This is the second of two parts. Last time we discussed lens sharpness in general, & learned how to find a lens’s sweet spot. Check out Part I. Did you do your homework? Hint: it was finding the sweet spot for your lenses! Remember all these images are copyrighted and not available for free download without my permission. Just click on them to check out purchase options on the main part of my webpage. If you can’t find something or have any other questions just contact me. Thanks for your interest.
Now we come to the meat of the matter. How much does all this matter? For one thing, you should realize that photography has changed with the advent of digital cameras, specifically the emergence of high-quality digital cameras. Sharpness and clarity are now expected by people. This is not generally a bad thing. But it is narrowing the range of images that people will look at for longer than a nanosecond. And that is a bad thing.
Notice I said sharpness and clarity, not focus. As an example take my post for Single-image Sunday, the Fog Returns. It’s an image that, while perfectly focused, is not particularly sharp. I’m using the word sharp in its broader sense here. It is encouraging that I a little push-back against this quest for sharpness in all images. But there are currents that are taking us in the other direction as well. For example focus stacking (where several images are captured and combined in Photoshop to have several focus points in the same image) is subtly changing the expectations of image viewers. To think I’m essentially being forced to do composites in Photoshop: ugh!
But let’s leave that aside and focus on sharpness vs. depth of field. You might be aware of all the tradeoffs in photography, and this is certainly one of them. But before we discuss that, here are a few givens:
- Some lenses are sharper than others, but that’s not your concern. Your concern is to get the best pictures possible with the equipment you have.
- Most images that are not as sharp as they could be are down to user error. If you don’t stabilize your camera on a tripod (or shoot at a fast-enough shutter speed if hand-holding), do not expect a sharp image. Use a cable release or timer delay as well. Mirror lockup, if your camera has it, has a lesser effect but is still worth doing.
- Atmospheric conditions, particularly at longer focal lengths, will also affect apparent sharpness.
- Some lenses are capable of being sharper stopped all the way down than other lenses, and can thus give you a greater apparent depth of field.
- The wider your focal length, the greater your depth of field will be. Though it’s a continuous change, think about 21 mm. as the cutoff between very short/wide focal lengths and just wide/longer focal lengths. Telephoto lengths (greater than 70 mm.) will yield much shallower depths of field.
- Despite the above factor, aperture is still the biggest influence on depth of field.
Shooting “Deep” Scenes: The Trade-off
Now let’s get to that tradeoff between sharpness and depth of field. If you want to maximize depth of field in your image (that is, sharpness from very close to very far away), you will be shooting at small apertures. So unless you are going the focus stacking route as mentioned above, you will be shooting a good ways past your sweet spot. Once you are two stops above the sweet spot (f/16 if your sweet spot is f/8, for example) you’ll notice a small drop in sharpness.
Let’s take an example. Say you are shooting a sunrise over a lake, with interesting rocks close by and beautiful forested mountains in the background. If you get low and close to those rocks, you might choose a very wide angle in order to get everything in. This will also help to maximize depth of field, but to really get there you will also use a small aperture like f/22. In order to show the fascinating detail in those foreground rocks, you will be focusing fairly close, perhaps only a foot or two past the closest rock.
This all sounds wonderful doesn’t it? But as with many things in life, there’s a hitch. One of photography’s tradeoffs has raised its ugly head! Depending on your lens the overall sharpness of your image will be just a bit less than what it is at the sweet spot (say that is f/8). This is because of diffraction, as mentioned in Part I.
But that’s not all. With most lenses, that image will also have its background slightly out of focus. If you’re lucky (rich?) enough to have a Nikon 14-24 mm. or other similar lens with a big curved front element, this effect is certainly minimized. But it is still there. You can focus deeper into the image, but then your foreground will be slightly out of focus. Shooting at a very wide angle and with a high-quality lens helps out with this tradeoff, but it will always be a balancing act.
I normally just accept some diffraction-related softness and go with f/22. But this is when I’m using my Tokina 16-28 mm. lens. With my Canon 24-105 f/4, I know it’s softer at f/22 than the Tokina and does not attain quite as large a depth of field. This is only partly because of the longer focal lengths; some has to do with the lens optics. The Canon does more things than the Tokina, so it can’t do the one thing as well. More tradeoffs.
Focal Point & Depth of Field
Let’s dive a bit deeper into the focal point: where to focus? It’s a question many photographers struggle with. For me, it not only depends on my desired focal length, but on the balance between background and foreground in the image. You should ask yourself, which is the dominant feature in my image: is it in the foreground or background? That main subject is what you should try to keep as sharp as possible.
Sometimes I will sacrifice and move back from my foreground, especially if my background subject is a strong one. This will increase apparent depth of field, but it might also force a longer focal length, which in turn decreases depth of field. Again, a balancing act.
Focal Point & Subject
Say you have a strong foreground and a less important background. It’s a seascape with a fascinating foreground and no interesting boat or other element in the background. You may just focus on the foreground and not care much about the background, even shooting at f/11 in some cases. This is how I handle those scenes. But I will often bracket my apertures, shooting at f/11 to f/22 (or whatever the minimum aperture is).
Now say you’re shooting a scene where your background subject is most important, yet you still want maximum depth of field. First off, definitely consider putting your foreground a bit further away as mentioned above. But this time, since the background is dominant, focus closer to it; about one third into the scene is the rule of thumb. Since your background is most important, you might increase focal length to make it bigger (longer focal lengths increase magnification). But careful! You could lose too much depth of field, putting your foreground out of focus. This is more likely if you’re tempted to shoot at wider apertures (smaller f/number) to get closer to that sweet spot.
You can always keep a very wide angle and crop later, thus helping to get better (apparent) depth of field and sharpness both. The tradeoff in that case is a smaller digital file, which is not really good if you’re thinking of printing the image large. If you’re using focal lengths of 50 mm. or greater, focusing one third into the scene should be your default point of focus. Just don’t get too locked into this, and always try to check focus right after the shot by zooming in on your LCD.
Okay, that’s enough for now. I’m willing to answer any questions on this somewhat convoluted topic, so fire away. If you’re not getting a quick answer it means I’m probably out shooting! Have a great weekend everyone.