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Thread: Are we diffraction limited?

  1. #1

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    Are we diffraction limited?

    Film vs. digital:

    I've started shooting 4x5 instead of digital APS-C and right now I'm going for a "f64" look, mostly with longish lenses for the subject matter (primarily landscape). This means some serious stopping down. Sometimes. On a 300mm lens, f45 and f64 are sometimes necessary...

    Since 4x5 is a little more expensive per shot ($6 vs free) than digital, I'm wondering if, with the next generation of dSLRs, 4x5 will offer no advantages--for my purposes.

    Here's the hypothetical that bugs me:

    Let's say I'm shooting the same scene with a Canon 7D and a 45mm tilt/shift lens as I'm shooting with a 300mm lens on 4x5. It's a pretty deep focus scene so I stop down to f64 with the 300mm lens.

    On 4x5 I'm diffraction limited at 25lp/mm. And 25lp/mm*2 lines/line pair*120mm (width of 4x5 film)=6000 lines of resolution. Thankfully most film can resolve this just fine with acceptable contrast. Unfortunately, since diffraction also softens lower frequency detail than the absolute extinction limit indicates, these are 6000 fuzzy lines of resolution. This is being generous. I've read repeatedly that in real world situations no one ever gets above 25lp/mm, except maybe in the very center, at any stop in 4x5.

    On the 7D, to get the same depth of field, I need to shoot at around f11. Now I'm diffraction limited at 141 lp/mm. And 141lp/mm*2 lines/line pair*23mm (width of 7d sensor)=6486. The difference between this and the figure for 4x5 film is just due to me using imprecise equivalents. It's really the same in both cases. Anyhow, since the 7D has a 5184 pixel wide sensor, we'd get 5184 semi-fuzzy pixels in this case.

    The resolution of both images should be closer to identical.

    With this generation of digital cameras, this problem only arises in extreme situations (300mm at f64, which is used but not often in landscape photography). It's still kind of humbling to think that an APS-C sized sensor matches 4x5 in some real-world cases, though.

    Megapixel counts are still increasing at huge rates. Will the quality advantage of 4x5 over dSLRs still exist in four years? Before you cry foul, there are some lenses which are diffraction limited at f4-f5.6 and live view allows you to implement tilt/shift with per-pixel precision.

    It seems digital sensors and modern lenses are getting so good that lens diffraction, not sensor size, will be the limiting factor in coming years, and instead of medium and large format we'll be dealing with full frame and APS-C digital virtually exclusively as "professional" formats.

    But my non-inflammatory question is this: it seems there's a given resolution limit for a photo, irrespective of format, based solely on FOV and DOF....so why are my eyes so much sharper than a 1:1 print? Maybe they really aren't.

  2. #2

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    Re: Are we diffraction limited?

    Firstly, the lens on the 7D might have a theoretical resolution of 141 lpmm, but realistically you might actually achieve 60 to 70 lpmm. Complex lenses with many glass elements can not be designed and manufactured to theoretical book resolution.

    The relatively simple lenses in large format will be very close to theoretical at the stops commonly utilized.

    Major Point - I have to enlarge the digital crop camera rough 7 or 8 X before I get to 4x5 inches which would be equal to the coverage of a contact print (1:1) for a sheet film camera.

    So the crop camera has a final resolution of 70/7 = 10 lpmm at the "final print equivalent" (and I'm being very generous).

    So digital optical chain has less than 50% of the large format system, but you miss the point.



    The great advantage of the view camera is we can conform the film plane to the subject so utilizing pure DOF to get everything sharp, is frequently not best approach. In almost everycase I can use a larger stop (area not numerical) so my actual resolution is many times your example. My most common f stop is f16 or f22. And I can certainly keep everything in focus in a landscape.

    There is a reason large format is so beautifully microdetailed.

    Being enlarged (roughly) 3X instead of 20x to make a 12x18 is why the tonality is so beautiful. Go bigger, the difference is even greater.

    Gotta think differently with a camera that can adjust the plane of focus.

    bob

  3. #3
    hacker extraordinaire
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    Re: Are we diffraction limited?

    Will the quality advantage of 4x5 over dSLRs still exist in four years?
    Photographs have many qualities. Which qualties were you talking about?
    Science is what we understand well enough to explain to a computer. Art is everything else we do.
    --A=B by Petkovšek et. al.

  4. #4
    Resident Heretic Bruce Watson's Avatar
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    Re: Are we diffraction limited?

    It seems I've said this about a thousand times in this and other forums. There's more to a photograph than simple resolution. A lot more. If all you want to do is count line pairs on resolution charts, go for it. It's your life; do with it what you will.

    But if you want to photograph in the real world, there's a lot to be said for control of the plane of focus through camera movements. And there's a huge amount to be said for image capture area. Especially if you like tonality, and smooth tonal transitions.

    The best reason to use LF IMHO is that it forces you to move slowly, and to think about what you are doing. There is very little "spray and pray" in LF. Interestingly to me, LF forces you to learn how to compose without a camera. You learn how to look at a scene and evaluate it without a camera stuck to your face. You learn how to walk the scene without the camera, to find the right spot with the right perspective to make the photograph; as opposed to "composing with your feet" and that camera stuck to your face. You learn also how to realize your vision on the GG as a final step, not the first.

