Zoom through wide angle adaptors

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MAGLINK
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Joined: Mar 8 2007

I have been thinking of getting a wide angle adaptor for my HPX301/371 and know that there are two types available zoom thru and non zoom thru.

I think I know this already but thought I would ask you guys as you may already have such things on your cameras.

I presume that zoom through means just that and you can find focus and then zoom with the lens like normal.

The non zoom thru mean that you can't zoom as normal but I assume you can still use the full focal length of the lens, it just means that if you zoom in you have to re-adjust the focus to suit the selected focal length.

Hope that makes sense, I rarely zoom for most shots and want to get a wide angle adaptor for live OB work as the std lens is just not wide enough for some of the work I do.

In time I am going to get a fujinon TH13 wide angle lens but wish to get an adaptor till then.

tom hardwick
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Joined: Apr 8 1999

No Gary.
A wide angle converter is a zoom through, a wide angle adapter is a non zoom through.

The zoom through simply means that you can use your entire zoom range once you've fitted the converter lens, whether you have a 6x or a 36x zoom If you fit a 0.5x converter (say) your 5 to 50mm zoom will be converted into a 2.5 to 25mm zoom with the same aperture.

Generally adapters are single elements, so are lighter and cheaper than the multi-element converters. They won't allow you to use your entire zoom range but then if you wanted to shoot at full tele you'd take off any wide converter, wouldn't you?

The non zoom through adapter I use with the Sony NX5 means that I'm reduced (!) to having a 16.8x zoom (down from the stock 20x). The lens holds focus up until the point where it immediately blurs. The lens speed remains unaltered.

tom.

Alan Roberts
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Joined: May 3 1999

For some lenses, a proper zoom-through converter simply isn't possible, it all depends on the design of the lens.

The instant loss of focus happens when the internal focusing elements bang up against the end-stop of their travel, won't go any further. Lens design is tricky.

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ChrisK
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Joined: May 11 2001

Having recently acquired a RedEye wide angle lens adaptor with a nominal magnification of 0.7, on first use I realised that it gave a wider field of view than a 0.43 zoom-through converter that I had previously used. This should obviously not be the case so I tried out several other wide angle and telephoto converters that were available.

I assumed that for a first approximation and certainly for comparative purposes, the magnification could be calculated from the width of the viewed field with no converter, divided by the width with one fitted. The results were surprising.

1. Zoom-through Wide Angle Converters

Sony VCL-0637A - Stated Magnification 0.6 - Measured 0.63
Sigma - Stated Magnification 0.5 - Measured 0.56
Neewer - Stated Magnification 0.43 - Measured 0.83
Vitalon - Stated Magnification 0.45 - Measured 0.76
'Professional Digital' - Stated Magnification 0.43 - Measured 0.81
'Digital Superwide' - Stated Magnification 0.45 - Measured 0.79
'HD Pro' Fisheye - Stated Magnification 0.18 - Measured 0.43

(The Neewer lens also vignetted to the extent that when zoomed in enough to see no cut-off, the field of view was less than the camera lens itself at full wide angle)

2. Non Zoom-through Wide Angle Adaptors

RedEye Aspheric - Stated Magnification 0.7 - Measured 0.74
Kenco Aspheric - Stated Magnification 0.6 - Measured 0.64

3. Telephoto Converters

Sony VCL 1437A - Stated Magnification 1.4 - Measured 1.32
Sony VCL 2046 - Stated Magnification 2.0 - Measured 1.67
'Vision Optics' - Stated Magnification 2.0 - Measured 1.52
'Digital HD' - Stated Magnification 2.0 - Measured 1.3

These are only checks on magnification with no assessment of image quality. The well known makes are obviously better than the others, and some are best avoided.

In several cases, it can be seen that taking the image areas rather than the linear measures brings the magnification much closer to the expected figure. I wonder whether some suppliers quote area magnification?

Chris

tom hardwick
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Joined: Apr 8 1999

Chris - you say you've got a Kenko aspheric? My single element Kenko is a 0.6x but the barrel distortion it gives suggests it's very spherical

Alan Roberts
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Joined: May 3 1999

It's not the field of view that's 'calibrated', it's the change of focal length. If a lens has a focal length of Fa, then it can be expressed in diopters as Da (=1/Fa). Then the equations get simple.

For a camera lens of focal length Fc, it's dioptric strength is Dc=1/Fc. When you screw in the WA adaptor, the dioptric strength of the two becomes Dc + Da. You can get back to focal lengths easily, F=1/Dc+Da).

If you don't want top invoke diopters, then 1/F = 1/Fc + 1/Fa, or F = (Fa Fc) / (Fa + Fc).

Get my test cards document, and cards for 625, 525, 720 and 1080. Thanks to Gavin Gration for hosting them.
Camera settings documents are held by Daniel Browning and at the EBU
My book, 'Circles of Confusion' is available here.
Also EBU Tech.3335 tells how to test cameras, and R.118 tells how to use the results.

ChrisK
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Joined: May 11 2001

Tom,

I imported the Kenco some years ago. It gives no specific information on the lens, but the white box it came in called it a Kenko 0.6 'Aspheron'. It does give some barrel distortion as does the camera lens at its widest and all the add-ons I tried including the RedEye which is listed as aspheric. I don't know how to tell by looking at the lens.

Alan,

Surely a zoom through converter is afocal which would give it a power of 0 dioptres. I have no means of measuring the focal length of an in-situ lens, but as I said for comparative purposes I would expect measurements across the object (a ruler) with and without the adaptorto be roughly proportionate to the changing focal length. In particular I would expect similarly specified add-on lenses from different suppliers to give similar results, not the widely varying results I am seeing

Chris

Alan Roberts
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Joined: May 3 1999

If it changes the focal length of the combination, then it has a focal length, and therefore has a dioptric strength. At any focal length Fc of the camera lens, the adaptor changes the combined focal length in the way I described. There's no other way it can work.

Surely, the camera lens has some indication as to focal length? I've never seen a lens which didn't.

Get my test cards document, and cards for 625, 525, 720 and 1080. Thanks to Gavin Gration for hosting them.
Camera settings documents are held by Daniel Browning and at the EBU
My book, 'Circles of Confusion' is available here.
Also EBU Tech.3335 tells how to test cameras, and R.118 tells how to use the results.

ChrisK
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Joined: May 11 2001

The Canon camera I was using has a focal length range of 6.1 to 61 mm and applying the same test to this zoom the ratio of object widths at wide and telephoto ends worked out exactly at 10 to 1. The non-zoom through adaptors are concave so have a negative power but i don't have any values. I was only looking at ratios, not absolute values and these ratios worked out about the same on the Canon video and a Sony still camera.

I have done a couple of 'measurements' on the barrel distortion and these work out at about 1.5% for the Canon lens alone at full wide angle, 2.1% with the Kenco fitted and 1.8% with the RedEye. I don't know if these are normal results for this amateur equipment.

I don't normally find this amount of barrel distortion objectionable, but on one occasion when I wanted to correct it, this was easily done in Premiere Pro,

Chris

tom hardwick
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Joined: Apr 8 1999

I tested the aspherical Red-Eye 0.7x and found that although it did indeed have an aspherical surface (and was beautifully coated) it gave too much barrel distortion for me. With sunlight reflecting off the surfaces you can 'rock' the lens in you hand and watch the reflection. The spherical side reflects the light in a direct relationship to your rocking, but the same is not true when the light reflects off the asymmetric surface.

My Bolex Aspheron and my Schneider Kreuznach UWL II are both aspherics andneither barrel distort the image. They're powerful, too, at about 0.55x.

tom.