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Old Jan 28, 2009, 8:19 AM   #1
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i just bought a smc-pentax A 1.7 50mm from ebay to use as a portrait lense

when i turned on the camera it asked the focal lenth, so i put it to 50mm

then i started wondering if i should have set it to 75mm, seeing how thats the dslr actual length?

only fired a few shots and the pictures are pretty good and sharp at 50mmso not sure, i know you guys will set me straight


and what will it do to the photos if you use wrong focal lenth?


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Old Jan 28, 2009, 9:31 AM   #2
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The actual focal length of the lens is 50mm, regardless of the camera body it's attached to. The only difference between a 35mm film body and an APS-C body is the angle of view. The only reason the camera needs to know the lens' focal length is so it knows how much the Shake reduction system has to compensate for camera shake. (That and entering the correct focal length in the EXIF data.)

When the camera needs to know the focal length of the lens, you should enter the actual focal length of the lens.
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Old Jan 28, 2009, 9:48 AM   #3
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thank you for clearing that up for me
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Old Jan 28, 2009, 2:20 PM   #4
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What TCav said. The Pentax cameras only need the information for the SR - the earlier cameras like the DS didn't ask for focal length (and there was no way to enter that information). The cameras are designed to compensate correctly when you enter the actual focal length. In your case, that's 50mm. The only time I've ever used something different was the occasional time I use an extension tube on a macro lens. Then I might fudge and put in a bigger number.

If you turn off SR, then turn on the camera, you'll find it won't ask for the focal length (which is annoying if you aren't using SR and want to know what lens you took a particular picture with).
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Old Jan 28, 2009, 5:30 PM   #5
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ah i see, thats why it didnt ask the first time i switched it on. i didnt have SR on.

i must say, im pretty impressed with lense so far.


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Old Jan 28, 2009, 5:51 PM   #6
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I have the M 50mm 1.7 and prefer it to the M 50mm 1.4 I also have. It'sa pleasure to use, in spite of the fact mine is all manual. Adding the A would make it a breeze to use also. You should get lots of good use out of it, a very nice lens for not much money usually. It's also a lens you can do a number of things with for macro - its sharp enough for adding macro TC and extension tubes. Some people will use old Pentax 50mm lenses reversed with another lens, even with non-Pentax cameras. So you have lots of things you can use it for besides portraits.
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Old Jan 28, 2009, 6:44 PM   #7
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TCav wrote:
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The actual focal length of the lens is 50mm, regardless of the camera body it's attached to. The only difference between a 35mm film body and an APS-C body is the angle of view. The only reason the camera needs to know the lens' focal length is so it knows how much the Shake reduction system has to compensate for camera shake. (That and entering the correct focal length in the EXIF data.)

When the camera needs to know the focal length of the lens, you should enter the actual focal length of the lens.
a good descript.. it's a wonder that there are way to many people that still think that a dslr has a magnification value instead of angle of view.. i've given up on explaining it a number of times..

roy
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Old Jan 29, 2009, 3:24 AM   #8
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I get lazy sometimes and just don't change the focal length when asked..I'm sure it should matter but I've yet to see a huge difference when I go from 50mm to 500mm.

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Old Jan 29, 2009, 11:26 AM   #9
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mtngal wrote:
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I have the M 50mm 1.7 and prefer it to the M 50mm 1.4 I also have. It'sa pleasure to use, in spite of the fact mine is all manual. Adding the A would make it a breeze to use also. You should get lots of good use out of it, a very nice lens for not much money usually. It's also a lens you can do a number of things with for macro - its sharp enough for adding macro TC and extension tubes. Some people will use old Pentax 50mm lenses reversed with another lens, even with non-Pentax cameras. So you have lots of things you can use it for besides portraits.
it was the 50mm 1.4 i was first looking for but even on ebay the A versions were fetching well over £100 which seemed quite a lot considering you can buy the smc FA 50mm F1.4 for £160, i did a little research and most people seemed to think there wasnt much difference between the 1.4 & 1.7 versions, so i had a look round and picked one up for £58 which seemed reasonable, id seen them going for more on ebay.
i also got a 28mm Pentax-M 2.8 which also seems very sharp for a meager£20 and a vivitar 2x teleconverter for just £5

so thats me with lots to play with for a while!
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Old Jan 29, 2009, 1:04 PM   #10
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bigdawg wrote:
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I get lazy sometimes and just don't change the focal length when asked..I'm sure it should matter but I've yet to see a huge difference when I go from 50mm to 500mm.
If you are able to hold the camera steady, such that the Shake Reduction doesn't kick in very much, then you won't. You only need to enter the correct focal length so the Shake Reduction works properly.
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