dkCambridgeshire Posted December 19, 2015 Share #1 Posted December 19, 2015 Advertisement (gone after registration) Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! I have posted this image previously on the Leica T sub-forum but did not fully mention how difficult the Samyang 800mm f8 mirror lens is to focus. The Leica T's APS-C 1.5 crop factor results in a 1200mm FF equivalent lens and required a very rigid tripod and mount to prevent camera shake. But overcoming 'shake' with the Samyang 800mm lens was simple compared to nailing accurate focus. The Samyang 800mm has no real fine focus because the focus helicoid is far too coarse. Thus there were rather too many 'just out of focus' images. The above image was one of the few with acceptable focus. I have since addressed the focus problem by making a fine focus accessory using rigid packing foam which cuts very easily using a bread knife. The accessory looks cumbersome but it works. Rubber bands provide sufficient pressure to attach and hold the rigid foam lever in place. The extra radius increases the focus travel and makes fine focus much easier. There is much prejudice against mirror lenses and although they have a bad reputation amongst photographers for unsharpness and inferior resolution, astronomers seldom complain - in fact they obtain wonderfully sharp and detailed astro-images. Astronomers are very aware that their reflecting telescopes require time for the rear prime mirror objective to cool down to the ambient air temperature. The rear prime objective lens' glass acts like a 'storage heater' when stored in a building and as soon as it's taken outside into the cooler atmosphere, the heat from the glass radiates into the cooler air stream inside the 'scope creating heat currents which can distort the image. Furthermore, the glass continues to radiate for up to an hour and as long as there is a temperature differential of just 5ºC between the glass and the air, imaging will be compromised. Astronomical reflector 'scopes often include one or more cooling fans to assist cooling and considerably reduce the cool-down time. Refracting camera lenses also need cool-down time after building storage but because a refractor's prime objective is at the front of the lens the temperature differential effect is minimal and they cool much quicker - compared to a reflecting lens where the prime mirror lens is at the rear of the tube and has a tube of air in front of it. Astronomers also use very rigid tripods and mounts compared to the average camera tripod - thus astronomers' 'apparatus shake' is minimal compared to the camera/lens shake of a photographic tripod. So … it's possible that camera/photographic mirror lenses are not the unsharp optics that many photographers purport them to be. A mirror lens' 'do-nuts' can seldom be minimised or eliminated entirely - but allowing sufficient time to cool down and using with a very rigid tripod could very much improve resolution and thus imaging results. Best wishes dunk Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! I have posted this image previously on the Leica T sub-forum but did not fully mention how difficult the Samyang 800mm f8 mirror lens is to focus. The Leica T's APS-C 1.5 crop factor results in a 1200mm FF equivalent lens and required a very rigid tripod and mount to prevent camera shake. But overcoming 'shake' with the Samyang 800mm lens was simple compared to nailing accurate focus. The Samyang 800mm has no real fine focus because the focus helicoid is far too coarse. Thus there were rather too many 'just out of focus' images. The above image was one of the few with acceptable focus. I have since addressed the focus problem by making a fine focus accessory using rigid packing foam which cuts very easily using a bread knife. The accessory looks cumbersome but it works. Rubber bands provide sufficient pressure to attach and hold the rigid foam lever in place. The extra radius increases the focus travel and makes fine focus much easier. There is much prejudice against mirror lenses and although they have a bad reputation amongst photographers for unsharpness and inferior resolution, astronomers seldom complain - in fact they obtain wonderfully sharp and detailed astro-images. Astronomers are very aware that their reflecting telescopes require time for the rear prime mirror objective to cool down to the ambient air temperature. The rear prime objective lens' glass acts like a 'storage heater' when stored in a building and as soon as it's taken outside into the cooler atmosphere, the heat from the glass radiates into the cooler air stream inside the 'scope creating heat currents which can distort the image. Furthermore, the glass continues to radiate for up to an hour and as long as there is a temperature differential of just 5ºC between the glass and the air, imaging will be compromised. Astronomical reflector 'scopes often include one or more cooling fans to assist cooling and considerably reduce the cool-down time. Refracting camera lenses also need cool-down time after building storage but because a refractor's prime objective is at the front of the lens the temperature differential effect is minimal and they cool much quicker - compared to a reflecting lens where the prime mirror lens is at the rear of the tube and has a tube of air in front of it. Astronomers also use very rigid tripods and mounts compared to the average camera tripod - thus astronomers' 'apparatus shake' is minimal compared to the camera/lens shake of a photographic tripod. So … it's possible that camera/photographic mirror lenses are not the unsharp optics that many photographers purport them to be. A mirror lens' 'do-nuts' can seldom be minimised or eliminated entirely - but allowing sufficient time to cool down and using with a very rigid tripod could very much improve resolution and thus imaging results. Best wishes dunk ' data-webShareUrl='https://www.l-camera-forum.com/topic/254561-overcoming-the-alleged-shortcomings-of-a-mirror-lens/?do=findComment&comment=2952047'>More sharing options...
