Fowl Mouth
Just a guy with dogs
What about a TeleVue Powermate? I wonder how they would operate photographically. Maybe no advantage regarding field curvature?!? At least they are only $300...(only)
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Comparison of teleconverters & barlows (1 Viewer)
- Thread starter Paul Corfield
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Paul Corfield
Well-known member
There's a few bird photos around taken with Powermates and they are pretty much the same as a good barlow as far is image quality.
Paul.
Paul.
Paul Corfield
Well-known member
When I re-did the money photos I noted that my small Antares Apo barlow and my 2" GSO barlow were a bit soft in the corners and I wondered if this was because I have them mounted very close to the camera. I moved them further away from the camera to the sort of position they would be if used with an eyepiece and that produced sharp photos in the corners. I had them mounted flush to the camera body but for these images I put a 40mm macro tube between them and the camera and this is roughly where they would be if screwed into the front of the MAX DSLR or T-ring/scope adapter.
Here's 100% crops of the bottom right hand corner of the photo and information written on each image. Range for all of them was about 7m (23 feet)
I think for the money the Kenko 1.4X is excellent. The 2" Antares 1.6X (2.3X on scope) is also pretty sharp to the corner. The other two are good value for the price, the Antares Apo is perfectly corrected for CA in the middle of the image but a little creeps in at the corners and the other three are possibly better in this respect.
Paul.
Here's 100% crops of the bottom right hand corner of the photo and information written on each image. Range for all of them was about 7m (23 feet)
I think for the money the Kenko 1.4X is excellent. The 2" Antares 1.6X (2.3X on scope) is also pretty sharp to the corner. The other two are good value for the price, the Antares Apo is perfectly corrected for CA in the middle of the image but a little creeps in at the corners and the other three are possibly better in this respect.
Paul.
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David Ellsworth
Well-known member
Thanks for the test. It's interesting that you were able to improve corner sharpness (field flatness?) in a couple of the barlows by modifying the camera distance.
Strange that the "apochromatic" barlow has much worse CA than the other Antares barlow.
The Antares "1.6x" seems to be the clear winner. The GSO 2x has lots more CA in the corners, but might be a tad sharper in that corner despite the lower magnification. Can sharp corner focus on the Antares "1.6x" be achieved, at the cost of loss of center focus (i.e. curved but sharp focal plane)?
I'd like to see the Antares "1.6x" pitted against a good 2x teleconverter. It's looking like an attractive choice, though I wish it used a brass compression ring instead of screws. (Not that I'm particularly fond of compression rings... they have their share of problems. But at least it would be consistent with what I already have and wouldn't be the only thing marring my 2" tubes.)
Strange that the "apochromatic" barlow has much worse CA than the other Antares barlow.
The Antares "1.6x" seems to be the clear winner. The GSO 2x has lots more CA in the corners, but might be a tad sharper in that corner despite the lower magnification. Can sharp corner focus on the Antares "1.6x" be achieved, at the cost of loss of center focus (i.e. curved but sharp focal plane)?
I'd like to see the Antares "1.6x" pitted against a good 2x teleconverter. It's looking like an attractive choice, though I wish it used a brass compression ring instead of screws. (Not that I'm particularly fond of compression rings... they have their share of problems. But at least it would be consistent with what I already have and wouldn't be the only thing marring my 2" tubes.)
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Paul Corfield
Well-known member
Imagine the image leaving the barlow as an ever increasing cone. By moving the barlow slightly further from the camera, the cone covering the sensor is enlarged and the softer parts of the image at the edges of the cone can be made to fall outside the edges of the camera sensor. This also has the effect of increasing the magnification because less of the image is now hitting the sensor. It works with teleconverters too. I found with my Kenko 1.4X teleconverter that I can put a 60mm macro tube between it and the camera and the magnification is boosted to 2X.
I'll do some more tests with the Antares 1.6X barlow just to be sure I had the focus spot on. Wont be able to post anything till Monday as I'm away doing a show this weekend.
Other big 2" barlows that I know of are the 2" Televue Big Barlow and the Orion 3 element Apo barlow. Also some made by Siebert Optics might be ok. All these are in a higher price bracket though so I've not been able to afford those yet.
