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convertible lenses

 
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Offset



Joined: 23 Jan 2003
Posts: 19
Location: Los Angeles
PostPosted: Wed Jan 29, 2003 10:35 pm    Post subject: Reply with quote
I have noticed several lenses on the internet that are convertible lenses. They have (2) focal lengths listed - such as 150mm f5.6 and 265mm f12. Does one change the focal length of these lenses with a screw-in adapter, or does the lens telescope outward? Is the image circle the same for both focal lengths, or does it change diameter? Does the lens compromise sharpness at one focal length? And how dark is it through the camera at f12 - are details clear, and can you focus okay? Thanks a lot!
offset
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Dan Fromm



Joined: 14 May 2001
Posts: 2144
Location: New Jersey
PostPosted: Wed Jan 29, 2003 10:44 pm    Post subject: Reply with quote
Quote:

On 2003-01-29 14:35, Offset wrote:
I have noticed several lenses on the internet that are convertible lenses. They have (2) focal lengths listed - such as 150mm f5.6 and 265mm f12. Does one change the focal length of these lenses with a screw-in adapter, or does the lens telescope outward? Is the image circle the same for both focal lengths, or does it change diameter? Does the lens compromise sharpness at one focal length? And how dark is it through the camera at f12 - are details clear, and can you focus okay? Thanks a lot!
offset
With the convertible Symmars, focal length is changed (from short to long) by unscrewing the front cell.

Don't know about image circle, but afaik at both FLs it is 'adequate.'

Sharpness? Again, don't know at first hand because I don't have one, but reports on rec.photo.equipment.large-format are to the effect that convertible Symmars aren't as sharp as non-convertibles.

My slowest lens is an f/9, next fastest is an f/8. In bright daylight, both are easy to focus. I've looked through all of my lenses at smaller apertures and, at least in bright daylight, the seeing isn't terrible at f/16. I don't focus with naked eye, I use a loupe. In my case, the Ednalite Magnifinder, which is roughly contemporary with my cameras.

Hope this helps,

Dan

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Nick



Joined: 16 Oct 2002
Posts: 494
PostPosted: Thu Jan 30, 2003 1:11 pm    Post subject: Reply with quote
Supposedly when fully assembled the sharpness is just fine. The problem is when you convert them even then adding a filter will supposedly help out some.

If you check your focus after stopping down to the final F/stop then F/8 seems fast-)) I'm usually at F/22. You'll need a good darkcloth. The darker it is under there the less light your eyes will need. Think about being in the darkroom. After a few minutes your eyes can almost see in the dark. Any little bit of light seems like a laser beam.
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Offset



Joined: 23 Jan 2003
Posts: 19
Location: Los Angeles
PostPosted: Fri Jan 31, 2003 4:51 am    Post subject: Reply with quote
Thanks danfromm and nick for your good advice, I really appreciate it! As an long time photog - but a new view camera user, your kind of help is wonderful!
offset
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