Aperture fixed when 35mm lenses used on small CCD's??

Hi Dave,

AFAIK "magnification factor" is a marketing term rather than an optical
term. As has been already stated, it is a cropping factor, which results in
a reduction of the FOV. If I cut off a tiny square of a 35mm film, I get a
"magnification", in terms of modern digicam marketing ;-) As far as the
f-stop is concerned, this will remain the same. Focal length and focal ratio
are characteristics of the lens - not the focal plane or whatever lies on
that. BTW the f-number is not defined through terms of power denity or
whatever interpretation anyone could come up with. In classical optics, the
f-number=focal length/diaphragm aperture, nothing more, nothing less. Try
http://en.wikipedia.org/wiki/F-stop and http://www.celestron.com/tb-trms.htm
for some terms.

I hope this helps :-)

dimitris

P.S. I remember well that f-number used to be the inverse of F-number, i.e.
f=1/F, I couldn't find any information on this on the web...

"Dave" wrote in message ...
I've often heard it said that a 100mm f/2.8 lens (for example) designed
for a 35mm camera if put on a smaller CCD sensor will become a 160mm f/2.8
i.e. the 'effective' focal length gets multiplied by some factor (1.6) in
my example and the aperture remains constant. I'm not so convinced the
latter is true.

I make a few observations.

1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm f/2.8
lens. The lens remains the same.
Yes it is still a 100mm lens - but it's field of view becomes the same as a
160mm. What this means is that the image you get from a 100mm lens on a 1.6x
digital will be pretty much the same as a 160mm lens on a 35mm cam.

2) The smaller CCD sensor means the focal length (as compared to 35mm) is
longer, as everyone agrees.
uhh... isn't this the opposite of what you said in 1? - the _equivalent_
focal length is longer. the lens is still 100mm. The behaviour of a lens in
terms of field of view is only relevant when considered in conjunction with
the sensor/film in use. a 50mm lens on a digital SLR is a mild telephoto,
on a 35mm cam it becomes a standard lens, yet on a medium format camera it
is a wide angle lens. on a 8x10 large format camera it would be a fisheye or
extreme wide angle, while on a compact digital it is an extreme telephoto.

3) The aperture whilst still f/2.8 is "effectively" larger, as much of the
light is thrown away, missing the sides of the sensor. So the viewfinder
will be darker than if fitted with a f/2.8 lens which filled the sensor
and no more.
The light is not thrown away, it is just that you are not using all of the
projected image circle. To have the equivalent of changing aperture, you
would actually have to throw away some of the light that would otherwise
have hit the sensor. Take one of your 35mm negatives or slides and cut it
down to 24x16mm. Did the negative/slide just become darker? of course it
didn't, it is still correctly exposed. This is because the light that hit
what's left of the negative was still F2.8.

At first the digital format would seem to allow long focal, fast
telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which would
be a very nice fast lens indeed.
Yep.
But I'm not so sure the lens would have the light gathering power of a real
f/2.8 lens, but instead be effectively an f3.5 (I think). I suspect if the
focal length is multipled by 1.6, the apeture will be multipled by
sqrt(1.6), although I might be wrong on the exact calculation.
Aperture is not effected at all. This is one benefit of digital - you can
have fast telephoto lenses at lower cost. Instead of having to pay for a
300mm F2, you can get a 200mm F2 for the same amount of zoom and light
gathering capability. 200mm F2 is a lot lighter and a lot cheaper to produce
than a 300mm F2. To take this to extremes, the Panasonic Lumix FZ10 has a
lens that is the equivalent of 420mm F2.8. A 420mm F2.8 lens for film would
be very heavy and would cost a fortune, however for the very small sensor in
a compact digicam, it is lightweight and compact.


