Need help please!

[Andrew76]

Can someone out there in our community - possibly one of our Engineers please create a 2x tele extender that does not 1/2 the aperture, or affect IQ, please?

That's all.

Oh, or please create an f/2.8 400mm lens that does not cost $15,000?

Either way, I'll be happy, and purchase one.

Thanks in advance!

[yauman]

Hmm... I don't know - it's a mathematical definition problem rather than engineering. Since f-stop = Focal Length/aperture diameter, then, of course doubling the focal length will half the f-stop.
eg: f4-200mm lens means 50mm diameter aperture opening, basically the diameter of the lens. (200/50 = 4)
Now, if you double the focal length to 400, but the aperture diameter stays the same (unless you can add more glass), you have 400/50 = 8, so you have an f8 400mm lens.

Now, let's see - a f4 400mm lens will have a diameter of at least 100mm - that's a huge lens which is a major engineering challenge to mold and grind with precision so I don't know if they can make it cheaper.

The only way to have a cheaper and maybe even smaller long focal length and low f-stop lens for some optical engineer to design an "active" lens - ie lens with photon multiplier build in, to intensify the light coming in to give a brighter image.

[Andrew76]

Yau-Man, you're not helping here!

We need someone to think outside of the box!!

Actually, I like your photon multiplier idea!

[HaseebM]

Yau-Man, you're not helping here!

We need someone to think outside of the box!!

Actually, I like your photon multiplier idea!

How about a Schmidt-Cassegrain scope with 8" objective and 2032mm focal length? It will be an f/10 but you can install "Fastar" which will turn the scope into a fast f/2.8. Camera can be attached and the tube will become a tele-lens.

[GrahamS]

Y'all need to get out more.....

[RustBeltRaw]

Oh, or please create an f/2.8 400mm lens...

Sure!

...that does not cost $15,000?

...Oh. Probably not, unless you're cool with LensBaby quality.

[rpcrowe]

Actually, Andrew, the higher ISO capability of today's cameras somewhat make up for smaller apertures.

[Andrew76]

Sure!


...Oh. Probably not, unless you're cool with LensBaby quality.

Common Lex, I have total faith in you!

[Andrew76]

Actually, Andrew, the higher ISO capability of today's cameras somewhat make up for smaller apertures.

It's true, Richard. I'm just complaining a little. I got to cover a pretty cool event yesterday in an arena, that was a motivational/concert/feel good-type event, so when the lights were low, even with a media access pass, my 200mm f/4 was pretty far away. I thought darn, I should have brought my 2x extender - BUT, I was already shooting at iso3200 to get a 1/200 shutter at f/4.

Then, of course, the guy from McLean's Magazine steps up beside me with THIS

Needless to say, I felt a little 'inadequate'!!

Regardless, the event was fun, and I got some great shots of some fantastic people - even Martin Luther King III etc...

[tao2]

Can someone out there in our community - possibly one of our Engineers please create a 2x tele extender that does not 1/2 the aperture, or affect IQ, please?

That's all.

Oh, or please create an f/2.8 400mm lens that d Andrew
oes not cost $15,000?

Eitiher way, I'll be happy, and purchase one.

Thanks in advance!

Hi Andrew,

It's a matter of physics rather than maths. Whilst the diaphragm opening is larger in the longer lenses, the light transmitting ability is the same. That's theoretically correct but it's accurate enough tae generalise. The greater volume of light is offset by the distance travelled.

A 50mm lens, focused in the distance, means light travels 50mm from the rear of the lens; similarly, a 200mm lens has its beam travelling 200mm. It's a law of physics that intensity of light decreases with distance travelled (inverse square law), therefore the 200mm lens must transmit 16 times as much light as a 50mm, given that it has a 4x longer light path. Ergo, it must have an aperture 16 times larger. As said in another post , at f4 a 200m lens has a 50mm diameter, c/f a 50mm lens has 12.5mm, this translates tae an area 16 times greater for the 200mm lens.

This applies tae all f numbers and all lenses; at any given f number, light-transmitting ability of the lens is mathematically the same, regardless of focal length.

"Why's he bothering me with all that"? Well, using yer tele-extender (which contains the negative elements), the converging rays of light are lessened, spreading the image over a greater area at a greater distance from the focal plane of the lens. The image is spread over 4x the area but with the same illumination-so intensity is reduced tae one quarter, therefore the f number has tae be doubled. A 50mm/f4 lens becomes a 100mm/f8. One advantage is that ye can focus closer since the tele doesn't affect the len's focusing scale.

