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Forum: Lens Clubs 03-08-2010, 01:30 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
I like the first image, the normally busy bokeh looks like a shimmering heat haze.
I agree that although they are rarely the sharpest lenses, mirror lense have their place and can provide images you wouldn't achieve any other way. As you obviously know how to get the best from your mirror lens, have you considered upgrading to a Tamron?

Here is one from my Tamron 55BB from yesterday, slight crop and I'm feeling Lucky'd in picasa.
Forum: Lens Clubs 02-20-2010, 10:39 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
I found the Tamron 55BB to be a really good lens too but took some getting used to. You have obviously got the hang of it already, nice photos, no obvious donuts in any of them. I especially like the close up one, no4.
Forum: Lens Clubs 01-29-2010, 12:31 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
The model 55BB didn't have a tripod collar so it can't be missing.

I now have two Tamron 55BB mirror lenses. The first one I got cheep because it had a dent in the filter ring and quite a lot of fungus on the optical surfaces. I cleaned it up and it has worked well for me. The second one I only got this week. It's previous owner had removed the front element to clean the mirror and acidentally dropped the front element on a tiled floor, taking some chips out of it, nd so I got it very cheep. I only really wnated it for the hood as mine didn't come with one but when it arrived it turned out that apart from the front element everything else was in very good condition. I have now rebuilt one imaculate lens from the best bits from both.
I think I will put some black paint on the the chips on the damaged front element. This should reduce the stray light and so prevent the damage redusing the contrast. It could then be a perfectly usable lens as only a small proportion of the total lens area is affected. It could then be a perfectly usable if rather ugly lens.
Forum: Lens Clubs 01-23-2010, 06:23 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Superb photos and not a donut in sight! What is this lens like to use in comparison to 500mm f/8 mirror lens?
Forum: Lens Clubs 01-23-2010, 05:01 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
It was made out of old bits from a cheep nasty tasco telescope. I modified an old T mount adapter so I could screw the telescope erecting prism to it and the eyepiece just push fits into this. It reaches infinity focus - more by luck than judgement - but this will only works with a T mount lens. It should be possible to make something similar for K or M42 mount etc but these have a shorter registration distance than T mount so it probably won't be possible to maintain infinity focus and have an erecting prism. I will take some close up photos showing the assembly when I get a chance.



I have not tried any of the Rokinon / samyang etc variants but I have both a Centon and the Tamron 55BB. Based on my experience and the information in this thread and elsewher the Tamron is markedly better than almost everything else. You should be able to get a second hand Tamron for a little more than one of the new Rokinon ones and I doubt you would be dissapointed if you did.
The mirrors are an aluminium film on glass as with astronomical telescopes and so thermal expansion should not be a significat problem unless the temperature there is very extreme.
Forum: Lens Clubs 01-01-2010, 11:04 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Ovim - That 300mm f/5.6 looks interesting, where did you get it from?

Hereis a photo taken earlier today from about 12 feet with the Tamron 55BB. Slight crop and re-size but no PP.
Forum: Lens Clubs 10-05-2009, 12:25 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
I haven't tried a commercial one but I made a T mount eyepiece from an old Tasco telescope erecting prism and an old T mount. I can use it with one of several cheap telescope eyepieces but at present it is fitted with an old microscope eyepiece. It actually workes remarkably well - your eye is much more forgiving of poor sharpness and contrast then film of digital sensors. I imagine that a commercial one will give better image quality than my lash up so if you want a spotting scope it's probably worth a try. Also if you intend to take your mirror lens with you anyway then you have a spotting scope for almost no additional weight.
Forum: Lens Clubs 10-01-2009, 12:42 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
A recent shot from the Tamron 55BB

User Photo Gallery - Nature
Taken whilst trying to enjoy my lunch in peace.
Forum: Lens Clubs 09-30-2009, 12:45 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
It is on my list of things to do! Once I have compleated it I haver two old spherical mirrors I managed to obtian to to re-figure. These may need a machine though as they are 24" and 38" diameter so will require a LOT of work. I think it will be along time before these are done though.
Forum: Lens Clubs 09-30-2009, 12:25 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Impressive. Any pictures of the finished contraption? What is the mirror size and F number? Did you make the mirror youself?
I started hand grinding a 6" f/8 telescope mirror about a year ago. I got the roughing out and first stage of fine grinding done in about 6 weeks of lunch breaks but got sidetracked and havent found the motivation to get back to it yet.
Forum: Lens Clubs 09-30-2009, 11:43 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Fantastic! Were these really together or did you edit Saturn in? Im only asking as the moon is usually a lot brighter than Saturn. What mirror lens was this taken with?
Forum: Lens Clubs 09-21-2009, 11:34 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Since the aperture is fixed (usually at f/8) these lenses are quite slow and due to the long focal length the depth of field is small. Between the relatively dark view finder image, shallow dof and the manual focus it is quite dificult to get good action shots. Some have managed it (see kristoffon's motorbike shot earlyer) but it would probably be easyer with a faster, AF refracting lens.
Forum: Lens Clubs 09-19-2009, 10:50 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
There are some photos of the back of this lens showing the internal filters in this ebay listing:
SMC Pentax M Reflex 13.5/2000mm Pentax Canon Eos Nikon on eBay (end time 06-Oct-09 15:41:15 BST)
Forum: Lens Clubs 09-13-2009, 02:49 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Usefull to know, I may have to try it again.
Forum: Lens Clubs 09-13-2009, 01:58 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Anyone considering getting one of these should beware that they may not mount on all DSLRs so try before you buy. I borrowed one from work about a year a go to try but unfortunately the rather large rear end fouled the protruding pop up flash housing on the GX20. The only way I could have mounted it would have been with either extension tubes or a teleconverter but this would have somewhat limited its use. It is undoubtedly a solidly built lens though.

