Pentax 1.7 AFA as a macro-adapter (1.7 X helicl converter): Why not ?

pcarfan · 03-01-2009, 05:53 AM

I had always thought that TC's will not allow for higher magnification at the closest focus distance, as I had imagined the focus distance will increase with the TC effectively cancelling the lens magnification. I was experimenting with shorting an M42 mount so that the AFA will auto focus. During those testing I found the focus distance of my Volna-9 didn't change with the afa adapter and gave me a magnified image.

So, why not use the 1.7 AFA as a macro extension tube (more like a macro helical converter).

Also, even though I got the afa to Auto focus with my M42 lens, it is not metering well and I have lost SR. Any suggestions ? (I shorted all the pins on the mount, can I do this selelctively to save SR and metering, thank you).

Now some sample images of the M42 1:2 macro lens alone and with the 1.7 AFA. (The lens is the Volna-9)

Volna-9 at F2.8 at the closest focus distance.

Volna-9 with 1.7 AFA at F4 and at the closest focus distance.

So, isn't this essentially what an extension tube does, OR am I missing something ? Other than the extension tube does it with no lens element by moving the lens away from the sensor, and the AFA does it with lens elements and magnification.

I like this better, as the lens is a 85mm lens now, so the bugs will be less scared. The only possible drawback over ext. tubes is IQ loss due to lens elements, but I think the 1.7 AFA adapter does well here...

The 100% crop with the 1.7 AFA and F4

P.S: I destroyed the lens element of a TC and made a 25mm extension tube, and took this shot for comparision. Even with thetripod and f16 (max) I missed the focus, the top of the image is much sharper.

Last edited by pcarfan; 03-05-2009 at 05:34 PM.

Ben_Edict · 03-02-2009, 09:25 AM

No, the AF-convertor is not an extension tube. An extension tube extends the distance between sensor and film, which allows to get nearer to your object and achieve a higher magnification.

A teleconvertor (any tc, not only the AF-convertor) does nothing to the sensor-lens distance BUT increases the lens' focal length by its factor. Thus you get a longer fl lens with the same minimum focusing distance, which gives you a bigger image size.

Its two different things - but the results are what counts!

Ben

03-01-2009, 05:53 AM	#1
pcarfan Veteran Member Join Date: Sep 2007 Location: Dayton, Ohio Photos: Gallery \| Albums Posts: 2,978	Pentax 1.7 AFA as a macro-adapter (1.7 X helicl converter): Why not ? I had always thought that TC's will not allow for higher magnification at the closest focus distance, as I had imagined the focus distance will increase with the TC effectively cancelling the lens magnification. I was experimenting with shorting an M42 mount so that the AFA will auto focus. During those testing I found the focus distance of my Volna-9 didn't change with the afa adapter and gave me a magnified image. So, why not use the 1.7 AFA as a macro extension tube (more like a macro helical converter). Also, even though I got the afa to Auto focus with my M42 lens, it is not metering well and I have lost SR. Any suggestions ? (I shorted all the pins on the mount, can I do this selelctively to save SR and metering, thank you). Now some sample images of the M42 1:2 macro lens alone and with the 1.7 AFA. (The lens is the Volna-9) Volna-9 at F2.8 at the closest focus distance. Volna-9 with 1.7 AFA at F4 and at the closest focus distance. So, isn't this essentially what an extension tube does, OR am I missing something ? Other than the extension tube does it with no lens element by moving the lens away from the sensor, and the AFA does it with lens elements and magnification. I like this better, as the lens is a 85mm lens now, so the bugs will be less scared. The only possible drawback over ext. tubes is IQ loss due to lens elements, but I think the 1.7 AFA adapter does well here... The 100% crop with the 1.7 AFA and F4 P.S: I destroyed the lens element of a TC and made a 25mm extension tube, and took this shot for comparision. Even with thetripod and f16 (max) I missed the focus, the top of the image is much sharper. Last edited by pcarfan; 03-05-2009 at 05:34 PM.

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03-02-2009, 09:25 AM	#2
Ben_Edict Veteran Member Join Date: Jul 2007 Location: SouthWest "Regio" Photos: Albums Posts: 3,309	No, the AF-convertor is not an extension tube. An extension tube extends the distance between sensor and film, which allows to get nearer to your object and achieve a higher magnification. A teleconvertor (any tc, not only the AF-convertor) does nothing to the sensor-lens distance BUT increases the lens' focal length by its factor. Thus you get a longer fl lens with the same minimum focusing distance, which gives you a bigger image size. Its two different things - but the results are what counts! Ben