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10-26-2010, 10:20 AM   #1
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Equivalent FoV

Hi Pentaxians,
My question is about the equivalent FoV on my K7, compared to a FF.

I have the Pentax 18-250mm (versatile, but needs an elder brother). When it reads 18mm, the FoV is equiv to about 27mm on a FF (hypothetically assuming the lens had the FoV big enough for FF), but 18mm on my K7. As for the lens on APS-C, what I see through the K7 is what I get, reading wise. But, for lenses that are made for FF, eg. when a FF lens reads 50mm, it would actually be a 75mm equiv FoV on my K7 (in other words the same FoV when my 18-250 would read as 75mm).

Here's what I don't understand. I was looking at the Tamron 70-200mm 2.8 at B&H the other day paired with my K7. At 70mm it had the exact same FoV of what my 18-200mm looked when it is at 70mm. The Tamron is a FF lens. So, the Tamron at 70mm should have looked more like what my 18-250 would have when dragged to 105mm. My question is, how is the FoV the same on both when each are showing their own versions of the 70mm reading? Where am I going wrong?

Cheers..

10-26-2010, 10:25 AM   #2
Raylon
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How the lens is built makes no difference to the focal length. All lenses are measured to 35mm film standards, not FF or APS-C. So 70mm will be the same for all lenses, regardless of what they were built for. But that 70mm would be a true 70mm on a FF, not a 1.5x crop on a APS-C, which is 105mm.

So basically a 50mm prime built for FF should be the same as a ~35mm prime on APS-C. But renaming all the formats would be very confusing, so everything is done to a single standard.


EDIT: It's kind of hard to explain just typing it out. Hopefully someone cane put it in the right words for you.
10-26-2010, 10:33 AM   #3
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QuoteOriginally posted by Raylon Quote
So basically a 50mm prime built for FF should be the same as a ~35mm prime on APS-C. But renaming all the formats would be very confusing, so everything is done to a single standard.
You were doing well until you got to this, and then it got confusing again.

I think what you MEANT to say above is:

"So basically a FF 50mm prime mounted on a FF camera will have the same FOV as a ~35mm prime on an APS-C camera. But renaming all the formats would be very confusing, so everything is done to a single standard."

But your basic sentiment was correct: 70mm is 70mm no matter what format the lens is intended for. It's the sensor that crops FOV, not the lens.
10-26-2010, 10:36 AM   #4
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QuoteOriginally posted by Raylon Quote
H All lenses are measured to 35mm film standards, .
Wrong. The focal length is measured through the (apparent) optical center of the lens. APS-C, FF or medium format has nothing to do with it.

70 mm is always 70 mm. So, a lens designed for a 35mm format will have a larger image circle than one designed specifically for an APS-C camera, but the focal length doesn't change.

10-26-2010, 10:39 AM   #5
Raylon
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QuoteOriginally posted by flyer Quote
Wrong. The focal length is measured through the (apparent) optical center of the lens. APS-C, FF or medium format has nothing to do with it.
Ahh, gotcha. Learn something new everyday. I simply thought because the 35mm format had been around for so long that lenses had been measured around them. That makes much more sense.

Like I said, I knew someone else would know more and word it better
10-26-2010, 10:47 AM   #6
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I still see thousands upon thousands of photographic websites, even reviewers, and many many ordinary amateurs repeat this stuff ad infinitum, parrot fashion, claiming they get extra reach when simply mounting a lens on a crop camera. Its become a legend that wont go away

The only way to get extra reach is with a Teleconverter, assuming we match like for like / pixel density between cameras

DCR Workshop: Digital Crop Factor Demystified

Field of View Crop Factor (Focal Length Multiplier)

DSLR Magnification
10-26-2010, 10:50 AM   #7
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QuoteOriginally posted by icypepsi Quote
Hi Pentaxians,
My question is about the equivalent FoV on my K7, compared to a FF.

I have the Pentax 18-250mm (versatile, but needs an elder brother). When it reads 18mm, the FoV is equiv to about 27mm on a FF (hypothetically assuming the lens had the FoV big enough for FF), but 18mm on my K7. As for the lens on APS-C, what I see through the K7 is what I get, reading wise. But, for lenses that are made for FF, eg. when a FF lens reads 50mm, it would actually be a 75mm equiv FoV on my K7 (in other words the same FoV when my 18-250 would read as 75mm).