The 1000mm is amazingly sharp for such a long lens - in general aberations scale with focal length so to acheive such small aberations with such a long lens is a reall acheivenet on Pentax's part. To have achived such impressive shots with such lens is quite an acheivement on Stephen's part - I particulary like the photo of the doves.

There have been a few impressive photos with Meade and similar reflecting telescopes but so far only the smaller ones. Has anyone tried one of the larger ones on ground based subjects? The Meade 8" f/10 Schmit Cassegrain for instance (2000mm f/10 in photographic terms) is comperable in size to the Pentax 2000mm f/11 but is cheeper new that the Pentax is second hand so it would be interesting to see how well it perfroms.

I took the Tamron to the beach yesterday to try it in a more open space. A couple of shots bellow.


User Photo Gallery - Abstract


User Photo Gallery - Sports
Forum: Lens Clubs 09-12-2009, 04:47 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
I took the Tamron out in the garden this morning for a play. It is really effective at close range but focussing needs to be really precise. At minimum focus distance (1.7m) the depth of focus is around 2.5mm!
However after taking quite a few shots I got some I am reasonably happy with which are presented bellow. They have all been croped a bit but ther than that there is no PP - jpgs straight from the camera.
Larger versions of some of them are in the user gallery, links bellow.


User Photo Gallery - Nature


User Photo Gallery - Nature

I really love this lens, it has barely been of my camera in the few days since I got it and it may not come off any time soon (provided the sun stays out):D
Forum: Lens Clubs 09-11-2009, 05:50 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
I have carried out some calculations and a quick test on the Tamron 55BB to determine its speed. Where as my Centon 500mm f/8 had a geometric F number (based on dimensions) of 8.3 and an effective F number (based on light throughput) of around f/11, the Tamron has a geometric F number of f/7.3 and an effective F number for f/8. So the cheeper ones have an F number based on their size wheras Tamron have compensated for the loses in the lens to get the correct effective F number. This is why the Tamron is larger than the Centon / Vivitar / Samyang versions (82mm filter thread on the Tamron, 72mm filter thread on the rest. The Tamron is therefore not only sharper and more contrasty but is effectively a stop faster then the cheeper ones.
Forum: Lens Clubs 09-10-2009, 01:03 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Sorry, didn't take any images other than the one showing the fungus. In actual fact the Tamron was the easyest lens I have repaired / cleaned yet. I didn't have to take much off, just unscrewed the retaining ring holding the front element in place and the front element together with the secondary mirror came out easily as one unit. This gave me access to everything I needed. The fungus was on the inside of the front element, and on the primary and secondary mirrors. Everything was cleaned with cotton buds and methilated spirits and the final clean was performed with a lens pen and a rocket blower before re assembling.
The mirrors in this lens are back surface mirrors so there was a sizable bit of glass between the fungus and the aluminized surface. This was just as well as cleaning the fungus of an aluminized surface wound have ruined the mirror surface and it would have been necessary to get it re coated.
If there had been fungus between the rear element group it would have been a lot hadrer to fix. I didn't even contemplate removing these as there was no reason to and it is a lot less obvious how they are mounted.
Forum: Lens Clubs 09-10-2009, 11:50 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
My new (secondhand) tamron mirror lens arrived yesterday. It was as described - has had a dent in the filter ring which has been straightened out quite neatly and a lot of fungus inside the lens. The extent of the fungus on the inside the front element is clearly visible in the second photo bellow.
The third and fourth photos are 100% crops of the same region of a nearby hill side taken with the centon and tamron mirror lenses respectively. Neither had a hood but I was shooting from the shade. This was the first photo I took with the tamron and it is was still full of fungus at this point. The Tamron lens is notisably sharper and more contrasty. The image from the tamron is also lighter even though the shutter speed was higher (1/180 sec for the centon, 1/250 sec for the tamron). The entrance pupil is a bit bigger on the tamron so this could account for it but I will work out the geometric F number later. The fifth shot is the uncropped but resized image from the tamron.
I took the lens apart and cleaned out the fungus, this was easyer than I thought and I had it clean and back together in just over 45min:) Fortunately, although widespread, the fungus hasn't been there too long and hasn't done any visible damage to the coating. The top image was teken earlyer today whilst testing the lens during my lunch break. None of the photos have had any PP.
Conclusions: The Tamron is quite a bit sharper and more contrasty than the Centon. It also seems a bit faster, I will have to test this more thoroughly though. The Tamron 55BB is a lot easyer to get good photos with than the Centon but is is going to take a lot of practice before I can take photos comperable the the rest of the thread.


