Depth of Field Myths: The Biggest Misconceptions Article✌✌

Depth of field causes more confusion among photographers — beginners and otherwise — than nearly any other topic out there. Many “common knowledge” tips about depth of field have some flaws, or are at least partially inaccurate. At a personal level, it took me far too long to separate the good suggestions from the bad, and I eventually realized that I had been relying upon some erroneous information for years without knowing better. My goal with this article is not to make the most controversial possible statements, or needlessly poke holes in things that are almost entirely true. Instead, my hope is to cover some of the basic, common inaccuracies that you may have heard about depth of field, in case you’ve been relying on faulty information for your own photography.

1. Is it True that Depth of Field Extends 1/3 in Front of Your Subject, and 2/3 Behind?

No, this one isn’t true. The 1/3-front, 2/3-behind suggestion is a fairly common one, but it doesn’t play out in practice.

In fact, the front-to-back ratio for depth of field varies wildly depending upon a number of factors. In very specific cases, it’s true that the ratio can be around 1:2 — but, more frequently, it’s something else entirely.

Which factors matter here? There are three: focal length, aperture, and camera-to-subject distance. As you focus closer, use wider apertures, and use longer lenses, the ratio starts to approach 1:1. When you do the opposite, the ratio quickly passes through 1:2, then 1:3, 1:10, 1:100, and onwards to 1:∞. The range where the focus is 1/3 in front of your subject and 2/3 behind (or the range where it’s close to that ratio) is quite thin indeed.

Where does this tip come from, then? My guess is that it started simply enough: There are cases where the depth of field behind your subject is twice as great as the depth of field in front of your subject. And, with certain lenses and apertures, that spot happens to be a very “medium” focusing distance away from your lens — in the range of 3 meters (10 feet). So, it’s not surprising to me that this morphed into a universal 1/3-front, 2/3-behind suggestion. And, it is indeed useful for beginners to know that depth of field takes longer to fall off behind your subject than in front.

Still, it’s quite a narrow window where the ratio is closer to 1:2 rather than 1:1.5, or 1:3, or 1:4, and so on. The ratio 1:2 isn’t some common figure that tends to occur when you focus at “medium” distances. It’s much more of a special case than a generalization.

1. How Do You Double Your Depth of Field?

It depends. But there is no simple thing you can do to universally double your depth of field for a given photo, so long as you’re defining “double” how most people do, and you’re not calculating polynomial equations in your head.

What about using an aperture that is two stops smaller? Or stepping twice as far away from your subject and refocusing? Or using half the focal length of your current lens?
This is easy enough to realize simply by doing a quick thought experiment. Say that you’re using a wide-angle lens, and your depth of field ranges from 1 meter to 15 meters. In this situation, infinity will be almost within your depth of field, but not quite; distant objects are probably only the slightest bit blurry. Still, they aren’t technically sharp enough to count within your depth of field.

In that case, you don’t need to do very much in order to get the farthest objects completely within your depth of field. Simply change your aperture by a fraction of a stop, or use a slightly wider focal length, or step back just a bit and refocus in the same spot.

In all of these cases, a minor change to your settings (focus distance, aperture, or focal length), will increase your depth of field from 14 meters (15 minus 1) to an infinite number of meters. Clearly, that’s more than doubling your depth of field! And, crucially, you don’t need to change your camera settings much in order to accomplish it.

(If you’re wondering about the exact values I used, it’s true that they’re a bit arbitrary. However, to make sure that they were realistic, I used this calculator with a 14mm lens, a subject distance of 2 meters, an aperture of f/5.6, and a 0.015mm circle of confusion. Feel free to use it and play around with your own values.)

That’s why there’s no merit to claims that you can “double your depth of field” by doing one particular thing for any photo. Sometimes, focusing twice as far away will triple your depth of field. Other times, doing exactly the same thing will increase it 10x, 50x, or infinitely. It all depends upon how much depth of field you already have.

1. How Many Variables Affect Depth of Field in a Photo?

Assuming a typical lens, there are three:

Focal length
Aperture
Camera to subject distance (how far away you’re focused)
From time to time, you may hear online that only two variables affect depth of field in a photo: aperture and magnification.

