Pentax camera/lens combo - practical?

"Serious_Practitioner" wrote:

Thank you all. I have to digest this and do some more reading, but this
raises another question. Why does the focal length of the lens "multiply"?


It doesn't change. The focal length remains the same. A 50mm lens is
a 50mm lens whether it is used on 35mm film, medium format film, full
frame digital or APS-size digital.

What *does* change is the angle of view. The APS-size sensor is about
a third smaller than 35mm in width and height. The result is that a
50mm lens used with an APS-size digital sensor gives a similar field
of view to a 75mm lens used with 35mm film (or with a full frame
digital sensor).

It is difficult for most people to understand the exact effect of a
change in the angle of view due to the APS-size digital sensor being
smaller than 35mm film. Because we are familiar with zoom lenses, it
is much easier for most people to understand the exact effect of
increasing the focal length by a factor of 1.5 (most DSLRs) or 1.6
(most Canon consumer DSLRs), hence the idea of a "focal length
multiplier".

"Serious_Practitioner" wrote:
Thank you all. I have to digest this and do some more reading, but this
raises another question. Why does the focal length of the lens "multiply"?


And not only that!
Also the depth of field increases!
infernal stuff ...

Thank you. Those are the clearest answers I've gotten to this question. Even
the answer I got at a good quality camera shop was less than useful. Thank
you very much.


Steve E.



"Tony Polson" wrote in message
...
"Serious_Practitioner" wrote:

Thank you all. I have to digest this and do some more reading, but this
raises another question. Why does the focal length of the lens "multiply"?


It doesn't change. The focal length remains the same. A 50mm lens is
a 50mm lens whether it is used on 35mm film, medium format film, full
frame digital or APS-size digital.

What *does* change is the angle of view. The APS-size sensor is about
a third smaller than 35mm in width and height. The result is that a
50mm lens used with an APS-size digital sensor gives a similar field
of view to a 75mm lens used with 35mm film (or with a full frame
digital sensor).

It is difficult for most people to understand the exact effect of a
change in the angle of view due to the APS-size digital sensor being
smaller than 35mm film. Because we are familiar with zoom lenses, it
is much easier for most people to understand the exact effect of
increasing the focal length by a factor of 1.5 (most DSLRs) or 1.6
(most Canon consumer DSLRs), hence the idea of a "focal length
multiplier".

Yes, Bernd is dead right here. The Depth of Field (DoF) increase with
the same effective AOV and same aperture number. To get the *same* DoF,
one will need to open up the aperture more by dividing with the crop
factor, e.g. for 43mm at f/4 on film SLR, to get the same AOV and the
*same* DoF on the *ist DS, you need to use a 28mm lens and set the
aperture to f2.6 which is calculated by 4/1.535.

For landscape photographers, the increase DoF is something desirable
sometimes but for portrait photographers, the decreased DoF is often
something they don't want. :-)

RiceHigh
http://www.geocities.com/ricehigh

"Serious_Practitioner" wrote:

Thank you. Those are the clearest answers I've gotten to this question. Even
the answer I got at a good quality camera shop was less than useful. Thank
you very much.


You're very welcome. Glad I could be of help.

In article ,
"Serious_Practitioner" wrote:

I am beginning to look for a digital SLR - never had digital. I want an SLR
because more things are eventually possible by varying lenses, not because I
have really great skills. Also, I'm going to Alaska this year so I want to
be able to take the best shots I can. Anyway...

Between us, my father and I have two Pentax film cameras and we like them.
He uses a Pentax model #SF1n and has a few lenses for it.

Steve-

At least rec.photo.digital.slr-systems is a good group for your question.
Although it seems that the majority of people here are Canon or Nikon
nuts, there are a few of us who like Pentax.

I wanted the *ist DS in order to use a 28-70mm f/4 zoom lens I used with
my film camera. It is a good thing I bought the DS with the kit lens. It
seems that my particular zoom lens has a chronic problem with lens element
separation, which I first noticed when it was attached to the new DS body!

In addition to the kit lens, I've only been able to find a couple of
Tamron AF zooms locally. However, they are working quite well. I still
hope to find a store that has a 50mm lens, although I can use an old
non-AF lens in a pinch.

So far I've been able to use ALL of my old pentax K-mount lenses. Manual
focusing can be tricky, since there is no split-image focusing aid, but it
is helped by using the camera's AF signal.

There is always a chance a new Pentax body will be announced at the PMA
show in a few days, so you might wait and see.

Fred

Will keep an ear out...thanks for the tip.


Steve E.


"Fred McKenzie" wrote in message
...
In article ,
"Serious_Practitioner" wrote:

I am beginning to look for a digital SLR - never had digital. I want an
SLR
because more things are eventually possible by varying lenses, not
because I
have really great skills. Also, I'm going to Alaska this year so I want
to
be able to take the best shots I can. Anyway...

Between us, my father and I have two Pentax film cameras and we like
them.
He uses a Pentax model #SF1n and has a few lenses for it.

Steve-

At least rec.photo.digital.slr-systems is a good group for your question.
Although it seems that the majority of people here are Canon or Nikon
nuts, there are a few of us who like Pentax.

I wanted the *ist DS in order to use a 28-70mm f/4 zoom lens I used with
my film camera. It is a good thing I bought the DS with the kit lens. It
seems that my particular zoom lens has a chronic problem with lens element
separation, which I first noticed when it was attached to the new DS body!

In addition to the kit lens, I've only been able to find a couple of
Tamron AF zooms locally. However, they are working quite well. I still
hope to find a store that has a 50mm lens, although I can use an old
non-AF lens in a pinch.

So far I've been able to use ALL of my old pentax K-mount lenses. Manual
focusing can be tricky, since there is no split-image focusing aid, but it
is helped by using the camera's AF signal.

There is always a chance a new Pentax body will be announced at the PMA
show in a few days, so you might wait and see.

Fred