Need some help with Canon Lens.

[paulg1979]

Hi all,

Not sure if i'm being stupid but i'm selling my Sigma 17-70mm lens to purchase a Canon 17-55mm IS USM lens but in the description of the lens it says "The focal length is equivalent to approx. 27-88mm in 35mm format." What does this mean exactly?

Paul

 

[lifeafter2am]

You are using a sensor that has a 1.6x crop. It is not the same size as a 35mm camera, so instead of having a lens that is 17-55, you have to mulitply that by 1.6 and get a range of 27-88mm.

http://www.millhouse.nl/digitalcropfactor.html

More information there.

 

[Big Mike]

It's only important when you are comparing it to 35mm film. 35mm had been the standard for many years, and many of the current lenses were designed with film cameras in mind. Digital SLR cameras use many of the same lenses and because most DSLR cameras have a sensor that is smaller than 35mm film...there is a disparity in the field of view that you get for any given focal length. Basically, 24mm was quite wide on a 35mm film camera....but it's not wide on your 400D.

That's all...it's just a comparison to the old standard.

The 17-55mm Canon lens will have the same view at 17mm on your camera, that your Sigma has at 17mm, on your camera.

 

[paulg1979]

Excellent. That explains it. So with the Canon at 55mm and the Sigma 70mm will the Sigma still have the better zoom. I'm hoping to buy the 40D body when it arrives I don't suppose this will make any difference either will it?

 

[lifeafter2am]

Excellent. That explains it. So with the Canon at 55mm and the Sigma 70mm will the Sigma still have the better zoom. I'm hoping to buy the 40D body when it arrives I don't suppose this will make any difference either will it?

Nope, only with a 5D or a 1D series.

 

[Big Mike]

Excellent. That explains it. So with the Canon at 55mm and the Sigma 70mm will the Sigma still have the better zoom. I'm hoping to buy the 40D body when it arrives I don't suppose this will make any difference either will it?

Well, I wouldn't say better...just longer. The Sigma at 70mm, only has a max aperture of F4.5...while the Canon at 55mm, still has a max aperture of F2.8. Also, the Canon is better optically...with a higher image quality.
But that's why it's so much more expensive.

And yes, lifeafter2am is correct. The 40D has the same size sensor, so the lenses will behave exactly the same on that camera. They would be different on a film camera or on a camera with a larger sensor...but of course, EF-S lenses will not fit onto film or larger sensor cameras...so you don't have to worry about it.