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12-02-2010, 05:05 AM   #1
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Question About Crop Factor With Pentax FA 50mm f1.4

Question: When I put my FA 50mm f1.4 onto my Pentax K-x body, does it function as a 50mm or am I using a different focal length due to crop factor. I'm hoping the K-x does something magic which means the 50mm does in fact remain so, but I'm assuming that this is not the case.

Badly worded question, but I hope you get the drift.

12-02-2010, 05:31 AM   #2
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Read this:

https://www.pentaxforums.com/forums/pentax-lens-articles/90477-crop-factor-fo...ield-view.html
12-02-2010, 05:33 AM   #3
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This is simply a matter of the APS-C sensor being smaller by a factor of 1.5 in height and width compared to the 35mm film frame. You get a hardwired center crop where the field of view is the same as it would be with a 1.5x focal length (75mm for 50mm) on a full frame camera.
12-02-2010, 05:35 AM   #4
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I'm sorry but I enjoy taking photographs but am totally uninterested in the science.

Could you answer my question with a simple 'yes the crop factor means your lens will not be performing as a 50mm or no your lens will perform as a 50mm'.

I must say though, I am impressed with your knowledge.

12-02-2010, 05:39 AM   #5
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QuoteOriginally posted by chrisrussell Quote
the crop factor means your lens will be performing as a 50mm
What changes is the Field of View.
12-02-2010, 05:44 AM   #6
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So will I be taking pictures at 50mm or something else?
12-02-2010, 05:53 AM   #7
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QuoteOriginally posted by chrisrussell Quote
So will I be taking pictures at 50mm or something else?
Yes, but you are seeing/capturing less than on a FF/35mm camera. It's like you were shooting with a field of view of a 75mm lens.
12-02-2010, 06:03 AM   #8
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That makes sense.

So I guess my next question is: what 50mm or similar do you recommend which captures full 50mm?

12-02-2010, 06:06 AM   #9
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QuoteOriginally posted by chrisrussell Quote
I'm sorry but I enjoy taking photographs but am totally uninterested in the science.

Could you answer my question with a simple 'yes the crop factor means your lens will not be performing as a 50mm or no your lens will perform as a 50mm'.

I must say though, I am impressed with your knowledge.
Short answer: no. Your wont get the same image you would with 50mm lens on film. Instead you get the center part of the 50mm film image, the rest falls outside the edges of the physically smaller sensor. As for as field of view - what fill fit in the frame height and lengthwise - you get similar results as you would with a 75mm (1.5x) lens and film, this is not the same as actually using a 75mm lens with film though as perspective and the depth of the area that is in focus (DOF) are different. The key issue is that the sensor is simply smaller than the film frame for which the optics were originally designed. It seems this is often presented as getting an actual boost in focal range ("zoom factor"), but that is marketing spin :-)
12-02-2010, 06:13 AM   #10
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If you want to work with "35mm equivalent" focal length values, then it's easy: divide the 35mm-equivalent focal length by 1.5. So, for a 50mm-on-a-35mm-SLR field of view, you will need a 33.3mm focal length lens. It'll still be a 33.3mm lens (or, shall we say, 35mm), but the effect will be of a 50mm lens mounted on an old film SLR.

By the way, it's common for point-and-shoot cameras to have their lenses specified in "35mm-equivalent" terms, partly because there's such a wide variety of P&S sensor-sizes/crop-factors - for instance I have a P&S camera whose zoom lens at 50mm gives an equivalent field of view of 270mm on a 35mm film camera!
12-02-2010, 06:23 AM   #11
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QuoteOriginally posted by chrisrussell Quote
That makes sense.

So I guess my next question is: what 50mm or similar do you recommend which captures full 50mm?
As said you'd need a 33mm (~ 50/1.5) lens or thereabouts to fit the same scene in the frame as you'd get with a 50 mm on film. In pratice a 28 or 35 mm lens should be pretty close.
12-02-2010, 06:31 AM   #12
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The closest you'll get to the actual equivalent field of view will either be the new DA 35mm F2.4 (msrp $219) lens, or the discontinued (but sometimes available used between $300-400) FA 35mm F2 lens. Those will be an equivalent of approximately 52.5mm, just slightly tighter than the 50mm FOV you're used to. Alternately there is also the 31mm F1.8 LTD, which is significantly more expensive and will give an equivalent of about 46mm, slightly wider than the 50mm FOV you seek!
12-02-2010, 06:56 AM   #13
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There have been, in my mind, a lot of really poor answers to the OP and his question,

the simple answer is YES his FA50 will behave as any other 50mm lens on his camera. That is all he asked.

The more indepth answer is

the focal length and aperture markings on a lens are physical properties of the lens itself, Focal length, (for a simple lens) is the distance from the lens to the focusing plane for an object infinitely far away. For the OP, this would be 50mm. The lenses used on cameras are comples optical devices, but the design is to yeild an effective focal length of 50mm (in the OP's question).

Aperture is simply the ratio of the focal length to diameter, and again this does not change as a function of the camera the lens is mounted on.

What does change, assuming, the OP has a film camera, and wants to talk about the differences in his shots using the same FA50 and both his film and digital cameras is the following:

- film has a frame dimension of 36mm wide by 24mm high
- digital has a frame dimension (for pentax ASP-C sensors) of approximately 24mm wide by 16mm high.

Using the same 50mm lens, shooting the same scene with both his digital and film bodies will result in the following:

The shot with the digital camera will be the center 24mm x 16mm crop out of his 36mm x 24mm film frame.

Enlarging both the digital and crop out of the film frame will yeild identical shots. the real difference is that due to the sensor size, the lens, on film offers a wider field of view.

That is ALL

If the OP does not have a film camera to refer to, there is no point at all discussing crop factor and references to different formats because they are totally irrelevant, We are introducing references to formats that he does not use, does not have experience in, and could care less about.

For the OP, what is important to know is that 50mm is 50mm and is format independant
12-02-2010, 07:22 AM   #14
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So essentially when I shoot with my FA 50mm f1.4 on my Pentax K-x - I'm actually shooing with a tighter 75mm equivalent but with all the bells and whistles of a 50mm.

Purchasing a modern 50mm which works with smaller censors will yield a 'wider' photographic view which will actually be 50mm?
12-02-2010, 07:29 AM   #15
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I hate discussions of crop factor and focal length multiplier.

QuoteOriginally posted by chrisrussell Quote
So essentially when I shoot with my FA 50mm f1.4 on my Pentax K-x - I'm actually shooing with a tighter 75mm equivalent but with all the bells and whistles of a 50mm.

Purchasing a modern 50mm which works with smaller censors will yield a 'wider' photographic view which will actually be 50mm?
No,

I believe you missed the point.

50mm is 50mm and nothing changes between old (film) and new (digital ASP-C), you will have the same field of view on your digital camera with any 50mm, old or new. That is why I provided you with the short answer above. Your FA50 will work identically to any digital 50mm lens.

Field of view on any camera is a function of focal length and sensor size. Since focal length is a physical property of a lens (and is camera independant), and your sensor size is a physical property of your camera (and lens independant), simply changing one 50mm lens for another will not alter AT ALL your field of view.
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