I'm confused *more questions added*

Toothie

Toothie

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Ok this is kind of a DoF / focus question.

I get the whole depth of field thing more or less.

and from what i gather from reading stuff on here i get the most lenses have an apature where they take the sharpest pictures.

But what i dont get it focusing on infinity so you can use the lower apatures on landscapes and get everything nice and sharp.

So how do you focus on infinity?

how do you make landscapes nice and sharp?

And why doesn't using a high f number achieve this?

As i said i am totally confused and might be in a total muddle for all i know.

Please can someone set me straight?
 
Well, if you want your landscapes to be all sharp, you need lot of DOF. To do this, you might want to use a wide lens and a high F number, which means smaller aperture.
To focus on infinity, set the MF mark on the lens to infinity could do the trick. I prefer focusing to about the third of the frame and setting the F number to F11 or F16. However, beyond the 'sweet spot' of the lens, diffraction kicks in, so the image is less sharp.
 
I think what the OP is thinking about is a trick called Hyperfocal distance. It allows you to use a wider aperture and still maximise the depth of field. This allows you to get faster shutter speeds, use an aperture that doesn't suffer drom any diffraction and still get the depth of field needed. The easy way i do this is choose a mid range aperture (Say F8) and focus a third of the way into the scene. The full version is a lot more complicated than that;

http://www.dofmaster.com/hyperfocal.html
http://en.wikipedia.org/wiki/Hyperfocal_distance

:)
 
Slapo said:
Well, if you want your landscapes to be all sharp, you need lot of DOF. To do this, you might want to use a wide lens and a high F number, which means smaller aperture.
To focus on infinity, set the MF mark on the lens to infinity could do the trick. I prefer focusing to about the third of the frame and setting the F number to F11 or F16. However, beyond the 'sweet spot' of the lens, diffraction kicks in, so the image is less sharp.
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so at f22 say its not the dof or the focus that makes the image soft its the diffraction. which is where more expensive high quality glass comes in??

and if u have the same conditions and equal quality lenses. and took a picture with a wide angle and a telephoto lens on the same settings you would get more dof on the wide angle lens?
 
Messiah Khan said:
I think what the OP is thinking about is a trick called Hyperfocal distance. It allows you to use a wider aperture and still maximise the depth of field. This allows you to get faster shutter speeds, use an aperture that doesn't suffer drom any diffraction and still get the depth of field needed. The easy way i do this is choose a mid range aperture (Say F8) and focus a third of the way into the scene. The full version is a lot more complicated than that;

http://www.dofmaster.com/hyperfocal.html
http://en.wikipedia.org/wiki/Hyperfocal_distance

:)
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i think you might be right i will check out the links. thanks
 
ok so have read the hyperfocal distance thing.

But how does that work practically with out measuring the landscape?? am i supposed to know how far the tree is from the mountain?
 
Toothie

Use the camera to measure the distance. Simply point the camera at the near object let it focus , and read the distance of the focusing scale. Mountains to all intents, are at infinity.

You'll need to use a DOF calculator to find out what aperture is needed to get both subjects sharp.

An alternative is to use the online D O F calculator to produce a chart based on a range of focal lengths and apertures. You'll probably just set it up on the hyperfocal distance.

http://www.dofmaster.com/dofjs.html


Simply put in the camera ,lens and aperture details . Put in a distance of 10 feet. It'll then calculate the hyperfocal distance. Now substitute that into the focus distance and it will calculate the near point. Transfer this to a card and carry it with you. You'll have a table that shows focal length, aperture, near point and hyperfocal distance. Use this to ensure the image is sharp.
 
Toothie said:
and if u have the same conditions and equal quality lenses. and took a picture with a wide angle and a telephoto lens on the same settings you would get more dof on the wide angle lens?
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If both shots were taken from the same position you'd have more DoF on the wider lens (less magnification). If you changed position, ie further away for the tele, so the shot was framed the same for each then the DoF would be identical.

Focal length has NO affect on DoF, only magnification and aperture. You get more DoF with wider lenses because they don't magnify as much and vice versa.

