Friday Foto Talk: Sharpness vs. Depth of Field, Part I   15 comments

Good morning Sunshine!  One more shot from my recent trip to Olympic National Park.  I needed maximum depth of field here and so things are not at maximum sharpness for this lens.  Is it enough?  I think so.

Good morning Sunshine! One more shot from my recent trip to Olympic National Park. I needed maximum depth of field here and so things are not at maximum sharpness for this lens.  Is the image sharp enough?

This is a bit of a sore subject with me.  One reason is that it’s one of those things in photography that is a trade-off, a limitation if you will.  When you’re going for either super-deep or super-shallow depth of field, a fall-off in image sharpness can occur.  This is a much bigger deal with the small apertures (big f/number) used to maximize depth of field than it is with large apertures (to throw background out of focus).  It’s also more noticeable with some lenses, and generally speaking the higher quality your lens the less falloff in sharpness.

But there’s another more important reason I’ve avoided this topic on Friday Foto Talk to this point: I think it’s an overdone subject, at least the way it’s discussed on so many photog. forums.  Too many folks obsess too much over the sharpness of a particular lens or lens/camera combination.  In my experience, images that are not as sharp as I would have liked are not the fault of my equipment. They’re my fault!

An example of a shot with foreground so close it is difficult to get everything sharp front to back.

An example of a shot with foreground so close it is difficult to get everything sharp front to back.

The Sweet Spot: Testing your Lenses

You might have heard of this before.  The sweet spot of a lens is that aperture where sharpness is at its peak.  It is generally about two stops above (smaller than) the lens’s maximum aperture.  So for example with a 24-70 mm. f/2.8 lens, the aperture that will yield the sharpest images is about f/5.6 give or take.  Since the sweet spot varies quite a bit by lens, you need to take each lens and experiment to find it.  Once you find it, it’s a good go-to aperture for images where depth of field is not a concern, particularly if you’re printing very large.  But don’t be like so many others and over-emphasize this.  Photography is about making pictures; it’s not a sharpness contest.

If you want to make this a real test, one where you can check your lenses’ real-world sharpness at the same time as finding their sweet spots, you’ll need to pick a day with clear air.  Dawn is usually clearest.  Go up high or out away from pollution, on a mountain or out on the prairie or desert is good.  Hey, you might as well have fun doing this!  Pick a mountain or hill at least a mile away with some good detailed features.  Trees backlighted along a ridge-line are perfect!   You want everything at infinity, and you want details at a variety of sizes.  If you’re testing a long tele lens, heat waves or dust will ruin the test, so a clear day is key.  If you’re testing a macro lens, print out a focus test chart from the internet and set it up carefully (google a tutorial).

A simple shot where it's easy to have everything in focus, thus the choice of f/8 for aperture.

A simple shot where it’s easy to have everything in focus, thus the choice of f/8 for aperture.

Back up on the mountain, put your camera on a tripod and use a shutter release or timer delay, plus mirror lockup if your camera has it.  Put it on aperture priority mode and focus by using Live View and zooming in.  Focus is of course critical.  After you focus, look at the focus indicator on the lens.  That is the point of focus for infinity, a good thing to know for each lens.  (I use this knowledge, for example, to focus for night shots of stars.)  It’s also good to check autofocus while you’re at it.  Just focus using AF then go to Live View and check the focus by zooming in.  If it’s off, you can adjust that on most DSLR cameras.  Check the manual or internet for directions on this.

Farmhouse in the Willamette Valley, Oregon.  Depth of field not a big concern, but shot at f/11 just to make sure.

Farmhouse in the Willamette Valley, Oregon. Depth of field not a big concern, but shot at f/11 just to make sure.