Advertisement Posted December 19, 2015 Posted December 19, 2015 Hi dkCambridgeshire, Take a look here Overcoming the alleged shortcomings of a mirror lens.. I'm sure you'll find what you were looking for!
dkCambridgeshire Posted December 19, 2015 Author Share #2 Posted December 19, 2015 EDIT: Astronomers are very aware that their reflecting telescopes require time for the rear prime mirror objective to cool down to the ambient air temperature. The rear prime objective lens' glass acts like a 'storage heater' when stored in a building and as soon as it's taken outside into the cooler atmosphere, the heat from the glass radiates into the cooler air stream inside the 'scope creating heat currents which can distort the image. Furthermore, the glass continues to radiate for up to an hour and as long as there is a temperature differential of just 5ºC 3º C between the glass and the air, imaging will be compromised. Astronomical reflector 'scopes often include one or more cooling fans to assist cooling and considerably reduce the cool-down time. … should read 3ºC … not 5ºC dunk Link to post Share on other sites More sharing options...
dkCambridgeshire Posted December 20, 2015 Author Share #3 Posted December 20, 2015 Just to show that reflecting telescopes can produce superb images, have a look at forum member Gary's moon image taken with his Leica T and 10" diameter reflector 'scope http://www.l-camera-forum.com/topic/226003-post-your-t-shots-here/?p=2952356 dunk Link to post Share on other sites More sharing options...
stuny Posted December 21, 2015 Share #4 Posted December 21, 2015 Lovely. Link to post Share on other sites More sharing options...
Iduna Posted December 21, 2015 Share #5 Posted December 21, 2015 Dunk, first of all, your capture is very nice and sharp enough, one can see the gloe in the eye of the deer. Your description of working with the mirror lens is most interesting. The Samyang lens is known and your experience is quite important to know before those who might think about purchasing one. You are right, a sturdy tripod is mandatory. Most people underestimate the danger of vibration and only go for lightweight products. The circumstances under which the lens produces the best results seems to be a matter of patience and trained skill. Thanks for your reportage. Link to post Share on other sites More sharing options...
dkCambridgeshire Posted December 21, 2015 Author Share #6 Posted December 21, 2015 Thank you Iduna. Another Samyang 800mm user has improved the fine focus by using a modified guitar string tuner with a rubber door stop on the shaft .. the door stop making contact and turning the lens barrel via the geared tuner. Optically the Samyang lens is fine but mechanically, the focus is far too coarse; its coarseness is well documented on other forums. My 'rigid foam' lever can probably be reduced in width (but not length) to make it less bulky. The foam grips the lens barrel firmly because it moulds itself around the circumference. But I may try making a neater balsa wood version and line the contact area with foam. Another 'to do' project is a comparison of the Samyang 800mm images with those of the Leitz Telyt S 800mm f6.3 … i.e. using the same subject and coupling both lenses to the Leica T. Best wishes dunk Link to post Share on other sites More sharing options...
Michael Geschlecht Posted December 21, 2015 Share #7 Posted December 21, 2015 Advertisement (gone after registration) Hello Dunk, Nice photos (Including Gary's). I think some of of the problems some people have when using Mirror Lenses come about because: Some people look at this small, light lens Which is effectively a big, light lens. And then treat this small light lens like a small, shorter focal length, light lens. Instead of treating it like the big light lens it is. Not sufficiently taking into consideration that: A big light lens is actually harder to get a good picture with than a big heavy lens. Best Regards, Michael Link to post Share on other sites More sharing options...