So, the long-winded answer is ...no, ye can't get a tele-extender which will allow ye tae keep yer aperture.

Re. the 400mm/f2.8, couldn't find any at reasonable prices but what about these? First one's a 500mm/f5.6 - £250+£40, respectively...



[IMG][/IMG]

[LocalHero1953]

I used to have one of them there Paragons, around 30 years ago. Used it on a Practica SLR. The light was never good enough to use it in the UK, but I hand held it for birds in brightest Africa, developing my biceps as I did so.

[ajohnw]

Yau-Man, you're not helping here!

We need someone to think outside of the box!!

Actually, I like your photon multiplier idea!

The only out of the box idea is to reduce the size of the sensor you might feel that it has already been done.

Photo multipliers come in 2 flavours. One flavour really One is bit loosely like a TV cathode ray tube and the other uses a sensor much like the ones in a camera. The former often produce green images and oddly get brighter when bigger lenses are put in front of them. The 2nd type have the usual sensor problems. I believe that night vision type things are available that use sensors but they need a lens to focus the image on to them. Bigger the lens ............... etc.

http://en.wikipedia.org/wiki/Photomultiplier

There was probably something suitable on StarTreck or maybe ET could produce an answer. Then there is gravitational lensing as well but as no one can detect gravity waves there isn't much idea how they could be generated. They would have unusual chromatic aberrations problems as well and attract everything around them including the user.

Perhaps the answer is software. It seems that certain guess who type organisations measure their lens aberrations very carefully and correct them with software. Maybe a single piece of glass could be used along side 50gbyte of ram, a 20ghz processor and a 40 amp supply from a tiny lithium battery or small nuclear generator. Pedal power might be more feasible.

John
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[Glenn NK]

Physics depends greatly on mathematics, so it's a physics-math problem (optics are loaded with complex equations).

One cannot get out of the physics-math "box", so there can be no solution to the OP's request - as long as we remain in the real world.

Might as well try for perpetual motion.

Glenn

[ajohnw]

There are some curious aspects to light but it's hard to see how they could be applied to normal photography. The amazing one might loosely be described as getting round diffraction problems. If that could be done there is no limit to the magnification that could be applied. The problem is that the distance between the "lens" and the object has to be short compared with the wavelength of light. It can be done by placing a glass bead on something and then examining that with a microsope. The field of view is a bit small.

There is also an interest in elements that have negative refractive indexes. Only one I have heard of is silver so exceeding thin layers of that can do strange things but I doubt if it would help with the original question.

I once saw a military lens that had white glass in it. Easy to look through and I understand it was radio active. Not sure what that achieved but no help again. Couldn't afford to buy it at the time either as the man wanted too much for it. I suspect it fell of the back of an aeroplane.

John
-

[yauman]

Ok, let me try thinking out of the box here! When I was in high-energy particle physics (40 years ago - Fermi Lab/CERN) I used a prepped a lot of photomultipier tubes. They were like oversize vacuum tubes - size of a thermos flask and outrageously expensive. To keep down the noise (signal, electrical), we have to house them in a box with dry-ice packed around them. Yes, they worked. Then, in 1978, I purchased one of the first precursor to today's camera sensor - a CCD (charge-couple device) sensor - resolution 25x25 pixels, overall size 1/4' square (1cmx1cm) - at the outrageous price of over $8,000 - paid for by the nice folks of AARPA aka Army Advance Research Program Admin (who also brought you AARPAnet, aka The Internet) - ok, actually all the nice tax payers of the US of A. (Wish I had kept the purchase order!)

Now, I'm thinking that perhaps 40 years later with all the great advances in solid state sensor technology, miniaturization, and nano-technology, someone would figure a way to embed that photo multiplying function into the glass itself. Yes, I dreaming about an "active" lens, ground/moulded with the proper optical geometry like any other prime/zoom lens, except, each lens element has the built-in "magic" to amplify the light, so the refracted light is brighter than the incident light - ie more photo exiting than entering. I've been reading about the newest thing in graphene technology, that they can make a transparent one - so it may not be that far fetch. Now, how about that for thinking outside the box Andrew?

[Andrew76]

OK, thanks guys!

I'm glad I sparked a little conversation here! Although, admittedly, my original post was kind of half in jest. I do understand the way an extender works, and I even almost understand why the lens I want is $15,000 - not sure I'll ever fully understand that.

I guess now I'll just either have to find someone who would be willing to pay for the lens for me, or track down the guys who built the time machine using a flux capacitor, and a Delorean to build me my extender. I don't imagine either will be very easy.