Where is infinite focus?

[Simonch]

Hey folks, time for a stupid question!! i have a 75-300 lens for my digital rebel, its the cheapest lens that CAnon do, and doesnt have any markings on the focus ring, where about is the infinite focus? or does this lens even hav such a thing?!

 

[EOS_JD]

Check out the depth of field calculators. Longer lenses do do lend themselves well to focusing on infinity. You need to focus on a subject many metres away in order to get the far limit to infinity. Depth of field is determined by focal length, subject distance and aperture.

At 300mm f22 you would need to focus on a subject 206m away and the near limit of focus would be 103.9 away with the far limit being infinity.

Using the calculator will provide you with more info on what you need.

 

[Big Mike]

Just focus on something very far away...that's your infinity focus.

Or, put it into manual focus mode and turn the ring all the way until distant objects are in focus.

 

[Simonch]

sounds complicated!! Mike, the reason im asking, is that i was tryin some night stuff, and was struggling to get it in focus, so couldnt really see what i was shooting, to focus it! tough times! lol.... i shall google the DOF calculator, and see what i make of it! thanks folks!

 

[WDodd]

Hey I have that lens and used it for fireworks in which case I used a larger aperture like F/13 etc. and focused on something in the distance (I used distant illumination) its a bit tricky but it worked out fine.

 

[Garbz]

The problem with setting it manually is that some lenses focus further than infinity. May sound stupid but if one of the lense elements moves because of a drop or something it can spare you a trip to the shop.

This is something you should try to figure out during the day. If your lens doesn't have markings for it use a white pen or something to make a line between the focusing ring and a stationary part of the lens. So in future you can go there straight away.

 

[Simonch]

yeah sounds like a plan!! thanks a lot guys! much appreciated!

 

[Mike_E]

Argh, I can't stand it anymore. It's 'second star to the right and straight on till morning.

I am truly sorry. LOLOLOLOL

 

[Chris of Arabia]

Surely, that's 'to infinity and beyond'....

 

[six-five-two]

Remember, if you are shooting landscapes, set it to auto focus to infinity. Also do this at night. I accidently used auto focus one time and all my photos were blurry (during a few night snapshots)

 

[Hertz van Rental]

if you are shooting landscapes, set it to auto focus to infinity.

This is not really the best way to focus for landscapes.
The depth of field of a lens extends both before and behind the point you focus on. If you focus on infinity then approximately half of the available depth of field falls beyond infinity and is wasted.
If you focus on the hyperfocal distance then the depth of field behind this point goes to infinity (or near enough) and the depth of field in front falls much nearer the camera. This optimises your focussing and gets far more of the subject in focus.
The hyperfocal distance is defined as the distance from the lens to the nearest point of the subject which is just acceptably sharp when the camera is focussed on infinity.
The hyperfocal distance varies with focal length and f-number so you have to work it out for each f-number and each lens. It's worth the effort.

(focal length)squared/f-number x diameter of the circle of confusion = hyperfocal distance.

The diameter of the circle of confusion varies according to negative size and expected degree of enlargement.
A reasonable figure for the c of c can be calculated:
0.25mm/degree of enlargement.
A 10x8 print from a 35mm neg has a magnification of x8. So the c of c would be 0.03mm.
A figure of 0.02mm or 0.015mm would probably be the maximum for general purposes. This calculation should also be adequate for digital.

 

[Helen B]

Not everyone agrees that setting the lens at the hyperfocal distance is always the best way of shooting landscapes. There are many cases when biasing the depth of field towards infinity, or simply focusing on infinity, may be preferable.

Depth of field is all about setting the limits of acceptable blur. If you set the lens to the hyperfocal distance the horizon will not be in sharp focus, but it will be out of focus - in fact it will be just on the limit of acceptable blur. Theoretically the blur will be undetectable to a human at the print magnification and viewing distance that was used for the DoF calculation. At closer viewing distances or greater magnifications it will look out of focus. It may even seem a little fuzzy to some people at the designed magnification, particularly if there are sharper parts of the picture. In many cases the horizon is the most important part of the picture to have sharp.

There is a good case for setting the lens to the hyperfocal distance for an aperture two stops wider than the one in use. This means that the horizon will not be at the limit of acceptable blur, but should be safely below the limit. The actual aperture can be used for determining the near limit of depth of field because it is generally less critical than having the horizon razor sharp. Every case is different and should be judged on the particular circumstances.

" (focal length)squared/f-number x diameter of the circle of confusion = hyperfocal distance."

Do you mind if I rewrite that?

h = f^2 / (NC)

f is the focal length
N is the f-Number (ie 2.8, 5.6, 16 etc, not 1/2.8, 1/5.6, 1/16 etc)
C is the diameter of the c-o-c.
The units should be consistent -ie everything in millimetres, or everything in metres, or everything in inches.

Best,
Helen

 

[Hertz van Rental]

Depth of field is all about setting the limits of acceptable blur.

Surely deciding on the diameter of the circle of confusion is setting the limits of acceptable blur. The depth of field merely tells you where the limit is on the subject.

The hyperfocal distance, like most things in Photography, is just a tool. So of course there are situations where it is not appropriate.
But in general it is the best way to get the most out of your lens - certainly with large format.
I don't know where you live but around here the horizon is 5 to 10 miles away. If you do the depth of field calculations you will find that an awful lot of the mid ground and all of the foreground could well be out of focus. These areas in most landscapes are more important than the horizon as they contain more detail. We are used to seeing the horizon as being slightly soft due to atmospheric conditions so having it soft on a print is not a problem.
If you read my post again you will see that I reduced the size of the circle of confusion - if you look at the calculation you will see that doing this has the same effect as moving the hyperfocal distance but is more accurate.

As for 'close viewing distances'. A lot of work has been done in this area and it has been established that people always view images to maintain the same aspect ratio. That is to say, the preferred viewing distance of an image is twice it's diagonal. If you spend time at a gallery watching people you will find it to be true. :mrgreen:

 

[Helen B]

Herz,

I was just trying to say that not everyone shares your 'one size fits all' opinion of post #11, and that there is an alternative to the conventional approach.

Interested folks might like to read Harold Merklinger's The Ins and Outs of Focus if they haven't already done so.

The technique I described has worked for me - it isn't particularly important how it is formulated, the basic concept is that there are times when the plane of best focus should be biased away from the hyperfocal distance. You could also look at it as accepting a smaller c-o-c for distant subjects than for nearby ones. It doesn't matter.

Though I have spent a lot of time in galleries, and a lot of time looking at people looking at prints, I have never observed that people stick to the rule of viewing photographs from twice the diagonal. Often they appear to be overcome with a sense of wonder, dragged in to the world the photograph represents, and get rather closer than that.

I'll repeat a sentence from my previous post: "Every case is different and should be judged on the particular circumstances."

Best,
Helen

 

[Mike_E]

Must be nice being able to see that far in sharp detail.

Personally, I am more interested in what is between me and the horizon as I feel that a soft horizon gives a better sense of volume than a well defined border and having detail from your feet (close enough) towards the horizon seems a better leading agent.

Ah, but what do I know, I mostly take pictures of people.