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PostPosted: Thu Sep 03, 2009 12:25 pm 
Interesting chart Bob. That would mean I should never go beyond f/5.6 if I want the sharpest possible image on my 50D.

edit: Just took a quick check of some lenses that were tested on the 50D over at Photozone.de. What I see with the MTF charts appears to back up Bob's claims. Lenses seem to peak at f/5.6 and get progressively softer after that.


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PostPosted: Thu Sep 03, 2009 1:18 pm 
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Note that the table is for a perfect lens. In theory, if you have a lens that is bad wide open, you might still get sharper overall results at higher f numbers in the diffraction zone than lower.

As part of my argument against those who cry at increasing MP, we could also take an example from the table. Ever shot at f/16? I have. But by that table, the limit for green is 3MP. Should we ask the manufacturers either go back and stop at 3MP, or not to bother implementing f/16 in lenses. Of course that would be silly. So why do people say the same things at f/11? At f/8? At f/5.6? At times, you may wish to trade off absolute pixel sharpness for more DoF for example or otherwise control light. True, a 18MP sensor might not give you any more detail than a 8MP sensor at f/16, but when you have that f/2.8 tele on the front, who's laughing then?

Off topic: note the diffraction is worse earlier for longer wavelengths (lower frequency) light. Things get rather blurry faster in infra-red...

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Thu Sep 03, 2009 1:32 pm 
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popo wrote:
I believe the Swiss product technical regulation is, by default and for practical reasons, the same as the rest of Europe. But I think the tax regulation is independent, and is classed as outside the EU regarding importing. Again, I'm not 100% sure on that. Definitely worth checking if that is the case, as you wouldn't want to be hit by possible taxes on rentering the EU.


well, it SHOULD be MUCH cheaper than in e.g. Germany, France or the UK since sales taxes in Switzerland are 7.6% compared to around 20% in the rest of Europe...

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PostPosted: Thu Sep 03, 2009 3:56 pm 
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Hi popo,

I agree, which is why I wrote "if you are viewing the image at a more realistic 50% crop where four pixels get binned down to one then f/11 is easily good enough" and also when I wrote, again referring to f/11, that "one might argue that you actually don't need the 7D's 18MP, though I think there's a case for having them available during post-processing". Don't forget that I enjoy 21 MP so I'm hardly a megapixel sceptic. 8)

So far as stopping down significantly into the diffraction limited zone in order to maximise depth of field is concerned I really don't believe you gain anything as all you do is add blur to the image which you can either see if the print (or screen image) is large enough or you can't if it's small enough. I'll try and explain why I take that view.

I'll start with the Diffraction Limit Calculator at the bottom of CambridgeInColour's Pixel Size, Aperture and Airy Disks tutorial. We already know from the table in my earlier post that any lens stopped down to f/8 or dimmer is diffraction limited so far as the 7D sensor is concerned: that's when viewing at a 100% crop. But the Diffraction Limit Calculator shows that, leaving the initial defaults (print size, viewing distance etc.) in place, and entering 18 MP and f/13 and clicking Calculate the resultant print is not diffraction limited. But double the print size to 20" and recalculate and the print is so limited. The fact that f/13 wasn't diffraction limiting in the 10" case is directly due to how much detail the observer can see when printing at that size and viewing from that distance. You can check that by changing the "Eyesight" field to 20/20 and recalculating. The 10" case does then become diffraction limiting.

Diffraction softening has a direct effect on the "circle of confusion" which is how depth of field is derived. You can play the same game as played with the Diffraction Limit Calculator using the Depth of Field Calculator at the top of this page. Accept the defaults already in place and plug in a 35mm lens at f/11 and focussed at 3m and "Calculate". The depth of field is 4.459m. Leave everything the same except the print size which now goes up to 20" once more. Recalculate and the depth of field drops to 1.719m, less than half what one had at 10". The hidden assumption in the calculation is, of course, that the sensor plus lens can provide enough resolution to sensibly fill the print but the principle stands: bigger prints produce shallower depths of field.

The effect of that hidden assumption in the Depth of Field Calculator can be seen if you plug f/64 into the aperture as you'll get an "infinite" depth of field. That's technically correct but with an Airy disk diameter of 85.9µm at f/64, some 20 times the width of the 7D's photosites, the resultant image will be a horrible blur at any reasonable print size, roughly equivalent to the image from a 43,000 pixel (0.043 MP) sensor!

That's an extreme case but I think the argument stands that stopping down significantly below the diffraction limiting aperture for a particular sensor doesn't actually get you anything but a dimmer image. The theoretical increase in depth of field is bought at the expense of blurring the whole image which means the image has to be viewed at a smaller size in order for the sharp bits to look sharp. But if you have to view the image at such a reduced size then you might just as well have left the aperture at the diffraction limit as the perceived depth of field will have increased because of the smaller image. Complicated, innit! :roll:

Just to reiterate, I'm a megapixel junkie but I think it's important to understand the limits physics imposes if you need to get best use of all those pixels for any particular image.

Bob.

P.S. My apologies to those, including popo, who undoubtedly know a lot of what I've just written. There was certainly no intention to try and "teach Granny to suck eggs" but I thought it important to try and justify my argument. Helps me think things through as well. :wink:

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Thu Sep 03, 2009 4:41 pm 
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Bob, in this area I think you have a better handle on the theory than I do. Taking a step back, do stop us if we're going a bit off topic for a 7D thread, as relevant as the theory is to the record setting pixel count for the sensor size.

