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Old Nov 6, 2006, 12:45 PM   #1
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Hi all,

This is an interesting one and I can't decide if it can be done and if my ideas hold water or not.

Now I'm not thinking of a wide converter to sit on the filter end of a lens but to go between the lens and the body in the same way as you would with a tele converter.

Here are my thoughts in no particular order (I'm sure diagrams would help, but you will have to stick with me).

I am going to base the assumptions on an APS-C size sensor and will assume this is giving a crop factor against 35mm of 1.5x.

1. With this size sensor each lens is behaving as if it is 1.5x longer which means not all of the image coming into the camera is being picked up by the sensor. By using a lens which takes all of the available image and then compresses it (for want of a better word) then it is all visible to the sensor which means you will get the full width of the lens. I guess this would be known as a0.66 converter (you would need a 0.625 for a Canon).

2. One effect of tele converters is how many stops you lose (f stops), a 1.4x will lose one stop making a f2.8 a f4 and a 2x will lose 2 stops making a f2.8 a f5.6. My question is would a 0.66 wide converter have the opposite effect and give back an extra stop (I am dubious about this as I don't fully know the physics as it has been too long since my degree).

3. Even if you don't gain an f stop back, the great thing would be you would have the same dof (depth of field) as you would expect from a full frame camera so it would make the camera better for portrait work as you could get the background thrown out more while still using the same aperture settings (apart from you would use a longer focal length while being the same distance which is what gives the reduced dof).

OK time to tell me where I am wrong and if any of this can or can't be done. Assuming it can, what is stopping people from making a 0.66x or 0.625 converter, I would buy one as it would make getting a wide lens much cheaper and give back reduced dof.

Looking forward to hearing peoples ideas.

Mark
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Old Nov 6, 2006, 3:23 PM   #2
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Very interesting idea. On the surface, I don't see why it could not be done. But my knowledge of optics is only enough to get me into trouble.
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Old Nov 6, 2006, 5:50 PM   #3
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Teleconverters work by magnifying the center of the image to give the extra telephoto effect. A wide converter would have to reduce the image. Since the image circle on most lenses ends just past the corners of the sensor you would end up with clipped corners or worse.
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Old Nov 6, 2006, 6:05 PM   #4
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Bob Nichol wrote:
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Teleconverters work by magnifying the center of the image to give the extra telephoto effect. A wide converter would have to reduce the image. Since the image circle on most lenses ends just past the corners of the sensor you would end up with clipped corners or worse.
Well yes and no..... only Digital lenses are just past the corners of the sensor but still the majority of lenses are designed for 35mm which is where the crop factor comes from so you can go back to being equal to 35mm before you get clipping otherwise the Canon 5D and 1D users would be very unhappy indeed.
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Old Nov 6, 2006, 6:43 PM   #5
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APS sensor would need a 29mm circle and a full frame would need a 43mm circle. 14mm isn't much to play with but my trig is too rusty to calculate the results.

In your example a 28mm would become an 18mm and there are plenty of those available at reasonable prices. Full frame wides shorter that that get expensive fast and you could get the reduced circle wide for the same or not much more.

The teleconverters are usually used to push a already fairly long lens up into a range where the real lens costs as much as a good used car.
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Old Nov 6, 2006, 7:01 PM   #6
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Not sure if I made it clear why I was asking the question as it's not about price of getting a wide lens. A 28mm lens is always a 28mm lens no matter what camera you use it on the difference between sensor sizes will make the lens appear to be longer than it actually is. Thiswill have affects on the dof as you are using a shorter lens to get the same framing and also at the wide end then the lens I just not as wide as it could be (making the assumption that it is not a digital lens which could never be used on a 35mm or full frame digital to give a full picture).

So basically I am posing the question as a lens can give coverage for the full 35mm then with a 'wide converter' you would then be able to turn your APS-C size dSLR into a camera which performs in a very similar way to full frame? I accept that there will be slight reduction in quality as you are adding some extra glass but you are going to get a bit of this back as you are using the whole lens rather than a cropped portion from the middle.

The other thing I am very interested to find out is if you would get a f stop back by having a 'wide converter' in the way you lose f stops when using tele converters. Part of me thinks youshould but then I can't work out exactly how..... this one is going to need to be answered by someone very clever in the lens dept.

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Old Nov 6, 2006, 7:55 PM   #7
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http://forums.dpreview.com/forums/re...essage=8028220
http://forums.dpreview.com/forums/re...essage=6974406
http://forums.dpreview.com/forums/re...essage=9990454
discussed this several years ago and found that Kodak was supposed to be sitting on the patents. They do exist for TV and cine applications and you do gain a full stop as you suspected.
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Old Nov 7, 2006, 4:15 AM   #8
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Thanks Bob, that raised some interesting points that I had not considered, namely the fact that although a 35mm lens is set up to hit the sensor at a set distance behind the lens the longer the lens the narrower the angle the light will be leaving it so you could not use just a single wide converter.... I knew that had to be something that I was missing in my rational. I would love to be able to get one for my 70-200mm f2.8 though as I would becomea very nice f2.0 zoom!!!
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Old Nov 7, 2006, 5:14 AM   #9
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Any time you throw more glass between the subject and sensor you have the possibility for more distortion. This is not to say the item you suggest couldn't be made of high quality glass, but at what cost? Not that, as you mention, cost is a factor in your analysis, but consider the camera/lens manufacturers. Are they willing to sink a chunk into R&D on a high-quality (and high-priced) item to turn a 28mm lens into am 18mm (while potentially adding distortion) when they may have a comparably priced 18mm lens in their stable already? Trying to sell the board on a redundant item, unless you can show that it will sell profitably, is a difficult task. They are, after all, businesses with an eye on the bottom line.
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Old Nov 7, 2006, 5:20 AM   #10
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After looking at the the links from Bob it seems that there will be very limited reduction in quality as you are using more of the main lens and the bigger gains rather than just making the lens act as if it is on a full frame camera are that you get the effect of a wider aperture which is worth a lot of money indeed!! I think they would have a go if Kodak hadn't tied this down under a patent.

It is well worth reading the links from Bob as a lot of points and questions are raised with many good answers. The patent does seem to be up in about 10 years so things might change then.
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