RB67 lens help

I recently made the jump into MF into digital and I am looking for a longer lens to compliment the 50 mm for my RB67. I really, really love the 50/1.8 for my Rebel XT and am trying to find something around the same focal length. Of course with the crop factor found with the APS sensors and the widening factor of lenses when put in front of a 6X7 plane, Im confused on the math. Here are my calculations.

50 (the digi lens)
x
1.5 (crop factor)
=
75

This is where things get a little fuzzy....


As far as I know lenses shot with a 6X7 neg are half their focal length.


So based on those calculations......


75
x
2 (double the length for 6X7)
=

150


So the 150mm would be approximately the same focal length as the 50mm digital ?

I hope I didnt give anyone a headache

It's easier than that You don't need to factor in the 1.5x when you do the conversion, since the focal length of your lens is still 50mm, regardless of what camera it's on. Your aps digital is merely cropping into the field of view of a standard 50mm lens- it's not really changing the focal length. So take your 50mm, multiply by ~2 and voila. Thus you want more like a 90 or 127mm for your rb. These are the near-normal lenses in the rb lineup.

The complication here is the big difference in aspect ratio between 6x7 and 35mm. They will feel quite different because the 6x7 format is so much more square.

Look at the mamiyausa site under products/rb67... for each lens they list a 35mm equivalent.

Note also there is a 6x8 back for the rb, with which the 90 and 127 both would feel a tad wider.

150 is a standard portrait lens and is available as a variable soft focus lens. It's more like short tele.

I would personally think of it this way: Apply the 1.6x sensor factor to the RB's normal 90mm lens, because you like a lens that has the AOV of the normal lens times 1.6. Easy. If you'd like, apply the factor to 2x50mm instead, for the theoretical 100mm RB lens that does not exist.

Here is a more detailed explanation: 50mm is the common normal lens for standard frame size on 135 format. The AOV of a 50mm on your camera is the same AOV as an 80mm lens on 35mm. Compare 80mm to 50mm. 80mm is 50mm + 3/5 of 50mm. Therefore, you like a lens that has the AOV of a lens 160% the focal length of a normal lens. Multiply the RB 90mm normal lens by 1.6, and you have your ideal FL: 144mm. So, I would look at the 150 (or 140 macro), based on numbers alone.

Since the RB frame is fatter, it appears wider, so I suggest looking at the 180mm as well. In fact, I am certain that the 180mm will indeed "feel" the most like the 50mm on APS-C.

Your figurin' is correct, though. The 2x factor is used for comparing 6x7cm to 24x36mm format, not for comparing APS-C to 6x7. Since you are using APS-C format, if you do what Keith says, and just multiply the 50mm FL by two, you don't get a similar AOV on 6x7. 90mm will be a normal AOV for 6x7, not medium long like your 50mm on APS-C. You have to 1) do what you did, and multiply 50mm by 1.6, and then by 2, or 2) condense the two factors ahead of time into one factor that goes directly from APS-C to 6x7 (a factor of 3.2).

Remember that the aspect ratio of a 35mm or digital Rebel is different than the RB, so you may want to favor either a horizontal or a vertical angle of view (probably horizontal if you shoot mostly horizontals, and vise versa), and compare lenses that way instead.

Oh now I see what he means, he wants an effective 75mm. Doh. Sure then it's a 140 or 150.

Glad I don't do APS any more

That's about right, a 50mm on a c or dx type sensor has the angle of view of about a 75mm lens on 35mm film. If you want this angle of view on 6x7 then that's about a 150mm lens on an RB67. You have a choice of either a 140mm Macro or the 150mm soft focus - which of course you can use as a normal lens if you don't put the little soft focus disks in. In fact you can get some good deals on 150mm where the seller has lost the little disks. Mind you, a 140mm macro is pretty versatile. Once you get into RB67's you'll want everything anyway, they're a bit addictive.

Thanks for the help folks, Im surprised my math was correct ! Im having a problem finding specs on both the 140 and 150 mm lenses. I went over to www.butkus.org and there is no specific info. I need to know what the minimum focusing distance is for these lenses, does anyone know where I might find that info ?

For info on those lenses you could go the Mamiya site

In general: the 140mm is a macro lens, the 150mm is a variable soft focus lens that comes with 3 aperture plates.
The 150mm goes from soft to sharp and contrasty.

Peter

R/D said:

Thanks for the help folks, Im surprised my math was correct ! Im having a problem finding specs on both the 140 and 150 mm lenses. I went over to www.butkus.org and there is no specific info. I need to know what the minimum focusing distance is for these lenses, does anyone know where I might find that info ?

Click to expand...

Here is the link to the manual for the RB67 Pro-SD which you can find on the Mamiya site. Check the section on close-up photography.

http://www.mamiya.com/assets/pdfs/6x7/RB67_Pro_SD_v7.pdf

Matt