If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
Bower 500mm f/8 Mirror lens
On 17/07/2015 10:10 a.m., RichA wrote:
On Thursday, 16 July 2015 16:54:56 UTC-4, Me wrote: On 16/07/2015 1:17 p.m., RichA wrote: On Tuesday, 14 July 2015 04:11:48 UTC-4, Eric Stevens wrote: Does anyone have knowledge or experience of these? How good are they? (For a particular value of 'good') -- Regards, Eric Stevens Even with focusing aids, 500mm with an APS or m4/3rds camera is a pain to focus absolutely accurately. Also, mirror lenses were never really meant for daytime use. They started out as telescopes. Reason is that light can (even with a baffle tube coming from the primary mirror centre hole) fall in the camera sensor from outside the light cone, reducing contrast. This can be dealt with pretty well in post-processing. Also, mirror lenses of that size are susceptible to heat waves, much more so than smaller aperture camera lenses. This means close-in (100ft or less) objects are generally ok to shoot, but anything beyond that that isn't high above the ground tends to blur. Ironically, the lenses are optimized to focus at infinity which means you get a small amount of spherical aberration when focusing on things close-in, but it's better than heat waves. Why would atmospheric refraction/shimmer affect a mirror lens more than a normal telephoto? Not unless the normal telephoto had a larger front lens diameter. It's all predicated on that. Over 4" diameter, heat waves are very bad, but even less than that, if you are looking through a significant atmospheric blanket, it's bad. Still doesn't make sense to me, as at the distances where shimmer is a common problem (more than 50m or so - but of course it depends) then the difference in angle between subject and the perimeter of the front element (or mirror) is so insignificant, I doubt you'd see it. Here's an example of some shimmer at only 300mm or so (100% crop), taken on cool clear calm winter morning before sunrise: http://i.imgur.com/NZBRL2w.jpg The small island is about 40km, already degraded by atmospherics, the low hills about 80km, the mountains about 150km. |
#22
|
|||
|
|||
Bower 500mm f/8 Mirror lens
On 2015-07-16 18:24, Me wrote:
On 17/07/2015 10:11 a.m., Alan Browne wrote: On 2015-07-16 18:05, Me wrote: On 17/07/2015 9:11 a.m., Alan Browne wrote: On 2015-07-15 18:57, Eric Stevens wrote: On Wed, 15 Jul 2015 18:52:55 -0400, Alan Browne wrote: On 2015-07-14 19:18, PeterN wrote: On 7/14/2015 10:52 AM, Alan Browne wrote: On 2015-07-14 04:11, Eric Stevens wrote: Does anyone have knowledge or experience of these? How good are they? (For a particular value of 'good') With some exceptions most people who buy reflex's relegate them to a shelf at some point after the initial blush of pleasure wears off. They are usually (always?) manual focus too ... which can be a challenge for the wearier eyes out there. Some cameras, at least my Nikons, have some type of in focus indicators available in the viewfinder. That doesn't always make focusing easy - esp. off centre. Modern higher level Nikons have the ability to select an off-centre focus point. Does that work with a manual focus lens? Yes. The accuracy of those focus points when using small apertures may not be very good. OTOH there's always CDAF in LV mode. 500mm f8 with no stabilisation is going to be a stretch unless in full daylight, unless high ISO is used, you're probably on a tripod anyway. How do you know it's in focus? What is the indication? Left or right arrow either side of a dot - o . Arrows light up to tell you which way to move the focus ring, dot lights up and arrows are off when focus is achieved. Fantastic. I have such indicators on my Sony a900 (dot on when the focus point is in focus - no indicator of which way however) - but I'm not sure if they will work with a cat lens. |
#23
|
|||
|
|||
Bower 500mm f/8 Mirror lens
In article ,
Alan Browne wrote: I have such indicators on my Sony a900 (dot on when the focus point is in focus - no indicator of which way however) - but I'm not sure if they will work with a cat lens. Alan- I think it depends on f/stop. I have the Minolta 500mm f/8 AF mirror lens, which auto-focuses on the Alpha 100 and 900. I expect the focus indicator would work with a 500mm f/8 manual focus lens. I'm not sure what determines the critical f/stop. I recall that the split image rangefinder on my old film cameras, could not be used with a Celestron C-90 mirror telescope (1000mm f/11 as a lens). One half of the split image would always be dark. I used the ground glass to focus, but it was extremely critical. Fred |
#24
|
|||
|
|||
Bower 500mm f/8 Mirror lens
On 2015-07-17 14:20, Fred McKenzie wrote:
In article , Alan Browne wrote: I have such indicators on my Sony a900 (dot on when the focus point is in focus - no indicator of which way however) - but I'm not sure if they will work with a cat lens. Alan- I think it depends on f/stop. I have the Minolta 500mm f/8 AF mirror lens, which auto-focuses on the Alpha 100 and 900. I expect the focus indicator would work with a 500mm f/8 manual focus lens. To be clear I'm referring to the off centre focus points, not the central point. blush I'd forgotten that the Minolta cat was AF. I'm not sure what determines the critical f/stop. I recall that the split image rangefinder on my old film cameras, could not be used with a Celestron C-90 mirror telescope (1000mm f/11 as a lens). One half of the split image would always be dark. I used the ground glass to focus, but it was extremely critical. I'm pretty sure contrast (ambient light) is the deciding factor. So an f/11 lens will have a 1 stop disadvantage. That's the difference between a sunny-16 day and moderate overcast. |
#25
|
|||
|
|||
Bower 500mm f/8 Mirror lens
On 7/16/2015 5:12 PM, Alan Browne wrote:
On 2015-07-16 06:46, PeterN wrote: snip Nope, not always. Manual Off center focusing can be a challenge. However, if off center focusing with MF is needed, the next best thing is to use a tripod and move the camera. Tt is rare that off center MF is needed. Unless you're into photography, I suppose that may be true. I should have been more clear. I rarely move into left or right focus mode. I do most of my MF focus for macro and landscape work, where I am more concerned iwith composition, and apparent focus. -- PeterN |
#26
|
|||
|
|||
Bower 500mm f/8 Mirror lens
On 2015-07-19 15:54, PeterN wrote:
On 7/16/2015 5:12 PM, Alan Browne wrote: On 2015-07-16 06:46, PeterN wrote: snip Nope, not always. Manual Off center focusing can be a challenge. However, if off center focusing with MF is needed, the next best thing is to use a tripod and move the camera. Tt is rare that off center MF is needed. Unless you're into photography, I suppose that may be true. I should have been more clear. I rarely move into left or right focus mode. I do most of my MF focus for macro and landscape work, where I am more concerned iwith composition, and apparent focus. Jeez, can't you see a cheap shot for a cheap shot and fire back. ;-) |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Opteka 500mm Mirror Lens | [email protected] | Digital SLR Cameras | 0 | May 21st 09 03:43 AM |
FA - Nikkor 500mm f/8 Mirror Lens | Paul Lester | General Equipment For Sale | 0 | December 10th 06 10:50 PM |
Vivitar 500mm f/8 Mirror Lens | William Graham | 35mm Photo Equipment | 7 | March 21st 05 01:00 PM |
Info on Vivitar 500mm f/8 Mirror Lens | Phil, Squid-in-Training | 35mm Photo Equipment | 0 | March 20th 05 03:07 AM |
Vivitar 500mm f/8 Mirror Lens | Matt Clara | 35mm Photo Equipment | 0 | March 19th 05 10:57 AM |