Lens: Sigma 50mm f/2.8 DG macro
Vintage: Current.
Lens Mount: Nikon F-mount (comes in various camera-specific mounts).
Needed Adapters: None.
Preferred Mounting: Normal, not reversed.
Filter Thread: 55 mm.
Street Price: About $370 new. Prices vary a lot online, shop around.
Controls: This lens has an aperture control, focus control and a focus limiting switch (close/full).
Aperture: 7 blades.
Basic Function: Autofocus macro lens.
Magnification | Working Distance |
0.26 | 186 mm |
0.49 | 95 mm |
0.72 | 62 mm |
1.00 | 42 mm |
Appearance:


Resolution vs. Aperture:
The sharpest aperture is f/5.6, typical for a lens of this focal length. The sharpness is nearly as good from f/4.5 through f/6.3.
The most resolving aperture is f/4.5 although f/5 and f/5.6 are virtually the same (within 1%).
This means that f/5.6 is a good aperture to shoot at for sharpness and resolution.
" alt="corner sharpness graph" title="corner sharpness vs. aperture graph" />Corner Sharpness vs. Aperture:
The corner performance of this lens is extremely good, even at large apertures, staying under about 15%, and typically less than 10% across the range.
(Testing done at 1:1 magnification. Numbers at each aperture setting will tend to worsen slightly as the magnification is decreased).
Sharpness and Resolution vs. Magnification:
This lens shows very high sharpness and resolution across the macro range. The numbers are only a touch below the Nikon 55 mm micro, the best performer that I have come across to date.
At 1:1 the MTF50 (sharpness) is about 1250 lw/ph, about as good as you are going to see. The lens outresolves the D200 detector across the board.
The price that you pay for this resolution is relatively limited working distance - only 42 mm at 1:1. Compare that to a Nikon 105VR micro at about 150 mm and the Nikon 200 micro at about 250 mm. More working distance means better lighting (at least for coins).
The corner sharpness is very good across the macro magnification range, less than 20% across the range.
Performance: Sharpness vs. Magnification:
I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.
The Sigma 50mm shows outstanding sharpness from 1:4 to 1:1, typical for a good short focal length macro lens.
Performance: Resolution vs. Magnification:
I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.
The Sigma 50mm shows outstanding resolution performance throughout the range from 1:4 to 1:1, typical for a good short focal length macro lens.
Resolving Power vs. Magnification:
This graph represents the smallest details that are able to be resolved by this lens at various magnificaitons. If the number doesn't get smaller as the magnification rises, there is little benefit to going up in magnificaiton with this lens. This situation is also called empty magnification.
Outstanding resolution, no problems.
Chromatic Aberration: This lens show minimal color fringing in the center (less than 0.10 pixel) and good on the periphery (0.70 - 1.10 pixels). Anything below 1.0 pixel is good.
Image Contrast: Image contrast is very good, typical for a high-quality macro lens.
Flare: This lens shows no significant flare during testing.
Distortion: This lens shows no significant distortion during testing.
Image Samples:
About 0.4:1 magnification, f/5.6, cropped and resized:

Pixel level crop from the image above:
There is very good pixel level detail, no problems.
About 1:1 magnification, f/5.6, resized:

Pixel level crop from the image above:
There is good pixel level detail, no problems.
Conclusion:
This is an easy to find autofocus macro lens that is available in most lens mounts. It will only autofocus on camera bodies that have an internal focus motor, but autofocus isn't all that useful with macro anyway. The lens provides the viewfinder with a nice bright image (f/2.8), making focusing easy.
This lens has very high sharpness and resolution across the macro magnification range and performs at its best at f/5.6. The high resolution and sharpness is countered by the relative limited working distance - you gotta pay somewhere. There is very little chromatic aberration and no visible distortion. Corner sharpness is very good across the range, so this lens should be good for any macro stitching you wish to do.
For me, I prefer to use shorter focal length macro lenses for imaging larger objects than coins. I tend to run out of copystand height with longer focal lengths and larger objects.
So, I wouldn't tend to recommend this lens (or any of similar focal length) as a primary coin imaging lens, although it will work OK if need be. This is mainly related to the limited working distance and the lighting limitations that it imposes. For larger objects, it works very well. For imaging above 1:1 magnification, it should work well when reversed on a bellows, although I haven't tried it yet.