What is this weird hatred of different focal lengths?

wrote:
On Mon, 10 Sep 2012 20:04:27 -0800, (Floyd L. Davidson) wrote:



The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Since you are only using the centre part of the focus plane on a non FF
sensor, the "bend" should be the same for the area used.

Trevor.


No matter what sensor you have, an 85mm focal length lens bends light at less of
an angle than a 58mm lens.

Not to cover a smaller sensor it doesn't.

You are wrong... the 58mm lens bends the light more to match the smaller size of
the smaller sensor.

http://www.pbase.com/flintstonestudi...me_things_work

Your cited page does not confirm your claims.

Please go to http://www.tawbaware.com/maxlyons/calc.htm
where you can scroll down and find a calculator for
"Angular Field of View". That is the angle at which the
lens bends the light!

Set the focal length to 85mm and the crop factor to 1.0,
which will give a horizontal angle of 23.9 degrees, a
vertical angle of 16.1 degrees and a diagonal of 28.6
degrees.

Now change the focal length to 56.667mm and the crop
factor to 1.5. Same angles!

That is because for a given framing, yes the shorter
focal length would bend the light more to get the *same*
size projected. But if the sensor is a smaller size,
then of course the angle will be less... That is exactly
what "crop factor" is. The "equivalent focal length" is
exactly where the angles *are the same*.

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)

wrote in message
...
On Sun, 09 Sep 2012 15:46:48 -0800, (Floyd L. Davidson)
wrote:
wrote:
On Sun, 9 Sep 2012 13:51:59 -0700 (PDT), RichA
wrote:
On Sep 9, 1:43 pm, David Dyer-Bennet wrote:
"Trevor" writes:
Which is all rather amusing when you consider the more serious
photographers
used an 85mm lens and a 35mm lens combination far more often than
anything
in the 40-70mm range. If anything a 58 mm lens was a little better
for
portraits than a 50mm one at least, even if not by much. A fast 50mm
is a
much better lens now on a non FF sensor DSLR however IMO.

A 58mm is great on a 1.5X DSLR for portraits :-)

But does it behave the same way as say an 85mm on a FF for the same
subject matter?


The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Unless of course you use the 85mm of a full frame
sensor, in which case it has to bend the light rays
*exactly* the same as a 56.7mm lens does on a APS-C
sized sensor.

No matter what sensor you have, an 85mm focal length lens bends light at
less of
an angle than a 58mm lens.

Right, but those which are bent more with a 58mm FF lens are not part of an
APSC sensor image, so become irrelevent. The comparison should only be made
with an 85mm APSC lens on an APSC sensor of course, or an 85mm FF lens on a
FF sensor.

Trevor.

wrote in message
...
On Mon, 10 Sep 2012 09:59:23 +1000, "Trevor" wrote:
wrote in message
. ..
On Sun, 9 Sep 2012 13:51:59 -0700 (PDT), RichA
wrote:
On Sep 9, 1:43 pm, David Dyer-Bennet wrote:
"Trevor" writes:
Which is all rather amusing when you consider the more serious
photographers
used an 85mm lens and a 35mm lens combination far more often than
anything
in the 40-70mm range. If anything a 58 mm lens was a little better
for
portraits than a 50mm one at least, even if not by much. A fast 50mm
is a
much better lens now on a non FF sensor DSLR however IMO.

A 58mm is great on a 1.5X DSLR for portraits :-)

That was my point.


But does it behave the same way as say an 85mm on a FF for the same
subject matter?


The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Since you are only using the centre part of the focus plane on a non FF
sensor, the "bend" should be the same for the area used.


No matter what sensor you have, an 85mm focal length lens bends light at
less of
an angle than a 58mm lens.

You really need to specify which you are talking about, which are APSC
lenses and which are FF lenses as the image circle and thus the "angle" the
light is bent is different for both types of lenses at the same marked focal
length.
And for FF lenses only, the image area actually used is smaller on an APSC
sensor, and thus the part which is "bent" the most is outside the sensor
area and becomes irrelevent.

Of course if you ignore all specifics, almost any statement can be claimed
to be correct! :-)

Trevor.

