DOF

John1964

John1964

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John
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Hi,

I am confused..........

I was playing today and took the same shot at:

F29, 1/25th, ISO250 and F4.5, 1/1000th, ISO250 both taken in aperture priority.

to see what happened with the depth of field and both came out exactly the same.........

What am I doing wrong.
 
Good point........
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[/url] DSC_5673 by Johneyre1964, on Flickr[/IMG]

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[/url] DSC_5674 by Johneyre1964, on Flickr[/IMG]
 
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It depends on where your focus point was and the focal length of the lens, on a wide angle even at f4.5 you are going to get a lot of DOF. Also at f29 you are well into the area of diffraction thus impairing the quality of the image, so that could end up giving you similar results.
 
The F4.5 background is definately softer than the F29 background (when viewed full size)
The focal lenght is 38mm and the subject distance 4.2m for both images.

I would have thought the F4.5 one back ground would have been a lot less sharp than the F29 shot.

So probably Ken is right.
 
It's not a great example of front to back focus, but it's a great example of how large DoF is with a wide lens and a medium distance subject (the background is much sharper on the 1st - see the bird poo on the roof).

It's great for those people who see 'large aperture' as the be all and end all for shallow DoF, which it clearly isn't. Great too for those people wondering about using tiny apertures for landscape pictures.
 
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Your problem is you cannot achieve a shallow DOF since the focal length of your lens is too short for the subject at hand. The subject is quite large so keeping it in frame while at the same time keeping the DoF short is going to be difficult. You can move in closer to the subject and achieve a shallower DoF but at the cost of not getting the entire subject in shot. Basically you need a lens with a much larger focal length, so use a telephoto.

This way, you can stand back from the subject, far enough so that use the entire focal length of the lens and yet still manage to keep the whole subject in shot. Use a fairly wide aperture, F5.6 to F7.1. The result should be a moderate blurring of the background. Sensor size also has an effect on the DoF achievable. A APSC sensor will produce a longer DoF generally.

Hope this helps.
 
Yes.

DoF is hard to grasp at first but when you think of the mechanics of what the lens is actually doing, it makes sense.

This is why a telephoto makes quite a good macro lens if you can stand back from the subject enough. The minimum focal distance is usually 1.5m or so.
 
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IkariWarrior said:
Your problem is you cannot achieve a shallow DOF since the focal length of your lens is too short for the subject at hand. The subject is quite large so keeping it in frame while at the same time keeping the DoF short is going to be difficult. You can move in closer to the subject and achieve a shallower DoF but at the cost of not getting the entire subject in shot. Basically you need a lens with a much larger focal length, so use a telephoto.

This way, you can stand back from the subject, far enough so that use the entire focal length of the lens and yet still manage to keep the whole subject in shot. Use a fairly wide aperture, F5.6 to F7.1. The result should be a moderate blurring of the background. Sensor size also has an effect on the DoF achievable. A APSC sensor will produce a longer DoF generally.

Hope this helps.
Click to expand...

Need to get me a telephoto lens then! Struggling with a 55mm to get the blurry background and get the whole subject in as you say
 
Nikkon 55-300mm DX can be had for £160 ish.
 
topcat07 said:
good guide ross , but how do you learn all of this without a computer working it out ? is there a way or is the best way to take multiple shots and just experiment until you get it right
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After a bit of practise, you'll just get to know how much DoF you'll get in most situations. A bit of basic knowledge and practise will speed things along, so...

There are numerous factors affecting DoF, but quite a few of them are fixed and that leaves only three that you can control at the shooting stage. They are focal length, shooting distance, and f/number.

The first two are both aspects of magnification and if the subject is made larger on the sensor, DoF is reduced - ie use a longer lens, subject gets bigger, DoF is reduced; by the same token, move closer and the subject gets bigger and DoF is reduced. These two factors are equal and cancel each-other out, so if you use a longer lens, but also move back so the subject remains the same size in the frame, DoF stays the same.

In practise, the most useful DoF control is lens f/number, because most times the focal length is set by the framing you want, distance is fixed by your position, so f/number is the only option left. Low f/numbers like f/2.8 give less DoF, high f/number like f/16 gives more DoF.

One of the things that's a bit confusing is DoF is not linear. At close distance, it is very shallow, eg macro photography it is just a few mms, whereas in a distant landscape DoF is massive, can be miles. Try these three experiments to get a feel for what's going on, with kit lens at about 35mm. Use high ISO to avoid camera-shake.

