f-Stop's - What's in focus?

For this article, I'm going to assume you know the following:

In rough terms what an 'f' stop is. (it decides how much light per second falls on the sensor, or in the old days, on the film)
If you take in too much light - even when taking a picture of a black bear in a dark forest, everything will come out too light or even white
If you don't get enough light - even when taking a picture of a polar bear skiing down on snow, everything will come out too dark or even black
To take a picture, there are FOUR (4) things that decide how much is 'too much' light, how much is 'not enough' light and how much is 'just the right amount of light'

What do you want the picture to look like. (How dark or light do you WANT it to look.)
What is the ISO setting of the camera (or in the olden days, what ISO is the film that you are putting into the camera)
What is the speed setting (how much TIME are you going to allow light - things like 1/500th of a second, 1/4 second, 1 second, 30 seconds
What is the f-stop - how big a hole is there to allow light in

So now we get to the topic of 'what's in focus'.

On older lenses, especially non-zoom lenses, there was often a diagram that showed you what was in focus at each f-stop. It would be a narrow band for an f1.2 (often just a single line) then a little wider as you go to F2.8, F4, F5.6 and so on, and a very wide band when you get to f22 and perhaps f32 or on very few lenses, even higher than f32.

First let's talk focus in general terms:

There are several things that affect how much is in focus

What is the size of the hole. All else equal, the larger the hole (the lower the number of the f-stop) the less will be 'in focus', all else equal.
What is the mm of the lens. All else equal, the bigger the mm (typically in the range of 16mm to 600mm) the more will be 'in focus', all else equal.
How far away is your focal point. All else equal, the further away the perfect focus point, the more in front and behind will be 'in focus.

Through the years a few people have told me "And the length of time the shot was taken, the longer the lens was open, the more out of focus it is", but this is not really the case - that is blur, not a lack of focus. It is a subtle difference but you can prove it to yourself by using a very sturdy tripod, keep everything except the ISO and the speed the same - you'll see that the time doesn't make a difference to what is in focus. More to the point consider a pinhole camera:'

When I was a kid, I had built several 'pin hole' cameras. These had f stops as small as f-1000. I had to use really thin metal.(I think if I remember I used copper sheets) and then to get it thin enough at the hole* I had to sand it down with fine sand paper. But it let me take pictures of my model cars a few inches away from the camera and trees in the background with everything in very sharp focus - but these pictures took something like 3 to 30 minutes each. I have never tried a pin-hole camera, but I suspect with sensors there will be a problem, much worse than film, with the amount of time it takes to get the picture, I suspect the sensors will introduce a lot of so called 'noise' - things that don't look right because of the amount of time it took to get enough light.

* Technical detail: With very small f stops (very high number like f100 or f1000) the thickness of the sheet the hole is in becomes very large compared to the diameter of the hole. Think of it not as a pin 'hole' but a pin 'tunnel' - so I had to sand the metal down to make it a pin 'hole' again instead of a long tube that light will reflect off and not allow light to go straight through to the film. This is also a limiting factor for f-stops in lenses. Up to f22 or f32 the thickness of the blades that make the hole in the lens is not an issue because the whole is 'so big'. But if you start getting to f45 or f64, now the thickness of the blades that make the hole is starting to cause a problem. That is why lenses don't routinely have f-stops smaller (higher number) than f22 or f32.

Then, some will say "The ISO affects the focus". Again, not really. the ISO affects the 'noise'. Both film and sensors start acting funny when they don't get enough light. Higher ISO film needed less light - but tended to be 'grainy', because it used larger crystals to capture the light faster. So in the film days, think of it as being 'fewer mega-pixels'. With digital sensors - it means that the individual sensors will act funny for a number of reasons, i'm not an expert on WHY - I understand heat, electrical signals, angle of light hitting the sensor and other factors become significant when there was very little light hitting the sensor or hitting the sensor per unit of time. So while this is very relevant to your ultimate picture, it doesn't affect the focus itself.

So what is actually in focus?

Let's suppose you see a chart that says at f2.8 from 4'8" to 4'9" will be in focus, but at F22, things will be in focus from 3' 6" to 7' 2" (taken from my Nikon f1.2 50mm lens focused at 4' 8.25")

Does that mean that at f22, everything is really sharp starting at 3'6" but at 3'5.5" everything is a blur? No! It means that everything that is 4' 8.25" away will be in PERFECT focus, and the further you get away, the less it will be in focus.

The focal charts use a generally agreed on definition of what is 'in focus' and what is 'not' in focus.

