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How many megapixels do you really need?

Discussion in 'Open Discussion' started by MadMarco, Jan 7, 2015.

  1. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    TLDR summary: 16MP is probably enough for what most people do with their images ***

    I often see discussions related to the sensor resolution on digital camera forums and I've been meaning to write an article on a forum giving my take on the subject.

    I’m not a professional photographer and never have been; I would describe myself as an amateur with a good technical background. What I am is an IT professional with years of experience, a good chunk of which was spend working for a company that provided digital imaging and printing solutions for large corporations to resell to both professionals and end users. My role was to develop and build the back-end server systems that handled the front-end web services, storage and processing of the images. As a result I know my way around (although I am certainly not an expert) a variety of professional printing solutions from wet film minilabs (C41b chemistry), inkjet minilabs through to warehouse printing labs.

    So anyway; onto the main point of the discussion, how many megapixels do you need?

    My first caveat is that I am not considering the usage case whereby the source image has been heavily cropped to produce the final image. This has the potential to change the maths in favour larger megapixel sensors. Secondly I am aware that some printing solutions can be configured to accept and input resolution of greater than 300ppi, it’s just that most aren’t configured this way (you may have a friendly lab owner that might do this for you).

    Viewing on the screen
    For viewing on a screen; the highest resolution commonly available to consumers is "Ultra High Definition" 3840 X 2160 which is roughly 8MP. So unless you are pixel peeping, there is really no benefit going much over 8MP, you only risk increasing noise unnecessarily. This is probably why certain mobile phones such as the Sony Z series 20MP sensors have a default output of 8MP, it also makes a lot of sense of the Sony Alpha 7S and its 12MP output.

    Lab Printed Images
    Printing images is a little more complex, although the maths is pretty simple. The first thing to know is that most lab solutions have a default input resolution of 300ppi. This would mean that any camera image with an output resolution of greater than 300ppi will be down-sampled either before reaching the lab or by the lab during the processing.

    To make the sums simple, let’s consider a 12MP output sensor with a 4/3 aspect ratio. This would produce an image with a resolution of 4000x3000. For a 300ppi input file your maximum image size without a reduction in output quality would be 13.33”x10”. Anything less than 13.33”x10” is fine, down-sampling is not likely to reduce the quality of the output. Anything more than 13.33”x10” will result in a reduction of potential quality of the output.

    If you have a m43 camera such as the Olympus E-M10 with a 16MP sensor, then your maximum output resolution is 4608x3456 which should be good for 15.36”x11.52” before you lose any output quality from the printing process.

    In reality you can push the image much further than this and still get good results. Quite a few years ago I printed a 6MP test image from a Fujifilm FinePix 6900 (this is a 3.3MP sensor interpolated output to 6MP) on a professional inkjet printer at 4ftx3ft which was nicely laminated for me by the lab. The poster looks great to my eyes (I still have it at home); sure if you stand 6 inches away from it you can see the resolution isn’t infinite, but standing a few feet away it looks great. That’s at roughly 60dpi output…

    Home Inkjet Printing
    Just a few words about home inkjet printers, which often advertise fantastic resolutions of 4800dpi or greater. It’s important to realise that an inkjet printer has a fixed number of colours available; typically 5 for a basic model (black and nothing should be considered as colours in this case), with 9/10 colour models commonly available (my Canon inkjet has CYM + Light CYM + Black + Grey + None + Text Black).

    With Inkjet dpi it’s important to remember that there is a big difference between input ppi and output dpi. With an input of 300ppi and an output of 4800dpi, each input pixel will be mapped to an output consisting of 16x16 dots of ink. That’s 256 individual dots of ink, each with a possible of 9 different values in the case of my Canon printer. That’s an argument for increasing the input resolution to 600ppi in order to obtain a better quality output. You might get some mileage out of this, but remember the downside will be reducing the dots of ink per pixel on the output.

    Summary ***
    For Aunty Agnes who prints 6x4 for the family album and views images on a 1080pHD screen, 2-3MP is really all that is required.

    For enthusiasts and many professionals, I suspect that 16MP is more than enough for what most people do with their images. Any more than 16MP will be tech jewellery (I love tech jewellery).