    LF forces you to actually think about what you are doing; if forces you to understand why you are doing it. It can be the difference between snaps and art. If you have the talent and work your tail off.

    Bruce Watson

  5. #5

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    Re: Are we diffraction limited?

    You have got to realize, each sheet of 4x5 film is basically like a 4x5-sized sensor. To elaborate:

    I have a Canon Digital Rebel XT, which is 8MP. I have a friend with a 10MP point-and-shoot. While my dSLR is theoretically inferior by sheer resolution, the QUALITY of the 8MP is much higher than the 10MP point-and-shoot, mainly because of the sensor-size. The larger sensor affords better contrast and less noise.

    Digital IS a cleaner, more efficient format than film. It's cold and crisp, and has a different feel well-suited to some things, and poorly-suited to others. But if you're just looking for a sheer resolution race, 4x5 black-and-white ISO100 film (especially Ortho film), scanned even on a simple Epson 700, can produce an acceptable "digital negative" between 80-100MP. Drum scan the negative and you have even more available resolution.

    Likewise, I've scanned color 4x5 film, Portra VC160, at resolutions approximating 40-50MP. And HERE is where digital may soon at least catch up—in color MEDIUM FORMAT digital. Those crappy little APS-C sensors will fall apart with noise and poor contrast, even if you could cram 40MP into them! The large Kodak sensors in the Hassys, Mamiyas (sp?), etc., with sensor sizes measured in inches not millimeters, may soon be able to go toe-to-toe with 4x5 color film...

    ...assuming you want to sink $60,000 into an ultra-high-end medium format setup. $60K is a LOT of color film + processing + scanning... Just sayin'...

    Of course, 5x7 is almost twice the effective "sensor size" of 4x5, and 8x10 is four times the size! It will be a long while yet before anything digital costing less than $100,000 will be able to TOUCH 5x7 or 8x10. And I feel as though it's safe to say it will be at least 10 years before digital quality catches up to 4x5 in the sub-$1,200 dSLR market, which is about the price-bracket for some of us "starving artist" types.

    Oh, and let's remember, you also need a MASSIVELY powerful computer and the latest software to deal with these huge digital files. If you print out your 4x5 traditionally, the biggest processing/darkom problem you will have is dust...

  6. #6

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    Re: Are we diffraction limited?

    I can't afford the digital camera I want...I'm just speculating that because digital sensors are so far superior to film over a given surface area, maybe maybe we'll see a massive shift from LF to digital among landscape photographers. Maybe we already have...

    Anyhow, in theory every given FOV/DOF choice is equally diffraction-limited irrespective of sensor size, not taking shifts and swings (which are available, for a price, in small formats) into account. But the sensor can't always match that resolution. For the example I cited (gentle telephoto, extreme deep focus) 8X10 should have just slightly more resolution than a 7D. But it's an extreme example of diffraction limitation...

    Just wondering what the future holds for current large format shooters. The same image quality (diffraction limited) may soon be available with tiny digital sensors, but not the same workflow or aesthetic.

  7. #7
    runs a monkey grinder Steve M Hostetter's Avatar
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    Re: Are we diffraction limited?

    I think large format shooters will continue to multiply...

  8. #8

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    Re: Are we diffraction limited?

    Thomas Kinkade's paintings are show a higher resolution than Van Gogh's. Is Van gogh a worse painter than Thomas Kinkade??? Given that Van Gogh paints in such low resolution does that make his pictures worse and of less value than any painter who paints with more detail? There is much more to a photo than resolution, although just for the record I have a 5d MkII as well as 5x4" I've never done any regimented testing because the resolution advantage of 5x4" is very obvious.

    As an aside digital is not free unless you think that depreciation is not a cost. When I bought a 5d MK1 and in about 18 months it went from a £1600 purchase price to a £300 trade in value. Second hand large format barely depreciates if you buy well. So in the same time period if you spent less than £75 a month on film and developing you'd be ahead of the same digital purchase.

  9. #9
    Cordless Bungee Jumper Sirius Glass's Avatar
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    Re: Are we diffraction limited?

    Quote Originally Posted by Policar View Post
    ... I'm just speculating that because digital sensors are so far superior to film over a given surface area, maybe maybe we'll see a massive shift from LF to digital among landscape photographers.
    Stop there! Fact: Digital sensors are so far inferior to film over a given surface area ...

    By the laws of logic, if the hypothesis is wrong, ...
    where will it lead you??

    There is no reason for me to read any further.

    Steve
    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

  10. #10

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    Re: Are we diffraction limited?

    Okay I knew this would get me in trouble...although I still think digital is better per square millimeter (for color, not black and white):

    1mm^2 from a Canon point and shoot:



    1mm^2 of velvia (with a sharp lens and scan that resolved all the detail):



    Even the most generous estimate puts color 135 film at 25 megapixels or 80lp/mm. The 5mmx7mm sensor on that point and shoot outputs over 10 megapixels, somewhere around 300lp/mm. That's where dSLRs are going...

    I guess I'm just frustrated by the concept of diffraction-limited resolution. You can't do anything about it no matter what format. (Except focus blending?) If my subject matter is going to be diffraction-limited maybe I'm wasting time with a bigger format?

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