Focal
 

I set up in manual, my focal, iso, etc, using a tamron 3.5 28-300 lens, set it to 3.5 and locked it, I thought, when I went from 28 to 300 mm the focal went to 6.5, how can I lock this to 3.5, all other settings don't move

Many zoom lenses, including those on video cameras, must reduce their aperture when zooming telephoto. A few lenses have only one number as their f/stop rating such as f4 or f2.8 and those lenses keep the same aperture throughout the range. There are no lenses with the zoom range as your lens that are constant aperture.

Look at the rating of your lens and you will see that it is marked like 3.5-6.3. This is the range of minimum f/stops.

Correct me if I'm wrong, but aren't f-stops ratios? Focal length divided by aperture diameter, I thought. The number will go up as focal length increases even if the aperture stays the same size. A 50-100mm zoom that's an f/2-f/4, for argument's sake, will have a 25mm diameter aperture at either end of its range. Assuming that's correct, zooms with only one f-stop printed on them would actually have to open the aperture as they zoom in to maintain the same number, wouldn't they?

Someone indicated this lens does not have a fixed focal at 3.5 due to zoom, guess it makes sense

Hello Hugh,

Your Tamron 28-300mm lens is not an f/3.5 lens. It's actually an f/3.5-6.3 lens, meaning it has a variable aperture which is a maximum of f/3.5 at the wide end and a maximum of f/6.3 at the telephoto end. Check out the official Tamron product page for your lens: AF28-300mm F/3.5-6.3 XR Di LD Aspherical (IF) Macro; Tamron USA, Inc. -- you can see how it's listed as an f/3.5-6.3 variable aperture lens. If it had a constant f/3.5 aperture throughout the entire zoom range, then it would be listed as an f/3.5 lens instead of f/3.5-6.3 -- hope this helps,

BTW, Hugh, I have this lens in the VC/IS model and I plan to use it only during the day since it is a bit slower. I find that it is acceptable even indoors when the sun is out. I plan to compensate for the variable aperture and changing light conditions outdoors with a variable ND "Fader ND" that I recently ordered. I like shutter and aperture settings to remain as constant as possible and even this slower Tamron lens is more than fast enough with the 5D2 outdoors to utilize an ND filter. The variable ND should allow quick exposure compensation when zooming with a non-fixed aperture lens.

thanks, I went back to the tamron site, but did not find the right info, on their older site that info was readily available, because I checked the lens out before I bought it, I have a tamron 28-75 2.8 lens from tamron, bought 5d because I had the lenses, which will do for now, and to get an adaptor and supporting gear for my V1 was not in the budget, so the V1 had to go. I believe these dslr's are the cameras of the future, want a EX 3 but that surly ain't in the budget either

A workaround to any of these variable F-stop zoom lenses is to set your lens to it's widest focal mm and then choose the higher F-stop. In this example, set your lens to 28mm and F6.3. Then when you zoom the lens will stay at 6.3 the entire time. If you do it the other way around and set it to the widest mm and lowest F-stop, that's when the aperture will change as the zoom changes usually around the mid-point of the zoom. So for your lens it will move from 3.5-6.3 as you zoom if you set your F-stop to 3.5 at 28mm.

Sorry if this all seems obvious but just wanted to put this out there.

Also if the F-stop range isn't printed on the barrel of the lens it is always printed on the ring around the front glass element. There you should see 3.5-X (X being whatever the maximum F-stop variable is).

Christopher Lovenguth
www.christopherlovenguth.com