Increasing the light-gethering ability of a lens

Discussion in 'Beyond the Basics' started by superhornet59, May 2, 2006.

  1. superhornet59

    superhornet59 TPF Noob!

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    Hey guys, as soon as i make some money by sellin some good shots, i want to by myself sigma's 300-800mm lens. now heres where it gets cool, im gonna slap on a Kenko PRO 300 3.0x teleconvertor, plus mind you im using a D70s, which uses a non-full frame sensor. so i should be getting about 1.5x crop. in other words whatever the focal length of my lense (assuming its for 35mm film) gets multiplied by 1.5. so basicly, my not too shabby 300-800 is gonna turn into a much much MUCH more intruiging 1350- 3600mm lens. that, is just jaw dropping. okay so ya now i have this super fantastic lens where i can photograph my best friend halfway accross town.... but there is not a chance this will stand up with a max aperature 5.6.

    at 3600 mm remember the focal double triple (as i call it anyway) rule.
    shoot at focal speed (more or less) when handheld, double if your photographing a moving object, triple if your in a moving vehicle. okay so lets assume i just want to takea photo of a bird, im gonna have to use a shutter speed of 1/4000 at F5.6 (keeping in mind that is max at 800). hell i might as well shoot at the sun and even that will be underexposed. what i need: more light gathering power. i need some sort of crazy contraption to pump way more light into the lens. im thinking some kind of 'magnifying glass' that would go infront of the lens and concentrate all that extra light into the same size, or the other telescope route to use a reflecting mirror. i dont like tat cause it would have to be way too precise.

    so ya, i want a 3600 mm lense real bad :p, but i just need your help. Thanks -Matt
     
  2. Fate

    Fate TPF Noob!

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    LOL a 3600mm lens :D thats insane.... what do you plan to use it for? photographing pluto !

    but sorry, i cant help you on the technical aspect of it.....
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Don't forget that a teleconverter will steal light. I've never heard the specs for a 3x TC but it will probably steal about three stops. So even in very bright situations, you would need a fairly long exposure. Shooting hand held would be out of the question. Even on a tripod, you might have trouble keeping that rig steady enough.

    How about trying to attach your camera to a telescope?
     
  4. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Remember that the 3x teleconverter also reduces your aperture. A 2x teleconverter reduces the aperture by 2 stops. A 3x teleconverter is going to steal over 3 stops of light, so your f/5.6 is really f/16 or f/22.

    EDIT: Mike beat me by a minute! :)
     
  5. superhornet59

    superhornet59 TPF Noob!

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    yes lol thats what i meant when i said the aperature is mx 5.6.. at 800mm (my photo vocabulary is till a bit rough). well anyhow if you can stack them that would be wild.... think about it... a 7200mm lens... but that point light gathering and image degredation would be rediculous. oh and i dont feel like carrying a clunky telescope... slappin on a 5cm or so part onto an existing lens beats (in practical terms) carrying a scope. photographing plantets is a total possibility... but in that light i need a telescope. lets say this. im photographing something with an LV of 14. thats about normal for my shots.
    say my focal lenth is 3600, ill want to keep a shutter speed of 1/4000. now lets assume for the worst our aperture is F22, and im shooting at 400 ISO (good signal to noise ratio).

    okay so i just tried some exposure calculators, here are my results:
    LV 14
    ISO 400
    F22
    =
    1/125 shutter speed. not very good for a 3600mm lens (it didnt ask for the focal lenght. as long as i factor in that my aperature is now lowered to F22 maximum, focal lenght is irrelavant right?

    if i want to shoot
    ISO 400
    F22
    1/4000
    =
    EV 19. in other words, i can only shoot very very bright objects.

    at best i cant shoot ISO 800 and use NR later. ill loose some detail but oh well, this is a compromise. also with the size and weight of that lens, ill almost definately be shooting with a tripod, so its more or less safe to keep it at 1/2000 exposure as long as if the subject is moving, its moving slow. that means i can shoot at EV 17. that still only limits me to pretty bright days.

    so the only thing i can possibly do is get my camera to gather more light. so... any other suggestions?
     
