
just that making the front element considerably more expensive for
a half-stop in the corners wide open--that 95% of people will never
care about--is economically silly.

A simple thought experiment: Put a step-down ring on a lens so it
definitely vignettes wide open. For slower, wider lenses at least
there will be a patch in the center that won't be darkened by the
vignetting (though of course there'll still be regular falloff.).
The size of that patch is the format that only uses the chunk of
glass you've left usable with the step-down ring.

A tangent on this thought I'd like your opinion on... it would seem
that the width on the image plane of the ring from "no vignetting"
to "dead black" should be the width of the aperture--the
theoretical width, not the reduced-by-optical-trickery width. It
would also seem to follow that a lens that covers without
vignetting stopped down enough could have no more than 1 stop of
light lost to vignetting wide open. Or am I being totally clueless?
Click to expand...
 
Roger Krueger said:
ljfinger said:
Every single pixel "sees" the entire aperture (well, except for
that portion of the sensor that may vignette a bit at the edges).
Click to expand...
Which is not the same as using every bit of the front element. A
given pixel only uses that portion of front element it can "see"
past the aperture.
Click to expand...
Right. It would be the same if the entrance pupil were far forward in the lens which it is on long lenses, but not on short ones (usually).
Roger Krueger said:
A tangent on this thought I'd like your opinion on... it would seem
that the width on the image plane of the ring from "no vignetting"
to "dead black" should be the width of the aperture--the
theoretical width, not the reduced-by-optical-trickery width. It
would also seem to follow that a lens that covers without
vignetting stopped down enough could have no more than 1 stop of
light lost to vignetting wide open. Or am I being totally clueless?
Click to expand...
Beats me.

--
Lee Jay
(see profile for equipment)
 
The advantage of the Leica (and other mix-and-match-elements systems) is that the interchangable front elements are an integral part of the aberration correction of the assembled lens. With NO front element the lens is nowhere close to corrected.

Whereas with a TC, the TC itself has to correct any abberations it induces, because the main lens is already fairly well corrected. Good rear converters have seven elements to get near-full correction, whereas the TCON-14B is only 5 elements, and it's still huge and heavy. I'd guess smaller/cheaper front TCs don't even have 5.
 
Pyogenes said:
Yeah, you can collect the light before it enters the camera lens,
but then the camera wouldn't be able to focus because you've
altered the location of the imaging plane. You can add a second
lens element to fix that problem, but it'd make the effective focal
length longer and you'd be back to the smaller effective aperture.
Click to expand...
Right, if you've got a camera-lens combination that hits infinity without a converter, there's no way to insert a wide converter between the camera and lens and make it work. (Unless the lens was originally able to focus far, far past infinity--not completely unreasonable on, say, a view camera.)

But this isn't the only plausible configuration.

As cited in my other posts, the Nikon E2 (and E3) series used a wide converter built into the camera body. I believe they just made the lens registration distance smaller, and got infinity focus back. But I can't imagine they had much room to play with, even with the tiny 2/3 sensor, I'm guessing the crummy performance stemmed from not having enough room to have enough elements to get anywhere near fully corrected.

Also, you could take a lens from a system with a significantly longer lens registration distance than the body being used and probably cobble together something that'd work.
 
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