PS,
Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak?? It seems such a move would have a lot of technical
advantages (lower noise) and people with expensive 35mm lenses would get
the full benefit, and not throw much of the light away, which is what I
think would happen now.
The aps sized sensors are a compromise between production cost, image
quality and usability. 35mm is an arbitrary size - there is no rule that
says 35mm is "full frame". If 645 cameras were just as common as 35mm I
could just as easily ask when Nikon would bring out a camera with a 6cm x
4.5cm imaging sensor, so that I could use my old 645 lenses without having a
crop factor. It is a fact of life that every format change requires a
change in other equipment. by having APS sized sensors, people who use
their 35mm lenses benefit by having the equivalent of long focal length fast
lenses, but lose out because their 24mm extreme wide angle lens is now only
a 35mm mild wide angle. It is only at the extreme wide angles that the
smaller sensor becomes a big disadvantage, but at the extreme wide angles,
due to the different characteristics of digital and film, it is best to have
a lens designed specifically for digital anyway. Film doesn't care if light
hits it at an angle, digital does.

"Dave" wrote in message ...
I've often heard it said that a 100mm f/2.8 lens (for example) designed
for a 35mm camera if put on a smaller CCD sensor will become a 160mm f/2.8
i.e. the 'effective' focal length gets multiplied by some factor (1.6) in
my example and the aperture remains constant. I'm not so convinced the
latter is true.

I make a few observations.

1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm f/2.8
lens. The lens remains the same.
Yes it is still a 100mm lens - but it's field of view becomes the same as a
160mm. What this means is that the image you get from a 100mm lens on a 1.6x
digital will be pretty much the same as a 160mm lens on a 35mm cam.

2) The smaller CCD sensor means the focal length (as compared to 35mm) is
longer, as everyone agrees.
uhh... isn't this the opposite of what you said in 1? - the _equivalent_
focal length is longer. the lens is still 100mm. The behaviour of a lens in
terms of field of view is only relevant when considered in conjunction with
the sensor/film in use. a 50mm lens on a digital SLR is a mild telephoto,
on a 35mm cam it becomes a standard lens, yet on a medium format camera it
is a wide angle lens. on a 8x10 large format camera it would be a fisheye or
extreme wide angle, while on a compact digital it is an extreme telephoto.

3) The aperture whilst still f/2.8 is "effectively" larger, as much of the
light is thrown away, missing the sides of the sensor. So the viewfinder
will be darker than if fitted with a f/2.8 lens which filled the sensor
and no more.
The light is not thrown away, it is just that you are not using all of the
projected image circle. To have the equivalent of changing aperture, you
would actually have to throw away some of the light that would otherwise
have hit the sensor. Take one of your 35mm negatives or slides and cut it
down to 24x16mm. Did the negative/slide just become darker? of course it
didn't, it is still correctly exposed. This is because the light that hit
what's left of the negative was still F2.8.

At first the digital format would seem to allow long focal, fast
telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which would
be a very nice fast lens indeed.
Yep.
But I'm not so sure the lens would have the light gathering power of a real
f/2.8 lens, but instead be effectively an f3.5 (I think). I suspect if the
focal length is multipled by 1.6, the apeture will be multipled by
sqrt(1.6), although I might be wrong on the exact calculation.
Aperture is not effected at all. This is one benefit of digital - you can
have fast telephoto lenses at lower cost. Instead of having to pay for a
300mm F2, you can get a 200mm F2 for the same amount of zoom and light
gathering capability. 200mm F2 is a lot lighter and a lot cheaper to produce
than a 300mm F2. To take this to extremes, the Panasonic Lumix FZ10 has a
lens that is the equivalent of 420mm F2.8. A 420mm F2.8 lens for film would
be very heavy and would cost a fortune, however for the very small sensor in
a compact digicam, it is lightweight and compact.


PS,
Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak?? It seems such a move would have a lot of technical
advantages (lower noise) and people with expensive 35mm lenses would get
the full benefit, and not throw much of the light away, which is what I
think would happen now.
The aps sized sensors are a compromise between production cost, image
quality and usability. 35mm is an arbitrary size - there is no rule that
says 35mm is "full frame". If 645 cameras were just as common as 35mm I
could just as easily ask when Nikon would bring out a camera with a 6cm x
4.5cm imaging sensor, so that I could use my old 645 lenses without having a
crop factor. It is a fact of life that every format change requires a
change in other equipment. by having APS sized sensors, people who use
their 35mm lenses benefit by having the equivalent of long focal length fast
lenses, but lose out because their 24mm extreme wide angle lens is now only
a 35mm mild wide angle. It is only at the extreme wide angles that the
smaller sensor becomes a big disadvantage, but at the extreme wide angles,
due to the different characteristics of digital and film, it is best to have
a lens designed specifically for digital anyway. Film doesn't care if light
hits it at an angle, digital does.

