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Old Aug 17, 2010, 5:35 AM   #1
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Default 2x teleconvertor

Just recently purchased a teleconverter for my sony A100 its a 2 x vivitar I am using a minolta lens 75-300 AF zoom .

It will not focus without blur in either Manual or auto plus it does not allow the shutter to take any photos . Could someone point me in the right direction .

Thanks very much in deed

kindest regards Lesley
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Old Aug 17, 2010, 5:40 AM   #2
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I don't know the specs on that TC but generally you don't want to be using a 2x TC on anything other than a lens that is f2.8 throughout the range so that you maintain AF and have a chance of having the resolution capabilities to use a strong TC. Even a 1.4x TC would not be good on that lens with the max aperture of f5.6 at 300mm.

As for it not even taking photos, if you are in manual focus it should still allow the shutter to release. Hopefully someone has a little experience of this lens to help out more.
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Old Aug 21, 2010, 11:41 AM   #3
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Hey Lesley.

You lose 2 stops of light (only 1/4 the light gets through the lens to the camera's sensors) with a 2X teleconverter.

As Mark1616 pointed out, you'll usually need a lens that can maintain f/2.8 throughout it's focal range to retain Autofocus with that type of TC, since a lens with f/2.8 available would have a widest aperture of only f/5.6 when wearing a 2X TC. With a 1.4x TC, you'd want a lens with f/4 or brighter apertures available (since you lose 1 stop of light with a 1.4x TC, turning a lens with f/4 available into one with a widest aperture of f/5.6).

Keep in mind that your 75-300mm is already a relatively dim lens when zoomed in much (widest available aperture of f/5.6 on it's longer end). So, with a 2X TC on it, you'll end up with a 600mm f/11 on it's long end; and most cameras are not going to try to Autofocus unless they see a lens with f/6.7 or brighter apertures (which will vary slightly by camera model/system).

If the TC is reporting corrected aperture information to the camera, then it's probably not going to try to Autofocus with a lens/TC combination that dim (most cameras won't even try to Autofocus with a lens reporting f/11 as the maximum available aperture, as that's really too dim for Autofocus)..

If the TC is not reporting corrected aperture information to the camera (some do, some don't), then the camera may still try to Autofocus (since it may think it's looking through a lens with f/5.6 available, even though the effective aperture is much dimmer with the TC). But, unless light is very bright, it may not be able to lock focus because the Autofocus Sensors can't "see" well enough to focus with a lens that dim on it (effective aperture of f/11 when zoomed in much with that lens with a 2X TC on it).

So, you'll probably need to change the AF/MF switch on your camera to MF and use manual focus with that combination unless light is very bright and the TC is not reporting corrected aperture info.

But, accurate Manual Focus may be difficult without a split prism type viewfinder screen. Also, in addition to significant light loss through a 2X TC, you're going to have some optical degradation usinig one; and your 75-300mm lens is already a bit softer on it's longer end with it's aperture wide open. So, it's not a good idea to try and use one with that type of lens. Stopping down your aperture may help a little. But, that means even slower shutter speeds than what you're probably getting with a lens that dim (so, blur from camera shake and subject movement would need to be taken into consideration, and you may need to use higher ISO speeds and/or a tripod, especially since you'd have a longer 600mm effective focal length, giving you the same angle of view as you'd have using a 900mm lens on a 35mm camera).

IOW, I'd target 1/250 to 1/500 second or faster shutter speeds if possible to keep blur from camera shake to a minimum if you're not using a tripod, even using anti-shake (as you'd normally want shutter speeds of around 1/1000 second with a lens that long without SSS Enabled). For moving subjects, or if you're not good with smoothly squeezing the shutter button to keep camera shake down, you may need even faster shutter speeds.

If for some reason it's not seeing a lens on it when you switch to manual focus (unlikely) and still won't shoot, go into your Custom Menus and change "Priority setup" from AF to Release, which will let your camera take a photo without any focus lock. But, make sure to change it back to AF with other lenses (otherwise, you're going to end up with a lot of out of focus images).

Here's that screen:
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