Canon or Tamron?

[MiqsPix]

I am considering buying either the Canon EF 28-135mm or the Tamron 28-75. I don't fully understand the Tamron in that it is f2.8 full time. I acknowledge this could be beneficial in low light but as there is no image stabilisation on this lens exposure times will be required to be fast. My question is this. If it is f2.8 full time does that mean that the focal length is permanently short so taking a shot for example of a group of people, or a landscape, the shot will have more out of focus areas than a lens that stops down? Does the camera compensate for this? I hope this doesn't sound elementary (it probably is to the informed) but I genuinely can't understand this point.

[RichardTaylor]

F2.8 is the maximum aperture.
Its minimum aperture is F32.
That means your can have very shallow depth of field (zone of sharpness) at F2.8 or lots of depth of field at F32 for all focal lengths.
See the lens specs here.

Tamron 28-75mm f/2.8 XR Di Lens Specifications, Measurements and Features

[Elmo]

First let me say of the two lenses mentioned the Tamron will generally give better IQ (image quality).

It seems to me you are making some false assumptions the f:2.8 has nothing to do with the 28-xxx values of the lens. The constant f:2.8 means no matter what FL (focal length) you choose the aperture will remain constant(Tamron) With the Canon the aperture will change from f:3.5 at shortest FL(28mm) to f:5.6 at it longest FL (135mm). Both lenses allow you to reduce the aperture from its maximum opening (f:32 as i recall) independent of FL setting of lens.

As for IS (image stabilization) it is a nice toy but with proper camera technique it is not necessary at these FL's. IS is a new thing an most of the famous images you see were shot before IS was available. Also I find most of my images are limited by subject movement (IS does not help with this) rather than camera movement.

I might also suggest reviewing exposure, it might help you understand your question better.

[firebox40dash5]

Originally Posted by MiqsPix

I am considering buying either the Canon EF 28-135mm or the Tamron 28-75. I don't fully understand the Tamron in that it is f2.8 full time. I acknowledge this could be beneficial in low light but as there is no image stabilisation on this lens exposure times will be required to be fast. My question is this. If it is f2.8 full time does that mean that the focal length is permanently short so taking a shot for example of a group of people, or a landscape, the shot will have more out of focus areas than a lens that stops down? Does the camera compensate for this? I hope this doesn't sound elementary (it probably is to the informed) but I genuinely can't understand this point.

Your understanding isn't bad, it's your terminology.

What you're calling 'focal length' is depth of field- the distance from the camera between the near focus and far focus. A larger aperture will in fact allow you to get a shallower DoF. Like Richard pointed out, you can stop down to a smaller aperture to get a deeper DoF.

Pretty much any lens can be stopped down- 'constant aperture' means the maximum aperture is the same for any focal length in the zoom range. Meaning you can have f/2.8 at 28mm or 75mm with the Tamron, whereas many zoom lenses have a maximum aperture that varies with the focal length. For example, an 18-55mm f/3.5-5.6 lens has a maximum of f/3.5 at 18mm, and f/5.6 at 55mm.

Most constant f/2.8 zooms in that focal length range lack IS/VR, because the assumption is the larger aperture will allow you to get the fast shutter speeds you need without stabilization.

[autofocus]

I own the Tamron lens and can honostly say it's a decent performer, and it's always nice to have F/2.8 available throughout the zoom range when you need it. I personaly opt for fast lenses when given a choice, but it really depends on what your needs will be when shooting. You should make your decision based on that.

Vinnie