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01-24-2013, 06:23 PM   #1
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2.6:1 Macros with 67 and A 35-105 Zoom

When I bought my Pentax 67 back in 2009, it came with the Pentax 67mm Reverse Adapter. Over the years I've used it a few tomes with my 90mm lens for some flower close ups but I've always wanted to try it with a shorter focal length to get more magnification. I have a 45mm 6x7 lens but with an 82mm it's a bit of a pain to fit to the 67mm adapter.
For years I've want to try reversing my SMC Pentax-A 35-105 zoom.
The A 35-105mm zoom is a superb manual focus lens built for use on K mount 35mm cameras. I was quite worried that it would produce an image circle big enough for the 6x7 frame, but on the ground glass it appeared feasible at f/3.5 anyways.
The main thing I was worried about was trying to determine correct exposure for this setup, which until recently, I didn't understand focal lengths, FOV, and aperture well enough to attemt this. I figured that having fast color negative film would help in case my calculations were off a bit and since I had half a roll of Fuji Superia 200 left in my 67, now would be a good time to try.To determine exposure here's what I did:
With the camera on a tripod I mounted the lens in reverse, set focal length 35mm, set aperture to f/3.5
Then set the camera to nearly the same height as our dining table. With lens pointing across the top of table, we moved and object closer and closer to the lens to determine where the lens was focusing.
Using a small tape measure which we set across the FOV where the lens was focusing, we measured the width (22mm) visible in the WLF (100% frame coverage of a 58mm wide frame)
We then measured from subject to film plane (245mm), and using those numbers we figured the actual focal length of the reversed lens.
Once the focal length (177.6mm) was determined, we could determine the effective f/stop of the lens, which we figured as f/81.2 when the lens was set to f/16. I figured the f/81.2 to be roughly 5 stops off of f/16, so using my Oly E-P1 in Av mode for a meter, I took a test shot at f/16 and ISO 400 and added 5 stops to the 1/8s shutter speed.

I'm not sure if all that adds up or even makes sense, but it worked!
There isn't any vignetting whatsoever that I can tell of.
Only problem was the super super shallow depth of field.

Here's our setup (Book was for lighting purposes )


This is an uncropped 6x7 frame of a Hotwheels '67 Shelby GT500. Lens was set to f/22


These are grains of very course salt from a salt grinder. They are about 3-5mm in diameter.Again this is an uncropped 6x7 frame.


01-24-2013, 07:01 PM   #2
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That's crazy magnified!
01-24-2013, 11:45 PM   #3
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That turned out very well, especially the salt grains!

Phil.
01-25-2013, 04:48 PM   #4
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QuoteOriginally posted by gofour3 Quote
That turned out very well, especially the salt grains!

Phil.
Thanks Phil!

01-28-2013, 06:37 PM   #5
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So, I realized I made an error in my calculation
I used 58mm for frame width when it should be 68mm. This makes the magnification more like 3:1, and also changes the exposure slightly but I think I was close enough.
01-29-2013, 11:58 AM   #6
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Pentax has long said that the frame is 55mm x 70mm.
01-29-2013, 07:28 PM   #7
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QuoteOriginally posted by desertscape Quote
Pentax has long said that the frame is 55mm x 70mm.
Oh, I thought I read somewhere 55x68, but I trust your knowledge of the 67 system more than mine
01-31-2013, 03:24 PM   #8
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Measure the resulting negative for the frame size. I have a dial caliper and can do if you like when I get home.

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