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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA
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 Posted: Fri Mar 02, 2012 2:03 pm Post subject: |
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| TheLostVertex wrote: | So we have (1+1/(1.1*2.45))=~0.721
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Ah, I see the error, I assumed that it was (1+1)/2.695 instead of 1+1/2.695  |
LOL indeed! (1+1)/2.695 was one of the possibilities I considered, but that actually evaluates to 0.7421... So, perhaps a typo along with a parenthesis error?
A possibly interesting side note... When I teach algebra, I tell my students that after a while they're going to think they're in a PE class. That's Parenthesis Error, not Physical Education. Well over half the mistakes made always turn out to be grouping, one way or another. It's another of the reasons I'm always skeptical of computations.
--Rik |
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_sem_
Joined: 04 Aug 2011
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 12570
Location: Richland, Washington State, USA
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| Posted: Fri Mar 02, 2012 3:43 pm Post subject: |
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| _sem_ wrote: | Another disgression... Have you ever considered using a close-up lens in front of a single reversed lens as mentioned by BR for the AI-s Nikkors 35-70mm
http://www.naturfotograf.com/lens_zoom_02.html
I'd assume the diopter brings the focus plane of the reversed lens closer to the sensor? |
We need to be careful what "front" means. Rørslett writes "This lens can give an excellent macro setup, when a 6T close-up lens is added and the lens then is reversed using the BR-2/2A and BR-5 adapters." In the final configuration, the close-up lens sits between the main lens and the camera, so it's like my 55+135 stack with the close-up lens replacing the 135.
I haven't tested this configuration, but with a high quality multi-element closeup lens I'd expect results similar to what I've shown above. In both cases the 55mm is being used at or close to its designed focus distances. The output of the 55mm on front will be focused closer to infinity than when it's used alone, and the close-up lens between it and the camera will finish focusing onto the sensor.
--Rik |
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TheLostVertex
Joined: 22 Sep 2011
Posts: 58
Location: Florida
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| Posted: Fri Mar 02, 2012 4:08 pm Post subject: |
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| rjlittlefield wrote: | So, perhaps a typo along with a parenthesis error?
A possibly interesting side note... When I teach algebra, I tell my students that after a while they're going to think they're in a PE class. That's Parenthesis Error, not Physical Education. Well over half the mistakes made always turn out to be grouping, one way or another. It's another of the reasons I'm always skeptical of computations.
--Rik |
For anyone that knows me, they know that typos are my specialty  I think I pressed the 4 key and it didnt depress all the way(scissor style keyboards tend to do that).
I am sure I will be able to come up with new and creative mistakes in the future
-Steve
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-Steven
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johnsankey
Joined: 02 Mar 2012
Posts: 18
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| Posted: Tue Mar 06, 2012 6:13 am Post subject: Re: Question about aperture and extension tubes. |
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| TheLostVertex wrote: | | We start with a lens that is sharpest in its normal configuration @ F5.6 Am I correct in assuming this should remain the sharpest aperture when reversed? |
If it's a macro lens designed for the distances you are using it reversed yes, otherwise no.
Images get softer at higher magnifications because the lens's aberrations are being magnified too, also because the effective aperture is being reduced so diffraction increases.
I've some formulae with examples at
http://johnsankey.ca/tripod.html#E |
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ChrisLilley
Joined: 01 May 2010
Posts: 680
Location: Nice, France (I'm British)
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| Posted: Tue Mar 06, 2012 7:03 am Post subject: |
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| rjlittlefield wrote: | | In a quick test varying exposure times to match the histograms, my rig says that a full f-stop is too much, 1/3 of an f-stop is not enough, and 2/3 of an f-stop is just about dead on. I'm not delighted by the remaining disagreement of about 1/4 f-stop (between 2/3 and 0.94), but it's close enough that I'm willing to wave my hands and assert "measurement error". |
Since you mentioned
| rjlittlefield wrote: | | For the stacked configuration I've used as the rear lens an even more ancient Sears/Ricoh 135mm f/3.5. |
and guessing that the coatings on the rear lens would not be up to those on the best modern lenses - perhaps even single coated - I wonder if light loss due to reflections at air/glass interfaces might explain some of the discrepancy. In other words the difference between f-stops and T-stops. |
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ChrisLilley
Joined: 01 May 2010
Posts: 680
Location: Nice, France (I'm British)
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| Posted: Tue Mar 06, 2012 9:19 am Post subject: |
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| rjlittlefield wrote: |
We need to be careful what "front" means. Rørslett writes "This lens can give an excellent macro setup, when a 6T close-up lens is added and the lens then is reversed using the BR-2/2A and BR-5 adapters." In the final configuration, the close-up lens sits between the main lens and the camera, so it's like my 55+135 stack with the close-up lens replacing the 135. |
Referring to our close-up lens FAQ the Nikon 6T is a multicoated, two-element achromat with a filter thread of 62mm and +2.9 diopter.