If you proceed to take a picture at f22 that has a tape measure, you will not see that suddenly at 3' 6" everything is suddenly sharp, and everything until 7'2". No, what you will see is that at 4' 8.25" away, it is in perfect focus, and the further you get away, the more blurry it is.

You will see that if you take the same picture at f2.8, that it gets blurry really fast, but at f22 it gets blurry slower.

The definition of what is 'in focus' is based on several factors including:

how big will the image normally be 'blown up' to. Take the exact same image, and at 4x6" a LOT more will appear in focus if you stand 18" away from it than if you blow it up to 4'x6' and stand 18" away from it. But in our example above, everything that is 8.25" away will look in perfect focus no matter what size you blow it up to.(If you don't like feet and inches, replace 4cm for 4" and 4m for 4', and use 50cm instead of 18"- it isn't the absolute sizes that matter, it is the relative.)
what is the normal viewing distance. Take the above 4'x6' image and look at it from 18" away and then from 10' away. At 10' - a lot more will appear in focus than if you look at it from 18" away. But again, using our example, everything that is 8.25" away will look it perfect focus no matter whether you look at it at 18" or from 10' away. (again if you prefer metric, use 4m for 4', 50cm for 18" and 3 meters for 10')
Did you crop the image before printing? If you didn't crop - more will appear in focus than if you cropped (but of course, everything 8.25" will be in perfect focus no matter how you crop.)

So the charts that say how much in front and behind will be in focus is just a general rule that some people decided decades ago so that you can talk about the relative difference, it is not saying what will really be 'in focus' when you are viewing the final product, just relatively speaking what will be in focus.

Let's look then just at the f-stops and how that affects the picture

If you look at the charts that show a comparison of f-stop to what is in focus, they all show a range. The closer the point you focus on, the less in front and behind will be in focus. So if you have a large f stop (a small number like f2.8) and you take a picture of a person, and you focus on their eyes, the tip of their nose may be visibly out of focus. But if change nothing else and you have the person 100' away, their eyes, nose will look in focus and everything perhaps 50' behind them and up to 10' in front of them will appear 'in focus'. But note carefully, this means the person is much smaller in the final image. In the first image, you might only have their face and shoulders, in the second you have their whole body, the barn, the trees, the house, the hill and the patch of flowers in front of them in the image.

If you are used to letting your camera make all the decisions for you. Now is the time to find that 'Aperture priority' mode on your camera and take a bunch of pictures at different aperture settings. You will have to set your camera and lens both to Aperture priority for this to work. With some, the camera will instruct the lens, on others you have to tell both the camera and the lens. Look for a dial that has things like A, S, M, P (Aperture manual, Speed, technically the S stands for 'Shutter', manual, fully Manual or Programmed) or an a Canon: Av, Tv, M, P (Aperture manual, Time manual, fully Manual or Programmed). Set them to A to get control of the Aperture (Note: Some reading this will say "Or set it to M" - but if you know that, you probably know enough that this article is too 'beginner' for you anyway.) Then look on your lens and see if it has a "A" followed by a bunch of f numbers. If so, moving it off of "A" puts it into manual aperture mode. (This is NOT the AF - that is for Auto-Focus vs MF, Manual focus.) On some camera/lens combinations, like my Canon 20D with 2 fairly modern - newer than 2002, I change one dial on the camera and then set the aperture using a second dial after pushing a button. Check your camera manual for how yours works, but note that if you have a new camera and older lens, you may still have to set the lens separate from the camera.

But this is such a good trick, if you have always just shot in the automatic modes, you should try this and learn how to quickly go back and forth between Aperture priority (manual) mode and Programmed mode. You will gain a huge amount of creative control with very little extra effort.

In another article I'll write someday, I'll talk about shutter or speed or time priority and how you gain creative control in a completely different way using time priority. All I'll mention here is that if you want creative control of focus AND you want creative control of the shutter speed - you will have to use the third value - the ISO to let your combination of shutter speed and aperture work. Remember: these three, shutter speed, aperture and ISO in the right balance is what is necessary to juggle to get the perfectly exposed picture.

And now some very general, creative, tricks

When taking portraits, you often should use a big f-stop (a small number near the smallest number your lens can do, typically numbers like f1.2, 2.8 or 4.5.) That way you can put the background out of focus. If your lens can't go down smaller than f8 or f5.6, you may find it hard to achieve this 'easily' but there is a trick below "focus in front."

When taking pictures of flowers near by and mountains far away, you might want to use the smallest hole (biggest f-number) your lens lets you - based on how much light there is. Try changing the ISO setting up higher and higher, letting you have enough light if you need to to make this work without too much blur from your camera shot.