To a lesser degree sensor size, pixel count and the size of the viewed image also play a role as they define the size of the Circle of Confusion - which defines how small a blurred area can be before it becomes noticable.
 
It's a complicated subject and, to understand it fully, you need a basic understanding of optical physics.

But not to worry, the basics are easy. Depth of field (DOF) is a product of magnification and aperture.
Don't confuse aperture with f/number, they're not the same thing at all.

Aperture (AKA effective aperture or actual aperture) is the size of the hole in the lens that lets the light through.

f/number is a mathmatical calculation. Take, as an example, a 100mm lens (F100). Divide the actual aperture (say 9mm) into 100 and the answer is 11, so the f/number is f/11.
If it was a 50mm lens (F50) at the same aperture then the f/number would be f/5.6 (50/9=5.6 and conversely, if it was a 200mm lens (F200) at the same aperture the the f/number would be f/22 (200/9=22).

So, although wider lenses don't produce the same depth of field as longer lenses as longer lenses, they do produce the same depth of field at the same actual or effective aperture. A lot of people get confused about this, believing that wideangle lenses have more depth of field than longer lenses. A 50mm lens at f/5.6, a 100mm lens at f/11 and a 200mm lens at f/22 all have the same size aperture so all produce the same DOF.

So far so good.

Infinity is NOT a defined distance. In photography, it's just the distance at which changing the focus point will make no difference. At say f/2.8 with a 300mm lens, that will be an enormous distance because the aperture will be a massive 107mm and the image is being magnified, so depth of field is very limited. But with say a 18mm lens at f/11 the aperture is only 1.6mm and the wideangle lens creates negative magnification, creating an enormous depth of field so infinity will be very close.

Hyperfocal focussing is focussing at a point closer than infinity but at which (say) infinity will APPEAR to be sharp because the depth of field is enough to make it LOOK sharp. The advantage of this is that points nearer to the camera will LOOK sharp too. In fact, only one point or plane of focus can ever really be sharp, the depth of field tables just give an idea of what will LOOK sharp to the average person if viewing a print from the 'correct' distance - that's the magnification bit!

BTW, diffraction limitation (an overall loss of sharpness caused by using too small an f/number) is related to f/number and film/sensor size. Basically, with an APS size sensor, don't use less than f/11, with a full frame, f/16 is about the limit. It affects all lenses equally but is more obvious with better lenses because the lack of sharpness is more obvious. Some of the kit lenses are so unsharp that diffraction limitation really doesn't show. It's still there though!
 
i think i am following this although possibly not. i think i will keep reading it and coming back to it until i understand.
 
Toothie said:
i think i am following this although possibly not. i think i will keep reading it and coming back to it until i understand.
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This one is a mare isn't it. I've been shooting stuff for quite a while and I could bore you for hours on anything from anomalous reflectance to why only half a car is sharp on a panned shot..... this is about the last area of snapping tech that isn't embedded in the little grey cells.

Gary's answer is the clearest I've seen yet and makes a whole lot of sense out of a nonsense topic. :thumbs:

I think that once you've got your head around the bit where the actual size of the aperture changes with focal length for any given f number, and that using hyperfocal distance is nothing more than putting the back of your depth of field at the back of your picture... you'll have it nailed. :D
 
there no hope for me i found Gary's explanation the most confusing. Starting to think i live in this Circle of Confusion of which you speak, and i can't find the door.

Sorry Gary nothing personal, physics not my best subject and i have real trouble applying theory to practice.

BTW where's the focusing scale?
 
Toothie said:
there no hope for me i found Gary's explanation the most confusing. Starting to think i live in this Circle of Confusion of which you speak, and i can't find the door.
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Definition of Circle of confusion... A group of photographers talking about depth of field:lol:

Seriously though, it can be a difficult subject to grasp, then it just clicks.
Just remember that DOF is a product of actual (or effective) aperture not f/number and magnification (because the more you magnify something that's out of focus, the more obviously it's out of focus) and it all starts to make sense.

Back in the bad old days when I did my photography degree, we had to have a physics 'A' level. Now all that people need to be able to do is to write a cheque and sign their name, with help if necessary. 'A' level physics was probably overkill, but it did help...
 
