I need some help with extension tubes

[sergiob]

I want to buy an extension tube for my Pentax 645D that will allow me to fill the sensor as best as possible with a 35mm negative. I have 2 lenses I could use, the 75 2,8 or the 55 2,8.
I don´t know how to calculate the magnifications required so I don´t know which one to buy. There is a 13.3 mm and a 26.6mm one. Or should I buy both and couple them together?

[Bob Salomon]

A bellows would give you more versatility in cropping and sizing then one or two tubes will.

[sergiob]

I haven´t seen one for pentax 645. If I can find a 645 lens mount I could figure it out how to put the 645 on my view camera. Then I could use enlarging lenses I guess because they are flat fields. But I haven´t been able to find one anywhere.

[Dan Fromm]

I haven´t seen one for pentax 645. If I can find a 645 lens mount I could figure it out how to put the 645 on my view camera. Then I could use enlarging lenses I guess because they are flat fields. But I haven´t been able to find one anywhere.

Look and you will find. https://www.google.com/search?hl=en&...pe=&as_rights=

[Bob Salomon]

I haven´t seen one for pentax 645. If I can find a 645 lens mount I could figure it out how to put the 645 on my view camera. Then I could use enlarging lenses I guess because they are flat fields. But I haven´t been able to find one anywhere.

Check out the first two bellows systems here. One has tilts and swings front and back and the other doesn't:

http://www.novoflex.com/en/products/...llows-systems/

[sergiob]

If I use the 120 macro lens, that should give optimum image quality, instead of using a non-macro lens, right? If I need more a higher magnification than 1x, then extension tubes should do the trick. Basically what I´m trying to do is rig a setup that permits me to very easily ¨scan¨ 35mm negatives in less time with good quality. Any ideas or suggestions?

[Bob Salomon]

Then see this as well:

http://www.novoflex.com/en/products/...stel-cop-digi/

[sergiob]

I decided to go the macro lens route since I anyway need a short tele. I bought a 120 f4 macro lens. The lens, (still in the mail), should do 1:1 mag. If I shoot a 35mm neg at 1:1 I would be wasting sensor real estate. Now the question is, which extension ring should I buy, the 13.3mm or the 26.6mm to fill the 645D frame as best as possible with a 35mm neg?

[Jeff Keller]

You're trying to take 24x36 up to 33x44. So you need about 1.5x enlargement. To get a 1.5x enlargement, distance from lens to image needs to be about 2.5 x FL.

Note M = (Di - f)/f where Di = distance from lens to image

http://en.wikipedia.org/wiki/Magnification

Jeff Keller

[Jeff Keller]

For your Pentax macro lens you could use the calculator at
http://www.cambridgeincolour.com/tut...es-closeup.htm

Plug in native mag 1, focal length 120, and extension of 50mm gives a magnification of 1.42x
So you need an extension tube of about 50mm.

Jeff Keller