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12-10-2013, 08:21 PM   #16
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QuoteOriginally posted by i83N Quote
Is it possible to create an adapter for apc lenses that will "boost" their coverage area to an ff lens level?
As lytrytyr has replied already, a teleconverter does exactly that.

A teleconverter does nothing more than enlarge the image circle (it "boosts the coverage" as you put it).

If you add a teleconverter to a lens that already produces a full-frame image circle then the teleconverter crops out the centre portion, thus effectively increasing the focal length.

If you add a teleconverter to a lens that only produces an APS-C image circle then the same thing happens: The teleconverter performs an optical crop; the only difference being that no information gets lost because the APS-C lens did not produce anything that is cropped away.

This makes it obvious that an APS-C lens does not produce the same AOV (Angle Of View) as a full-frame lens with the same focal length.

To get what you want -- an APS-C lens producing a full-frame image circle but producing the same AOV that a full-frame lens would produce -- you would have to find a way of increasing the AOV the APS-C lens has, e.g., by using a wide-angle adapter in front of the lens.

There is no way you can simply change the size of the image circle of the APS-C lens to get the same image a full-frame of the same focal length would produce (because the APS-C does not capture enough of the scene to start with).

12-10-2013, 10:22 PM   #17
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To all above; what if you combine telconverter with speed booster?
12-10-2013, 11:13 PM   #18
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QuoteOriginally posted by i83N Quote
To all above; what if you combine telconverter with speed booster?
Chuck Norris, obviously.

The speed boosters are only for smaller format milcs and 4/3, I think?

Presumably, a 50mm lens with a 2x teleconverter and the speed booster being .71x would make the lens act like a 71mm lens, adjusted for whatever crop factor the milc has
12-11-2013, 01:31 AM   #19
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QuoteOriginally posted by i83N Quote
To all above; what if you combine telconverter with speed booster?
Considering they do the exact opposite of each other, if you can somehow combine a teleconverter with a focal reducer, you will end up right back where you started: an APS-C lens with APS-C coverage.

12-11-2013, 06:04 AM   #20
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QuoteOriginally posted by narual Quote
Chuck Norris, obviously.

The speed boosters are only for smaller format milcs and 4/3, I think?
No for apc, but mirorless
12-11-2013, 06:12 AM   #21
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QuoteOriginally posted by Cannikin Quote
Considering they do the exact opposite of each other, if you can somehow combine a teleconverter with a focal reducer, you will end up right back where you started: an APS-C lens with APS-C coverage.
Not so, one is making an image circle bigger, other is keeping focal length the same as original lens and reducing lost of f spot or two.
12-11-2013, 06:24 AM   #22
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QuoteOriginally posted by i83N Quote
Not so, one is making an image circle bigger, other is keeping focal length the same as original lens and reducing lost of f spot or two.
No, what a focal reducer like the Metabones Speed Booster does is take the FF image circle and shrink it by focusing it on a smaller APS-C sized area. This concentrates the light, giving the so-called "1 stop boost" in light intensity (the DOF/FOV relationship is the same as FF, because it is using the full image from the lens). Really, this is just marketing speak for what an "equivalent" lens would have to be if mounted directly on APS-C.

A teleconverter does the exact opposite, spreading out the light to a larger image circle, and thus causing a corresponding decreasing in light intensity, giving a drop in effective "equivalent" aperture.

If you put a FF lens on a speed booster and mount it on a FF camera, you will get "black circle" vignetting, because the focal reducer has concentrated all the light into the APS-C circle. A teleconverter will spread this light back out, going back to where you started. You can't get extra light for free.
12-11-2013, 06:34 AM   #23
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Ok ok, but is it optically possible to create teleconvertor without "tele" part, so we can get same focal length with a bigger image cycle?

12-11-2013, 06:50 AM   #24
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QuoteOriginally posted by i83N Quote
Ok ok, but is it optically possible to create teleconvertor without "tele" part, so we can get same focal length with a bigger image cycle?
I assume you mean you want something that can make the APS-C lens somehow cover the field of view of a FF lens. The thing that makes an APS-C lens give a black circle on FF is because the lens does not see anything outside its designed angle of view. The light from the FF FOV never goes through the lens in the first place.

There is such thing as a wide-angle converter, however, which goes on the front of the lens (as in the filter threads). What it does is gather light from a wider field of view, and then the lens behind it will see this wider angle projected by the converter. So yes, in theory, if you put that on the front and mount it on a teleconverter you could make an APS-C lens behave as if it were an FF in terms of FOV on a FF camera. Wide angle converters tend to have terrible IQ, however.

