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Analogue Lenses on Digital Body

 
 
phk
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      12-20-2005
There are some theoretical issues, but whether they become material
depends on how much of a perfectionist you are. Most rear film lens
elements are not coated, which potential creates more internal
reflection with the digital sensor. Also, the digital sensor needs
light to fall as vertically as possible on all areas. Film didn't care
about the angle, so older lenses might produce a little less light at
the edges.

On the other hand, some of the newer "digital" lenses do shown signs of
vignetting at wide angles, which is disappointing.

 
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Mark Roberts
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      12-20-2005
"CJB" <(E-Mail Removed)> wrote:

>Jessops (Strand Branch, London) tells me that all of the Pentax SLR
>analogue (aka traditional 35mm film) lenses that I have - zoom,
>fisheye, macro, etc. - are unsuitable for digital use.


He's lying.

>Is this a cynical con. to force photographers to junk their expensive
>lenses and buy new 'digital' versions?


Oh yes.

Of all my lenses, only one is of the latest "optimized for digital"
generation (the Tamron 17-35/2.8-4) and though it's a fine lens, it's
nowhere near as good as my Pentax zooms (28-70/2.8 and 80-200/2., both
of which pre-date digital by a good bit. My primes also work
wonderfully. I also have a 1980's-vintage Vivitar 70-210 Series 1 that's
a great performer on digital.

Get the camera and try out your lenses. You'll be very pleased.


--
Mark Roberts
Photography and writing
www.robertstech.com
 
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Paul
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      12-20-2005
That's interesting.

I did experience slight vignetting on my 17-85mm 'digital' lens that
originally came with the camera at the wide end (below 24mm). And no, it
was not due to the filter or lens hood, as it was the same with a 'bare'
lens.


"phk" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> There are some theoretical issues, but whether they become material
> depends on how much of a perfectionist you are. Most rear film lens
> elements are not coated, which potential creates more internal
> reflection with the digital sensor. Also, the digital sensor needs
> light to fall as vertically as possible on all areas. Film didn't care
> about the angle, so older lenses might produce a little less light at
> the edges.
>
> On the other hand, some of the newer "digital" lenses do shown signs of
> vignetting at wide angles, which is disappointing.
>



 
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Iraxl Enb
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      12-20-2005
I recommend checking out Robert's gallery for the
quality that the pentax is capable of... i was impressed...

--
-- irax
http://rumimmi.blogspot.com
 
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Kennedy McEwen
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      12-20-2005
In article <(E-Mail Removed). com>, Matt
Clara <(E-Mail Removed)> writes
>
>Film doesn't care what angle the light's hitting it from (provided it's
>striking the emulsion side, of course), whereas digital sensors want
>the light more or less straight on/perpindicular to the sensor
>surface.
>

***!!!! MYTH WARNING !!!!***
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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Deep Reset
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      12-20-2005

"CJB" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Jessops (Strand Branch, London) tells me that all of the Pentax SLR
> analogue (aka traditional 35mm film) lenses that I have - zoom,
> fisheye, macro, etc. - are unsuitable for digital use. Is this a
> cynical con. to force photographers to junk their expensive lenses and
> buy new 'digital' versions? If so I feel a sudden need to travel
> overseas and get some duty free equipment 'cos I aint paying the
> grossly inflated prices in rip-off Britain!! But I would really just
> prefer to buy a Pentax digital SLR body replacement and keep all the
> lenses I already have. Please can anyone advise me on this issue. Sorry
> for cross-posting. Many thanks - CJB.


Lemme see - 28, 40, 50, 135, 200mm Pentax primes, 'A' bellows, Tamron 500mm
cat, Tamron 28-200mm..

Yup, all work great with my *istDS.

And my 330FTZ flash, my Vivitar 283s...

Really, all work perfectly

I'd expect better of Jessops (particularly the old Fox Talbot shop)

Deep.


 
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Bruce Hoult
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      12-20-2005
In article <(E-Mail Removed) .com>,
"CJB" <(E-Mail Removed)> wrote:

> Jessops (Strand Branch, London) tells me that all of the Pentax SLR
> analogue (aka traditional 35mm film) lenses that I have - zoom,
> fisheye, macro, etc. - are unsuitable for digital use. Is this a
> cynical con. to force photographers to junk their expensive lenses and
> buy new 'digital' versions?


