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Forum: Pentax SLR Lens Discussion 10-24-2011, 02:58 AM  
Konica-mount Vivitar conversion?
Posted By MattGunn
Replies: 6
Views: 3,231
A couple of year ago I converted a Vivitar 55mm f/2.8 macro from Konica to Pentax mount, maintaining infinity focus and with a functioning aperture lever. As pointed out above this involved removing 5mm from the back of the lens + another 0.75mm for the flanged M42 to K mount adapter I fitted. This was done with a lathe which made the job a lot easier and quicker.
Vivitar like several other third party lens manufacturers make lenses for a variety of mounts. I believe that the basic lens is the same in each case with only the back end – mount and aperture actuator being different for different mounts. For this reason there was enough clearance between the back of the lens / mechanism and the mirror to make the conversion possible. A photo of the conversion is reposted from https://www.pentaxforums.com/forums/lens-clubs/64271-impossible-lens-club-3.html bellow:

Forum: Pentax DSLR Discussion 11-14-2010, 03:43 AM  
Astrophotography
Posted By MattGunn
Replies: 1,248
Views: 307,584
I am just getting started in Astrophotography and haven’t yet taken any pictures worth showing off. However I have just finished reading a book which I would recommend to anyone interested in astrophotography with a DSLR - "Digital SLR Astrophotography" by Michael Covington (I have no affiliation etc...). For details of the book and lots of useful links see here: Digital SLR Astrophotography .
In the book Covington shows that superb astro photos can be obtained with wide angle primes and no tracking at all – I have tried it and with a 28mm lens and 15sec exposures the stars barely streak at all. I don’t have a telescope worth putting my camera on but I intend to build a barn door tracker or similar for use with longer primes – my converted 36” reconnaissance lens (https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/64271-impossib...tml#post893159 ) should be a reasonable telescope substitute
A recommendation for those who suffer from light pollution: didymium glass filters (Hoya red intensifier) will block out the sodium D lines from street lights but will pass all of the main emission lines from nebulae etc. There are dedicated nebula filters which will block other light pollution as well but I suspect they will be a lot more expensive.
Forum: Lens Clubs 06-03-2010, 11:31 AM  
Tamron 10-24mm club
Posted By MattGunn
Replies: 163
Views: 69,339
I have the tamron 10-24mm and enjoy using it. I have enjoyed looking through the superb photos in the sigma 10-20 club and have learned a lot from them. Here are a few of my earlyer attempts with the tamron:


Pentax User Photo Gallery - Landscape


Pentax User Photo Gallery - Landscape


Pentax User Photo Gallery - Landscape
Forum: Pentax SLR Lens Discussion 05-02-2010, 12:35 PM  
Advice on Polarizer filters
Posted By MattGunn
Replies: 28
Views: 5,722
Although as pointed out there is not an optical element in front of the metering sysyem which is designed to polarize light, the semi-transparent flip mirror which diverts light to the view finder will polarize both the light to the view finder and the light to the AF sensor (transmitted + reflected + absorbed light = total light entering the system from conservation of energy and absprbed light should be small). The information posted by Derridale is correct although Pentax cameras appear to have been designed so that the effect is not very pronounced. It is easy enough to explain from the Fresnel equations - Wikipedia, the free encyclopedia (See the graph showing the difference in reflectivity between S (perpendicular) and P (parallel to plane of insidence) polarised light about half way down the page).
Forum: Pentax SLR Lens Discussion 04-28-2010, 12:12 PM  
Sigma 400mm reflex mounting problem
Posted By MattGunn
Replies: 2
Views: 1,892
The "nib" is there to protect the aperture lever which that lens doesn't have anyway as the aperture is fixed. However the shroud that the nib is part of is also there to prevent stray light so contrast may suffer a bit without it.
Forum: Pentax SLR Lens Discussion 04-27-2010, 01:53 PM  
Is m42 55mm/2 same as 55mm/1.8?
Posted By MattGunn
Replies: 11
Views: 3,532
I have the super takumar versions of the 55mm f/1.8 and f/2 and I don't believe that they are exactly the same other than the max aperture. Testing with a geiger counter I found that the rear elements of the f/1.8 are radioactive and the f/2 arn't. This means that they use a different type of glass and so if their performance is similar then the optical design must be different. I havent done any serious comparisons of their performance yet though.
Forum: Lens Clubs 04-08-2010, 12:59 PM  
The impossible lens club?!
Posted By MattGunn
Replies: 486
Views: 175,674
The 300mm f/2.8 looks like it will be a lot of fun to use, good to see that you got it all working properly. I have deliberately left a few of my conversions with the aperture marks etc not at the 12 o clock position also, it makes it a whole lot easyer to see what your doing. I look forward to seeing more shots from it. What is it like with a teleconverter?
Forum: Pentax SLR Lens Discussion 04-01-2010, 12:48 PM  
Stacking Cokin NDs/What do you use for ND10?
Posted By MattGunn
Replies: 42
Views: 8,844
Accurate measurement of the transmision of a filter with an attenuation this high is likely to be difficult and will require a spectrometer with a high dynamic range. If someone wants to donate a small section (about 25mm square) then I will have a go at measuring its transmision with one of the spectrometers I look after at work but I'm not sure what signal to noise ratio I would be able to acheive without trying it.
Forum: Pentax SLR Lens Discussion 03-31-2010, 12:41 PM  
Stacking Cokin NDs/What do you use for ND10?
Posted By MattGunn
Replies: 42
Views: 8,844
Good quality short pass filters (IR block) will block around 99% of the IR light (I don't know the specs of the IR cut filters in Pentax DSLRs but I guess that their performance is around this) and so will pass around 1%. If the Baader film is passing aproximately 30 times as much light around 950nm as it does in the visible then the sensor will be recieving the IR around 950nm at aproximately 30% of the brightness of the visible wavelengths which could be significat enough to skwe the colour and could also account for the stray light issues. If this turns out to be the case then you might need an additional IR cut filter.
Forum: Pentax SLR Lens Discussion 03-26-2010, 01:56 PM  
DA* 300mm Aperature blades misaligned
Posted By MattGunn
Replies: 17
Views: 5,382
I have now dismantled (and sucessfully reasembled) a couple of dozen aperture mechanisms but have not seen one like this (except when I tried to resemble one under the influence of alcohol:eek:). If the ring which retains the blades was lose enough to allow them to move out of position like this then they would all have dropped out or jammed by now and if you had knocked the lens hard enough to do this then some or all of the glass ould have broken. I do not believe that this could have happened any time except during assembly (maby someone on the production line had a liquid lunch...). If it is under warrenty I would send it back for repair / replacement as it looks like a manufacturing fault.
Forum: Pentax SLR Lens Discussion 03-23-2010, 12:20 PM  
3 Cosina PK Lenses - Any good?
Posted By MattGunn
Replies: 12
Views: 7,498
I have cosina cosinon 24mm f/2.8, 50mm f/2 and 200mm f/4. They are well built (all metal and glass) and apart from the 200mm are quite sharp wide open. They all improve stopped down.
£60 for 3 seems fairly reasonable if they are in good condition. Look out for oily aperture blades, all of mine had this problem when I got them but they were all very cheap because of it and were quite easy to fix.
Forum: Lens Clubs 03-15-2010, 01:52 PM  
The tilt shift lens club
Posted By MattGunn
Replies: 164
Views: 70,647
That looks like it has very similar functionality to my converted Ilford Monobar but with front only movements and more restricted movement range. It seems like a lot of money for something wich like the monobar will only work with relatively long lenses which must restrict its aplications. Too bad its not available for Pentax mount...
Forum: Pentax SLR Lens Discussion 03-14-2010, 04:02 AM  
Lens Bokeh Research... Help?
Posted By MattGunn
Replies: 41
Views: 45,946
I think the term Bokeh covers it. Quote from the wikipedia page on Bokeh (here: Bokeh - Wikipedia, the free encyclopedia) Bokeh - "the way the lens renders out-of-focus points of light."
Forum: Lens Clubs 03-13-2010, 04:24 PM  
The tilt shift lens club
Posted By MattGunn
Replies: 164
Views: 70,647
I used two suitable sized holesaws to cut it out from a bit of 6mm thick brass plate left over from a previous job.
Forum: Pentax SLR Lens Discussion 03-13-2010, 01:03 PM  
Stacking Cokin NDs/What do you use for ND10?
Posted By MattGunn
Replies: 42
Views: 8,844
If the claims on their website are true then this stuff should provide the same optical quality as top of the range glass filters. It is not supposed to be pulled tight, just left slightly crumpled - counterintuative but it works. As it is so thin there is not much there to contribute aberations and having it not flat pretty much eliminates ghosting etc. We have some at work on a Meade telescope for observing sun spots and it workes great.
Forum: Lens Clubs 03-13-2010, 11:59 AM  
The tilt shift lens club
Posted By MattGunn
Replies: 164
Views: 70,647
Here are a few pictures of the conversion, posted previously in the https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/34323-zuiko-lens-club-2.html.