    For professionals, users who print very large format or someone who like to heavily crop images, a higher MP output might be more appropriate.

    It’s worth remembering that the difference between 16MP (E-M10) and 36MP (A7R) is 4608x3456 vs. 7360 x 4912 which equates to 15.36”x11.52” vs. 24.53”x 16.37” at 300ppi input resolution to a printing device. To me this doesn’t look as big a difference as the number would suggest, but that’s how headline specifications and the real world relate.
     
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  2. Paul80

    Paul80 Mu-43 Veteran

    254
    Jul 6, 2014
    `re your last point, its often forgotten that to double a censors size it needs to be doubled in both directions so to double a 16MP censor you need to be looking for a 64MP camera and there are not too many of those around at the moment are there.

    For the vast majority of Photographers 16MP is all they will ever need, my first Digital camera an Olympus E20 used to be able to produce stunning A3 prints and that was only a 5MP camera.

    But when it comes to buying new kit or tempting satisfied owners to part with their hard earned cash a few more Megapixels is all that is needed ;) and we have all fallen for that one at some point haven't we
     
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  3. Itchybiscuit

    Itchybiscuit Photon Mangler

    512
    Dec 10, 2013
    Glasgow, Scotland
    Ivor
    I'm by no means a technical kinda guy so this is more of a question than an answer.

    Does MP size have a direct correlation with sensor size? Can a photographer (of any skill level) effectively use say 20MP in a point and shoot? Or are those 'extra' mega-pixels wasted? It's something I've always wondered about but as I don't own a high-mega-pixel point and shoot, I can't verify one way or another.
     
  4. Venser

    Venser Mu-43 Regular

    48
    May 5, 2014
    The Nikon D3s has 12MP. It's still a workhorse of a camera.
    Here's a good video describing composition trumps all.

    video

    edit - whenever I include the hyperlink it wants to embed the video. Is there a way to keep this as a link instead?
     
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  5. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    That's a can of worms and a half type of question, anyone can take a great picture with any camera. I would however say that any PaS with more than 10MP is probably a waste and would be better served with less MP and improved low light noise handling. But that's just my opinion based on how I use my cameras.
     
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  6. swede

    swede Mu-43 Veteran

    277
    Oct 25, 2014
    I really dont want any more than 16... It was a relief coming from 26mp nikon down to olympus 16. Everything is just faster and easier. But thats me, and i have to accept and deal with the future and that there most certainly Will be more of the pixels in next gen m4/3 sensors.
     
  7. Itchybiscuit

    Itchybiscuit Photon Mangler

    512
    Dec 10, 2013
    Glasgow, Scotland
    Ivor
    I always have my patented 'can of worms' opener at the ready. :wink:
     
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  8. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    Unfortunately you are probably right and the marketing boys will win so we'll get more megapixels and worse noise/DR.
     
  9. MAubrey

    MAubrey Photographer

    Jul 9, 2012
    Bellingham, WA
    Mike Aubrey
    More megapixels doesn't necessarily mean more noise. Per pixel noise goes up but after down sampling, there's little change. Unless you're looking at absurd extremes in pixel pitch, I doubt you'd be able to see much difference at all. Your measurements are good though. 8MP, 300ppi is what DxO uses for their 'print' comparison.

    There's less than half a stop difference in noise between the Nikon D810 and the Sony A7s.
    There's a 1/8th stop difference between the D4s/Df (16MP) & the Sony (12MP).
    There's a 1/4 stop difference between the Sony A7 (24MP) and the Sony A7r).

    The generation of the technology usually matters a lot more: compare the 12MP sensor in older μ43 cameras to the modern 16MP ones, for example.
     
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  10. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    I agree that the generation of technology is certainly a huge factor in output quality.

    I also agree that down-sampling improves per pixel noise, although the relationship is not likely to be linear. Doubling the resolution will half the size of each photo sensitive pixel effectively reducing it's light gathering by a half, in theory increasing the equivalent ISO by around 1 stop (this is not likely to be exactly linear, but should be close).

    You will also likely lose some DR by increasing the resolution which will not be recoverable.
     
