Relationship between f/length and distance?

[Duncan.F]

Hi,
I am the proud owner of a canon d400! Came with 17-70 2.8 Sigma and 18-50 3.5-5.6 Sigma. The 18-50 seems to be unnecessary as the 17-70 covers a greater range? If so I could sell on I guess.I assume it was the kit lens originally supplied with the camera.All seems in excellent nick.

Its probably a numpty question but I am thinking of buying a telephoto lens and have followed a lot of the debates on the 70-200 and others but I don't understand a key thing. Whats the relationship between focal length, object size and distance? So If I say want a full frame shot of a swan at 30mts what f/l is required?

Incidentally camera etc [all s/hand] came off ebay paid 460 for the above. Also inc was small camera bag 1g card and spare batteries.
Left germany 25/7 and arrived 9am this morning!!
Thanks in advance.

Dunc

 

[Mr Perceptive]

Try the calculator on here, the 400D has a CF of 1.6x in the camera type

http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

Hope that helps

Mr Perceptive

 

[Steep]

The F number is a measure of how much light enters the camera and affects the shutter speed you can use and the depth of field (ie how much of the image is in focus) So the F number has no effect on how much of your swan is in the image but rather how much of it and it's surroundings are in focus.

At f2.8 only a very narrow band will be in focus though exactly how much varies with distance, so if say at 1m distance 5mm depth is in focus, at 50m distance 2.5m will be in focus (not entirly certain it's a linear increase depth for distance but just as an example)
At f16 if you get 3-4cm in focus at 1m distance you'd get 15-20m depth at 50m.

Does that make sense?

 

[Duncan.F]

I have not explained myself very well! A 200mm lens will be able to bring up something full frame from further away than say a 50mm lens. That being the case what is the relationship between say a 200mm lens and distance?

Dunc

 

[Mr Perceptive]

Duncan

Experiment with the calculator on the link in my post above.

All should become clear

Mr Perceptive

 

[Bob]

Makes great reading added it to my favorites list to refer to when I need it
Thanks Mr Perceptive
Bob

 

[Mr Perceptive]

Bob

Glad you found it useful

There's a whole a section of useful tutorials on that site. http://www.cambridgeincolour.com/tutorials.htm

Mr Perceptive

 

[whiteflyer]

Try the calculator on here, the 400D has a CF of 1.6x in the camera type

http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

Hope that helps

Mr Perceptive

Nice Link, I hadn't see that site before.

But to shot birds on my feeder from my kicken it says I'll need a 950 mm lens

 

[Ewan]

Bob

Glad you found it useful

There's a whole a section of useful tutorials on that site. http://www.cambridgeincolour.com/tutorials.htm

Mr Perceptive

Thanks for that, bookmarked Looks very useful.

But I think the general answer for birding is - unless you're a rich guy with a big white bazooka, you always need a longer lens

 

[Duncan.F]

Hi,
Thanks for the input. yes that site is really useful. Have added it to my favourites and will have a play!

Dunc

 

[IanC_UK]

But I think the general answer for birding is - unless you're a rich guy with a big white bazooka, you always need a longer lens

I used to have some money but now own a big white bazooka, and i still want a longer lens (than the 600mm)

 

[Rich]

i'd love a big white bazooka

 

[antc]

Bob

Glad you found it useful

There's a whole a section of useful tutorials on that site. http://www.cambridgeincolour.com/tutorials.htm

Mr Perceptive

Indeed a great site for tutorials there, i hadnt seen this site before either

 

[RichieRich]

That site is so good. I've found the section on Hyperfocal Distance to be of particular interest.

 

[Mr Perceptive]

Glad to be of help!

Mr Perceptive