Seeking advice on use of lens extenders: I have 2 lenses that I've been using on my Sony cameras for shooting the moon; a Minolta 500mm f8 auto focus mirror lens and a Sigma 170 to...

bgl Loc: Brooklyn,New York

I have 2 lenses that I've been using on my Sony cameras for shooting the moon; a Minolta 500mm f8 auto focus mirror lens and a Sigma 170 to 500mm f5.6 - 6.3 zoom. On these crop sensor lenses the effective reach is 750mm. Would I benefit from the use of an extender, and if so, which one(s)? Your comments will be appreciated.

MT Shooter Loc: Montana

Extenders (tele-converters) are pretty much a waste of time on slow lenses like these. They can be used in manual mode, but the light transmission is so low that its nearly impossible to focus them that way.

bull drink water Loc: pontiac mi.

I have a sony 500 f 8 reflex, I use it with a 1.4 vivitar and have auto focus. for moon shots you can use a 2.0x , set it for infinity and you should be ok. sure it's a slow lens, but up the iso and in sunlight you're ok. I spent Saturday at the zoo, with a 1.4 ans a2.0 tc and got some great shots. i'm going back this week with a 400mm and 2-2.0 tc's and stacking them together. I will post some shots.

bull drink water Loc: pontiac mi.

I have a sony 500 f 8 reflex, I use it with a 1.4 vivitar and have auto focus. for moon shots you can use a 2.0x , set it for infinity and you should be ok. sure it's a slow lens, but up the iso and in sunlight you're ok. here are 2 samples of the 500mm reflex with a 2x tc.

bgl Loc: Brooklyn,New York

MT Shooter wrote:

Extenders (tele-converters) are pretty much a waste of time on slow lenses like these. They can be used in manual mode, but the light transmission is so low that its nearly impossible to focus them that way.

I hear you. However, wouldn't shooting a bright moon be an exception?

MT Shooter Loc: Montana

bgl wrote:

I hear you. However, wouldn't shooting a bright moon be an exception?

Few cameras will AF past F7.1, and none past F8. Some odd combinations will drive the AF, but will not lock on focus.

bgl Loc: Brooklyn,New York

MT Shooter wrote:

Few cameras will AF past F7.1, and none past F8. Some odd combinations will drive the AF, but will not lock on focus.

I really appreciate the info - will save me a lot of grief and some money too! For my purposes I'm getting fairly sharp images of the moon with both lenses (at least compared to moon shots posted in the astronomy forum) and with judicious cropping and a little sharpening the final product isn't bad. I used to have a Celestron 8" mirror but it was destroyed during hurricane Sandy. That would be a much better tool for shooting the moon. In any case, the thought of stacking tele extenders sounds like a way to degrade the image. Thanks again for the advice.

pmackd Loc: Alameda CA

With the Nikkor 70-300 VR at 300mm and f 5.6 and the Kenko 1.4 x tele-extender (which loses one stop) even my Nikon D7000 has no trouble auto-focusing on the moon or other bright subjects. Only experimentation can tell you for sure what would be possible with a given camera, lens, and tele-extender combination in very bright light. There is no fixed autofocusing limit at f8 or any other f stop. The newest cameras need less light to autofocus than the previous generation.

bgl Loc: Brooklyn,New York

pmackd wrote:

With the Nikkor 70-300 VR at 300mm and f 5.6 and the Kenko 1.4 x tele-extender (which loses one stop) even my Nikon D7000 has no trouble auto-focusing on the moon or other bright subjects. Only experimentation can tell you for sure what would be possible with a given camera, lens, and tele-extender combination in very bright light. There is no fixed autofocusing limit at f8 or any other f stop. The newest cameras need less light to autofocus than the previous generation.

Unless I'm misunderstanding, I'm getting divergent opinions/statements from 2 smart, knowledgeable people over a technical subject - should I be surprised? After thinking more about the subject, it comes down to whether or not I'll actually use a tele-extender or will it languish in my camera bag(s). I won't invest in one just to take the very occasional moon shot and I already have the lenses to cover a wide range (from 17mm to 500mm, which on my cameras is effectively 25mm to 750mm).