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Author Topic: xti's sensor  (Read 2757 times)
wmchauncey
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« on: July 07, 2007, 07:08:34 AM »
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Canon rebel xti-am I limited to shooting f/11 or less because of my small sensor size?  If this is true, what other limitations does the smaller sensor place on the shooter?  I have a 17-85mm IS and a 70-200 f/4 ISL.
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John Sheehy
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« Reply #1 on: July 07, 2007, 08:04:44 AM »
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Canon rebel xti-am I limited to shooting f/11 or less because of my small sensor size?  If this is true, what other limitations does the smaller sensor place on the shooter?  I have a 17-85mm IS and a 70-200 f/4 ISL.
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Perhaps you're thinking of "diffraction limits", perhaps, which have more to do with pixel pitch than with sensor size.  Don't take them too seriously.  They really aren't very hard limits.  They just mean that with an extremely sharp lens, which is the biggest candidate to be limited by mainly by diffraction, things will get a hair softer at maximum sharpness when you start going to higher f-stops.  If you have something like an f/5.6 lens with a 2x TC, for an effective f/11, you still will get sharper results stopping down to f/16 to f/20, because the wide-open optics of the lens will probably be far worse than the effects of diffraction at these f-stops, and the bokeh may be a bit ugly wide open as well.
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wmchauncey
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« Reply #2 on: July 07, 2007, 09:23:17 AM »
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Perhaps you're thinking of "diffraction limits", perhaps, which have more to do with pixel pitch than with sensor size. Don't take them too seriously. They really aren't very hard limits. They just mean that with an extremely sharp lens, which is the biggest candidate to be limited by mainly by diffraction, things will get a hair softer at maximum sharpness when you start going to higher f-stops. If you have something like an f/5.6 lens with a 2x TC, for an effective f/11, you still will get sharper results stopping down to f/16 to f/20, because the wide-open optics of the lens will probably be far worse than the effects of diffraction at these f-stops, and the bokeh may be a bit ugly wide open as well.
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I'm new to photography and mostly shoot landscapes and have an interest in razor sharp images (front to back) with a 70-200 f/4 ISL lens.  Are you saying I cannot have my cake and eat it to?
« Last Edit: July 07, 2007, 09:33:57 AM by wmchauncey » Logged

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AJSJones
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« Reply #3 on: July 07, 2007, 03:34:25 PM »
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As John points out, there aren't really precise limits.  However, it is really simple to take a series of shots of a scene you think represents what you're likely to shoot, at smaller and smaller apertures.  f/8 is often the aperture giving the "sharpest" image, but not the desired DoF. Those images can be compared side by side, preferably on a print representing the size you'd normally want to print at.  You'll usually see loss of IQ (as you stop down) more easily at 100% on screen than you will when you print.  Then you'll be in a position the determine for future shots  the best balance between maximum DoF and IQ  for your prints from your lenses
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BJL
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« Reply #4 on: July 08, 2007, 04:03:03 PM »
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What is it you wish to achieve with apertures smaller than f/11 on the Xti? If the answer is "more DOF", then note that f/11 on the Xti gives the same DOF as about f/18 in 35mm format (comparing with a lens giving the same uncropped field of view and judged on prints of the same size viewed from the same distance etc.) And diffraction effects at f/11 on the Xti are the same as at f/18 in 35mm (comparing under the same conditions mentioned above).

The DOF/diffraction limited resolution trade-offs are the same for any format; they just shift to lower aperture ratios with smaller formats and smaller focal lengths.
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wmchauncey
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« Reply #5 on: July 08, 2007, 06:05:21 PM »
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What is it you wish to achieve with apertures smaller than f/11 on the Xti? If the answer is "more DOF", then note that f/11 on the Xti gives the same DOF as about f/18 in 35mm format (comparing with a lens giving the same uncropped field of view and judged on prints of the same size viewed from the same distance etc.) And diffraction effects at f/11 on the Xti are the same as at f/18 in 35mm (comparing under the same conditions mentioned above).

The DOF/diffraction limited resolution trade-offs are the same for any format; they just shift to lower aperture ratios with smaller formats and smaller focal lengths.
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I discovered with more educational surfing, you are correct, that at f/18 the diffraction increases dramatically.  So it's one of those things that I will learn to live with.  thanks guys
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ErikKaffehr
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« Reply #6 on: July 09, 2007, 12:31:10 PM »
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Hi!

If you use a very good lens I think you should avoid apertures smaller than 16. At 16 the loss due to diffraction would be unsignificant, I think. There are essentially two reason to use small apertures:

1) To increase depth of field
2) To increase exposure time (like photographing water)

The limitation is not because the small sensor, but because of the resolution in the sensor. There would be a similar limitation on the Canon 1Ds III (assuming 25 MPixels) but somewhat less on a Canon 5D with 13.5 MPixels. You would need to close aperture a bit more on a full frame camera to achieve the same depth field, so you gain some and you loose some.

Decent lenses have peak performance somewhere between aperture 5.6 and 11, best aperture is normally about two stops down. As far as I understand the lenses you have should be quite "decent".

http://www.photozone.de/8Reviews/lenses/ca...56_is/index.htm

http://www.photozone.de/8Reviews/lenses/ca...0_4is/index.htm

The tests above indicate that maximum sharpness is achieved at 5.6-8 rather than 11. That is very good. These figures do only apply to the plane of focus. using a smaller aperture you perhaps don't achieve maximum sharpness (in the plane of focus) but overall sharpness may be better. I wouldn't go beyond aperture 16 (or maybe 22) for optimal sharpness. Aperture 32 is bad anyway, so bad that it is hard to miss!

Best regards

Erik

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Canon rebel xti-am I limited to shooting f/11 or less because of my small sensor size?  If this is true, what other limitations does the smaller sensor place on the shooter?  I have a 17-85mm IS and a 70-200 f/4 ISL.
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