Infinity focus and DoF

[sdewal]

Hi Friends

I am an amateur photographer. I have been looking all around on internet to clear my confusion on infinity focus but all in vain. Couldn't find the information I am looking for anywhere, then I thought to ask it from the masters.

There are DoF calculators available online to calculate the DoF with the given set of Focal length, aperture and subject distance. I have two questions.

1. If, for example I am shooting a landscape at 28mm, f/11, what is the subject distance to calculate the Dof in this case because I do not have a subject in the FG. I want to get the far hills and trees in the sharp focus and I have grass land in between me and the hills but no specific subject to focus at.

2. If, now I focus at infinity with 28mm, f/11 then is there a way to find out the starting point of the DoF?


My questions may sound silly to the experienced but for me they are my biggest challenges at the moment.


Many thanks
Sdewal

[outwithmycamera]

The depth of field calculator on this site can help you figure that out, and if it is something you want to determine in the field there are apps you can install on a smart phone. It used to be that lenses had the DoF scale built into them and you could simply read that information off the lens barrel. That is an unfortunate loss I I suppose in the digital age as lenses no longer have aperture rings and DoF scales. What you might do is create a table of different apertures and the near focus point for infinity focus - that way you only have to refer to a table and not have to do any calculations.

[DanK]

I think the DOF markings disappeared because modern lenses rotate through a very small arc, I order to facilitate AF. There isn't room for DOF markings.

The app I use for this on my iPhone is called Field Tools Depth of Field Guide. There are lots, but this is the one I liked best of the few I tried. You set it up with information about your camera and lenses (the latter to set limits on the sliders). It will display near and far, hyperfocal distance, etc.

[ajohnw]

There have been a number of posts on this recently.

One method is to focus roughly 1/3 into the shot as seen through the viewfinder. What aperture? In real terms it's best to go out and shoot some landscapes anywhere of anything just to find out. The magnified preview on most cameras will show gross problems and the shots can later be looked at full sized. One of the problems with DOF calculators is that they are based on a certain final image size. It gets way too complicated to fully exploit in use and there is plenty of evidence that they don't hold up that well particularly when viewed on a PC screen anyway.

I think there is a comment in the tutorial on this site that it is really there to explain principles. We don't all walk about with laser range finders.

John
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[GrahamS]

There are DoF calculators available online to calculate the DoF with the given set of Focal length, aperture and subject distance. I have two questions.

1. If, for example I am shooting a landscape at 28mm, f/11, what is the subject distance to calculate the Dof in this case because I do not have a subject in the FG. I want to get the far hills and trees in the sharp focus and I have grass land in between me and the hills but no specific subject to focus at.

2. If, now I focus at infinity with 28mm, f/11 then is there a way to find out the starting point of the DoF?


Sdewal

You need to calculate the hyperfocal distance for the aperture and focal length you are using and set your focus on this distance. You can calculate the hyperfocal distance using the same AP on your smartphone as you use for calculating DOF.

[dje]

If, now I focus at infinity with 28mm, f/11 then is there a way to find out the starting point of the DoF?

Hi Sdewal

If you put a large number for the distance to subject into a DOF calculator, say 10,000 m, you will get a good idea of the starting point of the acceptable DOF range. eg for 28mm, f/11 on a 1.6x crop factor camera, the figure you get is 3.6m.

Dave

[Colin Southern]

Just focus about 1/3 into the scene. With a WA or UWA lens the DoF will be huge at medium to minimum apertures.

[sdewal]

Hi Sdewal

If you put a large number for the distance to subject into a DOF calculator, say 10,000 m, you will get a good idea of the starting point of the acceptable DOF range. eg for 28mm, f/11 on a 1.6x crop factor camera, the figure you get is 3.6m.

Dave

Hi Dave and all

Thanks a lot guys.

I guess I was looking at it from a wrong angle. I think the distance to the subject is just a guiding factor in getting the right DoF in the picture. If I am shooting a landscape then there is no subject 3 meters away from the camera but assuming that if there is a subject at 3 meters (f/11, 24mm on a FF sensor), I get the following results:


Depth of field
Near limit 1.09 m
Far limit Infinity
Total Infinite
In front of subject 1.9 m
Behind subject Infinite
Hyperfocal distance 1.72 m

Now I can manually pre focus my lens either at 3 meters mark (this will start the DoF at 1.09 meter from the camera) or at 1.72 meters mark (this will start the DoF roughly at 0.82 meters from the camera).


