<s>
The	O
M42	B-Algorithm
lens	I-Algorithm
mount	I-Algorithm
is	O
a	O
screw	O
thread	O
mounting	O
standard	O
for	O
attaching	O
lenses	O
to	O
35	O
mm	O
cameras	O
,	O
primarily	O
single-lens	B-Algorithm
reflex	I-Algorithm
models	O
.	O
</s>
<s>
(	O
The	O
M42	B-Algorithm
lens	I-Algorithm
mount	I-Algorithm
should	O
not	O
be	O
confused	O
with	O
the	O
T-mount	B-Algorithm
,	O
which	O
shares	O
the	O
42mm	O
throat	O
diameter	O
,	O
but	O
differs	O
by	O
having	O
a	O
0.75mm	O
thread	O
pitch	O
.	O
)	O
</s>
<s>
M42	O
thread	O
mount	O
cameras	O
first	O
became	O
well	O
known	O
under	O
the	O
Praktica	O
brand	O
,	O
and	O
thus	O
the	O
M42	B-Algorithm
mount	I-Algorithm
is	O
known	O
as	O
the	O
Praktica	B-Algorithm
thread	I-Algorithm
mount	I-Algorithm
.	O
</s>
<s>
Since	O
there	O
were	O
no	O
proprietary	O
elements	O
to	O
the	O
M42	B-Algorithm
mount	I-Algorithm
,	O
many	O
other	O
manufacturers	O
used	O
it	O
;	O
this	O
has	O
led	O
to	O
it	O
being	O
called	O
the	O
Universal	O
thread	O
mount	O
or	O
Universal	O
screw	O
mount	O
by	O
many	O
.	O
</s>
<s>
The	O
M42	B-Algorithm
mount	I-Algorithm
was	O
also	O
used	O
by	O
Pentax	B-Application
;	O
thus	O
,	O
it	O
is	O
also	O
commonly	O
known	O
as	O
the	O
Pentax	B-Application
thread	O
mount	O
,	O
despite	O
the	O
fact	O
that	O
Pentax	B-Application
did	O
not	O
originate	O
it	O
.	O
</s>
<s>
The	O
M42	B-Algorithm
mount	I-Algorithm
was	O
first	O
developed	O
by	O
Carl	O
Zeiss	O
at	O
their	O
Jena	O
plant	O
in	O
1938	O
at	O
the	O
request	O
of	O
the	O
KW	O
camera	O
company	O
for	O
their	O
Praktica	O
line	O
,	O
which	O
had	O
previously	O
used	O
M40	O
(	O
40mm	O
by	O
1mm	O
DIN	O
)	O
.	O
</s>
<s>
The	O
first	O
lenses	O
were	O
plain	O
stop-down	O
design	O
but	O
many	O
manufacturers	O
extended	O
the	O
M42	B-Algorithm
lens	I-Algorithm
mount	I-Algorithm
to	O
provide	O
extra	O
features	O
.	O
</s>
<s>
The	O
bar	O
on	O
Pentax	B-Algorithm
Spotmatic	I-Algorithm
cameras	O
is	O
operated	O
by	O
spring	O
pressure	O
with	O
timing	O
linked	O
to	O
the	O
shutter	O
,	O
but	O
these	O
cameras	O
also	O
had	O
a	O
separate	O
switch	O
for	O
the	O
light	O
meter	O
circuit	O
which	O
closed	O
the	O
aperture	O
and	O
gave	O
the	O
depth	O
of	O
field	O
preview	O
in	O
this	O
way	O
.	O
</s>
<s>
The	O
last	O
development	O
of	O
M42	O
lenses	O
was	O
the	O
introduction	O
of	O
a	O
link	O
between	O
camera	O
and	O
lens	O
to	O
transmit	O
the	O
lens	O
aperture	O
setting	O
,	O
which	O
allowed	O
light	O
metering	B-Algorithm
with	O
the	O
aperture	O
remaining	O
wide	O
open	O
.	O
</s>
<s>
The	O
means	O
of	O
doing	O
this	O
was	O
not	O
standardised	O
,	O
Praktica	O
's	O
method	O
was	O
to	O
use	O
an	O
electrical	O
connection	O
which	O
transmitted	O
a	O
variable	O
resistance	O
value	O
to	O
the	O
camera	O
's	O
metering	B-Algorithm
circuit	O
.	O
</s>
<s>
Asahi	O
Pentax	B-Application
developed	O
an	O
additional	O
lever	O
on	O
the	O
lens	O
which	O
