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Does the EF 40mm f/2.8 lens give sharpness not just at the center but also to the periphery most of the time..? In comparison with the 50mm f/1.8 STM which only very sharp at the center..?

I have done some research on the web and found myself ambivalent whether or not I can answer the above question to myself. Some people say the 40mm is consistently sharp across the frame so it has better IQ than the 50mm.

But, I also found that Canon USA official YouTube channel replied on a comment and said that not much difference between those two lenses. The comment is on this video

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    \$\begingroup\$ You need to define what you mean by "sharpness". Do you mean flat field correction needed for flat document reproduction (such as flat test charts)? Or the often less than flat fields of focus most lenses demonstrated? If you're doing flat document reproduction, neither of these lenses is very "sharp" on the edges and in corners. If you're shooting a three-dimensional world it's a bit different, as the sharpest points on the edges and in the corners will be slightly closer to the camera than the center. \$\endgroup\$
    – Michael C
    Oct 18, 2023 at 16:11
  • \$\begingroup\$ dxomark.com/Lenses/Canon/… says that it's sharpest at center than corner, but not dramatically so. \$\endgroup\$
    – dandavis
    Oct 18, 2023 at 22:12
  • \$\begingroup\$ @dandavis That assumes one is interested in flat field performance used imaging a flat test chart parallel to the camera's sensor. That's only one among several ways of looking at a lens' "sharpness". \$\endgroup\$
    – Michael C
    Oct 19, 2023 at 5:11
  • \$\begingroup\$ @dandavis The center isn't much sharper than the edges wide open at f/2.8. But by f/5.6 the center is much improved compared to f/2.8, while the edges/corners are not as sharp as they were at f/2.8, and also show increasing effects of astigmatism. Click on your link, then select Sharpness→Profiles→f/5.6. \$\endgroup\$
    – Michael C
    Oct 19, 2023 at 5:18
  • \$\begingroup\$ @MichaelC: true, you can't test everything, but this is at least repeatable and a consistent set of datapoints. I was contrasting the 40 to the nifty 50, which most folks like to shoot pretty wide, where it's dramatically inconsistent center-to-edge compared to the 40 even at it's worst, the 5.6 "donut hole" you mention... \$\endgroup\$
    – dandavis
    Oct 19, 2023 at 6:52

3 Answers 3

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According to what I see on this site 40mm is sharper, but not consistent over the frame.

OK, I am not optics expert, but AFAIK there is no lens which have same sharpness in the center and corners.

P.S. And in to the comment I do not see anything about consistent sharpens of the lens, I read:

...being a very compact, sharp lens that can...

which say (for me) this is sharp lens, nothing more, nothings less.

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  • \$\begingroup\$ In the video she says "very sharp from center to the periphery" \$\endgroup\$
    – Redsbefall
    Oct 18, 2023 at 14:26
  • \$\begingroup\$ @Redsbefall, did not watch the video, but this (for me, again) do not mean is consistently sharp \$\endgroup\$ Oct 18, 2023 at 14:34
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    \$\begingroup\$ @Redsbefall What one person considers "sharp" may not be what another expects for a "sharp" lens. Both may be "sharp" for some types of work and not "sharp (enough)" for other types of work. What kinds of photos do you wish to take with the 40mm f/2.8 Pancake or the 50mm f/1.8 STM? \$\endgroup\$
    – Michael C
    Oct 18, 2023 at 16:16
  • \$\begingroup\$ @MichaelC but at least it has better IQ at the center and the edges than the 50mm f1.8? If they are the same then I prefer the 50mm. \$\endgroup\$
    – Redsbefall
    Oct 20, 2023 at 2:51
  • \$\begingroup\$ @Redsbefall The 50mm is better in the center at f/2.8, the 40 is better on the edges and in the corners at f/2.8. In other words, at f/2.8 the 40 is more consistent from center to corners than the 50, but that doesn't mean it's also sharper overall. \$\endgroup\$
    – Michael C
    Oct 20, 2023 at 20:31
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When measured with the lens focused on the center of the field pointed at a flat test chart parallel to the sensor and both lenses set at f/2.8:

  • The EF 50mm f/1.8 STM is slightly sharper at the center than the EF 40mm f/2.8 STM
  • The EF 40mm f/2.8 STM is slightly sharper in the corners than the EF 50mm f/1.8 STM.
  • The EF 40mm f/2.8 STM has less difference between the center and the edges.
  • The EF 50mm f/1.8 STM has more difference between the center and the edges.
  • The average sharpness of the entire field is practically the same for both lenses

enter image description here

When measured with the lens focused on the center of the field pointed at a flat test chart parallel to the sensor and both lenses are set at f/5.6:

  • The EF 50mm f/1.8 STM and the EF 40mm f/2.8 STM are equally sharp in the center of the field
  • The EF 50mm f/1.8 STM is slightly sharper at the edges and corners than the EF 40mm f/2.8 STM
  • The EF 40mm f/2.8 STM suffers moderately more from astigmatism than the EF 50mm f/1.8 STM near the edges and in the corners
  • Both lenses are moderately sharper in the center than they are on the edges and in the corners

enter image description here

All of the above observations were made with the camera and lens carefully manually focused on the center of the frame. The difference in autofocus performance from one camera body to the next of the same model, and from one example to the next of the same lens model could very well be greater than the measured differences shown above.

The "less sharpness" on the edges and corners when imaging a flat test chart may be because the lens has less absolute resolution in those parts of the field, or it may be that the lens is just sharpest on the edges and in the corners at a slightly closer distance to the camera than the edges of the flat test chart. If the latter is the case, carefully refocusing the lens so that the corners are at their sharpest would cause the center to be "less sharp" because the point of sharpest focus in the center would then be behind the surface of the flat test chart. This is due to what is known as field curvature.

Roger Cicala, the founder and chief lens guru at lensrentals, has talked a lot at times about field curvature, what it is, and how different field curvature shapes can affect how different lenses can affect different types of photos.

There's a good overview of field curvature at this article he wrote for DP Review

He talks about various 50mm lenses in this blog entry at lensrentals

Keep in mind that often he is comparing the measured differences of two different copies of the same lens model!

Not all EF 40mm f/2.8 STM lenses will test exactly the same. Not all EF 50mm f/1.8 STM lenses will test exactly the same. Sometimes the measured differences of one copy of one lens model and another copy of another lens model are closer than the tested differences of two copies of either one of the lens models.

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Lenses are never consistently flat across the frame.

Lenses are real products, results of lots of compromises.

Manufacturers build them to have acceptable degradation for the intended audience and price range, which appear nearly always in the corners.

This degradation is maximum at high aperture. i.e will be much more noticeable at 1.8 than 2.8.

It's unlikely that a 1.8 lens will be sharper at 1.8 than another 2.8 lens at 2.8. And both can be near identical at 5.6

You can find a chart here and a comparison with the 50 1.8

Also be aware that a STM has a slow focus time which can result in IQ problem when taking pictures of moving objects.

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