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I have a D7000, and I want to buy a D800 sometime next year because of a "foolish idea" that FX cameras yield sharper images over DX cameras. I am quite new to photography and did my research. However, I seemed to have not come against an article/book that says you get sharper images with FX cameras. There is also a "foolish notion" that the bigger the megapixels, the sharper the images I can get, therefore, a 36MP D800 may give me sharper images than the D7000. So I need a little help in deciding if I should go FX or not if I'm only after the sharpness.

I want to shoot for stock photography, and I am astounded at how sharp the images are. I had the idea that if I wanted sharper images than the "soft" ones I get right now, I need to go FX, without having to do post processing. A sample of "sharp" image I see often in stock photos are:

enter image description here

A friend of mine yielded same results on a D5000 before but post-processed. I am only able to get "soft" images on my 80-200 lens, an image like this one: enter image description here

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4 Answers 4

The short answer is that all else being equal FX will give sharper images, at least in the centre of the frame (possibly not in the extreme corners with wide lenses). The long answer is here.

The typical argument that gets rolled out against full frame is that lens sharpness falls off toward the corners, so using the same lens on a crop body avoids the softest parts of the image circle. This is wrong on at least three and a half counts.

  1. Firstly centre sharpness will be 50% higher when using exactly the same central portion of the lens on a bigger sensor. A lens resolves a certain number of line pairs per millimetre on the film plane. When comparing images you should do so at the same resolution/output size. With a 16mm tall sensor a lens resolving 200 lp/mm in the centre will give an image with resolution of 3200 line pairs per picture height. With a 24mm full frame sensor the same 200 lp/mm translates to 4800 lp/ph giving you more resolution hence greater sharpness.

  2. Not all lenses are softer in the corners, this applies mostly to wide angle lenses. By the time you get to 85mm you can expect excellent sharpness across 90% of the frame or more, so with a little cropping you can get better resolution than a smaller sensor even at the borders.

  3. With a full frame sensor you can stop down whilst still achieving the same depth of field as a crop sensor (as to maintain subject size you have to get closer, which reduces depth of field). Stopping down often results sharper picture until you hit diffraction effects.

  4. Finally centre sharpness often has a much more dominant effect on perception of sharpness as the subject of the photograph is often at or near the centre of the image, and it is very rarely in the extreme corners.


Whilst that technically answers your question, I would not advise anyone to upgrade to full frame to fix the problem you are experiencing.

You are really not comparing like for like. The stock image you posted looks like a studio shot, with powerful studio lights allowing a very small aperture to be used. It has also been extensively retouched. That is totally different to a handheld outdoor shot you seem to have taken wide open at f/2.8

Compared to upgrading you'll gain far more in terms of sharpness by:

  • improving technique (eliminating subject/camera motion, meticulous focussing)
  • paying attention to lighting (certain lighting conditions enhance textures)
  • finding the sharpest aperture (usually between f/5.6 and f/11, depends on the lens, experiment!)
  • post processing (there are some advanced techniques, deconvolution, octave sharpening)

Finally, whilst it is good to strive for the best result in camera, I'm afraid you will never get the sharpest possible result without postprocessing - if you shoot RAW (which you ought to do, for stock). When shooting JPEG a lot depends on the camera's sharpness setting (which is incidentally akin to sharpening in post) so beware of this when comparing results from other people's cameras.

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+1. It should be noted just to be factual that lp/mm and l/ph are not directly translatable measures of resolution. Currently, no DSLR on the market offers 200lp/mm. Nikon's new 24.2mp sensors offer a little less than 129lp/mm spatial resolution, while most full-frame sensors offer less (sometimes considerably less) than 100lp/mm. A larger sensor can only offer better sharpness due to greater l/ph if the subject is framed identically, but that sharp image will usually not contain as much fine detail. A 24mp APS-C sensor will capture as much fine detail as a 57mp FF sensor. –  jrista Jul 9 '12 at 17:01
    
@jrista I'm not sure what you mean when you say the full frame image wont have as much fine detail, if you have the same number of pixels and similar framing then your full frame image should contain more fine detail in the centre. Resolution in terms of lp/ph is one of many measures of sharpness, MTF50 being a slightly better one. –  Matt Grum Jul 9 '12 at 21:45
    