operated	O
a	O
variable	O
resistor	O
in	O
the	O
camera	O
mount	O
.	O
</s>
<s>
These	O
mechanisms	O
spurred	O
the	O
use	O
of	O
electronic	O
shutters	O
linked	O
to	O
the	O
metering	B-Algorithm
circuit	O
,	O
allowing	O
for	O
automated	O
shutter	O
speed	O
selection	O
by	O
the	O
camera	O
(	O
aperture	B-Algorithm
priority	I-Algorithm
)	O
.	O
</s>
<s>
M42	O
cameras	O
with	O
wide	O
open	O
metering	B-Algorithm
facility	O
include	O
:	O
</s>
<s>
However	O
,	O
Olympus	B-Algorithm
FTL	I-Algorithm
lenses	O
and	O
Fujica	O
screw	O
mount	O
lenses	O
had	O
a	O
projecting	O
cam	O
which	O
means	O
that	O
they	O
cannot	O
be	O
fully	O
screwed	O
down	O
on	O
a	O
regular	O
screw	O
mount	O
body	O
.	O
</s>
<s>
Chinon	O
used	O
a	O
different	O
system	O
to	O
provide	O
aperture	B-Algorithm
priority	I-Algorithm
mode	I-Algorithm
with	O
standard	O
Auto-M42	O
lenses	O
with	O
the	O
CE	O
Memotron	O
(	O
1974	O
)	O
,	O
CE-II	O
Memotron	O
and	O
CE-3	O
Memotron	O
bodies	O
.	O
</s>
<s>
Chinon	O
used	O
a	O
then-modern	O
Silicon	O
(	O
Si	O
)	O
metering	B-Algorithm
cell	O
with	O
fast	O
reaction	O
time	O
compared	O
to	O
the	O
then-standard	O
CdS	O
cells	O
,	O
which	O
made	O
it	O
possible	O
to	O
close	O
the	O
aperture	O
,	O
meter	O
the	O
light	O
and	O
set	O
the	O
shutter	O
speed	O
automatically	O
in	O
one	O
full	O
press	O
of	O
the	O
button	O
.	O
</s>
<s>
While	O
this	O
method	O
did	O
not	O
offer	O
the	O
same	O
sensitivity	O
advantage	O
of	O
true	O
open-aperture	O
metering	B-Algorithm
,	O
it	O
was	O
much	O
more	O
versatile	O
as	O
it	O
did	O
not	O
require	O
specially-equipped	O
proprietary	O
lenses	O
.	O
</s>
<s>
Compatibility	O
problems	O
have	O
been	O
experienced	O
when	O
mounting	O
lenses	O
with	O
aperture	O
transmission	O
levers	O
(	O
e.g.	O
,	O
SMC	O
Takumars	O
)	O
on	O
older	O
bodies	O
such	O
as	O
Zenit-E	O
or	O
Mamiya	O
DTL	O
.	O
</s>
<s>
M42	B-Algorithm
mount	I-Algorithm
cameras	O
fell	O
out	O
of	O
general	O
production	O
during	O
the	O
late	O
1970s	O
and	O
early	O
1980s	O
,	O
with	O
the	O
exception	O
of	O
the	O
Russian	O
Zenit	B-Application
range	O
.	O
</s>
<s>
Pentax	B-Application
moved	O
to	O
the	O
Pentax	B-Algorithm
K	I-Algorithm
mount	I-Algorithm
from	O
1975	O
onwards	O
,	O
while	O
Praktica	O
adopted	O
the	O
electronic	O
B-Mount	O
in	O
1979	O
.	O
</s>
<s>
Due	O
to	O
the	O
simplicity	O
of	O
the	O
M42	B-Algorithm
lens	I-Algorithm
mount	I-Algorithm
and	O
the	O
large	O
selection	O
of	O
lenses	O
,	O
M42	O
adapters	O
exist	O
for	O
all	O
current	O
and	O
many	O
obsolete	O
lens	B-Algorithm
mounts	I-Algorithm
.	O
</s>
<s>
The	O
adapter	B-Algorithm
fits	O
between	O
the	O
camera	O
and	O
the	O
lens	O
,	O
making	O
it	O
possible	O
to	O
mate	O
an	O
M42	O
lens	O
to	O
a	O
body	O
with	O
a	O
different	O
lens	B-Algorithm
mount	I-Algorithm
.	O
</s>
<s>
M42	O
adapters	O
work	O
best	O
on	O
bodies	O
with	O
a	O
flange	O
depth	O
less	O
than	O
or	O
equal	O
