# How far can a lense see?



## SteveEllis (Jan 24, 2006)

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


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## DocFrankenstein (Jan 24, 2006)

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


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## markc (Jan 24, 2006)

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 &#8722; 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.


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## SteveEllis (Jan 24, 2006)

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?


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## SteveEllis (Jan 24, 2006)

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


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## markc (Jan 24, 2006)

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.


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## Dave_D (Jan 24, 2006)

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!


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## DocFrankenstein (Jan 24, 2006)

SteveEllis said:
			
		

> 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?


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## Big Mike (Jan 24, 2006)

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.


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## SteveEllis (Jan 24, 2006)

DocFrankenstein said:
			
		

> 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.


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## markc (Jan 24, 2006)

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.


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## SteveEllis (Jan 24, 2006)

Here is the product

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


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## 'Daniel' (Jan 24, 2006)

SteveEllis said:
			
		

> 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?


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## PlasticSpanner (Jan 24, 2006)

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?


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## Big Mike (Jan 24, 2006)

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.


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## PlasticSpanner (Jan 24, 2006)

And a hefty tripod to stop the mirror and shutter bounce when you take your shot! :er:


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## Fate (Jan 24, 2006)

man i hate maths


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## SteveEllis (Jan 24, 2006)

Daniel said:
			
		

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


 

Bloomin heck guys!!  Dont have a dig just cos I dont know how to calculate sine.  Yes I did have to do maths for this degree but I havent used the maths for 7 years, give me a break!!  Its not often you need advanced mathematics and when you do the functions have already been written, afterall everything nowadays is modular and reusable!!

With regards to the lens, I was actually browsing for something like a 300mm and spotted this, I have the spare cash so wondered if it would be a nice toy to have just in case.  The landscape in England is mainly flat with views for miles, unlike my homeland of Wales where its all hills and Mountains.  I figured if I spotted something nice way in the distance I would be thankful I had bought it and packed it for my hols 

Is the general concensus that the lens isnt worth it? It was on ebay that I spotted it.


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## 'Daniel' (Jan 24, 2006)

SteveEllis said:
			
		

> Is the general concensus that the lens isnt worth it? It was on ebay that I spotted it.



You will get an image that is of something far away but it will be terrible so It's not even worth it.


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## Rob (Jan 24, 2006)

SteveEllis said:
			
		

> Is the general concensus that the lens isnt worth it? It was on ebay that I spotted it.



You get what you pay for! 

As you're not paying a lot for a very adventurous focal length, you can expect poor quality glass, image aberrations, very long exposures, softness of image, extreme focussing difficulties... and that's _before _things are made twice as bad with a teleconverter! :mrgreen:

If you're not particularly fussy, and you're not in a hurry, then it might be acceptable for your needs. Don't be thinking you'll be springing into action and snapping that far away wildlife with it though.

Rob


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## SteveEllis (Jan 24, 2006)

So I would be better off putting the money into a better 300mm lens and taking PlasticSpanners advice of moving closer then


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## Rob (Jan 24, 2006)

SteveEllis said:
			
		

> So I would be better off putting the money into a better 300mm lens and taking PlasticSpanners advice of moving closer then



Definitely!


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## Rob (Jan 24, 2006)

Then you can get shots like this:
http://www.luminous-landscape.com/1photo-pages/t/tiger_2.shtml

That was a random sample picture I found which was taken with a 300mm f2.8 Canon prime. Nice lens. Bit pricey!

Rob


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## darich (Jan 24, 2006)

Steve

Have a look at this thread

it discusses another members question regarding the exact same lens


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## SteveEllis (Jan 24, 2006)

Decision made, thanks very much guys, even if some of you did pick on me


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