How far can a lense see?

Discussion in 'Beyond the Basics' started by SteveEllis, Jan 24, 2006.

  1. SteveEllis

    SteveEllis TPF Noob!

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    Hi Guys,

    I've been eyeing up a few lenses for my camera and came across a 1300-2600 lense and was wondering how far away an object could be for you to focus on it and take a picture of just that item.

    I suppose the best example would be a person, how far away could the person be and you still be able to take a picture of just them?

    And no, I'm not a peeping tom!! :lol:

    Cheers guys :)
     
  2. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    take a sine of the half the length of the sensor and the focal length and you'll find out the angle of view.
     
  3. markc

    markc TPF Noob!

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    Whee! Time for some more math.

    Since you want to take a picture of just a person, lets hold the camera in the vertical position and assume you are using 35mm film. A 35mm neg is 1" x 1.5", so the long side (height in the vertical position) is 1.5". Using a 6' tall person, we need to figure out how far away a 6' person would have to be in order for a 2600mm lens to make his image 1.5" tall.

    From Wikipedia, we get the formula for magnification:

    Or since we are looking for S:

    S = ( (M-1)f ) / M

    6' = 72"
    M = 1.5"/72" = -.0208 (negative because it's a real image, not virtual)
    f = 2300mm = 90.55"

    S = 4444" = 370'

    A 6' person 370' away would fill a vertical 35mm frame using a 2300mm lens.

    Someone check my math please.
     
  4. SteveEllis

    SteveEllis TPF Noob!

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    Um, pardon? I have an engineering degree, but not in maths :lol:

    You say Sensor, are you refering to digital? Mine isnt Digital.

    A rough distance will do, I'm just trying to get an idea in my head of what lenses are capable of. :)

    Going by the equation above I assume the focal length is the 1300-2600, how do I take a sine?
     
  5. SteveEllis

    SteveEllis TPF Noob!

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    Thanks Marc, you got your reply in before me there :)

    so 370' away, nearly 113meters.

    Will this change if it is really a 650-1300 lense with a x2 attatchment

    It is going for £200, think I might treat myself to that, I'm going on holidays in a few months, we've hired a river boat for a week, should be travelling through the open countryside of South East England, should be good for some nature shots. I want a few filters too :)
     
  6. markc

    markc TPF Noob!

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    That's at 2300mm, however you get there.

    You might want to make sure that the 2x multiplier will work with that lens. A lot of them work only with certain lenses.
     
  7. Dave_D

    Dave_D TPF Noob!

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    Take a good tripod with you. For that kind of focal length, it's not exactly a hand hold lens. Does it have an adjustable aperture or is it fixed? If the weather is cloudy, your exposure times will be longer especially with a 2x converter. You will loose a couple of stops with that....Just a few other things to consider. Have fun rafting!
     
  8. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    how can you have an engineering degree and not figure it out?
     
  9. Big Mike

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

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    Those lenses are not really practical for nature shooting (IMO)...you would be better off with a 300 or 400 lens, get a faster shutter speed and then just crop.

    I've used a 100-300 zoom with a 2x TC...giving me 600mm. On a 20D the field of view is about that of a 960mm (on 35mm film). Image quality was not great (cheep lens & cheep TC) but it was next to impossible to get sharp shots without a tripod or flash.

    There is a reason why pro wildlife photographers use lenses that cost $12000...it's what they need to get the shot. Cheap alternatives really don't cut it.
     
  10. SteveEllis

    SteveEllis TPF Noob!

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    Because I am a Software Engineer and in my field I dont use advanced maths. Poor I know.
     
  11. markc

    markc TPF Noob!

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    Yeah, the lenses with that kind of pull that are worth owning are usually out of the price range of the average Joe. A lot of the ones out there would turn out to be more of a toy. They'd be fun to play with, but you're likely to get tired of the poor quality images.
     
  12. SteveEllis

    SteveEllis TPF Noob!

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