Lens comparison

[BillW]

Hi,

A gorgeous night last night with the cold clear polar air sweeping over us. Excellent seeing.
I did a quick copmarison with the 12mm f0.8 and the 12mm f1.2.
This was just a quick subjective test as M45 was coming into the fov.

In the lab the f0.8 looked as though it was producing a brighter image in dim light but on the night sky it was a different story!

The f1.2 could cleanly "see" fainter than the f0.8, maybe a magnitude, perhaps a touch less but very noticeable and with better quality stellar images.
This might be to do with the IR correction and aberations we've talked about. However interesting to see the difference in action.

Cheers,
Bill.

[stewartw]

Hi Bill,

Many thanks for carrying out this "back to back" test - very interesting results. Obviously this is the opposite of what we'd expect (would expect the image from a 12mm f0.8 lens to be 2.25 (just under one magnitude) brighter than a similar focal length at f1.2).

As you know I have the following:
1 off Cosmicar / Pentax TV Lens EX ASP 12mm f0.8
1 off Cosmicar TV Lens EX 12mm 1:1.2

and I've ordered one of these: www.ebay.com/itm/43mm-Optical-UV-IR-CUT-filter-for-Camera-Lenses-Canon-Nikon-Sony-Pentax-/181191337861?#ht_6040wt_1161

in order to see if it is the absence of the IR correction that is leading to "soft" images. Can't explain the lower limiting magnitude, unless the optical aberrations from such a fast lens are blurring the fainter stars to the extent that they can be no longer seen ...

In the meantime, do you have AGC on or off - just wondering if the faster lens was transmitting more light to the sensor and the camera was adjusting the gain?

Will advise when I've done some experiments (still awaiting delivery of the filter).

Best regards

William

[BillW]

Hi William,
You had me worried there fora moment however I've just checked and the camera was on manual.
I did another test but had the idea of using a laser pointer to trigger the camera without obscuring the view.

This attachment is the 12mm f1.2:

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[BillW]

...and this attachment is the 12mm f0.8:

When you open them try and ignore the lines from the pointer but I think you'll agree the 1.2 has the edge quality wise.

cheers,
Bill.

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[BillW]

The conditions tonight are not quite as good as last night, a touch of haze as the humidity is higher but a good night nonetheless.

I also tried a BG40 filter. These filters come with a warning that they are hygroscopic. They are not joking! One night with a little dew and the filter surface has crazed and gone partially opaque , it's now in the bin! In practice it made little difference so keep us posted on how the photo UV/IR cut goes.

Cheers,
Bill.

[stewartw]

Hi Bill,

Thanks for posting these - as you say, the 1.2 does have the edge on image quality and limiting magnitude.

Will keep you posted on how my experiments turn out. In the meantime, UFO Capture has an "Optional Trigger" option whereby you can set it to automatically record a clip every x minutes.

Thanks again

William

[stewartw]

Hi All,

Finally got around to conducting the the experiment tonight with the uv/ir cut filter. The filter used was from Rocolax - the link is here: www.uvircut.com/43mm-optical-uvir-cut-filter-for-camera-lens-p-266.html

The camera used was a Watec 902H2 Supreme and the lens was a HS607EX-ASP Cosmicar / Pentax TV Lens EX ASP 6mm f0.75 CS. Capture was UFO Capture, set to auto-trigger once per minute.

20131112_194700 is with the uv/ir cut filter removed, 20131112_194800 is with the uv/ir cut filter fitted.

As you can see, fitting the filter does lead to smaller stars but at the expense of rendering the fainter stars invisible. Not clear at this stage if the advertising claim of "99% Transmission average across the visible spectrum" is accurate of if the sensitivity of the camera is down to sensitivity at longer wavelengths. Probably the latter.

Either way it appears to be of little use (to me anyway) as I'd rather retain the ability to detect faint objects, even if they are a little softer than I'd like.

Bill - am happy to send it up to you if you wish to experiment :-)

Best regards

William

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[stewartw]

And here's 20131112_194800 (uv/ir cut filter fitted)

How do you attach two images to the same post?

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[arp]

Hi Bill, William,

Thanks for the results of your tests.

My fast aspherical Computar lenses each came complete with a rear-cell B&W IR correction lens, giving nice sharp images.

The 6mm f/0.75 Pentax Cosmicar lenses mention a similar corrector in their documentation, but the lenses were sold without the correctors.

Is it worth contacting the original seller to see if they have the correctors in stock?

I noticed this listing

www.ebay.co.uk/itm/B-W-B-W-Lens-Corrector-35-5E-1-2-/301015090122?pt=UK_Lenses_Filters_Lenses&hash=item4615e5c3ca

which might not be suitable.

Alternatively, we could contact Pentax to see if they can help.

Cheers,

Alex.

[stewartw]

Thanks Alex,

I'll get in touch with Pentax to see if they have any more information / part number / stock.

Cheers

William

[arp]

Hi all,

Thanks to William for discussing this with his contact at Pentax.

This was most productive and William was able to order some rear-cell correctors and spacers for our Pentax Cosmicar lenses, including my 6mm f/0.75 models.

I'm pleased to report that they've done the trick, sharpening up the images in a similar way to the rear-cell correctors on the Computar aspherical lenses.

Here's a sample image without the Pentax corrector:-

Alex.

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[arp]

...and here's a sample image with the Pentax rear-cell corrector.

A notable improvement in image sharpness and limiting magnitude!

Clear skies,

Alex.

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[BillW]

Returning home after the festivities last night I had a look outside to get a breath of fresh air.
Turned out to be clear(ish) so decided to try out the f0.85 with the corrector.
The lens still has some spherical aberration but what a difference in the apparent magnitude.
Can see +5 stellar now. Very encouraging, looking forward to getting a decent spell with it on camera.
Cheers,
Bill.

[stewartw]

Hi Bill,

Excellent news - great to hear that you too have seen a significant improvement in LM.

As you know, UFO Capture allows the user to characterise lens distortions during the alignment procedure. I assume this is held somewhere as an equation that that corrects for position based on radial distance from the centre of the image. Just wondering if this equation could be utilised by yourself if you wanted to correct for aberrations to the spectral image ... ?

Best regards

William

[stewartw]

sorry, that should have read ...

" ... As you know, UFO Capture allows the user to characterise ... "