29 December 2009

Colour & Digital vs Film Resolution

Whenever digital photographers talk about large format, they end up getting obsessed about resolution. Most large format photographers know that this isn’t the main reason the large format experience is so enticing (i.e. perspective and focal plane control, the huge view, the luscious nature of film, etc) but being as it gets talked about so wrongly in so many places, I thought I’d get my own geeky view in. First of all let’s review some of the existing literature on the subject.

Probably the best known is Mr Reichman’s Luminous Landscape essay on the subject, where he compares a medium format camera with a Canon 1Ds, 11Mp, with a Pentax 67. He comes to the conclusion that the 11Mp camera has equivalent if not greater resolution than the digital. Well Michael gets a few things wrong, the most basic of which is effectively down sampling the medium format image to match the resolution of the DSLR.. duh! If you want a rough idea of where he’s going wrong, compare the images where he looks at noise content, you’ll see the reflections are more details and the edges of the window frames show 3d modelling, not blurred anti-aliasing. And this wasn’t using a drum scanner or medium format hires scanner (like a Nikon).

Moving on to someone who appreciates film and digital, hopefully who has a non-biased viewpoint, Ken Rockwell has an excellent article. Ken quotes a 25Mp count for 35mm film but having looked the Clarkvision website who has an enormous amount of information on the subject, it seems that 15Mp is about the limit for line resolution, read more about the capabilities of large format here. The surface area of 35mm film is 864mm2, 645 film is 2090mm2, 6×7 is 3920mm2 and finally 4×5 is 11520mm2 … This makes the scaling factor for 645 is 2.4x, 6×7 is 8.4x and finally 4×5 is 13.3x. So according to the very thorough research by Mr Clark, we should need 199Mp in order to reach the ultimate limit of high resolution slide film. However, bear in mind that various other aspects of the imaging chain mean that this ultimate resolution for 4×5 isn’t quite acheivable.

Charles Cramer does a comparison of 4×5 and a 39Mp digital back on the luminous landscape website where he says that a P45 ‘almost’ equals 4×5. However, his definition of almost isn’t quite almost the same as my almost definition. Take a look at the close up leaf comparison and look at the vein patterns and also look at the texture of the wall in the background. This difference looks to be more than just the difference between maybe 39Mp and 42Mp. It looks to me like the difference between 39Mp and 80Mp or more.

The problem with comparing film and digital is that they are effectively doing two completely different things. To understand why we need to know a little bit about how digital works. Each pixel in a digital camera can only be green blue or red. i.e. the pixels are essentially monochrome. The colour information comes from the fact that the pixels are distributed around the sensor in different colours and ‘clever’ algorithms try to work out the missing information. Below is an example of a bayer array (the name of the sensor pattern that is used in most cameras). Every four pixels has a block of two greens, a red and a blue pixel. The reason for two green pixel is because it was worked out that most textural, luminosity information can be worked out from green data and the colour information can be interpolated from the red and blue data (here for examples). This is pretty cool, it means that for each square of four pixels, you can work out the luminosity resolution using two of them and the colour information from the remainder. Oh! Hang on! That means that you only have half of the pixel count to work out the luminance data and only enough information for accurate colour every four pixels. So a 16Mp is actually an 8Mp luminosity camera combined with a 4Mp hue camera. But digital cameras look like they have more resolution than this! Well, sort of. RAW conversion software then uses various edge detection algorithms to try to increase the apparent resolution. So you probably get some symptoms of a 14Mp camera, maybe the occasional edge that looks like a 16Mp camera but mostly probably a 10-12 Mp camera. So perhaps digital does have some resolution after all. Well… no. What is effectively happening here is digital upsampling. You can apply this sort of edge detection and upsampling to a film shot just too.