Am I right here?


"Colin Southern

Re: Infinity focus and DoF
Just focus about 1/3 into the scene. With a WA or UWA lens the DoF will be huge at medium to minimum apertures. "

Thanks Colin. I think this is the quickest way to get better results without going into mathematical details however it makes a difference for a learner like me to understand the basics as well.


Thanks friends

[Colin Southern]

Put another way, what you're really getting at here is the hyper focal distance.

You can measure it manually, but in most cases it's about as hard to miss as the proverbial side of a barn at 5 paces with a shotgun.

It's only when you have to deal with wide apertures and short distances to foreground subjects that you need to break out the calculators.

[rpcrowe]

Colin wrote and I totally agree...
"It's only when you have to deal with wide apertures and short distances to foreground subjects that you need to break out the calculators."

I have never worried about focus when shooting with a relatively short focal length (28mm could be considered fairly short, even on a crop camera) and when shooting around f/11 or f/8 for my landscape shots. I certainly have never even thought about using a calculator to figure out my focus in cases like this. That is simply muddying the water.

[jcuknz]

If there was nothing in front of me to focus on I would look behind for something at that distance and continuing to hold half trigger [ or using AF Lock ] turn back to the scene I want to record and press the trigger. If you are using a single spot focus you place that area on the grass someway in front of you and the camera will focus at that point for you. The difference of the far hills between being really sharp and a little soft is so slight I wouldn't worry about it, but grass OOF in the foreground is a very discernable problem.
Colin's suggestion of focusing on something one third into the scene is probably how I would do it.
I only look at DoF tables in relation to questions on blogs, never to photography
I doubt if I would take a photo as you describe becuase I always try to have something in the near distance to focus on to give distance to the shot and infinity takes care of itself so long as that object is sharp.

2] If you can focus at infinity and I'm not sure how one does that with a digital camera you are wasting most of your DoF. When you have a lens with DoF markings [ old legacy lenses ] you place the aperture number to the infinity mark and the other aperture number shows the near distance.

Here the aperture is set to f/16 and above the infinity mark is positioned next to the figure sixteen and we can see that the lens is focused at about five metres or fifteen feet and the other figure sixteen , indicating the near limit of DoF is at about eight feet .... so for however the calulations were worked out by Pentax for a 35mm film camera the DoF at f/16 ranges from 8'<infinity when focused at 15feet. I suppose this means that the hyperfocal distance for the 50mm lens is 15ft at f/16 but then except for blogs I have never bothered about HF distances .... sorry to those who do

My reference book suggests I am wrong and it is 15ft at f/11 and at f/16 it would be a bit closer [ Ilford Manual of Photography 1948 edition. ] edit --- Looks like Pentax erred on the conservative side.

[Colin Southern]

Colin wrote and I totally agree...
"It's only when you have to deal with wide apertures and short distances to foreground subjects that you need to break out the calculators."

I have never worried about focus when shooting with a relatively short focal length (28mm could be considered fairly short, even on a crop camera) and when shooting around f/11 or f/8 for my landscape shots. I certainly have never even thought about using a calculator to figure out my focus in cases like this. That is simply muddying the water.

Here's a good example:



It was shot at F4; the background is a little soft, but not too bad ... and it's shot at F4 for goodness sake.

[ajohnw]

A comment on old lenses. In use it wasn't a good idea to make use of the markings on the lens for focal distance as they were not accurate enough. Focusing was done by eye. DOF preview was often a bit iffy as the light reduction was likely to prevent the focusing aids in the viewfinder from working. In fact some types would go pitch black. The depth of field indicators were of some use as a guide.

When the calculators do come out for certain types of shot there is also a need to calculate the blur in the background as well. What actually happens is that the photo is simulating human vision. Our central vision is sharp so if this is done correctly even exaggerated anyone will automatically look at the subject rather than the whole shot as soon as they look at it. I sometimes feel it's easier to do in black and white as colours in the back ground can be distracting. Also when I see good examples there aren't any. Examples can be seen in adverts in magazines, on the TV and all over the place.