https://www.pentaxforums.com/gallery/images/17955/1_SG209281_crop.JPG
Forum: Lens Clubs 09-08-2009, 05:59 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
There is a pentax 2000mm f/13.5 on ebay at the moment if anyone has money they feel they need to spend.
SMC Pentax M Reflex 13.5/2000mm Pentax Canon Eos Nikon on eBay (end time 06-Oct-09 15:41:15 BST)

edit: I just looked at the pictures again and it looks like its missing the same adapter part reported missing from various pentax 500mm f/4.5 and 1000mm f/8 refractors. He says he can supply an adapter for any of a large number of camera mounts though so mabe he has all the missing adapters...
Forum: Lens Clubs 09-06-2009, 09:19 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
I'm guessing you got this from Adaptall2.com? There is a lot of usefull info in this site. This is where I found out about the differences between the two versions of the lens. I was interested to see photos from both to see if there are any obvious differences in the IQ as they have not been compared directly on adaptall2.com.



Having spent my pocket money on a tamron mirror lens instead of a hood I have been looking for something to that can be easily converted into a hood instead. I have found an empty Golden Syrup tin which appears to be about the right diameter and am in the process of adapting it. What did you use or did you make it from scratch?
Forum: Lens Clubs 09-06-2009, 07:30 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
A quick test reveals that the Centon with its effective aperture of f/8.3 (based on dimensions only) gives about the same exposure as my Super Takumar 300mm at about f/10. The fact that the mirror lens is slower is not due to the central obscuration though, it is due to the losses from the mirror.
At normal incidense glass reflects about 4% of the light hitting it so 96% is transmitted (assuming it is good quality glass and there is no absorbtion, this is a valid assumption for most opticval glass in the visible region of the spectrum). A good antireflection coating will reduce the reflected light to less than 1% and so more than 99% is transmitted.
Aluminium which is used for the mirror coatings typically reflects about 90% of the light incident on it so at each reflecting surface you lose about 10%. In a mirror lens there are two reflecting surfaces, the primary and secondary mirrors. This means that about 19% of the light entering the lens is lost at the reflecting surface amd this is what givess the effective aperture of f/10-11
It is now possible to produce an enhanced aluminium reflecting surface with something exactly the oposite of an antireflection coating. These will reflect more than 95% of the light and so would give a significant improvement to the light transmission. This is probably not included in most mirror lenses due to cost, age etc. Reflecting surface do have a significant advantage over lenses though as they do not suffer from dispersion and so do not introduce chromatic aberations.
If, when I get my Tamron lens I find that it has fngus on a mirror surface I will get it re-coated with the enhanced coating and see if it improves things at all.
Forum: Lens Clubs 09-06-2009, 02:41 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Great stuff, this blog was great, motivated me to dig out my mirror lens and take some photos with it again.
For the last few months I have been using it as a spotting scope with a home made T mounted erecting prism and eyepiece. It makes a good spotting scope, much smaller and lighter than a similar refracting scopes.
Anyhow, the sun came out and so did the mirror lens. A couple of photots from it bellow, no PP other than cropping and resizing. Centon MC 500mm f/8, I think another re-branding of the Samyang / Vivitar.

Based on advice here and in Mark's blog I went searching for a suitable hood on ebay last night. I found a few but got sidetracked and brought a Tamron 55BB mirror lens in need of some TLC and instead:D If the faults are as described thn I should be able to get it back in good working condition, in which case I have a good mirror lens at an even better price:D If not:(...
Forum: Lens Clubs 09-05-2009, 03:12 AM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
Fungus is not the end for lenses, do not trash them! I have removed fungus from half a dozen lenses in the last couple of months and all of them are fine now. If it has been there too long it will damage the lens coating and so the contrast will be reduced marginally but the lense should be quite usable.



The central obscuration is taken into account when the f number is calculated. The entrance pupil of the lens is greater than 1/8 of the focal length so the open light collecting area of the lens is the equivelent of f/8. From measurements of my 500mm f/8 Centon and some back of the envelope calculations, the effective aperture is f/8.3
Forum: Lens Clubs 09-04-2009, 01:07 PM  
The Mirror Lens Club!
Posted By MattGunn
Replies: 1,239
Views: 467,218
There are some fantastic photos here!
I quite like the donut bokeh, especially in highlights in the water on your 5th shot abbazz.
I have a Centon mirror lens which as far as I can tell is physically identical to the Vivitar morror lens I borrowed from work. I need a bit more practice with it before I have anything worth posting though. I have had pretty poor contrast in most of the shots I have taken with it and have been considereing getting a hood for it. Has anyone used a hood with a cheep mirror lens? Does it help much (assuming not shooting into the sun)?
I have also been looking at the Tamron morror lenses which appear to produce superior results to pretty much everything else. Those of you lucky enough to have a tamron mirror lens, do you have the earlier 55B mode or the newer 55BB model? Apparently there is a significant difference between the optical designs.
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