Here's what I don't understand. I was looking at the Tamron 70-200mm 2.8 at B&H the other day paired with my K7. At 70mm it had the exact same FoV of what my 18-200mm looked when it is at 70mm. The Tamron is a FF lens. So, the Tamron at 70mm should have looked more like what my 18-250 would have when dragged to 105mm. My question is, how is the FoV the same on both when each are showing their own versions of the 70mm reading? Where am I going wrong?

Cheers..
What is the difference between focal length and crop factor? - Photography - Stack Exchange
10-26-2010, 12:11 PM   #8
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They have a good explanation. thanks,

10-26-2010, 12:17 PM   #9
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Gads, I wish the crop factor junk would just go away.

OK, focal length is focal length is focal length.

If the lens is marked as a 50mm lens (for example), it is a 50mm lens no matter what format it is built for, be it APS-C, 135, 645, 6x7, 4x5 or whatever.
What changes is the physical size of the imaging device behind it and how big an area of the image circle that the lens projects is captured.
10-26-2010, 12:31 PM   #10
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Amen
10-26-2010, 12:47 PM   #11
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QuoteOriginally posted by Wheatfield Quote
Gads, I wish the crop factor junk would just go away.

OK, focal length is focal length is focal length.

If the lens is marked as a 50mm lens (for example), it is a 50mm lens no matter what format it is built for, be it APS-C, 135, 645, 6x7, 4x5 or whatever.
What changes is the physical size of the imaging device behind it and how big an area of the image circle that the lens projects is captured.
but then we would have nothing to complain about.

Just think though. If pentax ever made a full frame camera people would be convertinig the ASP-C designed lenses back to full frame thinking for example the 12-24 was now an 8-16mm

Can you just imagine how screwed up that would be?
10-26-2010, 12:54 PM   #12
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Ouch
10-26-2010, 01:34 PM   #13
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Here is my zillionth iteration of this illustration:

Cut a picture from a magazine. Draw a 60x45mm rectangle on it. That's ~the frame size of a 645 MF camera. Draw a 36x24mm rectangle inside that; it's the size of a 135/FF frame. Draw a 24x18mm rectangle inside that; it's ~the size of the APS-C frame of your K7. Draw a 12x18mm rectangle inside that; it's ~the size of an M4/3 or 110 frame.

Notice: THE PICTURE DIDN"T CHANGE! Resolution and DOF and perspective remain the same. Each smaller frame just sees less of the picture, that's all. This is EXACTLY as if you used one lens, the same lens, the same focal length and aperture, on 4 different cameras at the same place. The lens projects the same image circle, but smaller frames get smaller slices of it.

Forget you ever heard of "crop factor", equivalences, any of that crap. Learn what a lens does on your camera. Don't think, "Well, the best portrait FL is 85mm, and with a 1.5x crap factor that's 58mm, so I need a Helios-44M". Think, "Hmmm, I'll try a 35-105 zoom and see which FL and shooting distance I'm most comfortable with for portraits". Constantly translating crap factors will drive you nuts. Just get out there and SHOOT!
10-26-2010, 01:50 PM   #14
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QuoteOriginally posted by RioRico Quote
Notice: THE PICTURE DIDN"T CHANGE! Resolution and DOF and perspective remain the same. Each smaller frame just sees less of the picture, that's all. This is EXACTLY as if you used one lens, the same lens, the same focal length and aperture, on 4 different cameras at the same place. The lens projects the same image circle, but smaller frames get smaller slices of it.
Hey...maybe it's time to go back to the old turret style of changing lenses like on old movie cameras. Except, this time, we'll pair it with a turret on the BACK of the camera so that we can rotate formats, too! That way, we can mix and match focal lengths and formats to get the FOV that we want.
10-26-2010, 02:20 PM   #15
Col
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QuoteOriginally posted by RioRico Quote
Here is my zillionth iteration of this illustration:

edit

Forget you ever heard of "crop factor", equivalences, any of that crap. Learn what a lens does on your camera. Don't think, "Well, the best portrait FL is 85mm, and with a 1.5x crap factor that's 58mm, so I need a Helios-44M". Think, "Hmmm, I'll try a 35-105 zoom and see which FL and shooting distance I'm most comfortable with for portraits". Constantly translating crap factors will drive you nuts. Just get out there and SHOOT!
CRAP factor! I think your true feelings are showing through Though I'm in total agreement with you...
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