In article , Dave wrote:
I've often heard it said that a 100mm f/2.8 lens (for example) designed
for a 35mm camera if put on a smaller CCD sensor will become a 160mm
f/2.8 i.e. the 'effective' focal length gets multiplied by some factor
(1.6) in my example and the aperture remains constant. I'm not so
convinced the latter is true.

I make a few observations.

1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
f/2.8 lens. The lens remains the same.

2) The smaller CCD sensor means the focal length (as compared to 35mm)
is longer, as everyone agrees.

No, the focal length is the same, but the field of view changes, as the CCD
is a smaller format. Think of 645 vs 35mm for good film analogy.

In article , Dave wrote:

1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
f/2.8 lens. The lens remains the same.

correct

2) The smaller CCD sensor means the focal length (as compared to 35mm)
is longer, as everyone agrees.

I don't agree; the angle of view changes, not the focal length.

3) The aperture whilst still f/2.8 is "effectively" larger, as much of
the light is thrown away, missing the sides of the sensor. So the
viewfinder will be darker than if fitted with a f/2.8 lens which filled
the sensor and no more.

You are missing the fact that film (or FF digital) NEEDS more light in
total to get the same exposure, because the same amount of light is
spread out over a much larger area. You have to look at exposure *per
square mm*, which remains the same of course, regardless of format.

Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak??

They have said numerous times they aren't. It has no advantages. BTW the
bodies with the most noise are the Contax N-digital and the Kodak 14n.
Now what does that tell you?

It seems such a move would have a lot of
technical advantages (lower noise) and people with expensive 35mm lenses
would get the full benefit.

The benefit would be they can use their old gear, which is an advantage
for users but not for Nikon. Their customers aren't the "brand-loyal"
people who have a dozen old Nikkors and a few bodies, their customers
are people who actually BUY new stuff.

Lourens

In article , Dave wrote:

1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
f/2.8 lens. The lens remains the same.

correct

2) The smaller CCD sensor means the focal length (as compared to 35mm)
is longer, as everyone agrees.

I don't agree; the angle of view changes, not the focal length.

3) The aperture whilst still f/2.8 is "effectively" larger, as much of
the light is thrown away, missing the sides of the sensor. So the
viewfinder will be darker than if fitted with a f/2.8 lens which filled
the sensor and no more.

You are missing the fact that film (or FF digital) NEEDS more light in
total to get the same exposure, because the same amount of light is
spread out over a much larger area. You have to look at exposure *per
square mm*, which remains the same of course, regardless of format.

Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak??

They have said numerous times they aren't. It has no advantages. BTW the
bodies with the most noise are the Contax N-digital and the Kodak 14n.
Now what does that tell you?

It seems such a move would have a lot of
technical advantages (lower noise) and people with expensive 35mm lenses
would get the full benefit.

The benefit would be they can use their old gear, which is an advantage
for users but not for Nikon. Their customers aren't the "brand-loyal"
people who have a dozen old Nikkors and a few bodies, their customers
are people who actually BUY new stuff.

Lourens

"Justin Thyme" wrote in message
...
"Dave" wrote in message
...
Aperture is not effected at all. This is one benefit of digital - you can
have fast telephoto lenses at lower cost. Instead of having to pay for a
300mm F2, you can get a 200mm F2 for the same amount of zoom and light
gathering capability. 200mm F2 is a lot lighter and a lot cheaper to
produce
than a 300mm F2. To take this to extremes, the Panasonic Lumix FZ10 has a
lens that is the equivalent of 420mm F2.8. A 420mm F2.8 lens for film
would
be very heavy and would cost a fortune, however for the very small sensor
in
a compact digicam, it is lightweight and compact.

PS,
Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak?? It seems such a move would have a lot of
technical
advantages (lower noise) and people with expensive 35mm lenses would get
the full benefit, and not throw much of the light away, which is what I
think would happen now.

The aps sized sensors are a compromise between production cost, image
quality and usability. 35mm is an arbitrary size - there is no rule that
says 35mm is "full frame".

True, except when using a film lens on a digital camera.