The Antares 1.6X beats my best 2X teleconverter. Don't know how it would compare to the Canon one though.
Paul.
David Ellsworth
Well-known member
Maybe I misunderstood, then. Does this change the focus of the corners that still fall inside the edges of the camera sensor, relative to the center?
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Paul Corfield
Well-known member
In theory, by moving the barlow further from the camera you are using more of the sharper central part of the glass. If the barlow produces the right shape cone then you could end up with a perfectly sharp image from corner to corner. Some barlows produce a small cone and some are bigger. A barlow with a small cone is harder to correct because you have to move it a very long way from the camera to see much of a difference. My 1.5X Apo barlow is a bit like that which is why there wasn't much improvement. The GSO produces a wider cone and so it corrected the sharpness much more than the 1.5X apo.
Paul.
David Ellsworth
Well-known member
As I understand it, moving the camera farther from a barlow or TC will result in more magnification. I'm rather foggy on whether anything else changes, and I'm not sure how to interpret your explanation as to answer my question of whether the corners are sharper because of the extra "cropping/zooming" or if it is an unrelated effect. (I would experiment myself, but I have no barlows yet, and I don't have any DSLR extension tubes so it would be really hard to experiment with a teleconverter and increase TC-to-camera distance while keeping a parallel focal plane.)
So, either for the sake of argument (a thought experiment) or as an experiment to try for real:
* Take a photo at minimal camera-to-barlow distance, and call this Photo A.
* Take a photo with some extra distance between camera and barlow, keeping all else equal, and call this Photo B.
Now, crop Photo A to match the magnification of Photo B; call this Photo A'. Downsize Photo B to match the magnification of Photo A and call this Photo B'. If necessary, sharpen Photo A' a little more than Photo B, to compensate for the extra effect of the camera's anti-aliasing filter in Photo A (or the reduced effect of the AA filter in Photo B, depending on how you look at it).
How does Photo A' compare to Photo B'? Does Photo B' have sharper corners?
Here is a test done a while back in which the Televue Powermate seems to lose by a huge margin: http://photo4fun.zenfolio.com/p231550804
Would the Powermate do a lot better with extra camera-to-barlow distance? And would those astigmatic corners be improved without being completely "cropped/zoomed" away?
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Paul Corfield
Well-known member
I remember when Yvan did that Powermate test but unfortunately I think he returned it before we could get to the root of the problem.
Powermates in the 2X and 2.5X varieties produce a very parallel light path so you gain very little in magnification by moving it further from the camera. Actually the 2.5X version produces less magnification by moving it further away. There's a good diagram on this link showing how distance affects the Powermate magnification. They also do a Powermate T-ring adapter which makes for a compact set up. There's a photo on this link. http://www.televue.com/engine/page.asp?id=42
Pretty sure that with moving barlows further from the camera you are just getting a cropping effect, just like putting a full frame lens on a small sensor dslr. Moving the barlow produces a bigger image circle which falls well outside the size of the sensor.
I'll do the test later today.
Paul.
Powermates in the 2X and 2.5X varieties produce a very parallel light path so you gain very little in magnification by moving it further from the camera. Actually the 2.5X version produces less magnification by moving it further away. There's a good diagram on this link showing how distance affects the Powermate magnification. They also do a Powermate T-ring adapter which makes for a compact set up. There's a photo on this link. http://www.televue.com/engine/page.asp?id=42
Pretty sure that with moving barlows further from the camera you are just getting a cropping effect, just like putting a full frame lens on a small sensor dslr. Moving the barlow produces a bigger image circle which falls well outside the size of the sensor.
I'll do the test later today.
Paul.
Chris B.
Well-known member
Paul and others -
Thank you for this thread!! I've been considering adding an "Astro" scope to my bird photography kit for several months and have narrowed it down to either a StellarVue or Tele-Vue. Since I have Canon gear and both Canon teleconverters (1.4x & 2x), it was helpful to know that I probably won't have to invest in Barlows or other in-line lens magnifiers right away.
Chris
Thank you for this thread!! I've been considering adding an "Astro" scope to my bird photography kit for several months and have narrowed it down to either a StellarVue or Tele-Vue. Since I have Canon gear and both Canon teleconverters (1.4x & 2x), it was helpful to know that I probably won't have to invest in Barlows or other in-line lens magnifiers right away.