There’s a similar (though slightly less common) argument, too, that two other variables are the only ones that affect depth of field: subject distance and entrance pupil size.

Neither of these claims is technically wrong, but there’s an issue: People who say that depth of field only contains two variables are merging two of the three together. That’s perfectly fine, but the individual components still matter, and they still affect your depth of field.

Magnification merges together focal length and subject distance. (It’s the size of an object’s projection on your camera sensor relative to its size in the real world.)

Entrance pupil size merges together focal length and aperture (focal length divided by f-number).

Most of the time, it doesn’t make things simpler to combine these variables together. No one in the field spends time calculating entrance pupils. The same is true for magnification, unless you’re doing macro photography.

To put it simply, all three components matter — focal length, aperture, and focusing distance. If you change one without compensating by also changing another, you’ll alter your depth of field every time.

1. Do Crop Sensors Have Greater Depth of Field?

This one has a lot of controversy around it, and I don’t want to add to that. The reality is actually quite straightforward.

The short answer is no, crop sensors don’t inherently have more depth of field than large sensors, although it can seem that way — in order to mimic a larger sensor, you’ll have to use wider lenses, which do increase your depth of field. (You also could stand farther back, which again increases your depth of field, although that does alter the perspective of a photo.) But the sensor itself does not directly give you more depth of field.

When it comes down to it, this shouldn’t be too surprising. A crop sensor is like cropping a photo from a larger sensor (ignoring individual sensor efficiency differences and so on). Unless you think that cropping a photo in post-production gives you more depth of field, this shouldn’t cause any confusion (indeed, if you crop a photo and display the final images at the same print size, it’s even arguable that you will see a shallower depth of field in the cropped image, since any out-of-focus regions would be magnified; but now I’ve started diving into a different rabbit hole, and this is a complex discussion for another day).

Still, the claim that small sensors have more depth of field isn’t entirely unfounded. Imagine that you have two cameras — one with a large sensor, and one with a small sensor — as well as a 24mm lens on both. Because the crop sensor will have tighter framing, you might choose to step back or zoom out in order to match what you’d capture with the larger sensor. Both of these options — stepping back or zooming out — do give you more depth of field.

So, the result of using a smaller sensor might indeed be that your photos have more depth of field, if you don’t do anything else to compensate for it. But this is an indirect relationship. The smaller sensor itself is not what causes the greater depth of field; it’s the wider lens or greater camera-to-subject distance.

1. Does the Sharpest Focusing Distance Depend upon Output Size?

No, although it’s a nuanced argument.

Here’s the starting point: If you’re making tiny, scrapbook-sized prints, you have way more leeway in terms of what looks sharp compared to something like a large, 24×36 inch print viewed up close. You won’t notice errors very easily in the small print. Even when the original photo has some major flaws, they won’t be visible if the print is small enough (or far enough away).

But does that mean the sharpest possible focusing distance changes as your print size does? No, not at all.

Indeed, there is only one focusing distance that will provide you with the most detailed possible photo of your subject (or the most overall detail from front to back, if that’s your goal instead). Just because you can get away with focusing on your subject’s nose rather than their eyes, for example, in a small print, doesn’t mean that the “best possible focusing point” is on their nose. Whether you’re printing 4×6 or 24×36, and whether or not you can even see a difference, it’s still technically ideal to focus on their eyes.

Small prints let you mess things up more without noticing a huge effect; that’s very true. But they don’t alter the position of the best focusing point. So, the sharpest focusing distance does not depend upon output size (which is the impression you might get if you follow hyperfocal distance or astrophotography calculators too literally).
Conclusion

Hopefully, this helped shine a light on the depth of field myths that you’ll see so frequently today. This is an important enough subject that accurate information is valuable, even if it isn’t always easy to find. And, of course, some of the tips in this article are suggestions more than pure, mathematical debunking. If you want to have defocused backgrounds, for example, go for it! Photography is all about your own creative vision, and that’s not something for me to determine.

Depth of field is a huge topic, and there certainly may be myths I haven’t covered yet. For space purposes, I also didn’t go into all the little nuances of some of these individual points, since this article already is quite long. So, if you have any questions about depth of field, feel free to let me know in the comments section below. I’ll do my best to answer them, or clarify anything I’ve written above.
Thank you for reading💟