The original OM mount removed from the lens together with the flanged K mount and brass blank which combined to make the new K mount:


The new K mount after silver soldering but before final machining:


The K mount centred on my small lathe being machined to final dimensions:


The completed K mount next to the original OM mount:


The tolerences are not really that tight and so it is quit possible to make the conversion without machine tools as several people have done with other Zuiko lenses.
Forum: Pentax SLR Lens Discussion 03-13-2010, 05:10 AM  
Super Tak 300 F4
Posted By MattGunn
Replies: 6
Views: 1,808
The Super Tak is as beautifully crafted as all of the other takumars. I have not used mine that much since I got a Tair 3 but the Super Tack is definately smaller and lighter. I found with mine that it is a bit soft and has quite a lot of purple fringing wide open, anyone else find this?
The main hastle I had with the Super Tak 300mm is mounting and removing it as the tripod mount fouls on the cameras built in flash and so has to be loostened and the lens screwed on slowly. I eventually got fed up with this and fitted a flanged K mount adapter and adjusted the infinity focus distance. This is the only Takumar I have found so far which has enough adjustment to get infinity focus with the flanged adapter. You don't even have to dismantle this lens to adjust the focus as the locking screws are on the focus grip and easily accessible.
Forum: Pentax SLR Lens Discussion 03-13-2010, 04:48 AM  
Lens Bokeh Research... Help?
Posted By MattGunn
Replies: 41
Views: 45,946
I think that point spread function is not the correct term for these measurements. The point spread function of an instrument is usually what the instrument sees when it is setup and used under optimum conditions. The point spread function of a lens would therefore be what the sensor sees when a lens is perfectly focussed on a point source. For a normal lens this would be a combination of uncorrected aberations and the airy patern caused by diffraction. As with the bokeh measurements the point spread function will change with aperture due to the outer zones of the lens being cut off and the diffraction patern increasing as the lens is stopped down. The term for the Bokeh is really the Circle of Confusion as it is caused by defocussing the system.

Here are a few more Bokeh measurements. Still taken from 5m but this time the LED was coupled into a 1mm plastic fibre which provides a smaller and more uniform source. Extension tubes were used for these lenses to increase the size of the Bokeh.

