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Clinton1
12th January 2008, 12:42 PM
Hey all,
I have gone and bought a Nikon D80 and have the bog standard single AF-S 18 - 55mm, F3.5 - 5.6 lens.

It was enough of an outlay without getting any other lenses, although the VR 18 - 200mm lens and something else is on the cards for later in the year.

What I'd like to do is to experiment with Depth of Field and f-stops with a goal of educating myself about that.

I have gone through the www.dofmaster.com (http://www.dofmaster.com) site and have a fair understanding, which now needs to be backed up by a practical experiment to cement the process in my mind.

Could some of the more experienced members help me set up/design an experiment with this camera and lens that will assist me in really learning about DoF?

So far I've come up with this:
At 18mm my f-stop ranges from 3.5 to 22, and at the other end (55) it ranges from 5.6 - 22. I'll work out the f-stop range through the lens and the hyper focal distances and put it into a ready reference.
Take a series of photo's at 18mm focus, and change the f-stop....

That is where the experiment ends with me.... help me out?

Big Shed
12th January 2008, 12:51 PM
Whilst your experiment will give you a better understanding of depth of field at various f stops, this calculator (http://www.dofmaster.com/dofjs.html) will come in handy and it will be quicker than compiling your own table.

There is a very good entry in Wikipedia (http://en.wikipedia.org/wiki/Depth_of_field) on depth of field, although somewhat on the technical side.

Googling Depth of Field will give you heaps of links as well.

bitingmidge
12th January 2008, 01:01 PM
Clinton,

Nice camera :D, I hope you enjoy it as much as I enjoy mine. You've certainly taken a slippery photographic ride since buying the second hand point and shoot a year ago!

Try manual focus too. You can see the DOF quite easily.

You could do worse than buying one of the excellent 50mm f 1.8 lenses for not many dollars.

Cheers,

P

dennford
12th January 2008, 01:18 PM
Clinton,

Why not just download the DOF calculator from the site, it can then be carried with you. Are you familiar with using these calculators that have dual aperture scales? at one time all lenses had them on the aperture barrell.

Denn

malb
12th January 2008, 02:52 PM
As you know, depth of field varies with the f stop (aperature) setting of the lens, but it also varies with the focal length as well. Wide angle shots have a greater DoF than tele ones, as tele forshortens the image in providing optical magnification.

For your experiment, conducting it only at wide angle setting will be extremely limiting, you need to try varying focal lengths as well.

To be meaningfull, you also need good repeatability, so a tripod and remote release would be very usefull.

Ideally, you set up with an primary object near something that is fairly continuous over the test range, but with discernable/measurable variations. By shooting your test shots parallel to the continous border, you can judge the resultant shots on computer afterwards and use the variations to determine distances.

Primary object size needs to be adequate for auto focus to lock to and the object needs to have decent detail, unless you have true optical SLR viewfinder with focus screen for manual focus. Do not trust LCD images for manual focus in tests of this nature.

Clinton1
12th January 2008, 07:02 PM
Thanks Big Shed and Denn, I have a DoF chart from that site printed out... till I remembered that I need to convert the D80's lens range to 35mm format.
Anyone know how to do that?

P - yes, I have one of those 50mm lenses down as a possible... need to work out what I want and what lens will be suited first. :rolleyes:

Malb - If I think about what you are saying, then this is my version of the "best test':
1. Put the camera on a tripod,
2. the test area is bordered with the fence, like the one in the attached photo,
3. In the perfect world I'd place identical items (like soccer balls) on poles in a dirverging line along the fence,
4. At 18mm, take a series of photos from the lowest to highest f-stop values, repeat with the focus on each soccer ball in turn,
5. repeat this for the full focal length range.
6. print the photos (or more likely view them on my 'puter and do a 100% crop) and find the distance on each photo (for each focal length) from where focus is sharp to where it drops off again.
7. compare this to the DoF chart and make any corrections where needed.

What I'd like to have happen is for me to know where focus both 'comes in' and 'drops out'.

I'm a little confused as the dofmaster site talks about focus being sharp from 1/2 the hyperfocal distance to infinity.... and I fluked a shot on my film camera that had the background out of focus and the foreground in focus (this is something I really like to achieve). I did this when fiddling with settings and have now forgotten what I was doing to achieve the shot. (this is the second pic, with the tree)

I'm not sure if I understand correctly in that I can use my f-stop setting to achieve a loss of focus behind the object that I want in focus?
Can anyone shed some light on this?
http://farm3.static.flickr.com/2055/2186409823_6d2daa259b_m.jpg

Here is what I mean by "background out of focus", focus is on the tree, and the building is "out of focus':
http://farm2.static.flickr.com/1322/1291820630_034264feec_m.jpg

Clinton1
12th January 2008, 07:18 PM
just worked out the 'theory' behind the near and far points on the DoF plane...now I need to work out 'how to do it' :? :rolleyes:

Stuart
13th January 2008, 01:00 AM
Here's another 2 cents...

An 18mm lens is not a good one (as mentioned above) for learning about DoF - they already have a DoF from close to near infinity anyway. The larger the lens, the narrower the DoF (eg a 200mm lens (small telephoto) has a much more limited DoF than a wide angle lens). My 11mm-18mm lens hardly needs to be focused at all - almost everything is always sharp as its DoF is massive.