Another way to put it would be, if a lens projects a fine element of detail onto the sensor at 4 microns in size, and a 24mp APS-C sensor could resolve it at about that size. A 24mp FF sensor could not resolve an element of detail that small, as it would be limited to around 6 microns or so in terms of the smallest (finest) grade of detail it could resolve. Assuming identical sensor pixel dimensions and framing, yes, a FF sensor would resolve the same fine details larger, and I would expect the outcome to be roughly the same (except for DOF.) –  jrista Jul 10 '12 at 0:18
    
I just wanted to note that lp/mm and lp/ph are not the same, and the former (when measured for a sensor) puts a hard limit on the finest level of detail you could resolve from a physical standpoint. The spatial resolution of a sensor is effectively your minimum CoC. From a portraiture standpoint, its probably a moot point, however from a bird photographers standpoint, a cropped sensor tends to offer more desirable traits due to their higher relative spatial resolution compared to full-frame sensors. I.E. To capture the same detail at the same distance, I'd need a 57mp FF or a 24mp APS-C. –  jrista Jul 10 '12 at 0:20
    
@MattGrum - I'm not meaning to be contentious (or, no more so than is absolutely necessary ;-) ) BUT great care needs to be taken in such comparisons to compare truly equivalent cases. Things like lens settings, actual subject area imaged and more affect the result and there needs to be a clear understanding of what assumptions have been made when making comparisons. I expect that we'd both hold the same opinion if we sat down and worked through the assumptions being made - but it's easy to come to apparently different conclusions due to apparently quite minor differences in starting points. –  Russell McMahon Jul 10 '12 at 1:09

Summary:

A FF (Full Frame)sensor has about a 50% advantage in resolution compared to an APSC sensor for equal sensor pixel density. For meaningful copmpaarison consider the case where the identical FF lens with the same settings (focal length, aperture) is used to photograph the same scene using a FF and an APSC camera, such that the identical scene area is reproduced in the out-of-camera image in each case. In this scenario the FF sensor uses essentially the whole of the lens area and the APSC camera uses half of the lens area, mostly in the centre of the lens. To achieve this comparative result with the same focal length settings in each case the FF user must be proportionally closer to the subject. Adjusting focal length to equalise image sizes invalidates the comparison.

If lens sharpness / quality / contrast / MTF gets progressively worse on average towards the edges compared to the centre, as is the case with all lenses affordable by mere mortals, then a FF sensor is more affected than an APSC sensor, as the FF sensor uses the whole lens image and the APSC sensor uses the higher quality middle portion.

Whether the FF's ~= +40% dpi advantage over the APSC offsets the degradation in lens quality at the edges depends on the lens parameters and aperture and focal length settings. With extremely high quality high cost lenses the FF sensor will be sharper at all locations under all conditions. With more ordinary lenses a FF sensor will be substantially sharper in the centre and less sharp at the edges than the APSC in absolute terms, and especially so in the corners.

As a lens is stopped down the image size remains the same but the outer portions of the lens are not used. This means that the APSC "centre of lens advantage" decreases as aperture gets smaller and a FF sensor should be sharper across the range at small apertures.

The above summary can be confirmed by looking at the Tamron FF SP 70-300mm f/4-5.6 MTF charts at the end of this post. In these Tamrom show the results for APSC & Full Frame sensors and you can scale the curves by whatever crop factor applies. It can be seen (as expected) that at the centre the Full Frame is clearly superior, while at the corners or edges the result varies with lens setting and in some cases, especially at large apertures, the APSC results will be superior across a significant portion of the image.


In the diagram below from here

The black outer circle represents the image area represents the image formed by a FF lens. The blue rectangle = the FF sensor and is almost touching the image circle. Clearly the diagonal corners of the sensor are a lot closer to the image edge than the outer extents of the vertical or horizontal axes are.
The green rectangles = the APSC sensor area are very comfortably inside the FF lens' image area and while the diagonal corners are closer to the corners than the extents of the vertical or horizontal axes are.

enter image description here

Assume that the FF sensor is exactly twice the APSC sensor area and that they both have equal pixel density per area, so that the FF sensor has twice has many pixels. The linear pixel density if square troot of two as great or about 41% higher for the FF sensor. ie the FF sensor has 40% more sensor cells in a straight line to assist it in obtaining the best possible line pairs per mm (or per inch).
For a lens that is equally good across the whole lens area this gives a clear advantage to the FF camera. Very expensive high quality lenses are therefore liable to give a substantially improved result with a FF sensor.