  6. Rob

    Rob TPF Noob!

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    Telescope - mine's 2000mm f11. :wink: Oh, and no, you can't hand-hold it.

    Rob
     
  7. 2framesbelowzero

    2framesbelowzero TPF Noob!

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    ???? are you agraphobic ?
    ;)
     
  8. superhornet59

    superhornet59 TPF Noob!

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    lol i hope im not the only one who didnt get that...

    and Rob, the only time you ever get to use a 2000mm F11 is during REM sleep :lol:
    how much are those things anyway, like 50,000 bucks?

    besides do you need to buy a special telescope for use with cameras, or just use an attachment? suppose i decide to get a telescope instead, what are some good companies?

    i still want the teleceonvertor just cause of the fact that if i ever have a 300-800, and that just aint gonna cut it i can grab i out of my bag and set up the shot in a minute. good luck carrying a telescope in a Tamrac :p

    EDIT: im lookin around now... and all im seeing is these crummy 1200 mm :(
    aint there any like 4000 mm telescopes out there i can mount on my room, at a relatively cheap price (like, not gettin too far over 5,000)
     
  9. Digital Matt

    Digital Matt alter ego: Analog Matt

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    The real question is, what are you planning to attempt to photograph handheld at 2400+mm, and why?
     
  10. superhornet59

    superhornet59 TPF Noob!

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    lol not handheld. im just saying its for portability. ill attach it to a tripod if im gonna use it at full zoom. lets put it this way, everyone wants to be longer. it makes surprisingly little difference in performance, and it can even get in the way. but every guy wants one thing. to be the biggest and the longest ;)

    jokes aside i just found this not so little beauty: http://www.meade.com/rcx400/
    16"... do the math. actualy ill do it for you, thats a freakin 3251mm at F8! I love it! :hail:

    at ISO 600, still acceptable noise levels on my D70 even though NR will of course help, at aperture set to F8 i can get a sweet 1/3100 shutter speed on a bright day. in other words, its perfect. i wont have to worry bout going higher cause i aint gonna hold htat jumbo machine lol, and ya its pretty much perfect. except one thing, well actualy, $16,999 THINGS :(:grumpy:

    oh well... time to pump out some better photos...

    oh and as for what im photographing? stars/ planets/ the moon. mabye get a few laughs by trying to photograph my friends who live a mile away then show the photo to them and watch there jaws drop :p it be a wicked paparzzi device and stuff but im not that much into the whole invasion of privacy thing. other then that, catch shots of planes and stuff. hell, the list goes on my friend... anyway i dont realy know what magnification a given focal length will give you, so figure me these examples so i can truley understand the power of what im using. at what distance will these things will a typical digital slr frame (i dont know if the telescope adapter will compensate for my smaller sensor. if not, were lookin at a 4876 focal length yay :D). so ya, at what distance will these fill my D70s frame:

    Me (im 6"1')
    a bird (lets say its a foot tall on a branch)
    a car (i dont know how long an 'average' car is lol)
    a plane (Boeing 747 from front. i forgot the wingspan sorry)
    and a typical 2 story house

    thanks for your effort, Matt
     
  11. Digital Matt

    Digital Matt alter ego: Analog Matt

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    ;to a point, and without sacrificing quality. I want sharp, smooth pictures. I don't think a Sigma mega zoom with 3x teleconverter is going to give you a sharp picture, and neither is a cheap telescope. If you have $6k + to spend on a telescope, then you'll get a nice telescope, but if you want a lens that is made to take sharp pictures, then for that price, you can get a 600mm f/4 IS prime.
     
  12. superhornet59

    superhornet59 TPF Noob!

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    like i said, the bigger the mm the nicer it sounds so far, but other then that those numbers dont mean much to me. so a 500 mm might be able to do everything i want from a lense, and i wont need a 3000. but then again, i dont have much experience in the supertelephoto range so i have an idea of what i want, i just dont know what focal length will get me there, which is why i asked earlier
     

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