How far can a lense see?

[SteveEllis]

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!!

Cheers guys

 

[DocFrankenstein]

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

 

[markc]

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:

M = f / (f − S)

where f is the focal length and S is the distance from the lens to the object.

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.

 

[SteveEllis]

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

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?

 

[SteveEllis]

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

 

[markc]

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.

 

[Dave_D]

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!

 

[DocFrankenstein]

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

how can you have an engineering degree and not figure it out?

 

[Big Mike]

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.

 

[SteveEllis]

how can you have an engineering degree and not figure it out?

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

 

[markc]

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.

 

[SteveEllis]

Here is the product

http://www.opteka.com/productdetail.asp?ProductID=4316

 

['Daniel']

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

Too many engineers nowadays. There's a refuge engineer (binman), culinary engineer (dinner lady). Surely you needed to do maths for this engineering degree?

 

[PlasticSpanner]

Quite often see the Opteka lenses on ebay!

You gotta ask yourself what are you going to use it for, will it do the job and why is it so cheap when others are over 800GBP?

Do you need to have such a long lens or can you just move closer?

 

[Big Mike]

The aperture is F8 - F16....probably F8 at 650mm and F16 at 1300mm.

With an aperture of F16 you are going to need a lot of light to get a shutter speed even remotely fast.