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by Aditya

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I asked a question a while ago here about the "sweet spot" of a lens, which led to a really interesting discussion of "sharpness", the physics of light, and general image quality.

1st Question

In the film world, larger format films are known for providing a certain quality of sharpness, dynamic range, and fidelity that surpass the smaller formats.

The end result is clearly noticeable, but difficult to describe.

In the digital world, do larger sensors provide the same advantages over their smaller counterparts as larger format film did to theirs?

Clarification Of The Question, and Caveats (please read)

  • This question is for current DSLRs only, so disregard "medium format" hasselblad type sensors or experimental sensors.

  • The question says "all other things equal", so please disregard the idea of "it's not the tool, it's the photographer", or "it's all about the lenses and the glass".

  • I know "sharpness" and "image quality" are problematic notions, especially when trying to measure them. Please try and think about the relationship of film sizes, and apply those to the question.

2nd Question

I traditionally shoot with film, and love it's "quality". But I want to upgrade from my digital Nikon, my D50.

With the first question in mind, will I "gain" that high fidelity image quality from the D700, or will the newer D7000 suffice (in case size doesn't matter, and the tech is newer after all)

cheers!

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3  
Could you next time create 2 posts if you have 2 questions? Please? –  stevenvh Sep 19 '10 at 10:41
    
it's a two part question no? I think the 2nd would be closed as a duplicate no? –  andy Sep 19 '10 at 11:36
    
The first part is a general question that excludes brand-- does an APS sensor provide a tangible quality benefit over an APS-C sensor? The second is very specific-- will I notice a quality difference between the d700 and the d7000? I don't know the answer to the first, but the second you can measure and test with samples of each for your own particular style, ie, it's a much more easily answered question. –  mmr Sep 19 '10 at 16:19
    
agreed mmr, although the real question I'm interested in the definately the first –  andy Sep 19 '10 at 22:18
    
I find it interesting claim that when using film, size gives any other advantage than resolution. If the emulsion is the same, how can it have more dynamic range? –  Karel Sep 20 '10 at 13:07

3 Answers 3

up vote 20 down vote accepted

Clarification:

Firstly, you are correct in stating that sharpness is subjective, but the ability of a camera system to resolve small details can be measured, and this measurement is strongly related to the perceived sharpness. As you image black and white lines that get closer together they will eventually merge into a grey blob. By measuring how close lines can get before a losing a certain amount of contrast between the black and white, you get a measurement of sharpness. Expressing the distance between the lines relative to the picture height removes the final output size from the equation, this the sharpness measure is maximum line-pairs (i.e. one white one black) per picture height that have enough contrast to be distinguished.

1st question:

All things being equal (same lens, subject, settings, final output size) the larger sensor will produce an image with higher peak sharpness in the centre of the frame, and higher average sharpness across the fame. It may have softer corners than the smaller sensor image as the sharpness of the image circle projected by the lens can drop off significantly the further you move from the centre, and the larger sensor captures the extremes of the lens image circle.

If you take for example the 8mp Canon 30D and the 22mp 1DsMkIII, the 30D has almost exactly the same pixel size so it's image is akin to cropping the middle out of the 1Ds image. As you can imagine, cropping the middle out then blowing the image up to get the same output size, you will have to upscale and lose sharpness.

If you take for example the 12mp 450D and 12mp 5D, the smaller 450D has the same number of pixels but they are much smaller and together take up less of the image projected by the lens so it's sort of like blowing up the centre of the image projected by the lens, again losing sharpness. The bigger pixels of 5D sensor effectively view the lens image from further away and are less demanding of the lens resolution and will thus produce a sharper image.

You can see this trend for larger sensors to produce sharper images reflected in the dpreview lens tests, which allow you to see the MTF (a measure of lens resolving power, or sharpness) of the same lens on different sensors:

http://www.dpreview.com/lensreviews/nikon_50_1p4g_n15/page3.asp

Compare the test results of the Nikon AF-S 50mm f/1.4G lens for the DX (1.5 crop) format compared to the FX. At f/4 the DX (smaller) sensor resolves 1500 line pairs per image height in the centre, and 1250 in the corners. The FX sensor, at the same aperture resolves over 2200 lp/ph in the centre and over 1500 in the corners!

2nd question:

For your second question increasing the number of megapixels for the same sensor size will improve sharpness (then thus image quality). However the improvements get smaller as the megapixel counts increase, thus at some point you need to move to a larger sensor. It's hard to say whether the D7000 will meet your sharpness needs, you'll have to try and borrow one!

There is another factor which makes much more difference than sharpness between sensor sizes and that is depth of field. Again, all things being equal (most importantly framing) you will get shallower depth of field with a larger sensor, around 1.3 stops. For most people this is a major reason for using a larger sensor 35mm DSLR. To get the same image as a FF camera shooting at 50mm f/1.4 on an APS-C (like the D7000) you would need something like a 30mm f/0.9, which doesn't exist!