SMC Takumar 135mm f/3.5
@ f/3.5


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22


Super Takumar 150mm f/4
@ f/4


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22


Topman MC 135mm f/2.8
@ f/2.8


@ f/4


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22


Cosina Cosinon 200mm f/4 showing a fault at the bottom left hand edge wide open. I don't know what is causing this as I hadn't noticed it untill I took these measurements but I will investigate further.
@ f/4


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22
Forum: Pentax SLR Lens Discussion 03-12-2010, 01:19 PM  
Stacking Cokin NDs/What do you use for ND10?
Posted By MattGunn
Replies: 42
Views: 8,844
Have you come across / considered Baader solar film? (Details here: Baader Planetarium - tech information about Baaders AstroSolar™)
It is designed for photographing the sun and so should provide a good attenuation across the spectrum. The density is only 5 but stacking it should not cause any problems. It is pretty cheep and you can make up any size filter you desire.
Forum: Pentax SLR Lens Discussion 03-12-2010, 12:45 PM  
Tamron (esp. adaptall) 90mm owners? (CCD flare)
Posted By MattGunn
Replies: 28
Views: 8,940
It is very unlikely that the coating on the lens has cracked in any way, it will be less than 0.000001mm thick and therefore relatively flexible and should only crack if the lens is broken. It is most likely to be fungus but providing it is kept dry it shouldn't spread, progress or cause any problems.

I have never observed the problem you mention with my 52BB but have never looked for it. I will try to test over the weekend.
Forum: Pentax SLR Lens Discussion 03-10-2010, 12:47 PM  
Peleng 8mm Problem
Posted By MattGunn
Replies: 19
Views: 5,932
Are you sure that the lens is supposed to be T mount? How difficult was it to screw the T mount adapter onto it?
I did a quick search on the Peleng 8mm and none of the results I got were for a T mount version. T mount and M42 mount have similar threads - same diameter different pitch - and I have seen T mount adapters forced onto an M42 thread. If this is the case then the T mount will have shifted the lens so far forward that you will not get focus on anything further than a few mm from the lens. Try taking the T mount adapter off and screwing the lens gently into the m42-K mount adapter. If it screws on easily then try this on the camera, it may work better.
Forum: Lens Clubs 03-10-2010, 12:30 PM  
The tilt shift lens club
Posted By MattGunn
Replies: 164
Views: 70,647
Nothing wrong with cross posting, please keep showing us the results. As tilt shift stuff is quite expensive, DIY tilt shift is definately the way to go. Your sucess with FF slr lenses is well worth showing off as there is a popular opinion that tilt shift won't work without medium or large format lenses. Your results go to show that popular opinion is wrong. Plese keep up the good work and show off your developments.



How much tilt did you use on the film & lens respectively?
Do you setup your large format camera by calculation or trial and error?
With SLRs and DSLRs, tilt usually appears to be used to put much of the image out of focus (for minature effects ets) but with large format it is used to put most of the image in focus and control distortion. Your shot shows clearly how effective this can be, a very LBA inducing shot:)
Would you care to show us a photo of the camera you took this with?



I believe Rense recently got the 35mm Arsat tilt lens, was this what you meant?
A thread with some results was started here: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/90885-some-fir...5mm-2-8-a.html
Forum: Pentax SLR Lens Discussion 03-09-2010, 06:36 AM  
Oh deary me.....!
Posted By MattGunn
Replies: 15
Views: 2,975
Using basic (if slightly skewed) logic it should be possible to argue that by using all those photons to from images in our cameras we must be capturing them and so preventing them from contributing to global warming. Therefore the greatest contribution we can make to the prevention of glabal warming is to buy more big fast lenses - a 300mm f/2.8 must collect more photons than a kit lens.
Forum: Lens Clubs 03-08-2010, 02:22 PM  
The tilt shift lens club
Posted By MattGunn
Replies: 164
Views: 70,647
Nice setup! $350 may not be all that budget but for two adapters and a medium format Zeiss Flektogon it seems very reasonable. 50mm is still quite long on APSC but for tilt it should be very effective and its still a lot shorter than my tilt setup. Nice shot, thanks for posting. I look forward to seeing more from this setup.
Forum: Pentax SLR Lens Discussion 03-08-2010, 01:44 PM  
500 MM Mirror lens-are they worth it ?
Posted By MattGunn
Replies: 22
Views: 12,896
Yes, with the right T mount adaptor it will work on any SLR or DSLR.
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