Focal Reducers - Squeeze all your FOV and Light on your GH1 Sensor

MatzeB

MatzeB

Active member
Astro guys use focal reducer / focal compressor / field flattener to concentrate the wide field of view on their small CCD chips.
The cool thing is that you not only widen your field of view, but also brighten your image, because the whole light is compressed to a smaller area.

check out this page:
http://www.isomedia.com/homes/cvedeler/scope/focal_reducers.htm

Compressing your full frame 35mm Lens to the 2x crop GH1 sensor should theoretically gain 2 stops of light, because the sensor area is 4 times smaler.
That would turn a 50mm f1,4 into a 25mm f0,7 or a 24mm f2,0 into a 12mm f1,0 ! :shocked:

I don't know, if a 0,5x compression is possible while keeping the overall quality of the lens. But one could use a weaker 0,75x reducer - and gain at least the half field and one stop of light.


For a quick experimental observation of the effect take a Macro Diopter and hold it - behind - your lens. Catch your image with a piece of paper or some kind of ground glass. A +10 Diopter is not strong enough - but you will see the effect.
 
Very interesting point. Plus your 50mm f1.4 would be a 25mm f0.7, or 50 mm equivilent. Because the image circle would be reduced to fit the image circle of the MFT sensor, resulting in the same field of view.

Less wasted light, same field of view as 35mm.

There's also a fair bit of room between the sensor and the back of a 35mm adapted lens in which to put these optic.

Your light gain from this, though, would be offset by any light loss due to the focal reducer optics... but it sounds like a spectacular idea, all in all!
 
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From the notes about this Meade focal reducer, it seems that is not suitable for conventional photographic imaging, but others are. They are only x0.5, but that is not bad if they work. One stop gain, and the approximative equivalent FOV of a FF35mm camera is fantastic.
I also understand that there is a resolution loss, but test will be required.
 
Who is going to try it and let us know how well it works?

I wonder if zooming in on the GG will negate the increase in light because of the increase in f-stop. Also, grain will become larger and possibly more apparent.

I still think its a cool idea.


J
 
The amount of de-magnification is variable, depending of the adapater/spacer length.

It looks that these adapters are not very expensive and trying to 'play' with a series of contraptions of different lens combos would be cool.
 
pix2pixels said:
From the notes about this Meade focal reducer, it seems that is not suitable for conventional photographic imaging, but others are. They are only x0.5, but that is not bad if they work. One stop gain, and the approximative equivalent FOV of a FF35mm camera is fantastic.
I also understand that there is a resolution loss, but test will be required.
Click to expand...


0,5x is the maximum. The GH1 Sensor has a crop factor of ~ 1,9 in 16/9.
1/1,9 = 0,53...
With a 0,5x reducer you actually should gain 2 stops, because the image AREA
is reduced to 1/4.
 
AwakenedFilms said:
Who is going to try it and let us know how well it works?

I wonder if zooming in on the GG will negate the increase in light because of the increase in f-stop. Also, grain will become larger and possibly more apparent.

I still think its a cool idea.


J
Click to expand...

I don't think it's a good idea to use it with a 35mm Adapter.
There's nothing you can improve if you already have the Full Frame.
It will only degrade your image.

I'm talking about using it on the smaller GH1 Sensor.
 
pix2pixels said:
The amount of de-magnification is variable, depending of the adapater/spacer length.

It looks that these adapters are not very expensive and trying to 'play' with a series of contraptions of different lens combos would be cool.
Click to expand...

It's definitely worth some experiments.
 
This might become my new DIY project... goodbye Ground Glass, hello GH1 Sensor! :love4:
 
Kholi said:
Again, because i don't understand the concept, if someone tells me how I'll try it?
Click to expand...

What the focal reducer does, the lens is already doing relative to the target sensor/film size. The real point of a focal reducer for a dSLR adapter is to reduce the image circle from a 35mm full-frame, for instance, to a 1.5x crop or 2.0x crop; from its original target size to a smaller one (which is what the OP I think is suggesting).

There are technical problems to this, primarily the focal flange distance being too short to accommodate the necessary optical elements. This is why focal reducers are primarily used on large astronomical telescopes that are simpler to implement. The Borg [SIZE=-1]77EDII Telescope I posted on the other thread for instance[/SIZE] can accommodate a focal reducer.

http://dvxuser.com/V6/showpost.php?p=1626017&postcount=67

To make it really work on a dSLR, you will essentially need an entirely new lens assembly that refocuses the image circle. This obviously will be expensive and would reduce overall optical image quality. The patent below demonstrates how this will need to be accomplished for a dSLR (the "50" area representing the focal reducer):

Picture3-2.png
 
I understand that we would be adding more elements in between the lens and sensor....but being that this camera (GH-1) does not use a mirror like traditional DSLRs, how bad would the degradation be? Maybe slightly softer? And gaining some light to boot.
 
A mirror wouldn't have any effect on degradation, as the mirror flips up out of the way when shooting anyway... it should have the same effect on a GH1 as it would on any SLR...
 
Without the mirrorbox there should be enough space for an adapter.
I doubt it would be good enough for 12MP photos, but downscaled to 1080 or 720 it might look good.
Here is a quick test of a DIY compressor - just to show, that it should be possible. Looks like it's compressing ~0,7x or 0,8x
Don't judge image quality. It's only to show the effect.

50mm Canon FD at f1,4 :
1241775171.jpg


50mm Canon FD at f1,4 with compressor:
1241775273.jpg
 
WOW MatzeB,

Did a similar thing yesterday.
I went to a local telescope shop and they had a Meade telecomprssor on the shelf. Just attached a 16-35mm lens from my Canon camera and got a similar image to yours. I didn't have another camera with me to take a photo.

I was told to look at Optec, but they are available only from US for less than $300. I hope to hear from a local guy that manufactures telecompressors and telescope adapters for DSLRs.

From further reading, I gather that the NextGen 0.5 needs 55mm between the sensor and flange distancel to get 18mm an image circle where they are optimised for best image performance.
 
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