George de Fockert - Some stuff I posted in the early years of pentax discuss

I have done a simple lens test.
Tested on ektachrome elite 2, 100 iso film. 10 minute exposure of cygnus (milky way) at 2.8

There were some high thin clouds, so not all lenses have had exactly the same conditions. :(

M 40mm/2.8 : darkest of all (is it realy 2.8?)
==> bright stars look a little blue (traces of chromatic aberation ?)

55mm/2.0 : Brightest of all. I liked this one the most, maybe caused by the extra 10% focal length.
==> Neutral sky background.

M 50mm/1.7 :
50mm/1.4 :
M 50mm/1.4 :
==> cannot see a large difference between these, all have a (dark) green sky background.

A 50mm/1.4 : in between the M 1.4/1.7 versions and the 55mm/2.0
50mm/1.2 : partly clouded exposure, but otherwise like the M 1.4/1.7 versions.

But what supprised me, was the quality of the 55mm/2.0. This design dates from the early sixties I believe.


George de Fockert - Some additional stuff I posted in the early years of pentax discuss

. . . I did some lenstesting. Maybe this is interresting for others.

The K-mount pentax lenses I compared for astrophotography purposes are:

SMC PENTAX 1:3.5/24mm
SMC PENTAX 1:2.8 24mm

SMC PENTAX 1:3.5/28mm
SMC PENTAX-M 1:3.5 28mm

SMC PENTAX-M 1:2.8 28mm (older type, very compact)
SMC PENTAX-M 1:2.8 28mm (newer type, larger and similar to A version)

SMC PENTAX 1:2/35
SMC PENTAX-M 1:2 35mm

SMC PENTAX-M 1:2 50mm
SMC PENTAX 1:2 55mm

SMC PENTAX 1:3.5/135
SMC PENTAX-M 1:3.5 135mm
SMC PENTAX 1:2.5/135

SMC PENTAX-A 1:4 300mm

Tested on fuji g 400 100 iso film. Each similar focal length pointed at the same sky location, so a direct comparison is possible. I used an equatorial mounting for earth rotation compensation.

Comparisson done on 10*15 cm prints.

exposures times : lens F 2.0 F 2.5/2.8 F 3.5/4.0

3.5/24 10min
2.8/24mm 5min 10min
3.5/28 8min
M 3.5 28mm 8min
M 2.8 28mm (o) 5min 10min
M 2.8 28mm (n) 5min 10min
2.0/35 2.5min 5min
M 2.0/35mm 2.5min 5min
M 2.0/50mm 2.5min 5min
2.0/55mm 2.5min 5min
3.5/135 8min
M 3.5/135mm 8min
2.5/135 5min 10min
A 4.0/300mm 10min

The two 24 mm lenses :

3.5/24 at 3.5 : 'v' shaped coma wings in the corner,
center is good.
no vignetting.

2.8 24mm at 2.8: '+' shaped coma wings in the corner.
center is a bit unsharp.
no vignetting.

at 4.0: '+' shaped coma wings in the corner now smaller.

overall a bit better (not much) than the 3.5

version at 3.5

The four 28 mm lenses :

3.5/28 at 3.5: very small coma wings in the corner.

very good sharpness.

M 3.5 28mm at 3.5: almost identical performance.

M 2.8 28mm (o) at 2.8: large coma wings in the corner.

center is a bit unsharp.
some vignetting.

at 4.0: large coma wings in the corner.
center is good.
no vignetting.

M 2.8 28mm (n) at 2.8: overall a bit worse than the (o)
version but some less vignetting.
at 4.0: almost identical to the (o) version.

 

The two 35 mm lenses :

2/35 at 2.0: extreme large coma wings in the corner and edge
centre is unsharp.
vignetting.

at 2.8: small coma wings in the corner,
no vignetting
sharpness is good.

M 2 35mm at 2.0: even larger coma wings in the corner and edge,
with blue halo.
centre is unsharp.
vignetting.

at 2.8: similar to the 2/35 at 2.8, but overall a bit less sharp.

 

The 50 mm and 55 mm lenses :

M 2 50mm at 2.0: unsharp, and vignetting.
at 2.8: no vignetting, but still unsharp in the corners.

2 55mm at 2.0: sharp in center, small coma wings in the corner.
some vignetting.

at 2.8: very small coma wings in the extreme corners.
very good sharpness.

The three 135 mm lenses :
3.5/135 at 3.5: from edge to corner very sharp.
some vignetting.

M 3.5 135mm at 3.5: unsharp edge and corners.
some vignetting.

2.5/135 at 2.5: good sharpness
vignetting

at 4.0: very good sharpness
no vignetting

And the 300mm

A 4 300mm at 4.0: not very sharp, clearly visable
chromatic aberations.
some vignetting

Conclusion for astrophotography purposes:

Best 24mm : 2.8, because its faster and smaller
than the 3.5 version, and performs equally.

Best 28mm : the M 3.5 version, performs similar as the non M version, but is smaller.
And at 3.5 it performs even better than the 2.8 versions at 4.0.

So, the cheapest Pentax 28 mm is the best !

 

Best 35mm : the non M version simply beats the M version in performance, but full open it is not very usefull.

Best 50/55mm : well, the M 50mm is simply a bad lens.
The 55mm a very good one.
It is a pitty they replaced the 55mm by the M 50mm.

Best 135mm : the 2.5/135, and if you don't need vignetting free
performance at 4.0, then the 3.5/135.
Both are a lot better than the M 135 version.


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