The exception to the digital cam scam (a.k.a. When megapixels actually matter)

Last week, I pointed to David Pogue's proof-points (registration may be required) that the lengths to which digital camera vendors are going to sell you more megapixels is mostly bull-wocky. I took what Pogue did, and ran with it, proving why most people can get away with 4 or even 3 megapixels.

Last week, I pointed to David Pogue's proof-points (registration may be required) that the lengths to which digital camera vendors are going to sell you more megapixels is mostly bull-wocky. I took what Pogue did, and ran with it, proving why most people can get away with 4 or even 3 megapixels. In fact, many of the pictures I take for ZDNet with my 6 megapixel Nikon D70 are taken at its 1.5 megapixel setting! That's because most people (including me), in most cases, will never get to use all the pixels that cameras in the 6-plus megapixel range generate when the shutter button is pushed. Most cases.

Then, there's the primary exception to the rule: when you're using a digital photo editor like PhotoShop or iPhoto to crop.  I'm not talking about a trim the edges crop. I'm talking about when you're going to do something like what I've done below: take an image that started at 3008x2000 pixels (6 megapixels), extract from it a cropped image that's only 2.5 percent of the original image's size (469x312 pixels), and share it with others as a close-up or an enlargement. Shown below are two images. The first is a slightly touched-up version of a 6 megapixel photo that has been resized to fit within the column borders (470 pixels wide) of ZDNet's blog page design. This photo is of an immature bald eagle from about 150 yards away. I took it yesterday near my home in northern Massachusetts with my Nikon D70 and a 500mm (effectively 750mm) telephoto lens (more on the effectively part in a moment):

Unfortunately, this young eagle is slightly out of focus and although I tried to sharpen it up with Photoshop, neither photography nor Photoshop are my strong points. As displayed here, this photo demonstrates one of the most important points that I made during my last post on the megapixel myth issue: Like most photographs that most people take with most cameras, most of the time, this was a 6 megapixel image with no cropping that was simply downsized to a 469 pixel wide x 312 pixel high image (to fit comfortably in the blog) for sharing purposes, the result of which was the loss of over 97 percent of the original pixels. 97 percent!!! Gone!!! Thrown out!!!

But to demonstrate the point about cropping (and when megapixels do count), I went back to the original 6 megapixel image and extracted from it (cropped) a 469x312 "frame." This way, when it's reproduced here on the blog, there's no loss of pixels. In other words, if you were to view the original image on your computer's screen at 100 percent of the image size, this is how big the eagle would be (and the entire image probably would not fit on your screen. As you can see, the net visual result of the cropping (and presentation in a frame that's the same size as the frame above) is an enlargement:

There are other examples below. The key here is that the more megapixels I have to work with, the closer up I can get without pixel loss. For example, if my camera was a 13 megapixel camera (more than twice the pixels of the camera I currently own), cropping a 469x312 image out of one of that camera's original photos would probably fill the entire frame above with just the eagle's head (or maybe less) with no loss of detail. 

So, the major exceptions to the megapixel myth are when you're going to be be cropping, enlarging (or producing the equivalent effect of enlarging as I just did above).

So, what does it mean in terms of buying digital camera? Well, for starters don't expect to do what I just did above with a typical point and shoot (P&S) camera. You can obviously still crop a photo that's taken with a P&S. But, in order for you to successfully zero in on 10 percent or less of the original image (as I did above), one of the following two things would need to be true (a) you were standing close enough to the subject for 10 percent of the original image to actually be useful or (b) the camera has a really good optical zoom on it.  

In my case, I'm using a Nikon D70 digital Single Lens Reflex (dSLR) camera. While most dSLRs are very good at pointing and shooting, the key advantage (at least to me) is the lens interchangeability. My reason for going with the Nikon D70 was because I already had an investment in Nikon mountable lenses (including the Sigma 170-500mm telephoto used to take the above pictures). At the time the D70 was out, it was Nikon's only consumer-grade dSLR (today, Nikon has others). But the bottom line given my existing investment in lenses was that I couldn't go to a non-Nikon body.

Matching my existing lenses with the new D70 produced another unintended consequence. Give or take 10 percent, when lenses intended for use on a film body are attached to a digital body (as I was doing), you end up with a 1.5x effect focal length. So, in the case of a standard 50mm lens that comes with many dSLRs, 50 x 1.5 = 75. In other words, the same lens that's a 50mm lens on a film body is a 75 mm lens on a digital body. Why is this? It's pretty simple. Lenses designed for film bodies expect to be shining an image on a 35mm sized frame of film. If instead, you use a digital body and those images are shined onto a digital sensor that's much smaller than a 35 mm frame of film (which they all are), then some amount of the image (from the outside edge in) will go right past the outer edges of the sensor. Put another way, the sensor isn't big enough to capture the entire image that the lens is casting upon it. The net result is a zoom because, to the human eye, it looks like the camera is zeroing in on something in the middle of the image (and, physically speaking, it is).

This is why a 500mm film lens in a digital body effectively produces a 750mm telephoto effect. In my case, I like these consequences. But let's say you purchased a 22mm wide-angle lens for your  film body and now you want to use it with your digital camera. Unfortunately, most of the wide-angle effect will get lost because the effective focal length is 33mm (22 x 1.5) which is decidedly not wide-angle. This can make things tricky if you're looking to use one type of each lens with both film and digital bodies. If you drop down to a 18mm lens so that it gives you a decent wide-angle effect on your digital, you may find that 18 is too much wide-angle for your film body.

Anyway, I digressed a bit here from the megapixel issue.  But it's important stuff given what I said in my last post about the importance of optical zoom (vs. digital zoom) if you're thinking of cropping and/or enlarging your photographs. Finally, here are some other examples of the principles discussed in this post. As you can see, I need more practice with the zoom lens. But, if you're cropping and enlarging, megapixels can make the difference:

Above, an immature eagle in flight from about 500 yards with 750mm of zoom. The image was reduced to fit in a 469x312 frame so most of the 6 megapixels that were captured were lost in the  process of size reduction in order to display in this blog.  Prior to reduction, I used Photoshop to apply an automatic color adjustment and to remove some blurriness (with unsharpen mask). Below, a 469x312 "window" cropped from the same Photoshopped-photo (so, now you're looking at a cropped version of the photo at 100 percent of its size). 


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