Pixels

[Dr PR]

CAUTION: esoteric camera nerd stuff!

 

We sometimes obsess about how many pixels our camera sensors have (mine is bigger than yours ...). But are more pixels ever worse than fewer?

 

I have two principal camera bodies, the Nikon D850 and the Nikon D5600. The D850 camera body sensor has almost twice as many pixels as the D5600 sensor. It would seem the D850 should create higher quality images. But I was curious about the difference in actual image sizes for subjects photographed with the D850 FX camera body and the D5600 DX camera body, using the same Nikon Micro Nikkor 105 mm macro FX lens (a "prime" or non-zoom lens).

 

The full frame (FX) D850 image sensor produces images with 8256x5504 pixels, or 45.44 megapixels. The crop frame (DX) D5600 produces images with 6000x4000 pixels, or 24.00 megapixels. So the D850 images have 1.89 times as many pixels as the D5600 images. This seems like an obvious advantage for the D850, producing images that can be cropped smaller to get the same number of pixels as a similar D5600 image that is cropped much less. But this isn’t the end of the story!

 

 

Full frame (FX) lenses can be used on crop frame (DX) camera bodies. Because the DX sensor is smaller, if used with a FX lens only the center 75% of the image produced by the lens is used (for Nikon cameras - other manufacturers have different full/crop fame ratios). This narrows the view angle, and  increases the effective focal length of the FX lens on the DX camera body by about 1.5.

 

 

 

 

 

This means you can take a picture of an object to fill the DX picture area from about 1.5 times the distance as needed to fill the FX picture area. So from the same distance the image of an object will be much smaller in the FX camera picture than in the DX camera picture (using the same FX lens on both camera bodies). Which camera and lens combination actually gives more pixels for the object being photographed? A simple 1.5:1.0 comparison doesn’t work. The calculations are complicated by the fact that the pixel density on the FX and DX image sensors is different (the D5600 sensor has about 1.2 times as many pixels per mm² as the D850 sensor).

 

To answer this question I took some pictures of a test pattern with the D850 and D5600 camera bodies mounted on a tripod. I used the Nikon Micro Nikkor 105 mm macro FX lens on both camera bodies. On the DX D5600 body the lens is equivalent to a 157 mm lens.

 

 

First I positioned the tripod so the test pattern image filled the D5600/105 mm picture vertically (left). Then I cropped the picture to include only the test pattern image (the outside of the outer black rectangle). The resulting test pattern picture was 5180x3904 pixels, or 20,222,720 pixels (20.22 megapixels).

 

 

 

 

 

 

Then I shot the test pattern with the D850/105 mm setup on the tripod at the same position as for the D5600 shot. As you can see (left), the test pattern image does not fill the picture. After cropping the image to include only the test pattern, the resulting picture was 4632x3496 pixels, or 16,193,472 pixels (16.19 megapixels). This is 0nly 80% (0.800756) as many pixels in the test pattern image as the D5600/105 mm body and lens combination.

 

 

 

 

From the same position, with the same lens on the two different camera bodies, the DX/FX D5600/105 mm setup produced 1.25 times as many pixels for the same object as the FX/FX D850/105 mm combination! So for more distant objects the DX body and FX lens gives greater magnification and more pixels for the objects being photographed.

 

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But what about close-up and macro photography? This is normally of more interest for model photography. For these situations you can position the camera close to the subject and fill the picture area with the subject. How much closer does the D850/105 mm combination need to be to fill the entire picture area with the test pattern?

 

For this test I measured the distance from the initial tripod position where the test pattern filled the D5600/105 mm image picture. The distance from the test pattern to the focal plane was 146 cm (57.5 inches). Then I moved the tripod closer until the test pattern filled the frame on the D850/105 mm setup. The distance was 108 cm (42.5 inches). The DX body/FX lens combination fills the image with the subject at 1.35 times the distance for the FX body/FX lens combination.

 

To fill the picture with the subject the FX/FX body/lens has to be only 0.74 times as far from the subject as the DX/FX combination. The D850/105 mm cropped test pattern had 7344x5499 pixels, or 40,384,656 pixels (40.38 megapixels).

 

At this closer distance the 40.38 megapixel test pattern image in the D850/105 mm has twice (1.99699) as many pixels as the 20.22 megapixel picture on the D5600/105 mm at the longer distance when photographing the same object. So the D850 and 105 mm lens combination is much better for macro photography than the D5600 and 105 mm lens setup.

 

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I normally carry the MUCH lighter D5600 camera body (and FX telephoto lenses) when I am hiking and photographing distant subjects like birds and animals. The D850 is used in the "studio" (my kitchen) when I am doing macro photography of ship models, wildflowers and such. However, on special wildflower expeditions I will carry the D850 and the 105 mm macro lens into the field.

 

EDIT: There is one special situation that is important for close up photography. Every lens has a minimum focus distance - the distance from the subject to the focal plane in the camera. This is a property of the lens, and not the camera body. You cannot move the camera closer than this minimum focus distance and get a sharp picture (at a given f-stop). At this minimum distance the image size produced by the lens is the same for both FX and DX camera sensors. With the Nikon 105 mm macro lens the image is the same size (1:1) as the real object. In this case the higher pixel density of the D5600 DX sensor produces 1.25 as many pixels in the image of the object as the D850 FX sensor.

 

Macro photography is an art. But the best images are the result of applying the science of optics with the art of composition.

 

Hope this is of interest to someone.

Edited June 29 by Dr PR