Should I buy the Canon 100-400 L F4.5-5.6?

[reg]

It is more of a, buy this 150-500 and it is just like a 240-800 on your FF camera.

Absolutely false!

A 150-500 on FF is.... well, a 150-500.

 

[darich]

No it doesn't. All lenses suffer from crop factor on a crop sensor.

All cameras with smaller sensors experience a crop factor and the focal length is lengthened as a result.
I have a 5D with a full size sensor and therefore any lens i fit does exactly what it says on the tin - 24-70 is exactly that.
Certain lenses are designed purely for APS-C sized sensor and don't fit other Canon cameras - eg designed to fit the 20D or rebel and called EF-S fitment.
Those lenses will not fit film SLRs or full frame DSLRs.
On the other hand though all EF lenses will fit all DSLRs

What you say is correct - "All lenses suffer from a crop factor on a crop sensor".
My earlier post was because i do not have a cropped sensor.

Looks like the lens is good for the 5D - thanks for the post to flickr reg.
i know i can ask in the shop but I'd like to find out before i go.

Still thinking about the 150-500 sigma though!!!

 

[usayit]

All cameras with smaller sensors experience a crop factor and the focal length is lengthened as a result.

Incorrect.

http://en.wikipedia.org/wiki/Focal_length

I don't see the size of the format in the formula for focal length.

 

[Overread]

Rule one - wiki is not good for specifices

Note that some simple and usually inexpensive cameras have fixed focus lenses which cannot be adjusted[

Whilst the formula is lacking the factor of cropping due to sensor size this is included in the last line from the site

Use of a 35 mm equivalent focal length is particularly common with digital cameras, which often use sensors smaller than 35 mm film, and so require correspondingly shorter focal lengths to achieve a given angle of view, by a factor known as the crop factor.

 

[usayit]

Correct... angle of view changes on the format but focal length of a lens remains the same.

 

[darich]

I should have said that the effect is as if the focal length is increased because the smaller sensor has a narrower field of view.

Perhaps I simplified it too much!!

Oops!