A thorough paper from the Stiftung Warentest institute measured the colour resolution response (effectively separating out the saturation component of the picture and working out it’s resolution) and produced the following graph showing how colour resolution falls as the frequency of detail increases. The blue line shows a foveon sensor (which works like film) and the red line shows a bayer mosaic’ed sensor. As you can see, the camera has a half of the linear resolution of the film/foveon equivalent. This equates to a quarter of the number of pixels. You can see the full paper here

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What is very important about that last paragraph, and something that is often missed when looking at comparisons like this, is that the colour resolution of a digital camera is only half of the luminosity resolution. For most instances, this doesn’t matter as the ‘upsampling’ algorithm uses all sorts of cleverness to make sure colour ‘looks’ good. However, it does show up along colour edges and also, most importantly for landscape photography, in high frequency colour details, i.e. colour texture. For example, a distant field of grass and flowers or the tecture of moss on wood. The colour of leaves in an autumn tree, etc., etc. How does this lack of colour resolution affect real world photographs? Well, a good example is in this show below where we compare a 5×4 large format transparency with 5D file. Now I’m not comparing resolution here, the 4×5 transparency is way, way better than the digital so for these purposes the 4×5 is equivalent to the ‘real world’ view.

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This is a pretty extreme example and I plan to do some further tests to see how this effects some more modern cameras (hopefully a test of a phase one back against a large format transparency at some point). Anyway, how does this lack of colour resolution translate into real world landscape photography. Well, for one, don’t expect very fine colour texture to appear correctly in digital images. If you do have fine texture, you will have to live with the fact that your pictures won’t blow up as well as they might otherwise.

As far as how many megapixels does a colour large format transparency have? Well, if we’re looking at a ‘digital friendly’ picture, probably around 80Mp for the typical shot, possibly 100+Mp with a good lens and great technique. With a highly textural picture, we could be talking about 180+Mp. This probably means that for an Imacon scan of a large format transparency, you probably have around 50Mp ish (but lacking in some colour detail).

We can also learn a little from the work at the Gigapixl Project who worked out a couple of things (along with an estimate of 12Mp as the equivalent of a 35mm picture), one of which was that 360dpi is the maximum a typical human can resolve in print. This is handy as it gives us an ‘optimum, eye resolution limited’ print size for various sources. The Imacon scanned print is optimum when printed at 17×21 inches.. which is pretty handy given the size of most affordable printers! But it also means that if you get a good transparency and drum scan it, you can get an eye resolution limited print of 20×25 inches. Of course, we’ve also learned that digital cameras, ‘cheat’ to get a better resolution and it ‘works’ fairly well. So we can do the same with our digital prints to get an increase in maybe 30% linear resolution (for ‘most’ pictures where things like ‘edge detection’ can work).

This post will be continued when I get a chance to play with a digital back… :-) Let me know if you’ve learned anything from this or if you think I’ve missed something.

UPDATE: A good example of raw comparisons and how they go wrong here and here

UPDATE: Ian Scovell has pointed me towards an excellent film comparison with digital on Marco Boeringa’s website which supports the 15+ Mp for a 35mm transparency idea – possibly even more.

UPDATE: I found a good example of the difference between pixel level resolution and bayer array resolution at the Foveon website. Foveon uses a technology where each pixel records all three components. The comparison is here and the background on Foveon is here

UPDATE: Another example for a Luminous Landscape reader that falls into the trap if testing monochromatic, line images where raw converters and bayer sensors are at their best here

UPDATE: A great example of the resolution capabilities of a film camera here. 35mm vs digital M7 vs M9 using Spur Nano Edge black and white film here

Comments (skip to bottom)

12 Responses to “Colour & Digital vs Film Resolution”

  1. On January 2, 2010 at 5:51 pm Simon Miles responded with... #

    With regard to the objectives vs subjectives, Guy’s point is well made. The image, and not the equipment, will always be paramount. But technical quality still matters. I am sure Guy does not use a phone camera for his landscape photography and nor should he. This would simply be inadequate to do justice to the amount of time and effort he expends in making his photographs. I think Tim has it just right when he says that photography is a wonderful blend of art and craft. In my experience, the best photographs usually include a healthy does of both. I was also going to say that I like the fact that Tim is never afraid to get into the technical nitty gritty, but that might make me sound like a geek… too late to worry about that now I suppose!

  2. On January 12, 2010 at 2:43 pm Chris responded with... #

    Hi Tim,

    Interesting reading. Although without sounding rude, there have been many a comparison since the introduction of digital cameras. It hasnt got anyone anywhere, and hasnt got the manufacturers to give us better cameras! Ive been using both for the past 5 years, film for another 5 on top, and nothing beats it. I use digital for my wedding business purely because the processing of film is very expensive now, it renders the use of it completely impractical. The obvious problems I always suffer, especially for landscapes are;

    Lousy definition, colour gradation, seriously bad DOF

    For portraites;

    Very bad skin tones and WB

    Post production can fix some bugs, but certainly not the main one, being, digital cameras are terrible! Digital cameras have done nothing but introduce another wave of things to fix, process, and compromise on. It has NOT made the photography process any easier at all. It may save a few quid on film processing, but the amount of work that goes into a very well taken image technically, is still nothing but a pain.