I just try and gauge it by eye from time to time and not very often. I did see an example worked long hand. It gets complicated and even accounts for the final viewing size. I also once submitted a slide to a competition where I had used a slight extension to the idea. Basically focusing ever so slightly short of the subject. Flamingoes in this case. It can give a very slightly soft effect. The judge was very impressed, best he had seen in a long time. A long lens so the background was taken care of as well. He then blew it up to an immense size to see how I had done it and concluded that I didn't know how to focus the camera. A friend pointed out how long it took me to set up the shot and that I intended to do just that and he looked rather embarrassed. The fact that I got it right has to be part fluke.

John
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[jcuknz]

When I used my Leica's I always used the lens markings but when I got an SLR I rarely did. The catch to that statement is that until I got the SLR I normally estimated distances, rarely using the rangefinder and with the SLR I had aids built into the screen to help me which are not normally found in DSLRs which are intended to be used with AF rather than MF.

[Downrigger]

As a still-a-lot-to-learn about making a passable image beginner I have gone geek on this as follows:

Here is a way to estimate (emphasis: estimate) the hyperfocal distance assuming your system has a circle of confusion of 0.03. (full frame DSLR). It provides rough estimates, but that’s all one needs to have an idea of what they have available in DOF to infinity, if they want it.

One multiplies the square of the focal length times four for f8, by three for f11, by two for f16 and by one and one half for f22. I find these multipliers are sensibly stepwise, so not too hard to remember. The resulting number is in millimeters so you drop three decimals to get meters.

If focused on the hyperfocal distance, everything from half way to it, all the way out to infinity is in focus (theoretically).

For example, at f11 and 16mm focal length one gets three times 256mm = about 770 mm, or a little less than a meter –
For another, at f11 and 200 mm focal length one gets three times 40,000 mm or 120 meters.

Truly, the practical advice to focus one-third into the field or other tricks surely more often than not make more sense than the math. But as long as one can square those focal lengths in one’s head and multiply by four, three, two, or one and a half, one can get a rough, working idea about how to focus for maximum DOF that includes infinity. Additionally, for long focal lengths (as in the example above) the hyperfocal distance is great enough that estimating is fraught, and measuring foolish. But for short focal lengths, one gets returns for the hyperfocal distance in fractions of, or a few meters, and this is easy to estimate and add to your thinking about how to set up a shot.

[Stagecoach]

Arh, there's an interesting scene, lets take one at f11, one at f16 and one at f22....................... my rule of 3s.

Grahame

[Colin Southern]

Arh, there's an interesting scene, lets take one at f11, one at f16 and one at f22....................... my rule of 3s.

Grahame

On a WA or UWA I'd put a chocolate fish on there being no discernible difference to anything except foreground.

[Stagecoach]

Colin, you win the fish

On another considertaton I wonder how many people can actually judge distance such as a third of the way into the scene for a landscape? As a test, I have a view from my window well into the distance with some prominent objects/buildings between and tried to guess a distance of a third and half. I then checked this using google earth and my judgement is well out.

Grahame

[sdewal]

Here's a good example:



It was shot at F4; the background is a little soft, but not too bad ... and it's shot at F4 for goodness sake.

Hi Colin

It is a very beautiful shot. I don't know when would I be able to take shots like this. I believe you would have taken it at a fairly longer shutter speed.

Two questions

Where did you focus?

Where did you take the meter reading from?


Thanks

[Colin Southern]

Colin, you win the fish

That's the problem ... I've won too many sugary fish over the years

On another considertaton I wonder how many people can actually judge distance such as a third of the way into the scene for a landscape? As a test, I have a view from my window well into the distance with some prominent objects/buildings between and tried to guess a distance of a third and half. I then checked this using google earth and my judgement is well out.

It honestly doesn't make any difference as it doesn't affect the focus of objects in the "distance to infinity" range (assuming narrow aperture). I think most people underestimate the latitude in picking a focal point.