using a camera with ttl flash is the easiest way to get good results
JCO
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED]]On Behalf Of Brent Hutto
> Sent: Tuesday, November 27, 2001 8:10 PM
> To: [EMAIL PROTECTED]
> Subject: Extension Tubes for Closeup Photography
>
>
> I just got my Pentax Extension Tube Set B from KEH and have been
> playing around with them on my 50mm f/2 SMC-A and 135mm f/3.5
> SMC-M lenses. I can get to around 1/2 life size with the 135mm lens
> and have over a foot of working room between the front of the lens
> and the subject. I can get 1:1 life size by stacking the 26mm and
> 19mm tubes on the 50mm lens. That gives me 3 inches between
> front element and subject which is about the minimum I'm
> comfortable with without worrying about nicking the lens. Of course,
> I can put the 12mm tube on there too and to about 1.2:1 as my
> absolute maximum magnification.
>
> These tubes don't couple the aperture information into the camera.
> In fact, the camera can't stop them down so you just adjust the
> aperture ring like a purely manual setup. I want to use my (manual)
> flash bouncing off a white board to light my closeup subject. Is it
> pretty accurate to figure the effect f-stop as follows:
>
> Original focal length: 50mm
> Original f-ratio: 2.0
> Original aperture: 50 / 2.0 = 25mm
>
> Focal length with tubes: 50 + 25 + 19 = 94mm
> F-ratio with tubes: 94 / 25 = f/3.8
>
> and just round off to f/4 for figuring the amount of light I need?
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