When using a more typical FF lenses on either a Fullframe or APSC camera (same lens in boith cases) with the same subject area filling the frame, an APSC sensor is liable to give a superior result when the lens is "wide open" or at the low focal length end of its range.

Real world lenses tend to have inferior performance towards the edges compared to the centre with results usually but not always increasing with distance from centre. As the FF sensor is using portions of the lens firther from the centre than the APSC sensor is it has its resolution advaantage opposed by lens quality disadvantages. The relative diffierence betwwen the rea os the lens used by the APSC sensor and the FF sensor govern whether the FF gains or loses overall due to its superior resolution.

Also, if lens quality falls with distance from centre, the FF will tend to have a greater variation in vertical to horizontal edge sharpness than an APSC sensor using the same lens, because the ratio of diagonal to horizontal distances as a fraction of lens image diameter are larger for a FF than an APSC sensor. This means that is a lens softens progressively towards the edges the diagonal edges (= corners) will be relatively softer than the middle or the horizontal axes edges than with an APSC sensor. (The same applies to vertical axes edge to corner distances and softness.

When a lens is stopped down somewhat or zoomed in somewhat the FF sensor will benefit more with a typical lens and is liable to about equal results with a reasonable quality lens and superior results with a very good to excellent quality lens.

ie if you can afford Zeiss lenses then use a FF camera :-)

  • I'll use "APSC" below to mean "cropped sensor / Half Frame / smaller than full 35mm sized sensor.

  • I'll use FF for full frame / full 35mm sensor.

A Full Frame camera *with the same lens as a half frame will usually (but not always) produce a SOFTER image.*

  • To allow reasonable comparison assume a FF camera with exactly twice the sensor area of an "APSC" camera and equal pixel density per sensor area, so twice the megapixels. eg a 24 Mp FF and a 12 Mp APSC sensor.

  • For the cameras to be using the same lens, which is what was asked, the lens must be a FF lens. The FF camera will use essentially the whole lens area (by design) and the APSC camera will use a small more central area of the lens. While it is technically possible to make a lens which has close to equal performance across the whole lens area, in practice lenses which mere mortals can afford tend to be softer towards the edges. The FF camera must deal with these edges and include them in the image whereas the APSC camera has them automatically excluded.

  • If a photos is taken from the same position with the same lens and with the same lens settings in each case the APSC image will be of 50% of the area that is seen in the FF image as the APSC sensor is 50% of the area of the FF sensor and it is being exposed to the same optical image by the same lens.) If the FF image is cropped to the same as the APSC image then you have identical image content being processed by equal sensor area and the results are identical for cameras with equal pixel density per sensor area. The results are identical.

  • If instead, the FF camera image is recomposed by either changing the lens settings (eg focal length increase by a factor the crop factor) change) or by moving closer so identical images areas are produced, the FF camera will now have the same image on twice the sensor area. Lines per inch are improved by a factor of 1.414 (because, as the sensor is 2x area, the linear dimensions are square root of 2 larger for the same sensor aspect ratio). This taken in isolation would improve the sharpness. However, the whole lens is now being used. If the MTF (modulation transfer function = measure of lens quality / contrast resolving power / sharpness) is worse by a factor of ~ 1.4 in any location then the lens will be less sharp in those area. So, in all locations it will be liable to be more sharp due to sensor resolution gains but at the edges many lenses will be worse due to MTF dropoff. Note that MTF variation differs (often widely) at differing apertures and focal length settings (for zooms) and certainly between different lenses.

The diagrams below, from here were chosen NOT to cherry pick my point, but simply as the first useful one I found with a web search, and demonstrate the above point. The lens is not an overly marvellous one and is a "DX" (APSC) lens but illustrates the point well enough - probably better than some due to it not being an overly expensive lens. While it is a DX lens, it is legitimate for this comparison to think of it as a FF lens with the APSC sensor using the centre to middle ranges.

