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Old Sep 18, 2002, 10:11 AM   #1
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Default DSLR's Magnification Factor / Dumb question??

Hi all! Great site!

I have a question that perhaps has been answered before or perhaps is just plain dumb.
I've been reading all the reviews about DSLR's (I'm thinking of sinking some green on a Canon D60...) and the issue of the focal length conversion factor of 1.6x for attached lenses is very discouraging, specially for the wide angle end of the lens spectrum.

My question is, if there are so many Teleconverters for SLR's (1.4x or 2.0x) to multiply lenses power accordingly why there isn't (maybe there is one and I don't know) a Teleconverter for DSRL's designed to 'substract' the magnification factor of these cameras and use lenses at their normal rating? Say perhaps a 0.6x Teleconverter for the D60.

Any comments?

Thanks for your time.

Luis Andrade
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Old Sep 18, 2002, 10:30 AM   #2
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Not a dumb question at all. Probably the reason no one has done it is because of optical problems when doing that with a wide angle of view. Not so hard with a normal to telephoto design. With wide angle you have problems with falloff , vingetting and distortion not to mention increased flare from having more glass to air surfaces. Much harder to shade the front of a wide angle so it's worse than with telephoto.

Fact is existing wide angle lenses already have that built in, it's called retrofocus design. Wide angle lenses in some cases have such short focal lengths that they'd interfere with the normal mirror operation of an SLR. The lens is designed with what is essentially the opposite of an add-on teleconverter but it's built in rather than a separate component between the lens and camera. Improved retrofocus designs might be the salvation of current DLSRs using full size CCDs.
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Old Sep 18, 2002, 11:16 AM   #3
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Padeye's anaswer is incorrect.

The correct answer is that there is no "telephoto effect ' at all. This is a myth like "digital zoom."

The dSLRs simply result in a cropping of the image. If you do not like the resulting cropped image, you can buy a a shorter focal length lens (and waste most of its image too).

Using an adapter lens on a dSLR would be like spending a whole lot of extra $$$ for the big, interchageable lens camera and then thrwing it awya on cheapo optics.
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Old Sep 18, 2002, 11:42 AM   #4
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What most call a disadvantage is actually an advantage if you like telephoto focal lengths. You get 50% more telephoto without a smaller aperture restriction. If you put a 80-200mm lens on the D100 for instance it beomes a 120-300mm lens.

Another "plus" is that any lens that you put on a less than full-frame dSLR gains the benefit of using only the sweet spot of the optics. Lens quality gets less out towards the edges of the optics and these dSLRs only use the central portion.

But it will all be moot as we see more and more full-frame dSLRs appear on the market such as the new Canon EOS-1Ds with sensors the size of 35mm film.

-Steve
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Old Sep 18, 2002, 12:47 PM   #5
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Quote:
Originally posted by Steves
Padeye's anaswer is incorrect.

The correct answer is that there is no "telephoto effect ' at all. This is a myth like "digital zoom."
I'd appreciate if you try to understand my answer before deciding I'm incorrect. I was referring to traditional SLR teleconverters that go behind the lens not in front. Some are of extremely high quality and matched to specific lenses not like the generic tele and wide angle converters that thread to the front of digicam and video lenses. It's not unreasonable to assume that a retrofocus adapter could be made in the same way. This could for example reduce thesize of the film plane image circle of a wide angle lens allowing the full field of view from a 14mm lens on cameras without full frame sensors. As I said this introduces optical problems and a better solution would be to incorporate it into digitial specific wide angle lenses as part of the retrofocus design.

Yes, in a perfect world we'd have full size CCDs but the filter thickness of current CCD designes causes it's own optical problems, particularly with wide angle lenses.
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Old Sep 20, 2002, 10:07 AM   #6
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Padeye

No intent to offed .. forfend such behaviour!

On your other point, am not sure what you mean. Obviously the thickness of the chip is not an issue. The only issue can be the thickness of the detection layer.

I had not though abut this before ... how thick is the detection layer? Obviously one wants a layer as thin as possible.

BTW .. someday, ... there is a digital technology that essentially does away with depth of field. This is called confocal. In confocal microscopy, the image is taken from a scanned detector and math is used to create the image. Since the detector is smaller than the circle of confusion, in effect, depth of field is created mathematically.

Of course such a camera might be rather large!
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Old Sep 20, 2002, 2:37 PM   #7
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Quote:
Originally posted by sparhawk
Say perhaps a 0.6x Teleconverter for the D60.


Luis Andrade
Actually, you should say wideangle converter .625 to be exact, not tele converter which will extend the focal length, and several wide angle converters are available out there for you to choose, just look at the factor, may not get you to the exact number but close and helpful...
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Old Sep 20, 2002, 6:35 PM   #8
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The thickness is from the filter layer on top of the sensor. Would be great if we had a zero thickness electronic detector that acted like fim but it's not on the horizon. Not a problem where the rays are closer to perpendicular to the CCD in the middle of the frame but a problem at the edges, particularly with short rear focus lens. A retrofocus design puts the rear nodal point farther away from the film plane and changes the angle of the light reaching the edges.

Depth of field control is a *good* thing, a creative photographer's friend. I consider it to be the most serious shortcoming of non-DLSR digicams. With larger formats DOF is often a matter of not having enough but with 35mm there is the flexibility to have a lot or a little as the situation calls for. With digicams that have a scaled down frame, the smaller focal length compared to the subject distance gives large DOF even at large apertures.
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Old Sep 21, 2002, 10:10 AM   #9
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Default f8 ain't f64

Padeye ..

This the most useful exchange I have ever had on the forum!

I agree that the lwakc of DOF control (BITH WAYS) is a short comng of the smaller format cameras (I assume this INCLUDES the cropped dSLRS!) as far as the problme with small chips go . But then ... there is the ridiculous abascence of iris control on the prosumer cameras. The 5700, e.g. is limited to ~f8. While this may be the equivalent of f64 on a 4X5, the result is a loss of control of shutter speed!
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Old Sep 23, 2002, 4:43 PM   #10
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Quote:
Actually, you should say wideangle converter .625 to be exact, not tele
Thanks everybody for your opinions on this matter!

Regarding this specific message I know about Wide Angle converters since I own two Nikon Coolpix cameras (950 and 995) and I use those converters for these cameras. These converter lenses sit at the front of the main lens used in the camera. My question was more related to high end prosumer dSRL cameras like the Nikon D100 or Canon D60 and finding a converter, similar to the actual teleconverter used to multiply the focal length of SLR lenses. Those teleconverters sit between the lens and the camera (not in front of the lens). In the case of my question, that nonexistent converter would substract focal length to achieve, in a dSLR, the actual listed focal length of the lenses attached to the camera.

Thanks again,

Luis Andrade
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