Focus/Recompose Rule Of Thumb

[amolitor]

At some point pretty early you'll learn to use single point focus, to do that, and then to recompose the picture after you've locked focus. This works fine, a lot of the time. Geometry tells us that whatever you focused on isn't quite the same distance away after you recompose, though. So your focus isn't quite perfect when you recompose. The more radically you move the camera to recompose, the worse it gets. How can you tell how much you can get away with?

First you need to know, or remember, that your f-stop determines how much depth of field you have which is, roughly, how wrong your focus can be before it starts to look all unsharp and bad. You can look up more about Depth of Field elsewhere. What you need to know here is that the bigger your f-number, the more you can get away with.

Second, hold your hand out at arm's length, and see how wide your thumb appears to you. It should be about 2 degrees unless you're somewhat non-standard. That doesn't matter. Yes, this rule of thumb uses actual thumbs!

Here's the rule: You can recompose by one thumb-width per f-number. At f/2.0 you can move the camera's view from one side of your thumb to the other. Here's an example:

Let's say you focus on the model's eyes. Then you recompose so that same focus point has moved to the middle of her chest. Hold your thumb out at arm's length and estimate how many thumbwidths would cover the distance from her eyes to her mid-chest. Let's say it's 5 thumb widths. Then you better be "stopped down" to about f/5.6 or so.

CAVEATS AND WARNINGS:

- this rule falls apart for closer distances, under about 6 feet, or with lenses longer than about 100mm. It is useless for macro work.
- with closer distances and longer lenses, be more conservative
- with longer distances and wider lenses (say 50mm lens at 30 feet) this rule is pretty generous you can push it a fair bit
- don't use it for more than about 8 thumbs, that's a lotta recomposing there, find a better way to focus on what you want
- if you have multiple focus points, use the closest one to minimize the amount you have to recompose
- it's always possible i have botched the math, so, use with caution

 

[cgipson1]

- it's always possible i have botched the math, so, use with caution

LOL!

 

[Designer]

- it's always possible i have botched the math, so, use with caution

!

 

[table1349]

I have always applied the K.I.S.S. principal with focusing. Use the single focus point that lands on the area of the subject you most want in focus. Especially when shooting portraits.

 

[amolitor]

I am of course pretty confident that I have the math right. Presumably someone will check my work shortly, and then you can all be that much more confident.

 

[table1349]

Why Focus-Recompose Sucks

The Problem With The Focus-Recompose Method - Digital Photography School

For Sharp Photos, Should You Be Using the Focus-Recompose Method? at PhotoWhoa Blog | Discover how you can be a better photographer

 

[cgipson1]

With modern DSLR's and movable focus points... I think that people should seldom if even, have to focus / recompose. It can be done successfully, if you know your gear, and have a basic understanding of DOF. I usually recommend that those with little experience DON"T focus and recompose, because I have seen too many shots that were OOF from doing it.

If you can't find a composition where you can use the focus point adjustment.. then you need to recompose, and then Focus... (except in rare cases)

 

[cgipson1]

Why Focus-Recompose Sucks

The Problem With The Focus-Recompose Method - Digital Photography School

For Sharp Photos, Should You Be Using the Focus-Recompose Method? at PhotoWhoa Blog | Discover how you can be a better photographer

Exactly!

 

[Derrel]

One of the strengths of the pro Nikon bodies that I like is that they can focus extremely well using the off-center AF points, so I use the giant, oversized multi-controller on the back of the camera to move the AF point to the spot I want to focus on. I never use focus and recompose, since the equipment itself is so good these days that there's no need to do it.

If the camera can use a small group focus, like 9 out of the 51 AF points in a tight cluster, it makes it easy to select a "top" group [camera is being held vertically, naturally], and that allows the camera to track a person as they do an activity, so that sequences can be shot, and the camera keeps continually focusing as the subject moves and goes about his or her activity. In social photography situations, this allows the photographer to use the camera in a very fluid way, focusing on-the-go, as opposed to stop-let me focus-okay-I'll-shoot-a-sngle-frame-now. Using a multi-point AF cluster also allows active, moving subjects to be photographed on the go, like people walking or running, kids engaged in play, and so on. Focus and recompose is mostly for static subjects, and is pretty much useless on subjects that are moving.

Different cameras have different specific focus system controls and focusing protocols. Newer cameras have the AF-A option, whereas old models do not.

 

[hopdaddy]

Why would anyone want " how wrong your focus can be before it starts to look all unsharp and bad."? I thought one would want the best possible sharpness ? NO?

 

[amolitor]

Why would anyone want " how wrong your focus can be before it starts to look all unsharp and bad."? I thought one would want the best possible sharpness ? NO?

Photography, like so much else in life, is a series of compromises. Lots of people recompose and focus, and now anyone who cares has the tools to quantify how much recomposition is acceptable when, where 'acceptable' uses pretty universally defined standards of acceptable sharpness from DoF tables. The same DoF tables the naysayers in this very thread are rather fond of chirping about, when they're not busy mumbling 'oh, it's Molitor, quick, I need to get my penis out and piss on him, because that's a thing I just have to do'

 

[Josh66]

I am of course pretty confident that I have the math right. Presumably someone will check my work shortly, and then you can all be that much more confident.

My right thumb is 1 inch wide (wow - didn't think it would be that big!), at arms length, it is 27.5 inches from my eye.

You said it should be about 2°... At 27.5", 2° would cover 0.95975" of my field of view - that's pretty much dead on.

I was bored.

But, hey! At least now I have a handy way of measuring angles! My thumb at arms length covers 2°, almost exactly.

 

[Josh66]

One of the strengths of the pro Nikon bodies that I like is that they can focus extremely well using the off-center AF points, so I use the giant, oversized multi-controller on the back of the camera to move the AF point to the spot I want to focus on. I never use focus and recompose, since the equipment itself is so good these days that there's no need to do it.

I always select the closest focus point to where I actually want to focus, to keep the recomposition to minimal levels. None of my cameras have a ton of focus points though, so there is almost always going to be 'some' recomposition. Even on a camera with only 3 focus points though, one of them is almost always surprisingly close to the spot I want to focus on.

With rangefinders though, I think I'm stuck with focus/recompose. Unless I break out a tape measure...

 

[usayit]

I shoot with a 50mm f/1 on FF on many occasions. Its a real concern but not as bad as most would expect. Do the math to give you an idea....

For the most part, I simply lean back during recomposition. Takes a bit of practice but its not that bad and its true to KISS. Once you start shooting at smaller apertures f/2.8 and on, its really not that much of a concern IMO. Other's have told me they dial in a little front focus by "feel"... didn't work well for me but I'm sure it depends on the person.

Here's one of my test shots during practice as my way of getting a feel for the lens:

 

[amolitor]

If you stick to a single thumb-width, the change in target distance is something like 6/100ths of a percent, which is probably a lot less than the focusing error of whatever system you're using.

The trouble with "leaning back" is that the amount you have to lean back depends on the target distance. On the other hand, at greater target distances it matters less. I expect you can get used to it, and kind of feel the right amount, especially if you have a good viewfinder.