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+1 genius! great answer –  andy Sep 19 '10 at 7:50
1  
Thanks, if you feel this has answered your question, could you mark it as accepted by clicking the check box icon next to the question! –  Matt Grum Sep 19 '10 at 15:02
    
++ Excellent answer. –  jrista Sep 19 '10 at 18:57
    
haha, I just like to wait a while... it encourages others to answer which enriches the knowledge I think –  andy Sep 19 '10 at 22:19

Sharpness, or perceived sharpness, is influenced by a number of factors, but the effects of the sensor on that does depend a bit on some details around them. Typically, a full frame sensor has larger photo sites than an APS-C sensor which are, in turn, larger than a point and shoot. So, if we use this as the basis for discussion, understanding that there are exceptions to the "typical" case I described, then:

The larger pixel size provides some advantage in noise control which can assist in perceived sharpness (less detail loss), but it's bigger advantage is that it takes a smaller aperture to become diffraction limited as the increasing size of the airy disk can be contained inside the photo site for longer. For example, a Canon EOS 1D starts to become defraction limited at f/16 while a Nikon D70 hits it at f/11. Now, bear in mind that the physical dimensions of the sensor don't specifically effect this, it's simply a case that usually, though not always, the bigger sensor has bigger photo sites in current cameras. See this Cambridge in Colour article for much more detail.

Another factor in sharpness is contrast. The bigger sensors usually, though not always, have greater dynamic range and that usually leads to better contrast. If the contrast is better, the sharpness is better. Of course, this depends on the subject matter and if there is little to contrast in that, it won't really make a lot of difference what camera you use.

Now, on the downside, is that smaller pixel sizes may capture more detail at the same focal length, in effect, magnifying. This is where the crop factor, or focal length multiplier, comes into play. A 100mm lens on an APS-C (1.5) crop factor has the same angle of view as a 150mm lens on the full frame. Within that angle of view the APS-C, if not diffraction limited, will have captured more pixels worth of data than the full frame which gives it more detail and the potential to be sharper for the same region of the image assuming you shot using the 100mm lens on both. Which, by the way, is sometimes a disadvantage if you want shallow depth of field. It's one of the reasons that many struggle to get nice bokeh out of a point and shoot (though it can be done).

However, I started this reponse with an assumption of the "typical" condition of sensors, but the technology around this is improving dramatically and constantly. Sony (which generally supplies Nikon and Pentax) and Canon are making great strides in sensor technology and producing higher density APS-C sensors with improved dynamic range and substantially less noise. For example, cameras such as the Pentax K-x (Sony sensor) are getting rave reviews for their low-light noise control being in the same ballpark as a full frame sensors. So, while a full frame camera may some advantages in this area today, I don't think that's necessarily going to remain a constant truism. Now, these are "pro" cameras and will have a number of other features to go with them that others won't, so those may be compelling anyways.

On a side note, you excluded medium format digital, but they have one additional advantage over their full frame cousins from 35mm: no anti-aliasing filter. The filter helps with moire, but also introduces some blur. Users of this format would rather deal with the moire in post processing in order to get maximum sharpness on the sensor.

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+1 great answer john, thanks! –  andy Sep 19 '10 at 22:20
    
@andy: You're quite welcome. By the way, if you can afford the price, I'd go full frame. In the end, it's going to be more similar to film in handling. –  John Cavan Sep 20 '10 at 1:01
    
hey man, just re-read you answer, I just skimmed it first site, again, excellent stuff, especially the Medium format stuff extra –  andy Sep 20 '10 at 1:03
    
thanks dude... yeah, I think I will, moving from film to the pretty modest D50 was a real struggle... and it still doesn't feel right. before I splash out on a D700 I'm thinking of getting my first film Medium Format! –  andy Sep 20 '10 at 1:06
    
If you're looking medium format, you may want to look at Pentax. The 645D is headed to North America and is slated to be around $10,000 for a 40mp sensor in a medium format with dSLR features and body structure. In addition to some new lenses from Pentax, there's often quite a few lenses available through Craigslist, though I expect a price spike when the 645D hits here. –  John Cavan Sep 20 '10 at 2:07

1st question

What do you mean by "a larger sensor"? Larger in size, or larger in pixels?

  1. Larger in size. As the pixel size increases its sensitivity gets better, which means less noise at the same ISO-setting. Sharpness is the same.

  2. Larger in pixels. In this case the effect is reversed: you get noisier pictures, because pixels are smaller. At the same time sharpness increases, though you may have to shoot RAW instead of JPG to take advantage of this.

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3  
I disagree with point 1, if you use the same lens, and keep the same final output size then sharpness is not the same. The larger sensor will produce a sharper image (in terms of absolute resolving power) in addition to greater sensitivity - see the comparison between the Nikon 50mm f/1.4 on a full frame and APS-C size sensor I posted: dpreview.com/lensreviews/nikon_50_1p4g_n15/page3.asp –  Matt Grum Sep 19 '10 at 15:12
    
Q higher pixel density does note always equate to more noise. It can, but not always. For example, a Canon 5D mk II is going to be a lot sharper than a Canon 20D, even with the same lens etc. –  Rowland Shaw Sep 19 '10 at 16:24
    
Good answer for "all other things being equal". Pithy. I like it, and I would have said the same. +1 –  AJ Finch Sep 20 '10 at 12:46
    
@Rowland: sharpness and noise are two different things. You van have a noisier and less noisy picture while both being as sharp as the other. Noise causes deviation from the real color in individual pixels. –  stevenvh Sep 20 '10 at 13:55
    
@Matt: couldn't it be that less noise is perceived as being sharper? After all, if the number of pixels remains the same corresponding pixels in both sensors see the same color. –  stevenvh Sep 20 '10 at 14:30

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