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05-27-2010, 09:25 AM   #1
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Pentax TC AF 1.7x

Dear B&H specialist,
Does the product turn MF lens into AF lens but with aperture size 1.5x and zoomed 1.7x (e.g. turn A50/1.2 into FA85/1.8)?
Thanks!!

05-27-2010, 10:23 AM   #2
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QuoteOriginally posted by im57yan Quote
Dear B&H specialist,
Does the product turn MF lens into AF lens but with aperture size 1.5x and zoomed 1.7x (e.g. turn A50/1.2 into FA85/1.8)?
Thanks!!
The maximum aperture after using the TC is f/2.8.

So that will only convert the A 50/1.2 to FA 85/2.8.

The lens will need a rough focus before AF can work.
05-27-2010, 10:36 AM   #3
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QuoteOriginally posted by mz-3 Quote
The maximum aperture after using the TC is f/2.8.

So that will only convert the A 50/1.2 to FA 85/2.8.

The lens will need a rough focus before AF can work.
This is incorrect.

The 1.7x AF TC will make the combined lens 1.5 stops slower so a 50mm F1.2 becomes an 85mm F2.

In terms of converting MF to AF, when used with some lenses in the wide angle range, there is sufficient focusing throw in the TC to fully focus a lens with no manual rough focus, as lenses get longer, there will be a need to rough focus first. I am not sure where the limit of full AF capability is.

The TC also corrects the aperture for A series and later lenses so the camera reports the correct Aperture to in the exif data

also note that the TC is specified to be used with lenses faster than F2.8 although by experience of many years and many users, it will actually provide focus for an F4 lens.
05-27-2010, 11:12 AM   #4
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I am sorry, but I am not aware of a Pentax Teleconverter nor an item that will convert a manual focus lens to an auto focus.

In general, a teleconverter will multiply a lenses focal length by 1.4X, 1.7X or 2X depending on the model you use. You will also loose 1 -2 stops of light again depending on the strength of the converter used.

So if you were to use a 1.7X converter on a 50mm lens, it would have a combined focal length of 85mm's. Nikon's 1.7X has a light loss of 1.5 stops, so an f1.2 lens would be f2.

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05-27-2010, 11:24 AM   #5
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QuoteOriginally posted by Chuck-B&H Quote
I am sorry, but I am not aware of a Pentax Teleconverter nor an item that will convert a manual focus lens to an auto focus.

In general, a teleconverter will multiply a lenses focal length by 1.4X, 1.7X or 2X depending on the model you use. You will also loose 1 -2 stops of light again depending on the strength of the converter used.

So if you were to use a 1.7X converter on a 50mm lens, it would have a combined focal length of 85mm's. Nikon's 1.7X has a light loss of 1.5 stops, so an f1.2 lens would be f2.

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chuck, it is an old product, released in the early 1990's ans is called an AF adaptor.

specifically it is an SMC-F 1.7x AF adaptor.

Pentax appears to be either remanufacturing it or building units from spare parts and making them available again in asia.
05-27-2010, 11:33 AM   #6
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QuoteOriginally posted by mz-3 Quote
The maximum aperture after using the TC is f/2.8.
You don't need F/2.8. F/4 works fine.

I used the Pentax 1.7X AFA with an an SMC-A* 300mm F/4 and a Super Takumar 300mm F/4 (both lenses now sold) without any problem. The AF feature works just fine.

Now I'm using the 1.7X AFA with my DA* 60-250mm F/4. I also have the Tamron 2X TC and 1.4X TC, but use only the 1.7X AFA with with SDM lenses.
05-27-2010, 12:10 PM   #7
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Thanks for info.


Chuck
05-28-2010, 09:13 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
This is incorrect.

The 1.7x AF TC will make the combined lens 1.5 stops slower so a 50mm F1.2 becomes an 85mm F2.
This is inaccurate. Your calculation is right. But A 50/1.2 and F1.7X will only give you a maximum aperture of f/2.8, not f/2. This is because the maximum aperture of the TC is f/2.8.

I have the A 50/1.2 and F1.7X. I know I am right on this.

05-29-2010, 06:36 AM   #9
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QuoteOriginally posted by mz-3 Quote
This is inaccurate. Your calculation is right. But A 50/1.2 and F1.7X will only give you a maximum aperture of f/2.8, not f/2. This is because the maximum aperture of the TC is f/2.8.