I keep it very simple for my little brain cell and try not to think too much about the technicalities!!

As far as my brain works, if I am trying to do a landscape picture I use f8 to f11 and focus 1bout 1/3 of the way into whatever it is I am shooting so I know both foreground and background will (if I keep my hands from shaking) be sharp. If I want foreground only I use as close to f2.8 as possible, and if I want the furthest bit clesr but not the foreground I use one in the other direction. I can understand the reasoning and why behind it when I am sitting down without the camera, but for taking pictures I find they work better if stick to my simplified version as it saves brain power and lets me enjoy the camera more :lol:
 
Let me try a different tack. (Though first I should say that I *do* understand the maths and the physics - what follows is from choice, not from necessity.)

A key thing about DOF is that it's all a bit subjective. Always remember that there is only one ever wafer-thin plane which is *exactly* in focus, and for everthing else in the picture we're really asking whether the focus is "good enough".

All these complicated formulae will supposedly tell you the range that is in "good enough" focus, but it depends on how much you're going to magnify the picture, how closely you're going to look at it, and so on. The standard formulae assume you want to look at a 12x8 print at arms length, but if you change those assumptions you have to change the calculations and you get different results.

Plus, the transition from in-focus to out-of-focus is a gradual one. It's simply not possible to examine a picture and declare that the DOF extends from point A to point B.

In short, in practice it's not as scientific as the formulae make it sound.

I would suggest that for *most* photos, the photographer's interest in the DOF can be summarised in one of 3 ways:
(A) as little DOF as possible please;
(B) as much DOF as possible please;
(C) I really don't care too much.

For (A), you want the lens wide open. For (B) you want it stopped down to the point where diffraction becomes important - say f/11 or f/16. And for (C) you might as well work at the aperture where the lens is sharpest, which is usually around f/8.

Yes, of course there are situations that don't fit these categories. But how common are they, really? And is it worth over-complicating the subject when you can virtually always get away with applying simple rules like this?
 
I took another look at my first post this morning - it even confused me so I've edited it, mainly correcting typos. I really must remember that it's better to try to explain things like DOF before going to the pub, not after...

Mind you, the post by Stewart R (above) is a better explanation than mine anyway :-)
 
Firstly a big thank you to you guys for taking the time to explain this to me.

Secondly what i understand so far.

1. That aperture is not the same at the same f stop on lenses of different focal lenths. Because of the maginification.

2. DoF is really all about what looks in focus not what is in focus.

3. Infinity is just the point it no longer makes a difference.

4. Because of lens diffraction most lenses are sharpest around f8.

What this all means in the effort to get landscapes nice and sharp is.
compose the seen, focus about a third of the way in, use something between f8 - f16 on a suitable shutter speed, and take the photo.

That all sounds about right?
 
Toothie said:
Firstly a big thank you to you guys for taking the time to explain this to me.

Secondly what i understand so far.

1. That aperture is not the same at the same f stop on lenses of different focal lenths. Because of the maginification.

2. DoF is really all about what looks in focus not what is in focus.

3. Infinity is just the point it no longer makes a difference.

4. Because of lens diffraction most lenses are sharpest around f8.

What this all means in the effort to get landscapes nice and sharp is.
compose the seen, focus about a third of the way in, use something between f8 - f16 on a suitable shutter speed, and take the photo.

That all sounds about right?
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ps 5. this is something not to be explained or worked out after a night in the pub;)

ops i clicked quote not edit :bang:
 
Toothie said:
ps 4. Because of lens diffraction most lenses are sharpest around f8.


QUOTE]

More or less right except for your understanding of diffraction limitation.

It's probably true to say that most lenses are at their best about 2 stops down from maximum aperture, e.g. if the max aperture is f/2 then it will be at its' best at around f/4, if it's max is f/5.6 then it will probably be best at around f/11.

But that isn't the same thing as diffraction (actually diffraction limitation) which is a general loss of sharpness that occurs at small apertures.

Diffraction is NOT due to light passing through a hole, but rather to light passing over the sharp edges of a hole.

