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03-12-2010, 04:09 PM   #1
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pentax FA 50mm f1.4

Hi all,

i was wondering since the FA lens are designed for film camera, does it mean if i use a FA 50mm on my k-x, it would really be at 50x1.6=80mm?

thanks in advance

M

03-12-2010, 04:29 PM   #2
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QuoteOriginally posted by macky112 Quote
Hi all,

i was wondering since the FA lens are designed for film camera, does it mean if i use a FA 50mm on my k-x, it would really be at 50x1.6=80mm?

thanks in advance

M
No. It has the same Field of View as 75mm (50x1.5). The Kx IS a 1.5x crop, correct? A 50mm lens will always be a 50mm regardless of what camera you put it on. The crop does not magnify the lens in any way (if that's what you were asking).

03-12-2010, 04:38 PM   #3
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That's right. It's still a 50mm lens that is cropped so the magnification is the same as used on film but the field of view will look like a 75mm (x1.5). Canon is 1.6x
03-12-2010, 06:04 PM   #4
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Shot from the same place with the same subject-to-lens distance, with the same exposure (f-stop and shutter), the same lens will produce an image with the same DOF (depth of field) and perspective and magnification, regardless what size camera it's on. A bigger sensor will have a wider FOV, will see more of the picture, than a smaller sensor. The smaller sensor just crops the image.

Put an 80mm lens on a 645 body. Snap. With an adapter, put it on a full-frame 35mm body. Snap. Now put it on a half-frame body (frame size is about the same as APS-C). Snap. Now put it on a 110 SLR body (frame size is about the same as m4:5 system). Snap.

Now contact-print each negative. Each picture is exactly the same at the center, but the smaller formats lose area at the edges.

03-12-2010, 06:39 PM   #5
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QuoteOriginally posted by RioRico Quote
Shot from the same place with the same subject-to-lens distance, with the same exposure (f-stop and shutter), the same lens will produce an image with the same DOF (depth of field)
That's actually not accurate. The depth of field on the print will be shallower for the FF camera and lens combination, creating greater blur in the background. This means that if you want to achieve more blur in the background using an APSc camera (1,5x crop), you will have to shoot at a larger aperture.

Depth of field - Wikipedia, the free encyclopedia
03-12-2010, 08:43 PM   #6
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The crop factor can be nearly always ignored when you're using the camera. You can forget about it unless:
  • You were/are a film camera user and need to reference how a given focal length works on film and digital (APS-C).
  • You are comparing your lenses to some other camera's lenses, the other camera has a different sensor size and its lenses are described in 35mm equivalence terms.
  • Arguing on the internet about crop factors, sensor sizes, pixel pitch, etc.

Once you start to use your DSLR, it is much easier to concentrate on how a 50mm lens looks with that camera. Conveniently, any 50mm lens will look the same on your K-x no matter what system it was designed for.
03-12-2010, 10:17 PM   #7
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QuoteOriginally posted by Peter Zack Quote
That's actually not accurate. The depth of field on the print will be shallower for the FF camera and lens combination, creating greater blur in the background. This means that if you want to achieve more blur in the background using an APSc camera (1,5x crop), you will have to shoot at a larger aperture.

Depth of field - Wikipedia, the free encyclopedia
Your citation refers to enlargement. That's why I specified contact-printing the negatives. I stand by my statement (and it's backed by numerous sources) -- disregarding CoC and other presentation issues beyond the photographer's control, DOF for any lens/aperture/distance setting is the same regardless of format.
03-13-2010, 11:44 AM   #8
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To put it another way: all 50mm lenses have exactly the same field of view on your camera. It doesn't mattyer if the lens was made for film or digital.

The crop factor is to help you understand how the field of view will different for a 50mm lens between a digital camera and a film camera. If you don't own a film camera, then you can safely forget you ever heard about a crop factor. But FWIW, on pentax, it's 1.5, not 1.6.

03-13-2010, 12:24 PM   #9
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Original Poster
Hi All,

thank you all for very helpful responses. the main reason i am asking about the FA 50mm f1.4 is that i have a k-x camera with dual kit lens (18-55 and 50-200), and I was testing at 40mm on the kit lens just to get a feel of what the DA limited 40mm f2.8 lens will be like

now i want to know what focal distance i should set on the kit lens to get a feel of what the 50mm f1.4 lens will be like. do i just set to 50mm on the kit lens or 75mm?

thanks again

M
03-13-2010, 12:29 PM   #10
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QuoteOriginally posted by macky112 Quote
now i want to know what focal distance i should set on the kit lens to get a feel of what the 50mm f1.4 lens will be like. do i just set to 50mm on the kit lens or 75mm?
Use the kit, set it to 50mm. Don't try to figure crop factor; that only applies if you're experienced with 35mm full-frame cameras and you're mentally translating how much area the lens sees.

Also, the kit at 50mm (with a maximum aperture of f/5.6) will just give an approximation of the 50/1.4 -- you'll see the width of the picture, but not what happens to depth when the 50 is wide open. Have fun!

Last edited by RioRico; 03-13-2010 at 12:36 PM. Reason: correction
03-13-2010, 10:02 PM   #11
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I will repeat what I said: all 50mm lens have the same fieldof view on your camera. that incudes your zoom lenses when set to 50mm. crop factor is only for comparing 8cameras*, not comparing *lenses*. if you don't own a film camera, please forget you ever heard about crop factors. It has no relevance for you.
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