Cropping w/more megapixels vs. using multiple focal lengths

Started Sep 28, 2013 | Discussions thread
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The Sage Knows Senior Member • Posts: 1,102
Cropping w/more megapixels vs. using multiple focal lengths

By taking advantage of the generous cropping ability of a high megapixel, high quality sensor paired with an ultrasharp and properly focused lens, one can indeed use a shorter focal length (smaller and lighter) lens in order to pull in more distant subjects at a resolution that is still sufficient to avoid visible pixelation. The result would be comparable to selecting a longer focal length on a full frame sensor of lower resolution.

Let’s take one of today’s highest MP cameras to demonstrate why it is not illogical to want to use one - even if you never intend to print billboards but shoot mostly for the web and/or 8inx12in prints.

According to the specs for the Nikon D800, its max resolution is (7360x4912)px = 36MP

If we use the often quoted 300ppi as what is required for photographic print quality resolution, then the largest 300dpi print this camera can produce is:

4912px /300px/in=16.4in

7360 px /300px/in=24.5in

Suppose you make a print this big and then cut around the edges to crop out an 8x12 piece. At that same dpi, how many pixels would be contained in it?

(8inx300px/in)x(12in X 300 px/in)=8,640,000 px or 8.6MP

The crop factor in this case is calculated by taking the sqrt of 36/8.6 = sqrt of 4.2 = 2x

It would be the same quality in terms of dpi. It has to be since it came from the same photograph. It just would not be as big a print. Which is fine if 8x12 is all you want.

If the crop came from the center, it could, depending on the lens, be even sharper overall than an image on the same size print taken with an equivalent focal length over the full 36MP frame (ignoring the potential improvement due to oversampling). This is because cropping allows you to use only the sharpest parts of the lens' image, the part nearest the center of the frame. The corner softness at wider apertures, typical of all lenses used at their native format, is cut off when they are paired with a smaller format sensor, one with a crop factor.

I don’t like to carry long heavy lenses around. So I take advantage of cameras with even more pixels than what I need in order to crop and give me a print that could have been taken with a camera of lower resolution but with a longer focal length lens.

For example, by using a Nikkor 105mm f/2.8D Macro, which has very high sharpness along with low chromatic aberration even wide open, mounted on a 36MP Nikon D800 (perfectly) focused on a distant bird, the image can be cropped to an 8x12 print at 300ppi to look like (or framed like) it was shot on an 8.6MP camera with a 210mm lens without carrying around the larger and heavier lens.

I would argue that for most subject matter, a 300ppi is overkill. One could often get away with as low as 200ppi and still have a photograph that doesn't show pixellation, even when examined closely. So the crop factor in those cases could then increase to 2.7x using the same math.

One caveat: bad focusing technique using a shorter focal length and crop will result in the subject not being as sharp compared to using a true telephoto lens with no crop. This is because the camera (or your eye if using manual focus) will have a smaller subject projected on the sensor that it can focus on along with less movement of the lens ring to fine tune it. If the subject is far away, this should not be a huge problem if you pre-focus on infinity. You would need to confirm where infinity focus is on your lens since many actually focus just short of the infinity stop.

One other caveat: the sensor and image processing technology has to be the same or the comparison is void as it would introduce other variables that could well diminish the usability of the crop. This of course is impossible as no two cameras of differing megapixels are going to perform exactly the same.

So in practice, the full frame lens on a lower MP camera could provide the photographer an edge in image quality over the shorter focal length cropped from a higher MP image. This is due to easier focusing and potentially lower noise when using the full frame. But it would be at the cost of greater size and weight.

A counter argument would say that few zoom lenses equal the quality (sharpness, CA, distortion) of a prime at all focal lengths so the above technique of creating a 300dpi print using say, a 105mm lens, and cropping up to the 210mm equivalent (or 280mm at 200dpi) could achieve very similar results since only the best part of the lens image, which is closest to the center of the frame, would be used in the crop.

Thanks for reading.

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Nikon D800
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