to	O
the	O
M42	O
's	O
flange	O
depth	O
,	O
which	O
includes	O
the	O
popular	O
Canon	O
EF-mount	O
,	O
the	O
Pentax	B-Algorithm
K-mount	I-Algorithm
,	O
the	O
Minolta/Konica	B-Algorithm
Minolta/Sony	I-Algorithm
A-mount	I-Algorithm
,	O
the	O
Sony	B-Algorithm
E-mount	I-Algorithm
,	O
the	O
Samsung	B-Algorithm
NX-mount	I-Algorithm
,	O
the	O
Fujifilm	B-Algorithm
X-mount	I-Algorithm
and	O
the	O
Four	B-Architecture
Thirds	I-Architecture
System	I-Architecture
including	O
the	O
Micro	B-Operating_System
Four	I-Operating_System
Thirds	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
A	O
simple	O
mechanical	O
adapter	B-Algorithm
allows	O
the	O
lens	O
to	O
be	O
mounted	O
,	O
but	O
the	O
effect	O
is	O
similar	O
to	O
the	O
introduction	O
of	O
an	O
extension	O
tube	O
,	O
reducing	O
the	O
minimum	O
focus	O
distance	O
at	O
the	O
expense	O
of	O
losing	O
infinity	O
focus	O
.	O
</s>
<s>
Alternatively	O
,	O
an	O
adapter	B-Algorithm
with	O
an	O
optical	O
element	O
can	O
be	O
used	O
to	O
retain	O
the	O
original	O
focus	O
range	O
of	O
the	O
lens	O
,	O
at	O
the	O
expense	O
of	O
some	O
image	O
quality	O
.	O
</s>
<s>
Finally	O
,	O
it	O
is	O
possible	O
to	O
replace	O
the	O
M42	B-Algorithm
mount	I-Algorithm
on	O
some	O
lenses	O
with	O
the	O
desired	O
camera	O
mount	O
,	O
or	O
to	O
perform	O
the	O
reverse	O
change	O
on	O
a	O
camera	O
body	O
.	O
</s>
<s>
Some	O
bodies	O
may	O
be	O
operated	O
in	O
aperture-priority	B-Algorithm
mode	O
,	O
others	O
will	O
only	O
allow	O
full	O
manual	O
control	O
in	O
this	O
circumstance	O
.	O
</s>
<s>
Mounting	O
an	O
M42	O
lens	O
on	O
a	O
digital	O
SLR	B-Algorithm
with	O
a	O
sensor	O
smaller	O
than	O
35mm	O
film	O
results	O
in	O
FOV	O
crop	O
.	O
</s>
<s>
With	O
certain	O
adapters	O
,	O
metering	B-Algorithm
and	O
focus	O
confirmation	O
is	O
available	O
.	O
</s>
<s>
Focus	O
confirmation	O
is	O
available	O
,	O
focus	O
to	O
infinity	O
is	O
only	O
possible	O
with	O
adapter	B-Algorithm
with	O
additional	O
lens	O
.	O
</s>
<s>
FourThirds	B-Architecture
:	O
All	O
DSLRs	O
need	O
full	O
manual	O
control	O
.	O
</s>
<s>
With	O
certain	O
adapters	O
,	O
metering	B-Algorithm
and	O
focus	O
confirmation	O
is	O
available	O
.	O
</s>
<s>
Pentax	B-Application
:	O
All	O
DSLRs	O
allow	O
aperture-priority	B-Algorithm
with	O
focus	O
confirmation	O
and	O
infinity	O
focus	O
.	O
</s>
<s>
Metering	B-Algorithm
works	O
and	O
lens	O
can	O
focus	O
on	O
infinity	O
.	O
</s>
<s>
Some	O
SLTs	O
are	O
reported	O
to	O
work	O
fine	O
in	O
aperture-priority	B-Algorithm
mode	O
(	O
A65	O
confirmed	O
)	O
.	O
</s>
<s>
Sigma	O
:	O
SD9	O
allows	O
auto	O
exposure	O
with	O
aperture-priority	B-Algorithm
mode	O
,	O
infinity	O
focus	O
and	O
focus	O
confirmation	O
.	O
</s>
<s>
Flange	O
focal	O
distance	O
:	O
45.5mm	O
(	O
The	O
figure	O
45.46mm	O
is	O
also	O
commonly	O
seen	O
,	O
particularly	O
in	O
Pentax	B-Application
literature	O
.	O
)	O
</s>