To go to an extreme, a macro lens (a real one, with 1:1 ratio) has a DoF of about 1mm at maximum enlargement. Anything closer, or further away than 1mm from the point of focus will be blurred.

Next, and as you know, f stop has a huge bearing on DoF. f22 gives a wide DoF, f2.8 a much narrower one.

For your tree/shed example above, if you want the shed right out of focus, try as many of these as practical:

Choose the longest lens you have (eg 200mm), and get back from your subject as far as needed (or possible) that allows the composition you want.

Set the aperature wide open (is f2.8 or whatever the lens goes to). If it is a very bright day, choose the lowest ISO (ASA) setting your camera will allow so that you can get that aperature wide. You can even resort to a neutral density filter.

Use the hyperbolic focus point to get your subject (the tree) in focus. Using the calculators mentioned above, you will find your DoF is (eg) 10 metres. If you focus on the tree itself, and the shed is close to or within 5 metres of the tree, it will be sharp, or close to it. You know your DoF is 10 metres, so actually focus on a point say 3 metres in front of the tree. The tree will fall within the DoF and so will be sharp. The shed is now outside the DoF, and will be blurry.

A good way of learning about DoF is to set up a chessboard, with pieces in a line one behind the other heading away from you (at a slight angle so you can see ever piece). Your soccerball idea is the same concept - use whichever takes your fancy.

Put your camera on a tripod, focus on the middle piece, then practice taking a photo with the aperature wide, then stopped right down, and see which chess pieces are sharp, and which (both forward and back of the middle piece) are blurry. Change lenses, and see how that makes a difference.

Some cameras have a DoF preview - ie before the photo is taken, you can get the lens to actually set the f stop you have requested, and look through the viewfinder to see the (dark) results. A camera doesn't show this normally, as it keeps the aperature wide open so the maximum light passes through to aid composition, focusing etc. It only actually sets the chosen aperature just as the photo is taken.

Hope that helps some.

Dirty Doogie
13th January 2008, 01:14 AM
It has been a long time since I did photography but isn't the dOf marked on the lens? - little lines between the aperature numbers? Maybe they've changed all that but gee they were useful especially in flash situations also.

If you knew how far away your subject was and you knew what f stop to use you could prefocus your lens and then point and click.

Clinton1
13th January 2008, 01:38 AM
Stuart,
The 18 - 55 is the only lens I have, and it will be a few months till I can get anything else. Hence, I want to push this one to the limits and learn all I can, before getting anything else.

I'll do an experiment tomorrow... see if I can make it work out for me.

Doogie - my lens is pretty basic, does not have the f-stop and distance info on it.

clubbyr8
13th January 2008, 06:40 AM
It has been a long time since I did photography but isn't the dOf marked on the lens? - little lines between the aperature numbers? Maybe they've changed all that but gee they were useful especially in flash situations also.

If you knew how far away your subject was and you knew what f stop to use you could prefocus your lens and then point and click.


Not sure about the Nikon kit lens, but the Canon kit lens (and a few others) don't have a distance or DOF scale on the barrel. They don't have aperture rings for that matter.

Stuart
13th January 2008, 09:24 AM
yeah, all that sort of thing has gone out the window - must have considered that it made the lens look too confusing or messy for the point and shoot crowd.

Once upon a time the one-touch zooms (the ones where the barrel extends for zoom and twists for focus had multiple curved lines showing how the DoF decreased as you zoomed, as well as how it changed wrt aperature.

The main change was with the autofocus cameras, which made the single touch obsolete.


Clinton - use the lens at 55mm to minimise its DoF. Using it at 18mm won't help you learn anywhere near as much.

dennford
13th January 2008, 10:06 AM
Clinton,

I really think that the whole thing should be simpler than you are making it. Okay, forgive me if I am being over simplistic but lets go right to the start and see what factors affect depth of field (not to be confused with depth of focus which is a different thing.

First of all the smaller your aperture (higher f number) the greater your depth of field. Secondly the further from the camera your point of focus is the greater your depth of field. There are other factors that come into play but for all intents and purposes aperture and distance are the only two to worry about.

Now to work out depth of field, most older cameras had dof scales engraved on the lens barrel but because modern lenses have the aperture set electronicaly rather than by rotating the lens it is no longer possible/practicle. Why manufacturers haven't overcome this anomally I can't imagine! However by downloading the dof calculator and making up a set of say three discs at differing focal lengths you would be able to calculate with reasonable accuracy your depth of field. The scale works simply by placing the pointer on the centre disc on the distance focussed on, you will see two f stop scales - one going each way, if you read the distance opposite the two f stop readings coresponding to the f stop you are using, that is your depth of field.

In pic 1 I have given an example of a lens at 35mm focal length set at f16 and focussed at 5ft, you will see that everything is in focus from 3.15ft to 12ft - a distance of almost 9 ft

Now the interesting part is that of working out your hyperfocal distance and this can be done using the same scale. If you now set the scale so that f16 coresponds to the 5ft point as in the second pic, you will notice that although your five foot is still in focus that your actual point of focus is now 12 feet and that everything from 5ft to infinity is in focus - a huge increase - in fact you are now getting maximum dof whilst remaining focused at your 5ft.

Hope this helps

Denn