Chris
Paul Corfield
Well-known member
Stellarvue make some nice scopes. Which ones were you considering.
The fibreglass ones are interesting, quite light weight for their size.
Paul.
Chris B.
Well-known member
One of the only telescope shops in my area has a floor sample of a Stellarvue SV102ED for a reasonable price and I'm considering that one. I've looked at their SV80ED Raptor which is one of theirs which uses the carbon fiber tube but the focal length (560mm) is what I can get now with my 400mm f4 DO + 1.4x TC.... I'm really looking for a scope with will give me a high quality image with a magnification factor of 30 - 40x and I think the larger SV102ED will enable me to get there without too many more pieces in the "optical" chain.
David Ellsworth
Well-known member
Hmm, I would think that the 400mm f4 DO + 2x TC should give you almost as much resolving and light gathering power as the SV102ED. The main advantage of the SV102ED would be easier manual focusing (due to the dual-speed Crayford). But on a 1-series body you'd still have auto-focus with the 400mm f4 DO + 2x.
How sharp is your 400mm DO with 2x+1.4x TCs stacked? Could you show a 100% crop? Does stopping down 1/3 or 2/3 stop from wide open make it sharper?
Chris B.
Well-known member
I don't use any Series 1 bodies... the "DO" virtually lives on my 7D with the 1.4x and I also have a 40D. I do have a 2x but can only use it with manual focus, and I've never tried stacking the 1.4 with the 2x because the viewfinder gets very dark, and I'm not a big fan of "Live View". I try and shoot as close to f7.1 as possible because the diffraction index on the 7D is f6.8. This is a shot made with the DO and the 2x, and it holds up ok...
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David Ellsworth
Well-known member
Wow, cool shot! I hadn't seen Osprey nestlings before, and didn't know they had red eyes!
Could you please show a 100% crop so I can see how well the 400mm DO is holding up under this magnification?
I could be wrong but judging from the reduced picture, it looks like there was some front-focus... the wood looks sharper than the nestlings. So if that really is sharper, you could just show a 100% crop of part of the wood that poop stains on it...
That diffraction index only tells you when diffraction softness begins. But this effect is very subtle at first! It's not until about 1.3x that aperture that the softness will start being really noticeable, and that's only on a lens as sharp as the 100mm USM Macro. On a lens like the 100-400mm, which is still sharp but not as sharp as the Macro, I've found that it doesn't start to get much softer than normal until 1.6x or 1.7x that aperture or so.
I don't have a 7D, but going by that scale, the diffraction index on the 450D would be f/8.2. That agrees pretty well with tests I've done with an artificial star... but this is a very artificial test, and most subjects don't have as much contrast as a pinpoint star against a black background. Tests I've done on my 450D show that my 100mm USM Macro doesn't get really noticeable softer until f/11, and my 100-400mm doesn't get really significantly softer until f/14 or so. Note that I'm a pixel-peeper (I got a 2560x1600 LCD so I could better indulge my pixel-peeping).
Could you please show a 100% crop so I can see how well the 400mm DO is holding up under this magnification?
I could be wrong but judging from the reduced picture, it looks like there was some front-focus... the wood looks sharper than the nestlings. So if that really is sharper, you could just show a 100% crop of part of the wood that poop stains on it...
That diffraction index only tells you when diffraction softness begins. But this effect is very subtle at first! It's not until about 1.3x that aperture that the softness will start being really noticeable, and that's only on a lens as sharp as the 100mm USM Macro. On a lens like the 100-400mm, which is still sharp but not as sharp as the Macro, I've found that it doesn't start to get much softer than normal until 1.6x or 1.7x that aperture or so.
I don't have a 7D, but going by that scale, the diffraction index on the 450D would be f/8.2. That agrees pretty well with tests I've done with an artificial star... but this is a very artificial test, and most subjects don't have as much contrast as a pinpoint star against a black background. Tests I've done on my 450D show that my 100mm USM Macro doesn't get really noticeable softer until f/11, and my 100-400mm doesn't get really significantly softer until f/14 or so. Note that I'm a pixel-peeper (I got a 2560x1600 LCD so I could better indulge my pixel-peeping).