On Wed, 12 Sep 2012 14:30:30 +1000, "Trevor" wrote:


wrote in message
.. .
On Mon, 10 Sep 2012 09:59:23 +1000, "Trevor" wrote:
wrote in message
...
On Sun, 9 Sep 2012 13:51:59 -0700 (PDT), RichA
wrote:
On Sep 9, 1:43 pm, David Dyer-Bennet wrote:
"Trevor" writes:
Which is all rather amusing when you consider the more serious
photographers
used an 85mm lens and a 35mm lens combination far more often than
anything
in the 40-70mm range. If anything a 58 mm lens was a little better
for
portraits than a 50mm one at least, even if not by much. A fast 50mm
is a
much better lens now on a non FF sensor DSLR however IMO.

A 58mm is great on a 1.5X DSLR for portraits :-)

That was my point.


But does it behave the same way as say an 85mm on a FF for the same
subject matter?


The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Since you are only using the centre part of the focus plane on a non FF
sensor, the "bend" should be the same for the area used.


No matter what sensor you have, an 85mm focal length lens bends light at
less of
an angle than a 58mm lens.

You really need to specify which you are talking about, which are APSC
lenses and which are FF lenses as the image circle and thus the "angle" the
light is bent is different for both types of lenses at the same marked focal
length.

No

Whether or not a lens is marked for FF or C sensors has nothing to do with it's
actual focal length, if both lenses are marked 100mm that is what they both are,
and they both bend the light the same amount. The difference is the size of the
circle of light they project, which has to do with the size of the glass lens
and construction, not it's FL.

My D700 shows NO difference in images if I use a Digital 50mm or a FF 50mm lens,
but the circle of light shows smaller with the dig. lens.

If I set the crop factor to c type in the camera, it simply ignores the larger
area, if I set it to FF then I capture the circle, complete with it's dark area
for the D lens.

The pictures are of course identical except for the 'blow up' effect if I crop
to the c size, which is called the crop factor. I can also crop the FF lens and
get the exact same picture as the D lens.

And for FF lenses only, the image area actually used is smaller on an APSC
sensor, and thus the part which is "bent" the most is outside the sensor
area and becomes irrelevent.

Again, the sensor has nothing to do with the amount of bending of the light. A
smaller focal length lens bends the light to a smaller size image, and usually
creates more distortion.

If I draw a schematic of a lens system, and draw a line from the perimeter of
the subject through the objective of a long lens to the target, I will get a
shallow angle. Then I do the same thing for a short lens, I will get a steeper
angle. (This is the only thing BTW I said concerned me in my first post.) Now we
take two sensors of different sizes, and place them to gather the same image, a
large sensor for the long lens and a small sensor for the short lens, the fact
remains that the shorter lens bent the light more, the proof being that the same
objective size fills the smaller sensor.

On Tue, 11 Sep 2012 16:38:51 -0800, (Floyd L. Davidson) wrote:

wrote:
On Mon, 10 Sep 2012 20:04:27 -0800, (Floyd L. Davidson) wrote:



The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Since you are only using the centre part of the focus plane on a non FF
sensor, the "bend" should be the same for the area used.

Trevor.


No matter what sensor you have, an 85mm focal length lens bends light at less of
an angle than a 58mm lens.

Not to cover a smaller sensor it doesn't.

You are wrong... the 58mm lens bends the light more to match the smaller size of
the smaller sensor.

http://www.pbase.com/flintstonestudi...me_things_work

Your cited page does not confirm your claims.

Yes it does.

Please go to http://www.tawbaware.com/maxlyons/calc.htm
where you can scroll down and find a calculator for
"Angular Field of View". That is the angle at which the
lens bends the light!

No it ain't!

That is the combination of the sensor size and the lens, that creates an image
with a certain field or angle of view. These combinations have infinite
variables.

A short lens bends the light to a smaller size which can match the size of a
smaller sensor, giving possibly the same image (angle of view) as a long lens
which doesn't bend the light to such a small pattern and therefore covers a
larger sensor.