    Im gutted that Velvia50 Quickload has now ceased production, because thisonly goes to show that digital photography will all that we will be left with. This is terrible! If film disappears, I worry that my love of landscape photography shall disappear with it, because being left to use a DSLR (no one can afford MF backs) would be a compromise too far.

    Its so simple to work out that a 35mm film area will NOT hold the same amount of data compared to a 4×5". NEVER. It either relies on interpolation to make the taken image area bigger, OR, divide the area into smaller and smaller pixels that get redistributed to form the final image. Either way is rubbish, period! Theres no argument with this. Its silly to even have the debate! If and when we have a Foveon sensor that IS the same area as a 645 (which we still do not have, because PhaseOnes is NOT a full frame, despite the stupid advertising of it) then I may consider it. But hey..look..we have the Leica S2…its move forward, but its still well beyond anyones financial reach!

    The fact is, we have no choice. We CANT stop film disappearing, and we HAVE to put up with what the likes of Canon and Nikon give us. We cannot manufacture the ideal digital camera ourselves. LF digital for around £2K is seriously not going to happen…ever…certainly not in my lifetime…so I doubt I will ever benefit and keep enjoy doing what I love.

    The manufactures have simply done and are doing this; distroying and removing film and its cameras and replacing it with technology that gives second hand results. There was never a problem with film…they invented something that didnt really need inventing…however, it has appealled to the greater majority who are NOT photographers, and its us photographers that will take the bullet. Yes there are many digital pro photographers, however, if you compare the amount of photographers with the rest of the world who just take snaps…we are NOT the majority…and the majority will always be who the manufacturers will want to appeal to…its all down to money/greed…like everything unfortunately…

  3. On January 1, 2011 at 5:52 am James responded with... #

    Nice and clear, thanks for the information. I like the blog.

    I switched to film recently when I was trying black and white with my digital, and realised that shutting down the green and blue channels (red filter effect) was losing me a lot of image quality.

  4. On June 12, 2011 at 11:35 am Simon Miles responded with... #

    Hello Tim, an interesting analysis (as always). This is my first visit to the photoblogs after Christmas so my brain is not yet fully re-engaged. I will therefore just post my own observations, based on many years’ working with scanned MF and LF transparency film and about a year working with a 25MP digital camera. I find that both MF film and 25MP digital will print exceptionally well up to A2, but digital capture has the edge in most situations. The reason for this, I think, is the lack of grain. The MF film scans may be higher resolution, but the digital capture is purer, with almost no noise.

    However, I have noticed a tendency for the smearing of fine micro-detail in some of my digital captures. This is the sort of detail, most obvious in a wide view, which is not fully resolved, but where the perception of some detail contributes to an overall impression of sharpness. This is an aspect where film is perhaps (subjectively) better than digital capture. LF scanned film is at least twice as good as the digital capture by my estimate, so perhaps 50MP is about right. However, to my eyes this is only obvious in prints larger than A2. Since I rarely print larger than this size, there is no significant advantage to me, in terms of resolution, from working with LF film.

    For me, the most significant advantage of LF is the ability to use camera movements to control the plane of focus. In certain situations, this allows for a much sharper image than is possible either with digital capture or MF film. Where this matters, the difference is obvious, even in small prints. For this reason, I am interested in acquiring a digital system with tilt/shift lenses. However, this will have to wait until the economy improves! As an aside, there are now some interesting software solutions for digital capture (Helicon Focus etc) but these are not without their problems and certainly not the elegant solution that camera movements provide.

    In terms of colour, I think it is only fair to say that both film and digital have significant problems. You have mentioned some of the problems with digital capture. However, film is not without its problems either, such as the colour casts which affect all films in different lighting conditions and the colour shifts which occur as a result of reciprocity. Contrast and saturation are also more prone to overload when working with colour transparency film, which can create problems at the scanning stage. With experience, these problems can be addressed by filtration etc. Equally, with experience, many of the issues with digital capture can be addressed in the raw conversion. Having worked with both film and digital, I am not persuaded that one is necessarily better than the other. It is just that they have different problems, and so require different techniques to get the best from them.