mike_j Posted December 23, 2015 Share #8 Posted December 23, 2015 I can attest to the difficulty of focusing Dunk's 800mm mirror having tried it! However this thread spurred me into fiddling about in the workshop this morning and I made this gadget to work with my Samyang 500mm mirror which is very difficult to use on the Sony A7 and my rather lightweight tripod. I used to do some model engineering so have lots of bits and pieces. The lens gear is a section of cogged rubber drive belt and it, together with the matching red gear wheel, originally drove the slew mechanism of a model fire fighting boat monitor (water gun). The horizontal bar is an old flash bracket. The gear housing is just milled from a bit of engineering grade nylon, the rear post is fabricated from a bit of brass hex section and a spare QR plate completes the job. It works very well. Firstly it gives very fine fingertip control using the red gear and the bracket with its central tripod mount damps down vibration dramatically. I even found that it is just about usable hand held. I know this is LF forum so showing a Sony is a bit of a cheek especially since the picture was taken on a Sony A6000 BUT I used a 50mm Summicron and close focus adaptor. Incidentally, I have found that a collapsible rubber lens hood has greatly improved contrast and reduced flare on this lens. Link to post Share on other sites More sharing options...
Michael Geschlecht Posted December 23, 2015 Share #9 Posted December 23, 2015 Hello Mike, Welcome to the Forum. You can make your "light weight" tripod effectively heavier by looping a strong piece of nylon cord around the ball head & tying the other end of the cord to something heavy on the ground (ie: A rock or a log.). By pulling the nylon cord tight you can make your light weight tripod as sturdy as a substantially heavier tripod. By having a nylon cord like that in your pocket you are effectively carrying around a 20 pound rock. Best Regards, Michael Link to post Share on other sites More sharing options...
dkCambridgeshire Posted December 23, 2015 Author Share #10 Posted December 23, 2015 Michael J that's a very elegant solution for improving focus, stability, and with the hood, for better contrast and reduced flare too. You've seen the photo of my Arca Swiss Cube tripod head Next time I try the 800mm I'll be using the Cube as an aid to improved composition - it will further address the lens' shortcomings especially if the lens is used with the Leitz APO 1.4x and 2x extenders. Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Please excuse the very Q&D pics Regards dunk Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! Please excuse the very Q&D pics Regards dunk ' data-webShareUrl='https://www.l-camera-forum.com/topic/254561-overcoming-the-alleged-shortcomings-of-a-mirror-lens/?do=findComment&comment=2954737'>More sharing options...
gbealnz Posted December 26, 2015 Share #11 Posted December 26, 2015 Thanks for the link Dunk, and basic stuff for those astro inclined, but perceived as mystic to those not. Good to see the DIY coming out. I have tried a few "tangent" style systems, mostly works in progress, for the likes of the Pentax 6x7 lenses, but with astro only in mind, where a very small amount of focus adjustment is required. For day to day use I found that the tangent arm didn't really have enough travel. I also tried the rubber band system, actually a rubber "O" ring attached to a smaller pulley on the end of a Feather Touch motor focus motor. Again, purely for astro. It worked, but required quite a bit of adjustment to get the tension right, and almost stressed the motor and shaft as a result. Not as easy as I thought. Gary Link to post Share on other sites More sharing options...
gbealnz Posted December 27, 2015 Share #12 Posted December 27, 2015 And to further show the ability or otherwise of the mirror systems, I tried this just after lunch, on a rather hot sunny day (normally the bane of a long focal length and mirror system). http://www.l-camera-forum.com/topic/226003-post-your-t-shots-here/page-51?do=findComment&comment=2956504 Gary Link to post Share on other sites More sharing options...
dkCambridgeshire Posted December 27, 2015 Author Share #13 Posted December 27, 2015 And to further show the ability or otherwise of the mirror systems, I tried this just after lunch, on a rather hot sunny day (normally the bane of a long focal length and mirror system). http://www.l-camera-forum.com/topic/226003-post-your-t-shots-here/page-51?do=findComment&comment=2956504 Gary Left you a comment Gary … That's a really superb image considering the time of day. I have a Lunt 60mm solar 'scope but plan to use it for sketching more than photo imaging. dunk Link to post Share on other sites More sharing options...
gbealnz Posted December 27, 2015 Share #14 Posted December 27, 2015 Thanks Dunk, difficult with two threads but each has it's place. My saviour was wheeling the dob base to a nearby grass area, way better than the asphalt which was too hot to stand on (we got to 28º C at the time this was taken, bright sun) Yes, I've dabbled in Ha imaging too, (not with Lunt but with Coronado), and this shot would have been so much simpler with the DMK camera, and Autostakert II. Gary Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.