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I want to compare sharpness of two lenses, consider 50mm f/1.8G vs 35mm f/1.8G from Nikon on a crop sensor camera by taking some photos (from some small size text on a paper) and review the results. Here, I don't care the perspective issues (just sharpness is important).

To have the similar frame on both photos, I need to move closer to the subject as the second one is a wider lens. When I do this, the texts are more clear on the wide angle lens. I'm not sure if this is fair for comparison, because when you are closer to a subject, you can see better details.

So, how should I arrange the scene to be able to test the sharpness of these lenses?

scottbb
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S.Serpooshan
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2 Answers2

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I need to move closer to the subject as the second one is a wider lens. When I do this, the texts are more clear on the wide angle lens. I'm not sure if this is fair for comparison, because when you are closer to a subject, you can see better details.

This is absolutely fair. When you are measuring clarity, you are determining angular resolution across the field of view of the lens. For instance, half way from center to edge of frame is a lot wider angle on a 14mm lens than it is on a 70mm lens.

Wide angle lenses need to be close to see details of subjects. Longer lenses can (and should) move further away to see those same details. That is the very nature of, and reason for, lenses of different focal lengths.

So, how should I arrange the scene to be able to test the sharpness of these lenses?

Whether you buy or make a test chart, or use a scene with lots of details, transitions, etc. (such a populated bookshelf), you should try to frame the scene the same between the lenses. What you are doing is correct: to frame the same, you have to get closer with wide angle lenses.

Make sure you test with multiple apertures. Lenses usually aren't their clearest wide open, and certainly aren't their clearest fully stopped down. Usually somewhere in the range of ƒ/4 – ƒ/11 is the clearest, and that is highly lens dependent; all lenses are different, they are their own beasts.

You can perform your tests just focusing all shots at the center of the frame. But you can also try to focus off-center. Try to focus as close as possible at the same off-center spot(s) when using the different lenses, if you do test off-center.

Professionally, lens sharpness is evaluated using modulation transfer function (MTF) charts. These charts are provided by most manufacturers for their lenses. Some labs and reviewers test actual lens copies independently and publish their own measured MTF charts. These charts quantify the spatial/angular resolution of lenses across the the field of view of the lens.

If you want to compare two nominal lines of lenses, then you should examine the manufacturer's MTF charts for those lenses, or find a comparison / review site that has examined copies of both lenses (preferably, the more samples for each lens, the better, in order to minimize sample-to-sample variation that is inherent amongst all lens lines).

If you want to compare two specific lenses, you will need to create your own MTF test setup. Note that the error in your measurements will be higher than a lab's controlled, measured setup. But large sharpness issues should show up in your own testing.

MTF charts at Nikon's site for the lenses you mentioned (I assume you meant the AF-S DX 35mm f/1.8G lens, not the AF-S 35mm f/1.8G ED lens (which is a FX lens), but both are linked below:

To understand MTF charts, and how to create your own, see also:

scottbb
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  • Thanks for reply. So if i have a 18mm lens to compare with my 50mm, i have to go very close to subject to get same frame (Eg 40cm instead of 1.5 m). And then the details will be obviosly better. I think its not fair to do this way. You can test this with your smartphone as they have usually very wide angles. – S.Serpooshan Jun 19 '17 at 00:09
  • @S.Serp so in a way, you're right, in that comparing lenses with very different angles of view is to a degree apples and oranges. But you have to eliminate variables and try to control your experiment environment. Don't forget that shooing test charts, brick walls, and bookshelves is not the same thing as real world usage. So with very wide angle lenses in your test environment, you can expect to see perspective distortion and fisheye-like effects away front the center of the image. You'll probably also have sharpness falloff away from center, as compared to longer focal lengths.... – scottbb Jun 19 '17 at 00:21
  • Please read the last note i add to my prev question: https://photo.stackexchange.com/q/90190/63511 – S.Serpooshan Jun 19 '17 at 01:48
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I want to compare sharpness of two lenses, consider 50mm f/1.8G vs 35mm f/1.8G from Nikon on a crop sensor camera by taking some photos (from some small size text on a paper) and review the results. Here, I don't care the perspective issues (just sharpness is important).

You have an apple and an orange and you're trying to compare them.

I can't think of a useful purpose in comparing sharpness for two different focal lengths. Two identical focal lenses maybe (as a deciding factor in choosing one lens over another, maybe), but not different focal lengths, as the choice of focal length is decided by the shot, not how sharp you want it.

To have the similar frame on both photos, I need to move closer to the subject as the second one is a wider lens. When I do this, the texts are more clear on the wide angle lens. I'm not sure if this is fair for comparison, because when you are closer to a subject, you can see better details.

Sharpness for any lens will vary depending on aperture and what part of the frame you look at. That will be different for two different lenses.

And here's where the sharpness test is essentially useless.

Not only will two different lens designs have different overall characteristics, but two individual lenses of identical design can have slightly different characteristics - design tolerances are not zero. Worse still, any two lenses combined with any two theoretically identical models of camera will produce four different results. The combined engineering tolerance variations of these will pretty much guarantee they'll be different.

And you're not even comparing two focal lengths that even close to each other. !

They're both sharp lenses, but they're not sharp everywhere in the frame at any aperture. I can't think of a lens that is.

But these are wide aperture lenses. The most typical use of these is to shoot wide open, or close to it. Wide open they have very narrow depth of field. Which in practical terms means that unless you're shooting a perfectly flat target aligned to the focal plane (think wall), you're really going to only have an extremely small region of the subject in focus even in ideal conditions.

So why is sharpness so important ? I'd suggest it simply isn't.

All a sharpness test of these lenses does is distract you from the reality of using these lenses.

So, how should I arrange the scene to be able to test the sharpness of these lenses?

Flat surface, parallel with focal plane (that's fun to arrange !), even diffuse lighting ideally. Tripod, mirror lock up and timed exposure release to avoid vibrations causing blur.

Manual focus, use live view. This should avoid any possibility of focus shift when stopping down because when using Live View cameras typically keep the aperture closed down to the set aperture, whereas when not using Live View they'll open aperture wide for focus acquisition and then stop down to the selected aperture when shooting - in some lenses this can result in a slight focus shift.

How I test lenses is simple : I look for someone else who specializes in this sort of delicate work to do it for me. Photozone.de was/is a pretty good lens test website and I'm certain they've tested those lenses. Lenstip.com was another good one.

StephenG - Help Ukraine
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