  11. John M Flores

    John M Flores Mu-43 All-Pro

    Jan 7, 2011
    Somerville, NJ
    I've done 20"x30" prints with 16 and 24 megapickles. No discernible difference.
     
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  12. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    I'd have no problem believing that.
     
  13. Promit

    Promit Mu-43 All-Pro

    Jun 6, 2011
    Baltimore, MD
    Promit Roy
    Some counterpoints -

    500px-Bayer_pattern_on_sensor.svg.
    This is our first enemy, right here. Each "pixel" of our megapixels is not a pixel. It's a single color dot. The RGB pixels we get out of this array are a fabrication. The actual resolution of this array works out closer to 60% of the measured number. A 4K image, on the other hand, is 24 million dots - 8 million pixels times 3 dots per pixel. There's also the question of moire patterns. The finer the bayer dot grid, the less AA filtration we need and the less susceptible we are to false color artifacts.

    Then there's the cropping issue. 16 isn't so bad if you want to use the whole frame all the time, and that particular aspect ratio always works out perfectly for you. But extra pixels gives us flexibility to reframe and re-aspect in post.
     
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  14. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    It's certainly and interesting point that the input is made of individual coloured pixels and the output in an interpolated mass of pixels, although the basic maths still holds because we only ever see the output after it has been processed by the electronics. Doubling the sensor resolution will still half the size of each photosensitive area, with the negative impact that this might have.

    I did mention in my piece that cropping could have a significant impact, although I suspect that most people only crop the images lightly to correct for rotation issues for instance.

    I'm not saying that no-one should get a 36MP camera, I'm simply suggesting that most people probably don't really need one and would actually be better served with a lower MP camera that performs better.
     
  15. Biro

    Biro Mu-43 All-Pro

    May 8, 2011
    Jersey Shore
    Steve
    I've long said that 16mp is the sweet spot for APS-C and I stand by that as of now. Personally, I'm fine with 10-12mp and a think a lot of cameras with smaller sensors would put out images with much higher quality if they stuck to that - at least until the next exponential step in sensor technology.
     
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  16. MAubrey

    MAubrey Photographer

    Jul 9, 2012
    Bellingham, WA
    Mike Aubrey
    Yes.

    I'm just saying the difference is negligible...as DxO makes extremely clear in both DR and Noise.
     
  17. WT21

    WT21 Mu-43 Hall of Famer

    Feb 19, 2010
    Boston
    I found 12MP very good for OOC pics. I much prefer 16MP because I often end up cropping quite a bit. I very much enjoyed 20MP on my Canon 6D, but the pixels were big enough that extensive cropping was really fine. Not sure how well that would go on a smaller sensor.
     
  18. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    If I could expect all my lenses to resolve well enough to show a difference at higher resolutions than 16MP, I'd be happy to go up to 20MP or 24MP. But I don't think it's required.

    I do think with BSI sensors the quality ceiling at high pixel densities goes way up, as is evidenced by the great 20MP Sony 1" sensor, and the seemingly phenomenal 28MP Samsung APS-C sensor. Sounds like with present gen technology there's room for a 24MP M4/3 sensor with the same sensitivity as our current FSI 16MP sensors.

    The main reason I would crave more resolution is for PP cropping. I think it's a really useful tool to have, and can make a smaller selection of lenses on hand go much farther.
     
  19. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    I'd have to disagree with your analysis of the DxO results, they show that DR reduces as ISO increases for a given sensor. If you take a sensor technology and double the MP count then you half the surface area of each pixel, therefore they gather half the light. In reality it's actually worse than that, due to the gap required between photo-sites and the interconnects although this affect is most likely negligable. This means that for any particular sensor technology you have to increase the amplification for each site which causes a decrease in DR. I'm not saying that this is anything like double or half, but it will have a noticeable affect.

    The issue with using DxO as a source of knowledge is that no 2 sensors at differing resolutions are the same even for the same manufacturer, so you can't compare a 16MP and 36MP Sony sensor because they are unlikely to be produced with the same process.
     
  20. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    Your point about the ability of the lens to resolve the detail is a good one. I can only think that this brings more validity to my argument about how much resolution is really required for your average photographer (if there is such a thing as average).