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Digital 17mm Is Not Equivelent 27mm on 35mm Film



 
 
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  #11  
Old December 19th 04, 05:10 PM
Fred McKenzie
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It's the biggest misconception in digital photography. People think a
smaller sensor somehow changes the optics of the lens. Film or digital --
it's the same light projected through the same lens, you just use a smaller
piece of it with the digital.

Bob-

Funny you should mention that. Another related misconception is that sensor
size is not as important as the number of megapixels. In other words, it is OK
to have a small sensor if you have enough megapixels.

Thirty years ago lenses were rated in "lines per millimeter". I don't fully
understand the more modern modulation transfer ratio (MTR), but I can relate
lines to pixels. So, above a certain number, no matter how many pixels you
have, the image resolution is limited by the lens resolving power. Fewer
sensor millimeters means fewer "lines" in the resulting image. This is another
flaw in the claim of 35 mm equivalency since you would divide lines per
millimeter by the cropping factor.

Perhaps it wasn't a significant factor when sensors were no more than one
megapixel, but is certainly is in today's eight to ten megapixel world.

Fred

  #12  
Old December 20th 04, 12:00 AM
David J. Littleboy
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"A" wrote:

Just realised that even though manufactures give 35mm equivalents, it is

not
really true.

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.


No, you get exactly the same image you would have with a 27mm on 35mm. The
way the lens projects the image onto the sensor scales exactly. The only
thing that changes is the DOF, which is increased by the ratio of the
formats in the smaller format.

As Fred M. points out, the mognification from the sensor to the print is
larger, so for the same resolution lens, you also may have a lower
resolution print, but most people find that film's MTF and noise are so
grody that an 8MP 1.6x sensor captures about the same apparent detail as ISO
100 film in 35mm.

David J. Littleboy
Tokyo, Japan



  #13  
Old December 20th 04, 06:20 PM
Andrew Koenig
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"A" wrote in message
...

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.


I don't believe it. Can you post some pictures to prove it? Take a picture
with a 17mm lens on a digital camera, and take an equivalent picture with a
28mm lens on a film camera. Then post both images side by side and show us
why you think they're so different.


  #14  
Old December 20th 04, 06:20 PM
Andrew Koenig
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"A" wrote in message
...

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.


I don't believe it. Can you post some pictures to prove it? Take a picture
with a 17mm lens on a digital camera, and take an equivalent picture with a
28mm lens on a film camera. Then post both images side by side and show us
why you think they're so different.


  #15  
Old December 20th 04, 06:33 PM
Owamanga
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On Mon, 20 Dec 2004 17:20:41 GMT, "Andrew Koenig" wrote:

"A" wrote in message
...

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.


I don't believe it. Can you post some pictures to prove it? Take a picture
with a 17mm lens on a digital camera, and take an equivalent picture with a
28mm lens on a film camera. Then post both images side by side and show us
why you think they're so different.


This will never happen, because the image will be practically
identical.

--
Owamanga!
  #16  
Old December 20th 04, 06:33 PM
Owamanga
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On Mon, 20 Dec 2004 17:20:41 GMT, "Andrew Koenig" wrote:

"A" wrote in message
...

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.


I don't believe it. Can you post some pictures to prove it? Take a picture
with a 17mm lens on a digital camera, and take an equivalent picture with a
28mm lens on a film camera. Then post both images side by side and show us
why you think they're so different.


This will never happen, because the image will be practically
identical.

--
Owamanga!
  #17  
Old December 20th 04, 07:48 PM
Andrew Koenig
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"Owamanga" wrote in message
...

This will never happen, because the image will be practically
identical.


That's what I think too, which is why I'm asking for evidence to the
contrary.


  #18  
Old December 20th 04, 07:48 PM
Andrew Koenig
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"Owamanga" wrote in message
...

This will never happen, because the image will be practically
identical.


That's what I think too, which is why I'm asking for evidence to the
contrary.


  #19  
Old December 20th 04, 08:29 PM
Chris Brown
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In article ,
Owamanga wrote:
On Mon, 20 Dec 2004 17:20:41 GMT, "Andrew Koenig" wrote:

"A" wrote in message
...

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.


I don't believe it. Can you post some pictures to prove it? Take a picture
with a 17mm lens on a digital camera, and take an equivalent picture with a
28mm lens on a film camera. Then post both images side by side and show us
why you think they're so different.


This will never happen, because the image will be practically
identical.


Well, the depth of field might change, that depends on the apertures used in
each case.