It's rubbish, assuming that the old lenses physically fit the camera
(same mount).

Most digital cameras have a sensor that is about 2/3 the size of 35mm
film in each direction. So old lenses that are designed to make a nice
image over all of a bit of 35mm film are making a bigger image than the
digital sensor can use. A lot of it spills over the edge of the sensor
and is wasted. Which doesn't matter at all. The pictures will be fine.
They'll just be cropped compared to what you'd get using a film camera
with the same lens and settings. Which is perhaps a good thing with
telephoto lenses because they appear even more telephoto than with film,
but a bad thing with wide angle lenses.

So digital cameras work fine with old lenses, they just don't do as wide
angle as a film camera does with the same lens. If you want really wide
angle then you need a new lens with a shorter focal length than you used
for film. But film lenses already went as wide as was possible while
covering a 35mm frame with image. A lens with an even shorter focal
length won't cover the entire 35mm frame. But it will cover the digital
sensor, which is all you need.

Getting away from the wide angle issue: because a lens designed for a
smaller sensor doesn't have to produce as large an image, the lens
itself can be of smaller diameter while retaining the same F number.
Thus the lens can, in theory, be smaller and lighter and cheaper. This
doesn't seem to have happened all that much in practise yet, though this
may be because this whole thing is still so new that lens manufacturers
have so far concentrated their new "digital" lenses mostly on the wide
angle end where they are really needed.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------
 
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Matt Clara
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      12-20-2005

Kennedy McEwen wrote:
> In article <(E-Mail Removed). com>, Matt
> Clara <(E-Mail Removed)> writes
> >
> >Film doesn't care what angle the light's hitting it from (provided it's
> >striking the emulsion side, of course), whereas digital sensors want
> >the light more or less straight on/perpindicular to the sensor
> >surface.
> >

> ***!!!! MYTH WARNING !!!!***



Dear Ewen, if you're going to shoot down facts, let's have some back up
for it, otherwise you're just another idiot troll:

http://www.cleanimages.com/DDBlog-Sh...izedSensor.asp
http://www.photo.net/bboard/q-and-a-...?msg_id=00Djmn

>From the second link listed immediately above:


To understand how sensor optics can cause vignetting, it is useful to
refer
to some concepts in optics. The exit pupil of a lens is the apparent
position of the aperture diaphragm when viewed from behind the lens.
You can
think of it as being the apparent point through which all principal
rays
pass as the image is inverted by the lens for projection onto the
sensor. A
principal ray is the central ray of the cone of light that is focussed
by
the lens onto each point of the image. The apex angle of the focussed
cone
of light will depend on the aperture in use, and will be wider for
faster
apertures.
The distance of the exit pupil from the sensor/film plane defines a
triangle
whose base is largest across the diagonal of the image. The angle
between
the central lens axis and a side that extends from the centre of the
exit
pupil to the corner of the image is also the off-perpendicular angle at

which the principal ray strikes the sensor at the corner of the image
(by
opposite angles, for those who remember their Euclidean geometry). The
microlens layer which lies over the Bayer colour filter array on top of
the
actual sensor silicon is typically only capable of accepting light to
focus
onto the sensor photodiodes provided it lies in a cone typically with
no
more than a 25-30 degree apex angle - i.e., within 12-15 degrees of
vertical
before light falloff begins to be significant. Film, which lacks the
microlens layer, has no such limitations.


Of course, all wide angle lenses are susceptible to various forms of
vignetting - there is an excellent discussion of these that also
illustrates
some of the concepts discussed above here:


http://www.vanwalree.com/optics/vignetting.html


When using a fast lens wide open with a digital sensor, the cone of
focussed
light has a much wider apex than when using a narrow aperture. More of
the
rays in that cone will lie outside the acceptance angle of the
microlenses
at the corners of the image. Again, film will only be subject to the
normal
causes of vignetting discussed by van Walree.


A comparison with the performance on film will reveal the extent to
which
vignetting is caused by features of the lens design or by additional
limitations imposed by sensor optics. Of course, it is possible to
correct
for vignetting (and drawing/barrel distortion) in software.


