Canon Community

islandertim · ‎10-26-2013

I recently purchased a Bower SLY650T 650-1300MM F8.0 Long Range Zoome Lense. In AV or M mode, the camera will not adjust the Fstop setting. It can be adjusted with the Canon lenses that came with my t3i Rebel, just not this lense. You can highlight the fstop area, but the scroll will does not have any affect on the setting with this lense. It simply will not adjust. I called Canon support and they said the camera does not appear to be recognizing the lense.

Any ideas?? The T-Adapter is the correct one (EOS).

TCampbell · ‎10-27-2013

There is no defect... the lens and camera are working as intended.

It is actually possible to connect your camera to a telescope. On a telescope, there are no "aperture blades" so it's not possible to adjust the aperture... the focal ratio of the telesocpe is simply it's focal length divided by the width of clear aperture.

This Bower 650-1300mm f/8-16 lens is basically a small telescope. The "lens" does not actually have aperture blades.

At the "wide" end (650mm focal length) it's f/8. The lens has a clear aperture of about 80mm.

To find the focal ratio of any lens, you simply divide the focal length of the lens (in your case 650mm) by the diameter of clear aperture (80mm) to arrive at the focal ratio. 650 ÷ 80 = 8.125 (basically f/8).

When you zoom in to the 1300mm end, the focal length changes but the diameter of the lens does not change... that means you can divide 1300 ÷ 80 and you arrive at about 16 (16.25 but these values are rounded.)

On your camera you will be able to set the ISO setting and the shutter speed but you will NOT be able to set the f-stop... it is what it is and is not adjustable. You'll just have to know that it's f/8 at 650mm and f/16 at 1300mm.

To take an exposure of, say, the moon... you'd use the "loony 11" rule. That rule says that if you are at f/11, you can set the shutter speed to the inverse of the ISO (let's suppose you're using ISO 100) so that the inverse of 100 would be 1/100th sec. That would actually result in a perfectly exposed moon. But you're not at f/11... you're at f/8 (one stop faster) or f/16 (one stop slower). You can just compensate by changing your exposure by that one stop.

e.g. ISO 200, f/16, and 1/100th second exposure at the 1300mm focal length or use ISO 100, f/8, and 1/200th sec exposure at the 650mm focal length.

it will help to know the rules for finding "equivalent exposures" and how to set exposures manually. If you are not familiar with this, then the book "Understanding Exposure" by Bryan Peterson is a great starting place.

Tim Campbell
5D III, 5D IV, 60Da

View solution in original post

cicopo · ‎10-26-2013

If things are what I think they are & I'm basing this on a friends experience years ago with his telescope & my A 1. I'm assuming the lens is F8 & only f8 so the camera can read the light passing through it for metering. If you use Program mode the camera should be able to figure out a shutter speed based on that meter reading but you may have to fine tune using Exposure Compensation.

"A skill is developed through constant practice with a passion to improve, not bought."

ebiggs1 · ‎10-27-2013

This lens requires a lot of light and a high ISO setting, a sturdy tripod or monopod on the camera/lens but can deliver fairly sharp photos. It is a f8-f16 if memory serves me correctly and is similar to the olden days "Girl Watcher Lens". I still have the one I bought in the mid-1980's.

You might just not have enough light for the camera to take the picture. It should work in AV. I use a Canon f8 FD mirror lens this way and it works fine. (On a 1D Mk II)

EB
EOS 1DX and 1D Mk IV and less lenses then before!

TCampbell · ‎10-27-2013