Increasing the light-gethering ability of a lens

[Rob]

Rob, the only time you ever get to use a 2000mm F11 is during REM sleep

Erm... nope. They're about £1,500 really.

This is what I use: http://www.werbeagentur.org/oldwexi/fotos/meade_etx125ec.jpg

I think you should seek help for your size issue!

The short of it is (pun intended) that you'd be looking at shooting stuff that's over a mile away with that kind of focal length. The crap in the air over a one mile stretch is enough to render most shots fuzzy anyway. You won't be able to use auto-focus, you'll need a mega steady tripod, and with the field of view offerered, you'll be relying on tracking using the viewfinder as you won't be able to see your target.

In short, what you're trying to do, can't be done on the cheap effectively. As the Doc says, a 600 f/4 with 2x TC would give excellent length and good quality, but that's going to be expensive.

Why? What do you want to shoot? Wildlife?... nope - they move too fast for that kind of set up. People? ..ditto. Landscapes?... they'll be rubbish quality.

Rob

 

[bigfatbadger]

You could try some wideangle shots ;-)

 

[Torus34]

Go ahead and start with the Sigma. It's only $4,500, and you can always re-sell it on eBay for a quick $2,000 or so. At least, you'll have nicely scratched the itch. After you've worked with it for a while, you'll have a pretty good idea as to whether you wish to go for an ultra-long fl lens. I rarely have a need for my 400mm lens.

One solution for the problem of steadiness is the use of small sandbags. These can be placed on the top of a car and the camera/lens nestled down on them. Much cheaper and easy to carry than the studio-weight tripod otherwise required.

 

[Rob]

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. t

The magnification of lenses does not get multiplied, you just get 1.5x less picture. This is not the equivilent of using a longer lens by the way.

A 300-800 does not turn into a 1350-3600, it just has the same field of view. The items in the frame will not appear closer.

An interesting experiment for you. For 35mm film, use a cardboard slide mount, for an APS-C sized sensor, cut a 22.7 x 15.1mm hole in a bit of card. Then, take some cardboard tubes, like a poster tube or a kitchen-roll tube and tape the cut out over the end. Cut the tube to the length (focal) in mm i.e. 5cm for 50mm or 20cm for 200mm or if you're mad, 3.6m for 3600mm. Then try looking through it - that's what you'll see.

You get the idea about how crazy over 600mm is then. You'll be hard pushed to find 4m of tube for the experiment - and you won't be able to hold it still relative to the background. Try it to see what focal length you're actually after.

Rob