"Diffraction" itself is related to f/stop or aperture in that the GREATER the f/stop or aperture, the greater the edge surface area over which the light passes, and the GREATER the diffraction. The circumference is all that matters when calculating the potential diffraction.

But few people are concerned about diffraction on it's own. What is important is the amount of diffraction COMPARED to the amount of image forming light.

Diffraction is more prominent at smaller f/stops NOT because there is more diffraction - because there isn't, there's LESS diffraction at smaller f/stops - but due to the even smaller amount of image forming light getting through. What we observe at decreasing f/stops is actually an increase in the diffraction to "signal" ratio. That is, as we decrease our f/stop, even though the total diffraction goes down, at the same time the total amount of image-forming light is going down at an even greater rate at smaller f/stops. So the ratio between them changes in favour of the diffraction at smaller f/stops.

To translate that into practical effect, if you have a 'full frame' DLSR then diffraction limitation is likely to be visible at any aperture smaller than f/16 ; on an APS-size sensor, at anything smaller than f/11; on a point & shoot, which usually have tiny sensors, perhaps at anything smaller than f/2.8 (which is why mostly, they don't allow small apertures.
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Garry i think i get your last post just one thing, smaller aperture means higher f number right? just got a little confused in the last bit when you say smaller are you refering to the lower fnumber or smaller hole?
 
Toothie said:
Garry i think i get your last post just one thing, smaller aperture means higher f number right? just got a little confused in the last bit when you say smaller are you refering to the lower fnumber or smaller hole?
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Yes, smaller aperture = higher f number. Easy to remember, the f number is the actual aperture divided into the focal length of the lens...

So, F100 (100mm lens), 9mm aperture, 100/9=11 = f/11
F100, 4.5mm aperture, 100/4.5=22 =f/22
 
Or you could all use a nice neat bit of software I have from my microscopy days.

Basically a microscope uses optics much like a camera and suffers from the same technical limitations. Chromatic aberration, spherical aberration and THE most shallow DOF you have ever seen. At 100x objective on 20x eyepieces the DOF is so small it does not allow a whole CELL to be in focus.

How did we get around that one? Well we have a lovely technique called a Z stack. On a microscope movement is defined as X and Y for the stage movement and Z is the dimension that the focus operates in (Left and Right, Back and Forward, Up and Down to you and me.

So we had a nice piece of software where we programmed the microscope to make small movements on the Z axis and each movement would produce a digital image. So in effect we would have a stack of images. Then we had software that would identify the "in focus" pixels from each layer and combine the best focussed ones to give us a completely focussed image.

Cheating?

Damn right and priceless too! CCD cooled cameras and some smart software. :)

I must look for some images to post here. They are certainly different!

Sorry about the off topic post but DOF was a HUGE problem in microscopy and it was Garry's fault starting me on about physics! lol.
 
Interesting read Gary thanks :thumbs:
Ruthie
the smaller the (f) number the bigger the hole
The bigger the (f) number the smaller the hole
 
Garry Edwards said:
To translate that into practical effect, if you have a 'full frame' DLSR then diffraction limitation is likely to be visible at any aperture smaller than f/16 ; on an APS-size sensor, at anything smaller than f/11; on a point & shoot, which usually have tiny sensors, perhaps at anything smaller than f/2.8 (which is why mostly, they don't allow small apertures.
Click to expand...

so in this sentence diffraction limitation will be visible on a full frame at f16 and higher numbers

APS sensor at f11 and higher f numbers

and P&S sensors at f2.8 and higher f numbers

sorry i tend to think in f numbers cos its easier.

Gary a big thanks for sticking with me on this.
 
AliB said:
Sorry about the off topic post but DOF was a HUGE problem in microscopy and it was Garry's fault starting me on about physics! lol.
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Not a problem, it wasn't really off topic and its a good point could image stack in pp. ;)
 
AliB said:
Or you could all use a nice neat bit of software I have from my microscopy days.
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I don't think this is too off topic. I found a piece of software called Combined ZM which is similar I think to AliB software. Experimented on some Macro shots and once you get the hang of it works a treat.

Have thought of trying it on some landscape shots to get ridiculous depth of field, but simply haven't got round to it yet. I wonder how well it would work
 
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