If 645 cameras were just as common as 35mm I
could just as easily ask when Nikon would bring out a camera with a 6cm x
4.5cm imaging sensor, so that I could use my old 645 lenses without having
a
crop factor. It is a fact of life that every format change requires a
change in other equipment. by having APS sized sensors, people who use
their 35mm lenses benefit by having the equivalent of long focal length
fast
lenses, but lose out because their 24mm extreme wide angle lens is now
only
a 35mm mild wide angle. It is only at the extreme wide angles that the
smaller sensor becomes a big disadvantage, but at the extreme wide angles,
due to the different characteristics of digital and film, it is best to
have
a lens designed specifically for digital anyway. Film doesn't care if
light
hits it at an angle, digital does.

It would be misleading to say that the crop factor of digital capture
devices which simulates a longer lens length doesn't come without a price.

As Dave Martindale pointed out: "However, there's no free lunch. To produce
the same quality of image from the smaller sensor, a given size print needs
to be enlarged a factor of 1.6X more. That means that the lens needs to
deliver 1.6 times the resolution to the sensor to get the same quality
print. Thus, lenses that are marginal in sharpness for full-frame use may
look just plain unsharp with the smaller sensor, while very sharp lenses can
stand the extra magnification without strain."

Film best,
me

"Justin Thyme" wrote in message
...
"Dave" wrote in message
...
Aperture is not effected at all. This is one benefit of digital - you can
have fast telephoto lenses at lower cost. Instead of having to pay for a
300mm F2, you can get a 200mm F2 for the same amount of zoom and light
gathering capability. 200mm F2 is a lot lighter and a lot cheaper to
produce
than a 300mm F2. To take this to extremes, the Panasonic Lumix FZ10 has a
lens that is the equivalent of 420mm F2.8. A 420mm F2.8 lens for film
would
be very heavy and would cost a fortune, however for the very small sensor
in
a compact digicam, it is lightweight and compact.

PS,
Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak?? It seems such a move would have a lot of
technical
advantages (lower noise) and people with expensive 35mm lenses would get
the full benefit, and not throw much of the light away, which is what I
think would happen now.

The aps sized sensors are a compromise between production cost, image
quality and usability. 35mm is an arbitrary size - there is no rule that
says 35mm is "full frame".

True, except when using a film lens on a digital camera.

If 645 cameras were just as common as 35mm I
could just as easily ask when Nikon would bring out a camera with a 6cm x
4.5cm imaging sensor, so that I could use my old 645 lenses without having
a
crop factor. It is a fact of life that every format change requires a
change in other equipment. by having APS sized sensors, people who use
their 35mm lenses benefit by having the equivalent of long focal length
fast
lenses, but lose out because their 24mm extreme wide angle lens is now
only
a 35mm mild wide angle. It is only at the extreme wide angles that the
smaller sensor becomes a big disadvantage, but at the extreme wide angles,
due to the different characteristics of digital and film, it is best to
have
a lens designed specifically for digital anyway. Film doesn't care if
light
hits it at an angle, digital does.

It would be misleading to say that the crop factor of digital capture
devices which simulates a longer lens length doesn't come without a price.

As Dave Martindale pointed out: "However, there's no free lunch. To produce
the same quality of image from the smaller sensor, a given size print needs
to be enlarged a factor of 1.6X more. That means that the lens needs to
deliver 1.6 times the resolution to the sensor to get the same quality
print. Thus, lenses that are marginal in sharpness for full-frame use may
look just plain unsharp with the smaller sensor, while very sharp lenses can
stand the extra magnification without strain."

Film best,
me

In article ,
(Dave Martindale) wrote:

No. The smaller sensor means that the field of view with the same lens
is narrower. This is sometimes *described* as having the same effect as
using a longer focal length on the full-frame camera.

In fact a small sensor increases DoF. What decreases DoF is the fact
that you need a shorter FL for the same image.

Lourens

In article ,
(Dave Martindale) wrote:

No. The smaller sensor means that the field of view with the same lens
is narrower. This is sometimes *described* as having the same effect as
using a longer focal length on the full-frame camera.

In fact a small sensor increases DoF. What decreases DoF is the fact
that you need a shorter FL for the same image.

Lourens