As a footnote, the high level of barrel distortion at 24mm with the
24-105
is a consequence of trying to design the lens with the exit pupil
further
from the film plane to help with vignetting, coupled with the 4+ zoom
ratio
of the lens. Figure 2 in this van Walree article should give a basic
understanding as to why this is so:


http://www.vanwalree.com/optics/distortion.html

Have you got anything other than exclamation marks and capitol letters
to back up your claim, you annoying little prat?

 
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Martin Brown
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Posts: n/a
 
      12-20-2005
CJB wrote:

> Jessops (Strand Branch, London) tells me that all of the Pentax SLR
> analogue (aka traditional 35mm film) lenses that I have - zoom,
> fisheye, macro, etc. - are unsuitable for digital use. Is this a
> cynical con. to force photographers to junk their expensive lenses and
> buy new 'digital' versions? If so I feel a sudden need to travel
> overseas and get some duty free equipment 'cos I aint paying the
> grossly inflated prices in rip-off Britain!! But I would really just
> prefer to buy a Pentax digital SLR body replacement and keep all the
> lenses I already have. Please can anyone advise me on this issue. Sorry
> for cross-posting. Many thanks - CJB.


Cynical con. Most will work fine. I am surprised you got such dodgy
advice from Jessops - most of their guys are reasonably clued up. If you
are very unlucky and have a bad or marginal PK fit lens any lateral
false colour will be annoying (but even that can be fixed in software).

Digital takes a smaller on axis field of view than the standard 35mm
format. This means a lens optimised for the CCD can be smaller lighter
and a full illuminated 35mm frame lens has slightly more compromises,
but stopped down you will seldom notice the difference (apart from the
extra weight).

One of my old lenses works better with the smaller size of the CCD than
it does on the full frame 35mm (corner vignetting much reduced).

You can even use antique M42 threaded manual lenses with the Pentax istD
series with a PK adapter. Obviously if you use old lenses you may have
to use manual focus and/or metering, but it isn't a great hardship.

A few cheapo lenses may show bad false colour on a digicam at the edge
of field but they were just as grotty using film. The extra 1.4x scale
factor plays a part in making it more obvious. All my Pentax lenses from
fisheye 17mm out to a 1600mm SCT behave perfectly well with the istDS. YMMV

Regards,
Martin Brown
 
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Bart van der Wolf
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      12-21-2005

"Matt Clara" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>
> Kennedy McEwen wrote:
>> In article <(E-Mail Removed). com>,
>> Matt
>> Clara <(E-Mail Removed)> writes
>> >
>> >Film doesn't care what angle the light's hitting it from
>> >(provided it's striking the emulsion side, of course),


Issue no.1: Film also reflects light as the angle of incidence
devitates more from normal.

>> > whereas digital sensors want the light more or less straight
>> > on/perpindicular to the sensor
>> >surface.


Issue no.2: Although the silicon surface has a somewhat higher
reflectivity than most films, it also varies with angle of incidence.
A microlens structure helps not only to focus the incident light, but
it also serves to improve the apparent fill-factor. This is in
addition to the fact that DSLRs use a retro-focus lens design
especially for shorter focal lengths. The need for a large minimal
'exit-pupil to sensor/film distance' is forced by the mirror-box.

So, angles of incidence are already limited by retro-focus lens
design, and further restricted by the micro-lenses.

>> ***!!!! MYTH WARNING !!!!***

> Dear Ewen, if you're going to shoot down facts, let's have some
> back up for it, otherwise you're just another idiot troll:

SNIP

In addition to the two issues above, since aperture often has a
profound influence on the amount of light fall-off, it by itself
proves the lens design to be the main contributor.
For example, the EF 24-70mm @ 24mm at wider apertures is said to
exhibit less fall-off than the new EF 24-105mm @ 24mm at the same
apertures. Same sensor, same focal length = same angle of incidence,
yet different behaviour due to design.

The effect of lens design becomes apparent when the exit pupil is
observed by looking at it from an off-axis angle. If the almost
circular exit pupil changes to an oval, you are bound to have light
fall-off due to geometry of the exit pupil. Lens designs that can
maintain a circular exit pupil shape over a variety of angles will
exhibit less fall-off.

And then there is the slightly longer travelling distance of corner
rays to the focal plane. Magnification differences will produce lower
luminance because the energy is spread over a larger surface area.

Bart

 
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