At f/3.5 and 18mm the differences between centre/border/extreme border are so pronounced that when used in FF you might think someone had used purposeful softening around the edges.
At f/5.6 and 18mm the border with our example sensors is perhaps just sharper with FF and extreme border is still softer.
By f/8 and 18mm extreme border is still just down on ff compared to APSC.
By f/11 and 18mm the lens as a while is getting softer (still very good in the middle) and the MTF losses even on the extreme border are more than made up for by the FF's lpi gain.

ie with this lens, at 18mm focal length and large apertures the centre would be sharper on FF but the edges would be noticeably softer and by f/11 it would be much sharper in the middle and somewhat sharper at the extreme borders.

The following graphs show results at increasing focal length. At 35mm the APSC is still sharper at the edges at large aperture and by 80mm and above, where the FF is not using the lens edges, the FF is clearly superior.

enter image description here


Here's an example where Tamron have done the work for me. From here
This is for a Tamron FF SP 70-300mm F/4-5.6 Di VC YSD model A005 lens (!).

Graph curve colurs can confuse.
A given lp/mm count has a red curve (radial) and a blue curve (circumferential).

Tamron very helpfuilly show APSC and Full Frame cutoff lines.

Looking at the right hand graph - at 300mm f/5.6 the FF wins easily on radial results.
At 10 line pairs/mm the response is close to straight line radially and not much worse at 30 line pairs/mm. In fact at 30 lp/mm it's superior radially for FF than for APSC before the sensor resolution gain is allowed for.
Circumferentially (blue lines) the FF fades badly compared to the APSC - so much that the APSC will be superior even allowing for sensor increase. Reading Tamron's text they suggest that 10 lp/mm is a measure of contrast and 30 lp/mm is a measure of sharpness. In practice they are both closely related but that simplification is good enough as a first assessment.
Tamron are saying that for circumferential results at 300mm f/5.6 the lens has better to much better contrast with a FF sensor but will have superior overall sharpness with an APSC sensor. Overall = ???
You'd have to take it out and play, but it's not clear that either FF or APSC will be a certain winner overall.

The left hand graph = 70mm, f/4 is less kind to the FF sensor and the APSC has a clearly visible edge overall for sharpness and is similar for contrast (if you decide you can in fact split these two measures). This is not unexpected with the lens "wide open" and using all the glass in FF mode.

enter image description here



Older:

This is because the FF uses all of the lens area and the APSC uses the centre portion. It is hard for a lens maker to maintain equal quality across the lens surface and hardest at the edges. Using the centre of th elens tends to produce a sharper result. In some cases this "rule" is broken and a given lens may work better on a full frame for various reasons, but this is not usually what happens. Matt and I may aappear to disagree on this point but probably don't. Using the same lens as a reference is necessary for comparisons.

APSC cameras are on average much lower cost than FF cameras and lenses used with them are usually lower cost. This is of course up to the user and some people will buy very high quality high cost lenses and use them on APSC cameras, but in most cases a user will migrate to a FF as they buy 'more expensive glass'. An exception may be sports photographers using Canon systems who use Canon's cropped sensor cameras due to their higher frame rate and features which to some target high ISO high speed photography.

The biggest factors influencing softness are lens quality and aperture.

Almost all lenses produce their maximum sharpness when used at less than full aperture. There are exceptions, but they are rare, and cheaper lenses always benefit from "stopping down". Odds are you used a lens with maximum aperture of around f/3.5 and it may have been used at say f/5.6 in that image - maybe not. With a cheaper lens best results are usually achieved at f/8 or smaller aperture. Initailly the image sharpens as aperture is decreased (larger f number). Somewhere, usually in the f/11 to f/22 range diffraction effects start to soften the image again. Some lenses are starting to diffraction soften at f/11 and the very best may get to around f/22. (Some eg Ansell Adams images are up around f/40 but with large format cameras the 'rules' change.)

If you want a sharo image with a cheaper lens you need to experiment to find its optimum aperture. Also be sure that shutter speed is fast enough to not have motion causing softening due to motion blur.

What were the camera settings for your "soft" image. Can you provide a web link to some "sharp" images.


