They don’t.

Another post that I wrote just before this one (“Why Your 21MP File “Looks Softer” Than Your 12MP File at 100%“) explains why pixel-peeping* photographers might imagine that cameras with greater photosite density (e.g. – “more megapixels”) might produce “softer” photographs, based on what they observe when they compare 100% magnification images on the screen. These cameras do not produce softer images – the results will either be equal to or better than those from lower photosite density cameras in this regard. You can follow the link to read the original post.

A reader wrote and suggested that perhaps the images from the camera with greater photosite density really are softer, but the cause is a greater susceptibility to diffraction blur or motion blur.

No. Neither is the case.

These are two additional misconceptions that can be fed by (yet again!) pondering 100% magnification crops on the screen without thinking through the actual (non-) effect of what you see there when it comes to actual photographs.

Diffraction blur — Diffraction blur is an optical quality, not a sensor quality. (There is a component related to sensor/film size, but that is not relevant here since we are comparing cameras using the same format.) As you stop down a lens, diffraction blur begins to diminish the resolution of the image projected by the lens, and a “point of light” expands to cover a larger area of the sensor. On a cropped sensor camera you might become concerned about this if you stop down much past f/8 – at least if you are very critical about resolution and perhaps intend to make a very large print. On a full frame camera you might become concerned roughly two stops later, perhaps beyond about f/16. (The actual point at which diffraction blur might become a concern as you stop down varies depending upon several factors that I won’t go into here, including your personal notions concerning how much diffraction blur is too much. In any case, the differential between cropped sensor and full frame will remain the same.)

The idea that diffraction blur increases as you increase photo site density (i.e. – “more megapixels”) is wrong. It is rooted in the very same misunderstanding that I described in the original post – failing to take into consideration differences in the way that images are seen at 100% magnification on the screen versus how they are seen in real final images such as prints or on-screen jpgs. If you look for diffraction blur in a 100% magnification crop from a 21MP image on your computer screen and then look for it in a 100% crop from a 12MP image, the diffraction blur will appear to be “larger” in the first case than in the second.

But it isn’t.

Imagine some very gross diffraction in which (to use loose terminology) the “blur” from diffraction is 1% of the width of the frame. (This would be absolutely horrible blur, and it is far beyond what you’ll see in the real world – but 1% is a nice convenient value for this explanation.) Since the lens produces the blur, not the sensor, this “1% blur width” will be the same whether the image is projected onto a piece of 35mm film, a 8 MP full frame sensor, or a 21MP full frame sensor. In fact, for the thought experiment, imagine that you make photographs with all three media. Now make three prints at whatever size you prefer – let’s say 16″ x 24″ for the sake of having a real size in mind. The “1% blur width” will be 1% of 24″ in all three of the prints. In other words, there is no difference in the amount of diffraction among the prints due to different recording media or different photosite densities.

To make this real, here are some factual statements about diffraction blur and cameras with the same size sensors but different numbers of “megapixels.” Assuming that everything else is equal (same lens, same aperture, same subject, same sensor format, etc), the following are true:

• If you make 20″ x 30″ prints from a 12MP image and a 24MP image, the two prints will have exactly the same amount of diffraction blur. (The 24MP sensor may render the optical blur a bit more accurately, but it does not change the amount of diffraction blur.)
• If you make 40″ x 60″ prints from a 12MP image and a 24MP image, the two prints will still have exactly the same amount if diffraction blur.
• If you make  a 20″ x 30″ print from a 24MP image and a 40″ x 60″ print from a 12MP image the print from the 12MP image will have “more” diffraction blur.
• If you make a 20″ x 30″ print from a 12MP image and a 40″ x 60″ print from a 24mp image the print from the 24MP image will have “more” diffraction blur.

If all else is equal, when it comes to the visibility of diffraction blur in prints it is print size and not sensor resolution that is the variable.

Motion blur — The situation with motion blur is essentially the same. The crucial issue is over what portion of the image the blur takes place. If it is, say 1/10,000 of a frame width the blur will be 1/10,000 of the print width no matter what number of photosites you use – ignoring for a moment the fact that no current full frame DSLR can resolve 1/10,000 of the width of the frame. But let’s say the motion blur is grosser – perhaps 1/100 of the frame width. It will be 1/100 of the picture width in all three cases, independent of the film/sensor characteristics.

The place people seem to get confused is, yet again, when they look at 100% crops on their monitors and, again, forget that they are looking more closely at a smaller portion of the image when they view the image from the high photosite density camera. The blur would, indeed, cover a very slightly larger percentage of their monitor width but this is exactly counteracted by the smaller portion of the image they are viewing.

But, one of them says, “the blur” (whether motion or diffraction caused) “covers more pixel widths!” Right. It does. But that doesn’t change the fact that the size of the blur relative to the size of the print or other final display is no larger at all. That “pixel width” will cover a smaller portion of the print!

To those who imagine or suggest that higher photosite density creates a “diffraction-limit problem” or “need for greater camera stability to control blur” (some have gone so far as to suggest the use of higher shutter speeds on higher photosite density cameras!), there is a very good chance that you are accidentally thinking of something as a problem when in reality it is either neutral or an advantage. You will get exactly the same amount of difffraction blur or motion blur in a print of a given size with any of the available photosite densities. In the best case you might get slightly better resolution by opening up a stop or so if you are using a very good lens and you are extremely careful about focus and camera stability and if you make a really big print.

In terms of diffraction or motion blur, when it comes to the photographs you produce with your camera there is no bad news and perhaps at least a bit of potential good news when you move to a camera with higher photosite density.

Notes:

• This post compared 12MP to 24MP sensors. The same principles hold true for comparisons of sensors with larger or smaller numbers of photo sites, too. (As I edit this in August, 2015 I now use a 50.6MP camera!)
• I am not proposing that everyone needs higher photosite density, nor that everyone will seen improvements in their photographs if they move to a higher MP camera. There are many other factors here than might negate advantages, but few if any that are real disadvantages.)

Notes:

• Some readers may arrive here from links at DPReview. I was reply to one of the subsequent discussions there, including one criticizing this article, but DPReview has apparently banned me from their site for unknown reasons. (Their message is: “Due to abuse, you have been banned from posting. Please contact us if you have any questions.”) Repeated attempts to contact them to inquire about this were fruitless — no one ever replied to my polite messages, so I gave up. Ah, well…
• “Pixel-peeping” is often taken to mean engaging in the obsessive comparison of slight and frequently inconsequential differences between images from different pieces of gear and then assigning the supposed differences far more significance than they actually deserve in real photographs. In extreme cases, the obsessive comparison and the subsequent discussions/arguments about the minute differences take on an importance for some pixel-peepers that is orders of magnitude greater than their interest in making photographs. It brings to mind comparisons such as a would-be chef who obsesses endlessly about the perfect way to sharpen knives… but who rarely, if ever, actually uses the knives to cook –  or when he does cook, the goal is primarily to create a dish the demonstrates the excellent cutting ability of his knives. (Considering and understanding technical issues around photography equipment and processes is an important and useful activity, but mostly when this leads to the creation of more interesting and effective photographs.)

G Dan Mitchell is a California photographer and visual opportunist. His book, “California’s Fall Color: A Photographer’s Guide to Autumn in the Sierra” is available from Heyday Books and Amazon.
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