 

Now you’re finally ready to shoot.  Start with your lens wide open (max. aperture).  Keep shooting, stopping down one stop with each shot, until you come to the lens’s minimum aperture.  Then view each image on your computer monitor, zooming in to 100% to check sharpness.  Look at a variety of edges, from large shapes to small detail, and narrow it down to two or three to view in compare or survey mode in your software.  Don’t obsess, just make a call.  After you find the sweet spot take another look to see how the sharpness falls off in both directions from that aperture.  Don’t worry if you find some flaws in sharpness, especially if they’re in the corners.  It doesn’t mean your lens isn’t a good one.  This is just telling you its limitations, that’s all.  Always remember that sharpness is a relative thing and certainly not the most important thing in photography.  You’re just gaining information about your lenses, not seeing if you want to sell them!

A full-moon shot from the other night, the low light made me shoot at f/8.  I needed some depth of field here, and not everything has perfect sharpness.  But using my sharpest lens (a Zeiss) plus tripod sure helped.

A full-moon shot from the other night, the low light made me shoot at f/8. I needed some depth of field here, and not everything has perfect sharpness. But using my sharpest lens (a Zeiss) plus tripod sure helped.

You’ll see in your experiment that sharpness starts out pretty good, gets better to a certain point, then falls off (with some lenses quite dramatically) as you go to smaller and smaller apertures.  With every lens I’ve had, sharpness is much worse when the lens is stopped all the way down (minimum aperture) than when it is wide open (maximum aperture).  This is because of diffraction.  As light rays pass through a smaller and smaller opening, they are bent to a greater and greater degree.  Since your lens is the thing that’s supposed to do the bending of light rays, it’s obvious that if the rays also bend when going through the aperture opening then sharpness will be negatively affected.

Same place as previous shot but next morning.  I shot it at f/11 because the trees across the lake are much closer than the mountain.  So I needed good (not great) depth of field.

Same place as previous shot but the next morning. I shot it at f/11 because the trees across the lake were much closer than the mountain. I needed good (but not maximum) depth of field.

With large apertures you’ll probably see the softening coming in more at the edges (and especially the corners) of the image, not at the center.  Who puts their subject in the corner when shooting wide open anyway?  Still, better-quality lenses tend to minimize this.  Controlling diffraction at the small-aperture end, on the other hand, is a lot tougher. Some wide-angle lenses have large, curved front glass elements.  The Nikon 14-24 mm. f/2.8 and a few other ultra-wide-angle lenses account for diffraction at the small end, at least to some degree.  But diffraction is part of the physics of optics, so it cannot be eliminated, only controlled.

So now that we know what we’re dealing with, the question is: how does it affect our photography.  Let’s dive into that topic next Friday.  In the meantime, you have your homework.  Go out and determine where the sweet spot is for each lens in your bag.  Don’t worry if it’s hard to tell the sharpest point, say between f/5.6 and f/8.  If you shoot mainly for a lot of depth of field, write down the smaller aperture (f/8).  If you do a lot of portraits, wildlife and other shallow depth of field stuff, record f/5.6.  See you next time, and happy shooting!

The Willamette Valley.  Though I was not real close to the barn, I shot this at f/11 to keep the background trees in focus and the clouds from going too soft.

The Willamette Valley. Though I was not real close to the barn, I shot this at f/11 to keep the background trees in focus and the clouds from going too soft.

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15 responses to “Friday Foto Talk: Sharpness vs. Depth of Field, Part I

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  1. Pingback: Friday Foto Talk: Sharpness vs. Depth of Field, Part II | MJF Images

  2. Beautiful work.

  3. That sky in the last shot really grabbed me.

  4. What magic light! Breathtaking shots – the last one is very much to my taste.

  5. Thanks a bunch Siri & Selma!

  6. Congratulations to these extraordinary pictures and the educational text. Thank you!
    All the best from the sunny coast of North Norfolk
    Klausbernd and his happy Bookfayries Siri and Selma

  7. These are breathtaking shots! Thank you, MJF.

  8. Once again, thanks for the tutorial. 🙂

  9. Wow, beautiful shots. I particularly like the last one for some reason. Interesting comments too.

  10. Beautiful job.

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