Achromat Question

Keith Kline · February 14th, 2006, 09:55 PM

Okay I figured I'd just ask in this thread instead of starting a new one. I know alot depends on the camera but in general if two achromats are the same diameter which would be a better choice a 170mm FL or a 115mm FL? The 115mm FL would be a stronger diopter correct? Would there be any reason to use one over the other?

Wayne Kinney · February 15th, 2006, 05:25 AM

I would go with the 170mm. I think the other will be overkill and you may end up with soft edges.

Bob Hart · February 15th, 2006, 10:55 PM

Larry.

The 42mm hypotenuse gives you two 21mm half faces, which is all you get when you set them up in 90degree opposition for image erection. This may be too tight.

I'm using 56mm hypotenuse ( = 27mm half face) and I find this only just big eough to pass a 24mm x 18mm 4:3 image without edge fall-off on the side affected by the junction of the prisms. In my arrangment it happens on the left edge in pciture view.

From rear elevation view of the appliance, the physical arrangement of the prisms is up and to right.

These will work for 16:9 with a slightly larger image but only just and pick up a fall-off on one side edge or another depending on how well centred the camera and SLR lenses are with the most aggravated fall-off on the left.

I have tested the following combination into a Panasonic DVX100 in aerial image mode only and it worked :-

Sigma f4 - f6.3 50mm-500mm zoom lens.
Equivalent camcorder framing to 4:3 24mm x 18mm image off groundglass
2 40mm x 40mm x 56mm prisms in 90degree opposition.
Century Optics 7+ Achromatic Dioptre. 58mm thread, mounted via a 72mm to 58mm stepdown ring.

This was not an off-groundglass test. The setup was with an AGUS35 appliance with the disk removed. Success in eliminating the vignette from the aerial image with the Sigma lens suggests that the groundglass image with f1.8 lenses via this arrangement will work with the DVX100.

With the HDR-FX1, prisms with a longer hypotenuse are needed for an aerial image from the Sigma 50mm-500mm via the 7+ acromatic dioptre from a 16:9 image about 18mm high.

With the 56mm hypotenuse prisms, a successful image from the groundglass with f1.8 SLR lenses can be had but centres have to very exact and the prism alignment perfect. Even then, a slight fall-off can occur from the prism junction in certain lighting condtions.

February 15th, 2006, 05:25 AM	#17
Wayne Kinney Major Player Join Date: Jun 2005 Location: Brighton, East Sussex, UK Posts: 938	I would go with the 170mm. I think the other will be overkill and you may end up with soft edges. __________________ Thanks, Wayne.

February 14th, 2006, 09:55 PM	#16
Keith Kline Regular Crew Join Date: Aug 2004 Location: Akron, Ohio Posts: 153	Okay I figured I'd just ask in this thread instead of starting a new one. I know alot depends on the camera but in general if two achromats are the same diameter which would be a better choice a 170mm FL or a 115mm FL? The 115mm FL would be a stronger diopter correct? Would there be any reason to use one over the other?

February 15th, 2006, 10:55 PM	#18
Bob Hart Inner Circle Join Date: Dec 2003 Location: PERTH. W.A. AUSTRALIA. Posts: 4,477	Larry. The 42mm hypotenuse gives you two 21mm half faces, which is all you get when you set them up in 90degree opposition for image erection. This may be too tight. I'm using 56mm hypotenuse ( = 27mm half face) and I find this only just big eough to pass a 24mm x 18mm 4:3 image without edge fall-off on the side affected by the junction of the prisms. In my arrangment it happens on the left edge in pciture view. From rear elevation view of the appliance, the physical arrangement of the prisms is up and to right. These will work for 16:9 with a slightly larger image but only just and pick up a fall-off on one side edge or another depending on how well centred the camera and SLR lenses are with the most aggravated fall-off on the left. I have tested the following combination into a Panasonic DVX100 in aerial image mode only and it worked :- Sigma f4 - f6.3 50mm-500mm zoom lens. Equivalent camcorder framing to 4:3 24mm x 18mm image off groundglass 2 40mm x 40mm x 56mm prisms in 90degree opposition. Century Optics 7+ Achromatic Dioptre. 58mm thread, mounted via a 72mm to 58mm stepdown ring. This was not an off-groundglass test. The setup was with an AGUS35 appliance with the disk removed. Success in eliminating the vignette from the aerial image with the Sigma lens suggests that the groundglass image with f1.8 lenses via this arrangement will work with the DVX100. With the HDR-FX1, prisms with a longer hypotenuse are needed for an aerial image from the Sigma 50mm-500mm via the 7+ acromatic dioptre from a 16:9 image about 18mm high. With the 56mm hypotenuse prisms, a successful image from the groundglass with f1.8 SLR lenses can be had but centres have to very exact and the prism alignment perfect. Even then, a slight fall-off can occur from the prism junction in certain lighting condtions.