Canon or Tamron?
 

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. :confused:

Hi Miqspix,

The Canon 28-135 has a maximum aperture of 3.5-5.6. This means that the widest aperture the lens can achieve is f/3.5 at the 28mm end and f/5.6 at full zoom of 135mm.

The Tamron has a maximum aperture of f/2.8 across the entire zoom range, so whether you have it set to 28mm or 75mm, the maximum aperture will always be f/2.8. It can of course be stopped down to a smaller aperture (not sure what the smallest is but probably f/32 or something in that region)

The focal length is determined by the lens. What you are referring to is the depth of field, which is controlled by the aperture. The wider it is, the narrower/shallower the depth of field is, which creates the nice out of focus backgrounds. The smaller the aperture is the greater the depth of field and the more in focus the background becomes.

I hope that helps.

[QUOTE=MiqsPix;25566]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. :confused:[/QUOTE]


When a zoom lens is described as having an F stop aperture that is "full time" it means the lens can shoot at that specified value throughout the whole of it's zoom range. It doesn't mean that it has to, or can only shoot at that value. Any aperture size, beyond f2.8 in this case, can be dialled in at any time. You can choose what aperture you want, and set the variables of the shutter speed (time) and ISO (sensitivity) accordingly, depending on the amount of light available.

edit
AMK1977 is more eloquent than I and can think and type a lot faster than me too.