Focal reducer (1 Viewer)

Paul Corfield · Apr 22, 2011

Today I came up wit a very nice focal reducer. Not sure why I never tried it before but never mind. I've got quite a few zoom lens objectives collected from lenses I've taken apart. Mounting the objective into a macro tube (which is extremely simple) will then act as a focal reducer. Noting is lost at all in terms of sharpness and no CA is introduced. Corner sharpness is the only issue and it varies from lens to lens. Good ones are sharp almost to the corner and this is easily cropped if it shows in the photo which quite often it doesn't.

Here's a few images taken with an objective lens from a Canon 100-200mm lens. Another odd thing is that you don't need any extension in the scope at all and you can reach focus from infinity down to around 10m. Adding some extension then brings the focus down much closer.

This Canon lens adds 30% reduction and takes the 600mm SW80ED down to 420mm equivalent.

First two images are uncropped. Butterfly was from around 5m range and third image is a tight crop of the same photo showing that detail is very good.

Last image is of a wall, taken square on and you can see some slight corner softness. I need to check my other lenses as I think I have one or two that are sharper in the corners. For something that is basically free I can live with the corner sharpness, especially as the images are so sharp.

Also different lenses give different reductions, I have another lens that gives about a 40% reduction or 360mm equivalent and is equally as sharp with no CA.

Paul.

goldenarrow · Apr 22, 2011

So taking a lens that is usually in front and putting it in the rear will turn it into a reducer?? I am currently using my tamron (103A) objective as a handheld magnifying lens with original lens cap as a cover. Will have to try this neat trick.

Paul Corfield · Apr 22, 2011

They seem to work really well. I found that as a hand magnifier, the closer you need to hold the lens to something to get it in focus then the more it reduces the power of the telescope. One of the lenses I tried to was a Tamron 85-210mm and that takes my 600mm scope down to around 340mm. I found it to be a bit soft in the corners though. The best of my lenses is a Miranda 75-300mm and that is sharp all the way to the corners with only a slight loss in detail in the corner areas. I found all of them to be razor sharp across most of the image, slightly harder to focus if the subject is small due to the narrower depth of field but using live view can help. I mount them in the macro tube and hold it in place with some Blu-Tac.

Shall post some more photos later on.

Paul.

Paul Corfield · Apr 22, 2011

Here's a comparison, one with the reducer and one without taken from around 6m range (20 feet). I did a proper check and the Miranda 75-300mm objective gives exactly a 40% reduction which makes the scope into a 360mm f/4.5.

First two images are both uncropped, also posted the 100% crops of both images showing that no detail is lost with the reducer.

Paul.

Paul Corfield · Apr 22, 2011

Here's some bird images.

First one is uncropped from around 10m range (32 feet) and shows good sharpness to the edges, taken with the Miranda 75-80mm objective which makes the scope 360mm F/4.5.

The other two are crops from around 6m range (20 feet).

Paul.

Paul Corfield · Apr 22, 2011

Here's an uncropped photo of a wall shot with the Miranda lens on the SW80ED and the second image is a 100% crop of the top right corner. Not bad for F/4.5

Paul.

Paul Corfield · Jun 30, 2011

Here's the simple assembly steps for making the focal reducer. Nice thing about the Miranda lens is it's a perfect fit inside the ceap macro tubes available on ebay. Images are sharp right to the corner and they keep the original sharpness of the scope, no loss in image quality.

Paul.

alphan · Jul 5, 2011

The Cosina 70-210 gave me roughly 0.6X. Would that Objective also give similar focal reduction if I am to mount it on a Celestron C90? Celestron C5 are hard to come by and still cost close to new price. C90 comes cheap but if the Objective works the same, that would give me roughly 720mm f 8 which isn't too bad. Seldom needed a long FL anyway but with the faster aperture, it can be a handy lens for me to carry around in my back pack when I go travelling, which I did every month. stefano seems to be doing well with this concept.

Paul Corfield · Jul 5, 2011

No reason why it shouldn't give a similar reduction. Out of around 10 or 12 zoom front lens assemblies I've tried the Miranda one I posted above gives the flattest photo. When viewing at 100% it's hard to detect any corner softness.

Paul.

alphan · Jul 6, 2011

I have yet to try the Canon Objective but are not too keen on that objective as it is made of glass very heavy. Wonder if there are any noticeable improvement with glass over plastic element.

JvdM · Aug 6, 2011

Paul Corfield said:
Here's the simple assembly steps for making the focal reducer. Nice thing about the Miranda lens is it's a perfect fit inside the ceap macro tubes available on ebay. Images are sharp right to the corner and they keep the original sharpness of the scope, no loss in image quality.

Paul.

Hello Paul (and all the other astro-scopers)
First time poster.
What is the orientation of the objective when introduced into the optical chain?
Does it "face forward" like it did in the lens, or must it "face backward" ?
Thanks
Jaco

JvdM · Aug 6, 2011

Paul Corfield said:
Here's some bird images.

First one is uncropped from around 10m range (32 feet) and shows good sharpness to the edges, taken with the Miranda 75-80mm objective which makes the scope 360mm F/4.5.

The other two are crops from around 6m range (20 feet).

Paul.

Hi Paul

75-85=printing error?

Jaco

kurakura · Oct 4, 2011

Paul I am not sure I fully understand how it is possible to make the lens faster from f7.5 to f4.5. The lens should still only be able to take in that much light, can you or someone explain that too me?

Aegea · Oct 5, 2011

kurakura, it reduces the focal length and that lowers the effective f ratio. It works the exact opposite way to a teleconverter or barlow.

Paul Corfield · Oct 5, 2011

Yes, it's exactly as Aega says. Think when you add a teleconverter, the image gets darker and you have to lower the shutter speed to gain more light in the exposure. The F number has been increased along with the magnification.

With a focal reducer the F number is reduced due to lowering the magnification. This results in a brighter image and so a faster shutter speed is required for the correct exposure.

Paul.

kurakura · Oct 5, 2011

Gotcha - It came to me as a revelation earlier

Focal reducer (1 Viewer)

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