Set the focal length to 85mm and the crop factor to 1.0,
which will give a horizontal angle of 23.9 degrees, a
vertical angle of 16.1 degrees and a diagonal of 28.6
degrees.

Now change the focal length to 56.667mm and the crop
factor to 1.5. Same angles!

You think that parallel light waves coming through 2 different FL lenses have
the same angle of refraction??????

AND that they produce the same size images on DIFFERENT size sensors means the
same thing?????

Which means they would produce DIFFERENT size images on the SAME sensor, and
that proves they are the SAME angle?????

I think not.

wrote:
On Tue, 11 Sep 2012 16:38:51 -0800, (Floyd L. Davidson) wrote:

wrote:
On Mon, 10 Sep 2012 20:04:27 -0800, (Floyd L. Davidson) wrote:



The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Since you are only using the centre part of the focus plane on a non FF
sensor, the "bend" should be the same for the area used.

Trevor.


No matter what sensor you have, an 85mm focal length lens bends light at less of
an angle than a 58mm lens.

Not to cover a smaller sensor it doesn't.

You are wrong... the 58mm lens bends the light more to match the smaller size of
the smaller sensor.

http://www.pbase.com/flintstonestudi...me_things_work

Your cited page does not confirm your claims.

Yes it does.

Please go to http://www.tawbaware.com/maxlyons/calc.htm
where you can scroll down and find a calculator for
"Angular Field of View". That is the angle at which the
lens bends the light!

No it ain't!

In fact, it is the maximum angle. It is the angle that the edges of a scene
have to be bent at. Note that light rays from the center of a scene are not
bent at all for rays going through the center of the lens.

That is the combination of the sensor size and the lens, that creates an image
with a certain field or angle of view. These combinations have infinite
variables.

And that defines the maximum angle required.

A short lens bends the light to a smaller size which can match the size of a
smaller sensor, giving possibly the same image (angle of view) as a long lens
which doesn't bend the light to such a small pattern and therefore covers a
larger sensor.

If the long lens bends the light to cover a larger sensor then it *necessarily*
is bending it *more* than is required to bend a light ray to cover a smaller sensor.

The *fact* is that for the same framing on different sized sensors, the ratio
of the focal lengths and the sensor dimensions (crop factor) will be the same.

Set the focal length to 85mm and the crop factor to 1.0,
which will give a horizontal angle of 23.9 degrees, a
vertical angle of 16.1 degrees and a diagonal of 28.6
degrees.

Now change the focal length to 56.667mm and the crop
factor to 1.5. Same angles!

You think that parallel light waves coming through 2 different FL lenses have
the same angle of refraction??????

AND that they produce the same size images on DIFFERENT size sensors means the
same thing?????

No body said "the same size images"! Pay attention to
detail! They produce the same framing of the image, *on
different sized sensors*, which clearly also means different
size images.

This is not rocket science...

Which means they would produce DIFFERENT size images on the SAME sensor, and
that proves they are the SAME angle?????

I think not.

Then it's time you actually started thinking about it.

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)

wrote in message
...
On Wed, 12 Sep 2012 14:30:30 +1000, "Trevor" wrote:


wrote in message
. ..
On Mon, 10 Sep 2012 09:59:23 +1000, "Trevor" wrote:
wrote in message
m...
On Sun, 9 Sep 2012 13:51:59 -0700 (PDT), RichA
wrote:
On Sep 9, 1:43 pm, David Dyer-Bennet wrote:
"Trevor" writes:
Which is all rather amusing when you consider the more serious
photographers
used an 85mm lens and a 35mm lens combination far more often than
anything
in the 40-70mm range. If anything a 58 mm lens was a little better
for
portraits than a 50mm one at least, even if not by much. A fast
50mm
is a
much better lens now on a non FF sensor DSLR however IMO.

A 58mm is great on a 1.5X DSLR for portraits :-)

That was my point.


But does it behave the same way as say an 85mm on a FF for the same
subject matter?


The 85mm lens bends the light rays less than the 58mm lens does, that
might make
a differance... you need to try it!

Since you are only using the centre part of the focus plane on a non FF
sensor, the "bend" should be the same for the area used.