There is a myth that the choice of focal length has some sort of magic
effect on the perspective of a scene, and I think that's where a lot of this
squabbling over terms comes from. I can only suppose that those who relate
focal length to perspective this way have read a simplified explanation of
the effects of telephoto and wide angle lenses in composition, and think
that the differences they see are due to the focal length, rather than the
distance to their subject. Thinking about it carefully for a few minutes
should be enough to dispell this myth, but some insist on clinging to it.

Addressing the original point, the distortion from a wide-angle lens is a
direct consequence of the field of view represented in the image. Since the
17mm lens on the smaller format digital gives the same field of view as a
28mm lens (I'm assuming we're talking about so-called 1.6x digicams here) on
a larger format 35mm camera, the resultant images will show the same amounts
of rectilinear distortion.

To those who still don't get this, I suggest you think for a moment about
the focal lengths used in zoom compact digital cameras. These ofetn have
lenses that don't even reach 10mm. However, these cameras are perfectly
capable of taking telephoto images which do not show the sort of distortion
associated with ultra-wide angle photography with rectilinear lenses.

People who shoot multiple different film formats tend to understand this
stuff properly (they have to), and know, for example, that a 110mm lens on
6*9 medium format gives pretty much exactly the same results as a 50mm lens
on 35mm, or a 28mm lens on an EOS 20D, the only differences being in terms
of the detail in the final image, and the aperture they'll need to use to
get the same depth of field.

Once you understand this, you'll see why it doesn't make a blind bit of
difference whether you think about this phenomenon as a "crop factor", a
"focal length magnifier", or anything else. They're all equivalent.

Personally, I don't see why it's important to see everything in terms of
35mm equivalents anyway. For me, with my 10D, a 28mm lens is standard, a 50
is a short telephoto, suitable for portraits, a 20 is a wide-angle, and a 14
is an ultrawide.
  #20  
Old December 20th 04, 09:30 PM
Chris Brown
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In article et,
Robert Scott wrote:

"A" wrote in message
...
Just realised that even though manufactures give 35mm equivalents, it is
not
really true.

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor), you
still get distorted pics just like 17mm lenses on 35mm film cameras.



It's the biggest misconception in digital photography.


Indeed it is, and if you, like the original poster, think that you'll get
the same area distortions from rectilinear projection with a given lens,
then I'm afraid you're suffering from said misconception yourself. ;-)

Think about where the distortion comes from. Imagine a 50mm simple lens
focusing an image on to a piece of paper below it, the paper underneath the
centre of the lens will be exactly 50mm from the optical centre of the lens.

Now with a little basic schoolboy triganometry, we can get to the bottom of
this "crop factor" business. Let's do the maths:

The paper 10mm out from the centre point will be 51mm from the optical
centre of the lens (by Pythagoras - the light is coming along the hypotenuse
of a right-angled triangle 50mm by 10mm by 51mm). The paper 20mm out from
the centre will be 54mm from the optical centre of the lens.

This means that as we get further and further from the middle, we are
"stretching" the image, causing a distortion. This is essentially the same
problem that cartographers have when they want to map the curved surface of
the Earth on to a flat map. If you represent the area accurately, the
relative angles get distorted. If you represent the angles accurately, the
area gets distorted. You can't keep both.

We have the same choice in photography - you can use a rectilinear lens (the
angles stay the same, but the areas get distorted), or you can use a fisheye
lens (the areas stay the same, but the angles get distorted). You can't keep
both, and this is where distortion comes from.

*but* the amount of distortion varies according to the distance from the
optical centre of the lens, as we've already seen. With a 35mm camera, a
point at the centre of the short edge is 18mm from the centre of the frame.
With a camera like an EOS 20D, the point at the edge of that same line is
11.25mm from the centre of the frame.

Now think about what that extra distance represents. If you think of a light
ray coming through the centre of the lens and hitting the edge of the scene,
that ray will come from an angle off to the side of arctan(18/50) = 20
degrees (draw a diagram if you can't see why).

We'll get the same level of distortion from a lens that makes a 20 degrees
angle 11.25 mm out from the centre on out 20D (since that's where the edge
of the frame is).

A bit of triganometry says that the focal length is 11.25 / tan(20) (again,
draw a diagram if you can't see why), which is 31.25mm.

So we get the same amount of distortion from a 31.25mm focal length on a 20D
as we do from a 50mm focal length on 35mm.

50 / 31.25 is, surprise surprise, 1.6, so it all works out nicely.

People think a smaller sensor somehow changes the optics of the lens.


And, as we know, that's impossible. However, as we've seen above, the
smaller sensor *requires* different optical properties to get the same
level of distortion. Since we can't change the optics of the lens, we need
to change the lens to one that has the optics we need, that being one with a
1.6 times shorter focal length.

QED
 




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