    Of course, there are other reasons to work with a view camera, to do with the technical discipline, slowing down and adopting a more contemplative approach. These are some of the most valuable lessons I have learned from using a LF camera. Interestingly, however, I have never had any difficulty switching between a view camera and SLR (whether MF film or more recently a DSLR). It seems that after ten years of working with a view camera, I am just incapable of making a snap. This is not always a good thing, as I am absolutely useless at taking family photos!

  5. On June 12, 2011 at 6:15 pm Ian Haskell responded with... #


    LF and digital cameras i.e. Canon and Nikons – probably not a fair comparison yet. Would be interested when you go on to explain about the difference in 12bit-16bit on the CCD back compared to scanning and whether or not calculations prove that these are marketed figures rather than correct figures. Have read some interesting information from Mr. Percys site blog on quality from LF and comparing it to Mamiya 7ii type setups. When comparing will you mention something about sliding backs and also stiching (with cameras such as the new Alpa you can stitch to create a 100mp equiv images I believe – please don’t ridicule me if I’m wrong). I read somewhere Joe was using a P1 back with the new Linhof Techno? Would be interested in seeing some comparison on how he works added to this article. Maybe another video for the 5DM2?


  6. On July 12, 2011 at 10:47 am Ian Scovell responded with... #

    Have you seen this test: http://www.boeringa.demon.nl/menu_technic_ektar100_imagequality.htm

  7. On July 12, 2011 at 1:26 pm Guy responded with... #

    Some good points, Tim, but in my opinion, as one who worked exclusively in LF for years and has made thousands of 4×5 images, the technical discussion misses the point entirely for the reason you articulated so well in this sentence:

    "The problem with comparing film and digital is that they are effectively doing two completely different things."

    No, they do exactly the same thing – capture images; and a good image will rate above a mediocre one for the vast majority of users without regard to the technology that captured it.

    At the end of the day, the success or failure of an image is primarily determined by subjective factors, not measurable ones:

    It’s The Subjectives, Stupid

  8. On July 12, 2011 at 1:32 pm Tim Parkin responded with... #

    @Simon – thanks for the long comment and I totally agree that the reasons that I enjoy large format cover so many other bases that mean I am happy to use a roll film back on my Ebony even though the ‘resolution’ is probably only the same as my 5Dmk2.

  9. On July 12, 2011 at 1:39 pm Tim Parkin responded with... #

    @Ian – I hope it didn’t come across that I was trying to say that 4×5 is better than a canon 5d! The canon comparison was merely to show the effect of the colour data smearing on fine detail. As for getting down to the gritty with the newer digital cameras, I hope to be comparing my 5Dmk2 with Joe’s Techno-Phase monster (and also throwing in a 5×4 and 10×8 picture hopefully). I may have something to say about stitching (although Colin Priors recent podcast says enough I think)

  10. On July 12, 2011 at 1:39 pm Tim Parkin responded with... #

    @Mr Scovell – great link, very interesting analysis.

  11. On July 12, 2011 at 2:20 pm Tim Parkin responded with... #

    Hi Guy – I totally agree with you on the fact subject, composition, etc. are all more important than technical capability of your tools of choice. However, I can’t believe that you ignore those technical capabilities. There is useful information to be had from research such as this but only if you know how to use it.

    I also still think that, given the same subjective factors, the end result will be ‘improved’ through the use of appropriate craft (i.e. knowing what aperture to use, knowing when tilt movements will improve a picture, knowing how much detail you should expect, etc, etc).

    Photography is a wonderful blend of craft and art and whilst the art can be done without the craft and vice-versa. the best results come when someone has mastered both aspects.

  12. On December 6, 2011 at 3:14 pm Len Metcalf responded with... #

    I didn’t get it at first, the differences that is, until someone mentioned that the digital files look more plastic… Then last week I was re-looking at some the comparison images between digital and large format and suddenly saw it. The digital files are definitely more plastic… Tonight’s discussion with a friend also centered around the concept of random film grains or dye clouds verse the square pixel pattern. An unmistakable difference. For another good metaphor look at analogue verse digital sound. Thanks for a great article.

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