Need Advice on Focal Lengths

Hey everyone,

So I went waterskiing on my friend's boat this weekend, and I brought my XTi along for the ride. Unfortunately, I only have a Sigma 20mm f/1.8 lens on my camera at the moment (effective focal length would be about 32mm), so the following photos are kind of extra wide for the shot. I'm curious what focal length would allow me to capture closer images, say having the skier mostly filling the frame. Unfortunately I do not know the length of the rope, but I believe it was approximately 60 - 70 feet.

I am ok with primes, as that's all I've got now, and I enjoy low light shooting anyways so fast primes could be considered a plus. Zooms are ok too, but I've noticed they tend to be somewhat more expensive.

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The skier occupies about 1/13 of the height of the image @ 20 mm focal length. For him/her to use the full frame, you'd need to multiply that by 13, so 250mm should just about do it.

Perfect timing!! I'm going wake-boarding tomorrow. I'm bringing my gear and will be trying to photograph the action. I figure a 70-200 will fit the bill nicely. I'll let you know!!

The 250mm would be effective focal length (after including the 1.6x crop factor), correct? If that is the case, would a 135mm (probably the L model) be about the right focal length, maybe a little shorter?

woloi wrote in post #3804687
The 250mm would be effective focal length (after including the 1.6x crop factor), correct? If that is the case, would a 135mm (probably the L model) be about the right focal length, maybe a little shorter?

No, because the estimate was based upon the fact you were using a 20mm on the same camera, so the 'crop factor' was already there. If you switched to FF, the factor would be applied the other way and you would need a 400mm.

Primes are lighter and cheaper, but I'd still use a zoom for this type of shooting. More flexibility to change perspective for different shots. Like above, the 70-200 is probably an excellent choice for water skying.(and you'll find a LOT of other uses).

There are 4 to choose from. The f/4.0 versions should be adequate for sunny-day water skying. IS may help with the boat bouncing. Of course if you can afford it, get it all 70-200 f/2.8 IS.

Darn...the 70-200mm zooms are amazing lenses...but they're pretty expensive, especially seeing as I just started my senior year of college...

For low light shooting, is the f/4 IS equivalent to the f/2.8 IS (assuming a fairly static subject)? I've heard that the f/4 has better IS that can do 4 stops, while the f/2.8 IS can only do 3 stops (plus the one stop of aperture). I have a feeling that I'd wind up using whichever lens I eventually buy in lots of low light situations, so I'd rather get the "right" lens first (and yes, I know there's no way to designate one right lens for a particular situation).

Here's a similar type of shot with the cheapest of the 70-200s - perfectly well up to the job in this light.

200 2.8L would be good and it isn't terribly expensive. I think a zoom would work better though. The 70-200 f/4s are hard to beat for the money.

Fast zooms are spendy, but so are long primes!

Madweasel, that's an awesome shot, looks kind of painful for the wakeboarder though...any reason he's still holding onto the rope when he's half out of the board? Apart from being pretty hardcore...

I'm hoping to get an on campus internship position in the next week or two. If I do get it, it'll pay $1500 or so for the year (I think), which is just about right for a 70-200 f/2.8 IS. We'll see how that works...

woloi wrote in post #3804687
The 250mm would be effective focal length (after including the 1.6x crop factor), correct? If that is the case, would a 135mm (probably the L model) be about the right focal length, maybe a little shorter?

The ONLY time you need to consider the "crop factor" is when you are comparing what a particular lens on someone's "full-frame" digital or 35mm film camera would relate to on your camera.

What was stated above is that if you want to make the subject 13 times taller in a particular image, you need to multiply the focal length that you used for the original by the 13. If you need to double the height (or width) of a subject in an image taken with a particular focal length lens, you would need to double the focal length.

"Crop factor" is not something you should even be thinking about in any way shape or form for this sort of calculation.

70-200 f4 non IS rawks! these were taken from the shore, they are cropped afterwards but gets you closer.