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Author Topic: equivalent focal lengths?  (Read 3286 times)
Steve West
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« on: January 25, 2006, 08:47:01 PM »
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I'm having trouble agreeing with the way in which euqivlent focal lengths are calculated for non FF sensors.

It seems to me that cropping factors are erroneously applied to focal lengths.  It seems that a better way to describe equivalent focal lengths should be the height of a length standard at a standard distance on the focal plane.  So if I use a 500mm focal length lens (designed to be used on an FF sensor) on an APS sensor camera, the focal length should still be 500mm because the height of some length standard on the focal plane at a standard distance is exactly the same as the lens used on the FF camera.  The only thing that happens is that the APS sensor crops it--but we should not apply the x1.6 crop factor to manufacture an equivalent focal length.

Similarly for non-DSLR cameras, we should be looking at the size of the length standard at the focal plane to determine the equivalent 35mm focal length.  The whole idea that a 1-pixel wide detector produces an extremely large  crop factor (and thus we would say it produces an extremely large equivalent focal length is just plain wrong). Instead we should look at the length standard's image size on the focal plane without regard to the size of the detector.

Am I all wet here?

Thanks

Steve W
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Yakim Peled
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« Reply #1 on: January 26, 2006, 01:27:02 AM »
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http://www.luminous-landscape.com/tutorial.../dslr-mag.shtml

Does that help?
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Happy shooting,
Yakim.
crspe
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« Reply #2 on: January 26, 2006, 01:32:44 AM »
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It depends on your definition of "equivalent focal length"

... hopefully you would be happy with the statement "When using this 500mm lens on a 1.6x crop camera, you will have an equivalent angular field of view as you would get using an 800mm lens on a FF camera."  
This should be what people understand when they see "equivalent focal length", unfortunatly it is often not the case.  As you describe, the focal length of a lens does not change depending on what camera you attach it to, and as has been discussed seemingly endlessly in online forums, there are still many differences between using a 500mm lens on a 1.6x crop camera and using 800mm lens on a FF camera.

However, I find it very helpful to know which lenses on this sensor format provides the same field of view as the lenses I am used to from 35mm film.  This is especially important for compact cameras where it can be difficult to work out what FOV I will get from the 5.5mm - 11mm lens on a particular camera - here it is very helpful that they publish the "equivalent focal length".

So ... when you see "equivalent focal length" please read "provides equivalent field of view".  Do not expect them to be identical.
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hellofacanon
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« Reply #3 on: January 26, 2006, 05:59:44 AM »
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FOV crop does not increase the focal length of a lens.It just crops the full image, thus producing a smaller angle of view.
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Yakim Peled
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« Reply #4 on: January 26, 2006, 06:21:36 AM »
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And smaller angle of view "means" a longer focal length.
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Happy shooting,
Yakim.
Tim Gray
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« Reply #5 on: January 26, 2006, 09:00:21 AM »
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Quote
The whole idea that a 1-pixel wide detector produces an extremely large  crop factor (and thus we would say it produces an extremely large equivalent focal length is just plain wrong).
[a href=\"index.php?act=findpost&pid=56784\"][{POST_SNAPBACK}][/a]

I'm not sure resolution is relevant for the limited purpose of comparing the geometry of images taken at the same fl with different sized sensors.  I don't think anyone would claim that a 1 pixel crop produces a massive equivalent fl - but conceptually a "sensor" the size of a (today's) pixel would have an extraordinarily narrow fov when coupled with a lens designed for a 35mm capture surface.  The physical limitation of how you capture information from a surface that size isn't relevant to the gross geometry of what's happening.
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dbell
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« Reply #6 on: January 26, 2006, 11:44:55 AM »
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All of this business about "equivalent focal length" came about as a way to help 35mm photographers feel comfortable with digital cameras.

Photographers who use multiple formats have known about the effects of field of view and focal length forever, and they don't refer to their lenses in "35mm equivalent" focal lengths. They just know how long a lens they need to use to get a particular perspective in a particular format. Someone who shoots 6x6 might consider 85mm to be "normal" and someone who shoots 4x5 might consider 200mm to give a "normal" field of view. The focal lengths of those lenses are still 85 and 200, respectively, no matter what camera they are mounted on.

The confusion isn't helped by the fact that there are so many digicam sensor sizes available (many users have no idea what size they have) and that digicam marketing is based on number of megapixels with little mention made of sensor size.

None of this changes the definintion of  "focal length, " and I agree with the original poster in that regard.
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BJL
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« Reply #7 on: January 26, 2006, 01:35:47 PM »
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"35mm format equivalent focal lengths" serve one purpose, and one only: they describe the field of view that you get with a certain focal length on a digital camera in terms of the focal length that would give that same field of view on a 35mm film camera. Other properties like DOF and noise levels at a given shutter speed are not equivalent, at least not if the same aperture ratio is used in comparisons.

P. S. It is nonsense that formats smaller than 35mm film format always impose a crop: in particular, all-in-one (fixed lens) digital cameras with format factors of 4x and up are not cropping from a far larger 35mm film format image; they use lenses of shorter focal lengths that produce a far smaller image, with a sensor that records all of that image, not just a crop from it. Ditto for pairings of 4/3rds, DX or EF-S cameras with 4/3rds, DX or EF-S lenses.
« Last Edit: January 26, 2006, 01:37:30 PM by BJL » Logged
Anon E. Mouse
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« Reply #8 on: January 26, 2006, 06:01:10 PM »
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Angle of View = 2 arctan (dimension at the image plane / (2 x focal length))

Or as it is usually written:

W = 2 arctan (k/2f)

As stated above, "equivalent focal length" is not really a useful term. Focal length is focal length. And it is dependant on the optics, not the sensor/film size.
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Steve West
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« Reply #9 on: January 26, 2006, 07:01:54 PM »
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Thanks to all for replying.  I guess I was reading the "equivalent focal lengths" stated on websites too literally.  Still I'd rather that it always be stated as equivalent FOV for clarity.  Thanks again!

Steve W
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Steve West
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« Reply #10 on: January 26, 2006, 07:18:58 PM »
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Quote
http://www.luminous-landscape.com/tutorial.../dslr-mag.shtml

Does that help?
[a href=\"index.php?act=findpost&pid=56796\"][{POST_SNAPBACK}][/a]

Thanks much!  If I had found this, I wouldn't have had to make this post!!  It is a very description of the situation.

Steve
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