Which makes the focal length 1000/2.9 = 344mm. |
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naturephoto1
Joined: 13 Nov 2011
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Location: Breinigsville, PA
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 12570
Location: Richland, Washington State, USA
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| Posted: Tue Mar 06, 2012 10:03 am Post subject: |
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| ChrisLilley wrote: | | I wonder if light loss due to reflections at air/glass interfaces might explain some of the discrepancy. In other words the difference between f-stops and T-stops. |
That sounds like a good possibility, and it's one that hadn't occurred to me -- doh!
--Rik |
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Harold Gough
Joined: 09 Mar 2008
Posts: 5726
Location: Reading, Berkshire, England
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_sem_
Joined: 04 Aug 2011
Posts: 42
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| Posted: Tue Mar 06, 2012 3:01 pm Post subject: |
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| rjlittlefield wrote: | We need to be careful what "front" means. Rørslett writes "This lens can give an excellent macro setup, when a 6T close-up lens is added and the lens then is reversed using the BR-2/2A and BR-5 adapters." In the final configuration, the close-up lens sits between the main lens and the camera, so it's like my 55+135 stack with the close-up lens replacing the 135.
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| ChrisLilley wrote: | | Referring to our close-up lens FAQ the Nikon 6T is a multicoated, two-element achromat with a filter thread of 62mm and +2.9 diopter. Which makes the focal length 1000/2.9 = 344mm. |
I must admit I was being careless but I didn't think BR's recipe would be misunderstood. Though I didn't look at it as a stack of the 35-70 reversed on top of a 344mm. Rather as the 35-70mm appended with a close-up diopter to make it focus closer, then both together reversed on the camera I mean, with the lens set to its close-focus MFD of .7m or 1m reverse mounted without the diopter, the focus plane on the subject side when focused on the sensor must be "extended" far away from the designed flange distance, and the diopter lets it work closer to it (consider the "focus jig" shown at http://johnsankey.ca/tripod.html#E). |
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 12570
Location: Richland, Washington State, USA
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| Posted: Tue Mar 06, 2012 4:22 pm Post subject: |
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And I must admit I didn't realize the 6T was so weak. I'm used to working with Raynox DCR-250, which is 8 diopter = 125 mm.
But the basic principle is the same either way -- add optics in the rear to let the front lens work closer to its designed range.
My "need to be careful" caveat was prompted by my own initial confusion about what was being described, before I went off and read Rørslett's page. I just figured that if I was confused enough to notice, I'd try to make things clearer for other people reading the thread. There are always a fair number of those. As I write this, the thread is sitting at 877 page views.
--Rik |
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_sem_
Joined: 04 Aug 2011
Posts: 42
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| Posted: Thu Mar 08, 2012 1:21 pm Post subject: |
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| _sem_ wrote: | | ... Though I didn't look at it as a stack of the 35-70 reversed on top of a 344mm. ... |
I guess this would actually work as such a stack with the diopter placed 344mm away from the sensor? I haven't seen any recent hack where someone would try to replace a tele lens with a simple achromat diopter though, the quality probably sucks |
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 12570
Location: Richland, Washington State, USA
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| Posted: Thu Mar 08, 2012 8:58 pm Post subject: |
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| _sem_ wrote: | | _sem_ wrote: | | ... Though I didn't look at it as a stack of the 35-70 reversed on top of a 344mm. ... |
I guess this would actually work as such a stack with the diopter placed 344mm away from the sensor? I haven't seen any recent hack where someone would try to replace a tele lens with a simple achromat diopter though, the quality probably sucks |
I agree the quality would suck, but not that's necessarily because of the rear lens. The catch is that if you reverse a 35mm in front of a 344mm at infinity focus, you get about 10X, and then if you run the 35mm at its own f/4 or f/5.6 because that's where it is sharpest, the rear lens ends up stopped down to f/40 or f/56. A simple achromat can do pretty well stopped down that far, while covering only the narrow angle of an APS sensor at 344 mm. The problem is that because of diffraction you won't much like f/40 or f/56 on an APS sensor!
--Rik |
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