When taking pictures of toy dioramas along with life size backgrounds, you are going to have to get a pinhole camera or make a change to your camera to have a pinhole. There are several articles on the internet that show how to do this with a bellows or body cap. I've noticed that a lot of them have 'blurry' images. These are 'in focus' but blurry. I'm pretty sure the reason is that they are using things like black construction paper, and so as I discussed above, they have a tiny tunnel, not a tiny hole. Look for articles that show very sharp pictures. They will be using a sheet of very thin metal, and just like I had to do when I was 9 years old, they will sand down the metal around the hole so that the diameter of the hole is significantly bigger than the thickness of the material being used to make the pin-hole camera. I haven't done this since the late 1960's - but I'm pretty sure God hasn't substantially changed physics in the last 50 years, so find an article that shows you properly how to do this and, if sensor noise doesn't make it a failure (a camera that can take good multi-minute pictures at night will have a good low-noise sensor), you will be able to take some stunning pin hole pictures. Yes, I definitely have to put this back on my bucket list ... it has been too many decades since I did this.

"Focus in Front", "Focus behind", what is this? You may have noticed that when you set your focal point, there is substantially less in FRONT of your focal point that is 'in focus' than what is behind. This is because the further away an object is, the slower it loses focus. So this means that sometimes you are better of NOT focusing on your subject, but focusing a bit in front or behind.

If you are using a lens that will only go down to f5.6 or f8, or if the 'background' is too close to the subject but you still want the background out of focus, you can many times achieve this by focusing in front of your subject and relying on them being 'enough' in focus and the background being enough out of focus. With film cameras a lot of this was guess and waiting a week for your film to see if it worked. With digital you can just try focusing successively closer and closer to the camera. Indeed, just take a bunch and decide later which was best.Generally to do this what I do is let the camera initially focus, then I switch to manual focus and slowly move the focus further and further in front of the subject while taking multiple pictures.
If you don't have enough light to use a small f-stop (an f-stop with a high enough number) and you want the background to be 'more' in focus than it is, you can often use the 'focus behind' trick. This is the reverse of the above trick. Do it the same way, but this time move the focus more and more 'behind' the subject. With this second trick, note that you may not be able to see the effect unless your camera has, and you use, the 'preview' feature to 'stop the lens down' so you can see how much in focus the background is. But again, I often just take a bunch manually, starting with the focus on the subject and changing the focus slowly further back while taking a bunch of photos, then pick the best one later.

Some other tricks

Tilt and shift lenses, which are very expensive, allow you to use your digital camera like my large frame view camera. Depending on the features of the lens, you get to change the focal plane.

So what does that mean? On a normal lens, the focal plane is 'everything that is such and such a distance away from the camera.' Let's use 20' as our focal plane. On a normal digital camera (not digital view camera), with a standard lens - zoom or otherwise, everything that is 20' in front of the lens will be in focus.

But with a tilt and shift lens, you can play with this. Imagine a person lying on a hill with their feet close to you and head further away. On a normal lens if you focus on their face, their feet will be more or less out of focus. But a flower 10' to the right of the face and at the same height as their feet will be in focus.

With the tilt and shift lens you can change the 'plane' to be the plane that their head and face are in. Now that flower we talked about, even though it is 20' away (like the person's face) will be very much OUT of focus - but the person's feet will be IN perfect focus!

I have used this in real life to have flowers in front of me and mountains near the top of the shot to be in perfect focus, but the hills in-between were out of focus - but it wasn't obvious. I did this by tilting the plane of focus so that the flowers and mountains were both in perfect focus. The eye is tricked into thinking that the things in-between are in focus when they really aren't. Now, this specific 'trick' done exactly this way won't work if your main subject is a building in the middle. But I have several times used it for pictures that I have things close and far away that I want in focus.

Note the 'shift' part of the tilt and shift lens is used to have buildings look like they are standing up straight instead of looking like they are falling over. The shift part changes the perspective, not the focus.

If you have a tilt and shift lens, there are all sorts of creative things you can do with this. You can have things close on the left in focus and things far away on the right in focus. You can combine the two tilts and have things close in the bottom right and things far away in the upper left in focus.

A lot of architectural and 'fashion' magazine shots take advantage of the tilt (and shift of course) to change the focus (and perspective) of the shots. If you want to have a 'trick' lens - and you are trying to decide between buying a 'fisheye' lens or a tilt and shift lens - the tilt and shift lens will have all sorts of uses once you have it (I personally still use my view camera for this purpose) whereas the fisheye lens will almost never be used after the first dozen or so shots. The tilt and shift lens will be used for years once you find out how versatile it is.