Added:

Your f/2.8 cat photo MAY be very sharp in the original BUT over a very limited depth of field. DOF is a quite different issue to sharpness. When shooting at f/2.8 you either have all the subject in a very shallow distance range if you want it totally sharp OR you not only accept but usually intend that all except a small band of distances will be out of focus. This effect is usually sought after AND will be more pronounced on a FF camera all else being equal. The effect will be reduced with increasing distance to subject, decreasing aperture (larger f number) and shorter focal length.

The examples that you give from istockphoto MAY be sharp all over as you think but are too small (low resolution) to be sure and have been taken with settings aimed at ensuring subject sharpness overall.

Try taking photos at f/8 and f/16 and see what the result is. When focsing pay special attention to getting focus "spot on". If you have a focus magnifier feature in the camera use it.

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I have forgotten the settings for that photo but here is another example of "soft" photo taken at f/2.8 a8.sphotos.ak.fbcdn.net/hphotos-ak-ash4/… –  Ygam Jul 9 '12 at 12:30
    
also a more concrete examples of "sharp" are the "crisp", almost "HD" stock photos available from istockphoto.com, see this: istockphoto.com/stock-photo-20778507-cafe-girl.php?st=65f162b and this: istockphoto.com/stock-photo-20766167-pin-up.php?st=65f162b –  Ygam Jul 9 '12 at 12:32
    
I think you should definitely try lower aperture (at least f/8). –  zacharmarz Jul 9 '12 at 13:07
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@Russell I can say both from experience and a technical standpoint the that the full frame image will appear sharper. Yes it's true that the centre portion of most lenses is better, however due to having more line pairs per picture height your FF image is 50% sharper when using the same central portion of the lens (but may be worse at the edges). Given an image that is sharper in the centre, and one that is softer in the centre but sharper in the corners, guess which one will appear to be the most sharp! –  Matt Grum Jul 9 '12 at 13:10
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Finally as some people will think I'm attacking APS-C and lose all objectivity. APS-C systems are great and have many advantages, but I was specifically aiming for absolute maximum sharpness I'd go full frame every and get a good (doesn't have to be expensive) lens, such as a 50 f/1.8. But you can't put an $80 lens on a $3000 camera! Yes you can, and stopped down it'll produce images so sharp they'll make you eyes bleed. –  Matt Grum Jul 10 '12 at 14:56

Ignoring (for the moment) the many technical details that can get involved here, I think it's worth considering, for at least a moment, the two pictures you posted (the child and the cat).

In the picture of the child, I would posit that the "softness" we see is more from the lighting than anything else. The lighting itself is very "soft", which mostly means it's (effectively) coming from a relatively large area. This is often favored for things like portraits and "beauty" shots, because (among other things) it makes skin look extremely soft and smooth (though it makes a much bigger difference with people who are older and have more wrinkles and such).

The picture of the cat shows a different problem. In almost anything with visible eyes, you nearly need to focus on the eyes to get a picture that people will perceive as sharp. In your case, the cat's whiskers are actually pretty sharp -- but the eyes aren't. Most people will look almost immediately at the eyes and if they're blurry, dismiss the picture as blurry in general, without looking for the fact that it actually is sharp somewhere else. It's possible to put enough emphasis elsewhere to avoid that, but a simple rule of thumb is that eyes need to be in focus for the picture to look sharp.

Bottom line: my guess is that with the right lighting and focus, your current setup is probably capable of producing images you (and most other people) are likely to perceive as sharper than what you've shown here. Ultimately, yes, a D800 is capable of higher resolution than a D7000, but under the circumstances I doubt it's going to make much difference. A higher resolution camera requires even more careful technique to take (anywhere close to) full advantage of its capabilities and will do even more to expose any mistake(s) you might make.

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Unwanted softness in images usually comes from some combination of focusing on the wrong place, overly narrow DOF, motion blur from slow shutter speed, and diffuse, uniform lighting. Try shooting at f/8 with a remote shutter on a tripod in direct sunlight (for strong contrast and fast shutter time) and spot focusing on your subject and I think you'll see sharper pics right away. There are good reasons to upgrade to FX, but that's not your limiting factor based on your example.

Edit: Stock photos also tend to make heavy use of software-based sharpening. See Lee Varis' Photoshop sharpness tutorial for an excellent guide.

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