That's certainly an interesting point raised on the apparent benefit or not as the case may be for stopping well into diffraction zone. My gut feeling from experience says that the improvement to apparent image due to DoF is greater than the loss from diffraction, however that is considering acceptable sharpness of the whole image, not at pixel level. My personal "acceptable sharpness" is more relaxed than 100% pixel peeping level. Still, it is a simple test to perform and I'll put it on the "to do" list as I'm definitely interested in how that works.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Thu Sep 03, 2009 5:00 pm 
Bob Andersson wrote:
The effect of that hidden assumption in the Depth of Field Calculator can be seen if you plug f/64 into the aperture as you'll get an "infinite" depth of field. That's technically correct but with an Airy disk diameter of 85.9µm at f/64, some 20 times the width of the 7D's photosites, the resultant image will be a horrible blur at any reasonable print size, roughly equivalent to the image from a 43,000 pixel (0.043 MP) sensor!


Ah, but it still is an infinite depth of field as the "blurness" would be roughly equal throughout the frame :D.


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PostPosted: Thu Sep 03, 2009 7:23 pm 
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For those of you looking more closely at Switzerland prices, I found this page on the UK HMRC site. This confirms my suspicion that Switzerland is indeed not in the EU, therefore strictly speaking goods bought into the EU are subject to duty and VAT. However, if the land neighbours choose not to enforce that, sure make use of it. But do note the main postal services in the UK (Royal Fail and Parcel Farce) are hitting most eligible packages recently, and you'd still have to do the red/green channel thing in the airport if considering going there directly.

While the above applies to UK, I believe most EU countries have similar systems in place.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Fri Sep 04, 2009 3:45 am 
I can not wait for the 7D to release! I recently bought a 50D+Kit lens for 1250$ two weeks ago and decided to return it and wait for the 7D. I hope that was a good decision.

I mainly enjoy my DSLRs for macro and night shots and figured the new macro lens sensing mode of the 7D seemed interesting as well as the supposed less noise at higher ISO.

Also what do you guys think about the kit lens the 7D comes with? I am talking about the Canon 28-135mm f/2.8-5.6. Is that worth paying 200$ more for? Otherwise I can get just the 7D body for 1500$ USD.

(5DII is 2400$ for me but I think thats a little bit much for me atm. Though I would love to have a FF sensor for my work =/ )


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PostPosted: Fri Sep 04, 2009 5:59 am 
get a useful kit lens like the new 18-135mm IS - don't bother with the 28-135, that's too narrow for a crop frame camera.


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PostPosted: Fri Sep 04, 2009 7:21 am 
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I think the diffraction issue is something many photographers underestimate until they have to deal with it. The resolution war appears not to be over yet and if it continues like that digital cameras will produce duller pictures every year. I recently worked with several 60-year-old b/w photographs and they really fascinated me. Despite their age everything about them was perfect: clear black and white points, gradual shadows, etc. Comparing them to what most modern compacts produce clearly shows that photography - in technical terms - has not evolved as much as people would like to think.

As I recently stated in the Nikon forum I tried out the Nikon D3x, which has about the same pixel density as a 10 megapixel APS-C sensor. With high quality lenses that I went up to f/16 and still got outstanding results on my D3's 12MP sensor, I would not dare going higher than f/11 on the D3x.

I'm not sure about this, but aren't the microlenses on many sensors these days actually making diffraction worse than it already is?


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PostPosted: Fri Sep 04, 2009 7:29 am 
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For a given viewing size, more pixels will always mean more detail under good conditions. When diffraction softening kicks in, you don't get any less detail than with a lower MP sensor, only that any given pixel may contain less information than is possible. If you are looking at a pixel level, then to be fair you must enlarge the film image to similar proportions, where I'm sure you'll see it for all its faults.

In photographic systems, diffraction is an effect primarily caused by the aperture in the lens. Microlenses on the sensor don't impact that, all they do is increase the available light collection at each sensor photosite.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Fri Sep 04, 2009 8:39 am 
I find this talk about diffraction softening very fascinating. I've never thought about such a physical phenomenon before.
As far as I know sharpness in a photo is pretty subjective to the individual viewer.
But aren't one able nowadays to reduce the diffraction softening effect in a digital photo by using edit softwares like Photoshop or the like
or is it considered as an artificial manipulation of the "real" image?


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PostPosted: Fri Sep 04, 2009 9:09 am 
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Hi 3P-O,

As I understand it image sharpening can help a little if the image is just starting to get soft but with a potential trade-off in introduced artefacts. But full on diffraction softening represents a real loss of information. If the information isn't there then nothing can get it back. That's one reason why the Hubble Space Telescope is as big as it is - a smaller unit wouldn't have the resolving power as well as lacking light gathering power.

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Fri Sep 04, 2009 9:26 am 
It really looks like we are reaching the physical limit in terms of MP don't we?

I mean, F/8 is a very common aperture and yet we see in the chart provided by Bob that at such aperture we already can't take full advantage of the D7's sensor resolution. :?

Of course, I'm not going to argue that the full 18MP resolution will come in handy on occasion and that it's great to have such resolution at one's disposal. Still, it seems to me kind of silly to have such a high-res sensor and having to see my pics at 50% zoom max whenever I shoot any slower than F/5.6. What's more, why would anybody want to go around with huge 18MP image files when you have to see many of them at 4-5 (or even 10) MP size?

Having said all this, I must also say the 7D looks impressive, I'm certain that Canon's come up with yet another winner with this one. I'm anxious to see the D300s vs 7D round up.


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PostPosted: Fri Sep 04, 2009 9:27 am 
Thanx Bob. Your answer is crystal clear. 8)

I won't try to capture faint light that has been traveling many billions of light years. :lol:


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