Last edited by Cannikin; 12-11-2013 at 07:09 AM.
12-11-2013, 06:51 AM   #25
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QuoteOriginally posted by i83N Quote
Ok ok, but is it optically possible to create teleconvertor without "tele" part, so we can get same focal length with a bigger image cycle?
No, that is what full frame lenses are for.
12-11-2013, 09:43 AM - 1 Like   #26
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QuoteOriginally posted by i83N Quote
Ok ok, but is it optically possible to create teleconvertor without "tele" part, so we can get same focal length with a bigger image cycle?
As discussed above, if you put a 1.5x teleconverter on a crop sensor lens and stuck it on a full frame camera, you'd get roughly the same image that you'd get without the 1.5x teleconverter on a crop sensor.

Step back from what you want to see, and think about the very basics of how lenses work.

Light comes through a lens in a cone that is X mm in diameter.

A full frame lens has a bigger diameter cone than a crop sensor lens, which is why it's called a crop sensor -- it's effectively like cropping a full frame image down.

A crop lens has a smaller diameter cone which covers the whole crop sensor but won't cover the whole full frame sensor (depending on lens design, it may cover more or less of the sensor. That's also why 3 different lenses of the same focal length may have different viewing angles)

A 2x (to keep the math easy) teleconverter spreads that cone of light to twice its original size.

With a crop lens/crop sensor, you'd get full sensor coverage (plus lots outside the sensor), but only half the brightness you would have gotten before. With a full frame lens/full frame sensor, you'd get full sensor coverage (plus lots outside the sensor), but only half the brightness you would have before. With a crop lens on a full frame sensor, with that 2x teleconverter, instead of getting a circle of light in the middle of the sensor, you'd get full sensor coverage (plus a little bit outside the sensor), but only half the brightness you would have before.

Since most APSC sensors are 1.5x crops, a 1.5x teleconverter would expand the cone of light from a crop lens enough that it would cover a full-frame sensor and give the same basic photo you'd have gotten with the same lens on a crop sensor camera (after you adjust for the reduction in brightness)

The speed booster things are basically the same thing in reverse - they're making the cone of light smaller and more bright. If one existed for an APSC sensor lens mount, and had approximately a .67 multiplying factor, a full-frame lens placed on it, on a crop sensor camera would give the same basic photo that the full frame lens without it would have given on a full frame camera (after you adjust for the increase in brightness).

and no - they're not keeping the same effective focal length and just making it brighter. That would require magic. Both of the companies that make them explain on their sites that it's also making the lens have a wider field of view.

"Metabones Speed Booster Adapter Makes Your Lenses Faster, Wider and Sharper"
"Increase maximum aperture by 1 stop; increase MTF and sharpness; make lens .71x wider!"
"Mitakon Lens Turbo provides 0.726x magnification and increase aperture by 1 f-stop"
12-18-2013, 07:37 PM   #27
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I have found that DA lenses 20mm or longer will cover a full format. I tried it with my 55-300 and my 16-45 on my SF1 film slr. Below 20mm the the 16-45 would vignette some, getting worse as it got wider. I don't think edge sharpness would be very good though. I couldn't do the test on my FF Canon 5D, because the K-mount appeture lever hits the mirror. I guess that I could do it with the mirror locked up, but focusing would be a pain since it doesn't have live view. I couldn't afford the $3,000 5D III. I think that this idea would be a wast of time, because the DA lenses would perform much better on a K5 or K3. I just like to tinker sometime.
12-19-2013, 12:59 PM   #28
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QuoteOriginally posted by Parallax Quote
I think what he is asking is if there is an adapter to enlarge the reduced image circle fo DA lenses to cover full frame. With a teleconverter, say a 2x on a 21mm you would effectively have a 42mm lens. If you could just enlarge the image circle, it would still act as a 21.
No, that's not optically possible. When you "make the image circle larger", you are increasing the focal length. When you are "make the image smaller", you are reducing the focal length. The focal length changes cause the changes in image circle size.
12-19-2013, 01:00 PM   #29
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QuoteOriginally posted by Big Dave Quote
I have found that DA lenses 20mm or longer will cover a full format. I tried it with my 55-300 and my 16-45 on my SF1 film slr. Below 20mm the the 16-45 would vignette some, getting worse as it got wider. I don't think edge sharpness would be very good though. I couldn't do the test on my FF Canon 5D, because the K-mount appeture lever hits the mirror. I guess that I could do it with the mirror locked up, but focusing would be a pain since it doesn't have live view. I couldn't afford the $3,000 5D III. I think that this idea would be a wast of time, because the DA lenses would perform much better on a K5 or K3. I just like to tinker sometime.
The Tamron 17-50 is rather notorious for its large image circle as well. It nearly covers full frame even at the wide end.

Many full frame lenses have even larger image circles. A Pentax 35/3.5 throws an image circle 55mm in diameter, which should be sufficient for even 127 format, or circular 120 images.
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