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Old July 23rd, 2009, 09:39 AM   #1
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Telephoto conversion lenses by Sony

Hi,

I'd just like to ask, does the telephoto conversion lenses being sold by Sony for their camcorders can create a more shallower depth of field for my videos? Thanks!
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Old July 26th, 2009, 06:53 PM   #2
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The only way to really get a shallow depth of field is to have a camera with a larger sensor and lenses with a wide aperture. My guess is that you would need at least a 1/2" sensor to even start using DOF as a technique in your videos. Your best bet on a budget is to use a 35mm lens adapter like the Letus or Brevis to simulate using a 35mm sensor. You could also shoot with one of the new Canon SLR cameras that have HD recording. Both the 5D mark 2 (5Dii) and the Canon T1i can shoot video and have larger sensors than normally found in video cameras.
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Old July 26th, 2009, 07:06 PM   #3
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That being said, yes, you will have shallower DoF zoomed all the way in to a longer focal length than a shorter one.
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Old July 27th, 2009, 03:01 PM   #4
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Quote:
Originally Posted by Marcus Marchesseault View Post
the Canon T1i can shoot video and have larger sensors than normally found in video cameras.
Just be aware that the Canon T1i has a limitation with 1920 x 1080 that makes it essentially useless. It shoots at only 20 fps. Even if 24P is your "thing" and you love judder, 20 fps is too much. I despise the product positioning stunts that manufacturers do to sabotage lesser models by removing key functionality to force a customer to move up to the higher model - 5D MKII. There are many things a manufacturer can do to create differentiation between models without resorting to this type of trickery. 20 fps! What kind of game is that?

Since the 20 fps limitation in 1920 x 1080 on the T1i is a deliberate stunt by Canon, we may see a firmware hack to fix this little maneuver. If anyone sees it, please post info about it.
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Old July 28th, 2009, 08:16 AM   #5
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In a word; yes.

With a 2x converter, your depth of field (DoF) will be half that of the lens without the converter.

Also, you will be losing two stops of light, so your iris will open up. If you were working at f11, it will be f5.6.

And if that was not enough, the converter magnifies the central part of what the main objective lens is seeing. So if your main optic is poor, the converter will magnify the defects.

Then... many converters, and this is seen in my Sony 2x, there will be chromatic abberation towards the edges of the frame that increase in intensity as the iris opens to cope with lower light levels.

Unless you spend serious $$ on a multi element, multi coated converter, they are not a good investment in image quality.

(check the Nikon and canon 35mm converter prices as a comparison)

Ben
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Old July 28th, 2009, 08:28 AM   #6
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If you have a 300mm f/4 lens and add a 2x teleconverter, you'll have a 600mm f/8. The DOF is exactly the same. While the longer lens (higher magnification) will decrease DOF, the larger f/number (f/8 vs f/4) will increase your DOF. The net result is DOF is exactly the same.
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Old July 28th, 2009, 10:04 AM   #7
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Hey Jeff and Ben - I'm pretty sure Ian is talking about adding a zoom-through teleconverter to the front of his fixed lens video camera, though he doesn't say so of course.

In which case Ian the answer is yes - you will get shallower dof but only if you zoom in further (natch).

Look at it this way. If you get an acceptable blurry background when filming a girl in full tele so that the top of her head and bottom of her chin are just contained within your frame, adding the teleconverter will allow you to zoom more.

Zooming more (filming at a longer focal length) will mean the background gets even blurrier, but now you only have from her eyebrows to her top lip in frame.

You've also added to the flare levels, lost a little sharpness, added to the distortions, upped the weight and bulk and spent some dosh.

tom.
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Old July 28th, 2009, 11:43 AM   #8
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Hi Tom,

Given your example the teleconverter will increase the magnification and will change the DOF. However, as you point out the camera position will need to be changed to keep the framing the same. When you increase the camera to subject distance, the DOF will increase as well, to the point that when the subject is framed the same, the DOF will be the same.

Also as Tom pointed out, the optical quality will decrease as well as the resultant image quality.
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Old July 28th, 2009, 11:55 AM   #9
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Jeff's right - the picture quality will deteriorate slightly but if this gives you the images you're after, so what?

Zoom lenses are built with something like 14 or so elements, all tightly tolerances to a centre-line axis, so controlling a lot of optical aberrations. Then we come along and bolt on another 4 elements. This alone makes me wince a bit, but the big problem is centering. We attach these elements (all sightly misaligned due to production tolerances) via a filter thread or (just as bad) a plastic bayonet.

More wincing - the filter thread just happens to circumnavigate the optical axis, there's little need for the manufacturer to control its position at all accurately.

tom.
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