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06-06-2011, 01:30 AM   #1
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Lens' field/angle of view

Hi.
In description of Pentax-DA L 18-55mm written that its Horizontal FOV (APS-C) is 67-25 degrees and Diagonal FOV (APS-C) is 76-29 degrees.
In description of Tamron SP AF 24-135mm written - Angle of View: 84-18 degrees.
If I understand correctly, because Tamron's range starts from 24, its starting FOV should be narrower than DA's 18-55 at 18.
Am I missing something? Maybe I should divide 84 by 1.5 (factor of K-x) ?
Thanks

06-06-2011, 03:28 AM   #2
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Morning,

There are 3 field of view measurements - vertical, horizontal and diagonal. Also, you can compute them yourself. Go to In the case of the Tamron SP AF 24-135mm, it is a full frame lens, so the crop factor would be 1.0, thus the Angle of View: 84-18 degrees (on a full frame sensor) - and that is a diagonal angle of view, however when mounted on a APS-c sensor camera its Angle of View would be less - i.e., one with a crop factor of 1.5.

The case of the Pentax-DA L 18-55mm - its a lens for the APS-c sensor (crop factor of 1.5), and the values appear to be correct.

So the moral of the story is that you need to look at the lens and see if they support full frame - and if so, then their Angle of View is probably expressed as a function of the full frame sensor. However, they can also be mounted on a APS-c body, and therefore their angle of view would be reduced.

hope that helps...
06-06-2011, 03:42 AM   #3
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The FOV will be different for the tamron as interested observer noted. But it may not be as simple as applying the crop factor to the FOV you will need to recalculate this fir the new image circle
06-06-2011, 04:26 AM   #4
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QuoteOriginally posted by interested_observer Quote
So the moral of the story is that you need to look at the lens and see if they support full frame - and if so, then their Angle of View is probably expressed as a function of the full frame sensor. However, they can also be mounted on a APS-c body, and therefore their angle of view would be reduced.
if I understand you correctly, does it mean that (when using K-x) 24mm with the Tamron will have smaller AOV than 24mm with the DAL?

06-06-2011, 04:46 AM   #5
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QuoteOriginally posted by MK-x Quote
if I understand you correctly, does it mean that (when using K-x) 24mm with the Tamron will have smaller AOV than 24mm with the DAL?
No. Given a specific frame size, any focal length has ONE diagonal AOV, no matter what lens is used. A FF zoom at 24; a HF / APS-C zoom at 24; an MF or FF or HF 24 prime; all will have the same AOV on your Kx. My spreadsheet says it's 64 degrees. The formula is:
=2*ATAN(diag/(2*focal))*(180/PI())
where diag is the frame diagonal in mm
and focal is the lens focal length in mm
06-06-2011, 04:51 AM   #6
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QuoteOriginally posted by MK-x Quote
if I understand you correctly, does it mean that (when using K-x) 24mm with the Tamron will have smaller AOV than 24mm with the DAL?
Absolutely not. A 24mm lens is a 24mm lens, and any example should have the same AOV with any given sensor.

However, not all lenses will produce an image circle capable of covering (in this instance) a 'full frame' sensor (one equal in size to 35mm film in a 35mm film camera, which is what the Tamron lens was designed for). As the Tamron was designed for film, thats what is given in the specs.

Even if the Pentax D AL was capable of producing an image circle to cover 'full frame' (it isn't), that isn't what it was designed for, so the specs are given for its intended use.
06-06-2011, 05:04 AM   #7
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And I've already started thinking that I've done a mistake buying the Tamron 24-135. I can live with lack of 18-24 range, but don't want to "lose" more.
Thanks
06-06-2011, 07:47 AM   #8
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That's all you lose.

06-06-2011, 08:31 AM   #9
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QuoteOriginally posted by MK-x Quote
And I've already started thinking that I've done a mistake buying the Tamron 24-135. I can live with lack of 18-24 range, but don't want to "lose" more.
Thanks
You "lose" nothing in that you have an excellent walkaround versatile lens. If you want a wide angle you get a wide angle. At these FL's the visible "loss" is minuscule on APS-C.
06-06-2011, 08:46 AM   #10
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Here's a FOV table:


Adam
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06-06-2011, 04:50 PM   #11
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thanks guys
06-06-2011, 10:24 PM   #12
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I had to go in to work early, and I got home late.... Sorry.

The focal length of a lens is always the same - it never changes. The sensor upon which the lens focuses it light is the primary factor here. Think of a projection screen up on a wall. A large one will have a large scene projected upon it. If you put a smaller screen immediately in front of it, the scene will be smaller, hence the difference in the Angle of View of two differently sized sensors.

That is exactly what the algorithm that Rico provided shows, along with the table that Adam provided.

If you look at Adam's table - say the 45mm focal length, as the sensor gets larger, the horizontal Angle of View increases proportionally. Or correspondingly, just track the bold value of about 50 degrees (AoV) across the various sensor sizes, against the corresponding focal lengths.

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