No matter what sensor you have, an 85mm focal length lens bends light at
less of
an angle than a 58mm lens.

You really need to specify which you are talking about, which are APSC
lenses and which are FF lenses as the image circle and thus the "angle"
the
light is bent is different for both types of lenses at the same marked
focal
length.

No

Whether or not a lens is marked for FF or C sensors has nothing to do with
it's
actual focal length, if both lenses are marked 100mm that is what they
both are,

Who said otherwise? One is necessarily designed for a bigger image circle
though.


and they both bend the light the same amount.

NO, because one has a bigger image circle than the other.


The difference is the size of the
circle of light they project, which has to do with the size of the glass
lens
and construction, not it's FL.

If you rearrange that so as to realise the size and construction is a
*consequence* of the required image circle, then correct.


My D700 shows NO difference in images if I use a Digital 50mm or a FF 50mm
lens,
but the circle of light shows smaller with the dig. lens.

Exactly.


If I set the crop factor to c type in the camera, it simply ignores the
larger
area, if I set it to FF then I capture the circle, complete with it's dark
area
for the D lens.
The pictures are of course identical except for the 'blow up' effect if I
crop
to the c size, which is called the crop factor. I can also crop the FF
lens and
get the exact same picture as the D lens.

Right, so what?


And for FF lenses only, the image area actually used is smaller on an APSC
sensor, and thus the part which is "bent" the most is outside the sensor
area and becomes irrelevent.

Again, the sensor has nothing to do with the amount of bending of the
light.

But it DOES affect what part is used!!!!


A smaller focal length lens bends the light to a smaller size image, and
usually
creates more distortion.

Focal length has nothing to do with "image size" at the sensor. Pehaps you
are just being sloppy again with your terminology? And distortion of course
is not just about smaller focal lengths, unless you are talking about fish
eye's.


If I draw a schematic of a lens system, and draw a line from the perimeter
of
the subject through the objective of a long lens to the target, I will get
a
shallow angle. Then I do the same thing for a short lens, I will get a
steeper
angle. (This is the only thing BTW I said concerned me in my first post.)
Now we
take two sensors of different sizes, and place them to gather the same
image, a
large sensor for the long lens and a small sensor for the short lens, the
fact
remains that the shorter lens bent the light more, the proof being that
the same
objective size fills the smaller sensor.

Hard to even tell what you are trying to say here. But drawing the ray
diagrams for both size sensors on both size lenses is a good start. It would
certainly show you need a steeper angle for a given FL to fill a bigger
sensor image circle.

Trevor.

You really need to specify which you are talking about, which are APSC
lenses and which are FF lenses as the image circle and thus the "angle"
the
light is bent is different for both types of lenses at the same marked
focal
length.

No

Whether or not a lens is marked for FF or C sensors has nothing to do with
it's
actual focal length, if both lenses are marked 100mm that is what they
both are,

Who said otherwise? One is necessarily designed for a bigger image circle
though.


and they both bend the light the same amount.

NO, because one has a bigger image circle than the other.

WRONG! The bigger image circle contains MORE IMAGE AREA.

Anyway, you guys do what you want, I'm done wasting my time.

wrote:
Anyway, you guys do what you want, I'm done wasting my time.

Bob, you've been wasting your time is correct...

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)

RichA wrote:
On Sep 11, 3:19*pm, David Dyer-Bennet wrote:
Thre is no flattening effect or compression caused by focal length.

Perspective (which technically means the relationships between objects
in the rendered image) is controlled by camera location. *If you take a
photo from the same place with the center of the frame pointing exactly
the same direction with a 24mm lens and 600mm lens, and crop the 600mm
angle of view out of the center of the 24mm image, the perspective will
be the same.

So, if we have two objects in a frame, at different distances from
the camera and we just frame them in a 500mm lens and then a crop from
a 50mm lens shot, the appearance between the two subjects in the
images will appear identical in both shots? No "telephoto
compression" will be visible making the two objects in the 500mm crop
seem closer to each other?

Yes.

The composition will be the same. That is, the size of the two object
will have the same relationship. If the nearer object appears to be twice
as tall as the far one, that will be true for both images.

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)