I have the A 50/1.2 and F1.7X. I know I am right on this.
the calculation cannot be right and wrong at the same time.

Optics defines the final aperture.

When you say you own th e50mmF1.2 and the TC and it only gives F2.8, do you mean to say that the camera reports F2.8? What the camera (and TC report) may have no relevance to the actual aperture achieved.

The real question is how does the camera meter with the lens attached?

An F2 lens reporting back as F2.8 would tend , in my opinion to over expose.
05-29-2010, 09:21 AM   #10
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QuoteOriginally posted by mz-3 Quote
This is inaccurate. Your calculation is right. But A 50/1.2 and F1.7X will only give you a maximum aperture of f/2.8, not f/2. This is because the maximum aperture of the TC is f/2.8.

I have the A 50/1.2 and F1.7X. I know I am right on this.
The 1.7x AF TC manual doesn't refer to that F2.8 max restriction

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05-29-2010, 09:43 AM   #11
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QuoteOriginally posted by mz-3 Quote
This is inaccurate. Your calculation is right. But A 50/1.2 and F1.7X will only give you a maximum aperture of f/2.8, not f/2. This is because the maximum aperture of the TC is f/2.8.

I have the A 50/1.2 and F1.7X. I know I am right on this.
QuoteOriginally posted by Lowell Goudge Quote
the calculation cannot be right and wrong at the same time.

Optics defines the final aperture.

When you say you own th e50mmF1.2 and the TC and it only gives F2.8, do you mean to say that the camera reports F2.8? What the camera (and TC report) may have no relevance to the actual aperture achieved.

The real question is how does the camera meter with the lens attached?

An F2 lens reporting back as F2.8 would tend , in my opinion to over expose.
MZ-3 I apologize for the comment. I have misread the statement and I assumed it was meant for the A50mm F1.7 which fits. In fact that's true for all lenses faster than 1.7 which become 2.8 max.

Rui
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05-29-2010, 06:02 PM   #12
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this aperture issue must be a maximum aperture reporting only behavior.

the adaptor physically does not alter / restrict the light by more than the magnification ratio.

As a result, this can lead to metering errors specifically on K10D and K20D.

I can check this using my vivitar 85mmF1.4, which should open up to F2.5 when attached not F2.8.
06-04-2010, 01:19 AM   #13
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Just a bit more reference info on the AFA. . . The F 1.7x AFA also reports weird max apertures with at least some Sigma lenses. This has been explained by some Sigma users as being due to Sigma's reverse engineering of the data chips in their "A" and later series lenses, and I have no reason to doubt it. This can sometimes be cured by insulating selected pins on the AFA, usually some combination of the #1,#2, and #6 pins (with the #6 being the data pin, or the one closest to the AF screw shaft).

For example, without mods, my Sigma EX 180 f3.5 APO DG Macro reports f2.8 with the AFA instead of something like f6. I wanted to use this lens with a Metz 15 MS1, so it would have to work with wireless TTL which needs aperture information from the body to have a chance of exposing properly. The best I could get is f3.5 by insulating the #2 & #6 pins, and I settled for this as it allowed me to easily figure the correct effective Av with a simple 1.7x multiplication factor-- and this works fine with the Metz if I dial in between -1 and -2 flash Ev comp on my K20, depending on the subject (which makes some sense to me).

For some reason, this lens/flash combination (at 0 flash Ev comp) results in at least 2 stop underexposure with my K-7 (and a constant flash Ev comp setting doesn't give consistent results), so I only use the lens combo with this flash on the K20. I assume that Pentax made some subtle changes in the flash metering with the K-7's new 77 segment metering sensor, and this puts the Sigma's lens chip that much further from Pentax's current wireless TTL compatibility.

Strangely, something of the opposite is true when an external flash is used on-camera. The K20 exposes correctly with 0 flash Ev comp, and the K-7 nowoverexposes by at least a stop. . . I stopped trying to figure it out -- maybe this winter. . .

For reference, I also tried my D FA 100/2.8 Macro with the AFA and both the Metz and an AF 360 FGZ, and both bodies report f4.5 max aperture and expose correctly with either flash at 0 flash Ev comp.

Scott
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