1) Clothes pegs on a washing line, standing at a 3/4 angle looking down the length, focus on the middle peg about 2ft away and shoot at lowest f/number, then f/8, then f/16. Check and compare. DoF will be shallow and not easy to get everything sharp from near to far.

2) Get someone to lean against a fence or wall, same 3/4 angle, and frame them up from about 6ft with a half-body portrait. Same routine. You'll have more DoF control here, with options to make the background blurred or sharp.

3) Take a series of shots looking down the street, with some cars parked from near to far. Focus at about 4m, same routine. There won't be much DoF control here and everything will mostly be sharp.
 
HoppyUK said:
After a bit of practise, you'll just get to know how much DoF you'll get in most situations. A bit of basic knowledge and practise will speed things along, so...

There are numerous factors affecting DoF, but quite a few of them are fixed and that leaves only three that you can control at the shooting stage. They are focal length, shooting distance, and f/number.

The first two are both aspects of magnification and if the subject is made larger on the sensor, DoF is reduced - ie use a longer lens, subject gets bigger, DoF is reduced; by the same token, move closer and the subject gets bigger and DoF is reduced. These two factors are equal and cancel each-other out, so if you use a longer lens, but also move back so the subject remains the same size in the frame, DoF stays the same.

In practise, the most useful DoF control is lens f/number, because most times the focal length is set by the framing you want, distance is fixed by your position, so f/number is the only option left. Low f/numbers like f/2.8 give less DoF, high f/number like f/16 gives more DoF.

One of the things that's a bit confusing is DoF is not linear. At close distance, it is very shallow, eg macro photography it is just a few mms, whereas in a distant landscape DoF is massive, can be miles. Try these three experiments to get a feel for what's going on, with kit lens at about 35mm. Use high ISO to avoid camera-shake.

1) Clothes pegs on a washing line, standing at a 3/4 angle looking down the length, focus on the middle peg about 2ft away and shoot at lowest f/number, then f/8, then f/16. Check and compare. DoF will be shallow and not easy to get everything sharp from near to far.

2) Get someone to lean against a fence or wall, same 3/4 angle, and frame them up from about 6ft with a half-body portrait. Same routine. You'll have more DoF control here, with options to make the background blurred or sharp.

3) Take a series of shots looking down the street, with some cars parked from near to far. Focus at about 4m, same routine. There won't be much DoF control here and everything will mostly be sharp.
Click to expand...

Thanks for this very detailed explanation helped a lot I'll give those exercises a go :-)
 
If I got a lens with a focal length of 300mm could I stand quite far back and get all of the subject, use f5.6 and get a very blurry background?
 
topcat07 said:
If I got a lens with a focal length of 300mm could I stand quite far back and get all of the subject, use f5.6 and get a very blurry background?
Click to expand...

Potentially yes. Also depends how far away the background is from the subject too.
 
topcat07 said:
Thanks for this very detailed explanation helped a lot I'll give those exercises a go :-)
Click to expand...

You're welcome :)

Yes, give it a go, there's nothing like putting things into practise for learning. Three very simple exercises that'll give you a good grounding.

topcat07 said:
If I got a lens with a focal length of 300mm could I stand quite far back and get all of the subject, use f5.6 and get a very blurry background?
Click to expand...

Now you're complicating things beyond a straight question about depth of field. Anyone still trying to get to grips with basic DoF and how to control that should ignore this next bit for now.

What you're probably talking about here is subject isolation, making the subject stand out from a blurry background, and words like bokeh get thrown in too.

When you move back and shoot with a longer lens so the framing is the same, depth of field will stay pretty much the same too, but it doesn't always look that way because the field of view is reduced. Easy to see this effect for yourself with the kit zoom if you frame up say a portrait at the wide end, then move back, and zoom to the long end so the subject fills the frame the same.

Compare how much background is included in the two shots and there will be a lot less with the longer zoom shot and the background will appear to be more out of focus even though technically it's not. But the background appears bigger relative to the subject, shapes are more bold and simplified, less busy, less distracting.

For maximum subject stand-out, the trick is to use a longer lens, at a low f/number for shallow DoF, and also get as much distance between the subject and the background as possible to maximise the DoF differential.
 
Wow, thanks to everyone that has had an input here.

Lots to take on board.

John
 
topcat07 said:
good guide ross , but dhow do you learn all of this without a computer working it out ? is there a way or is the best way to take multiple shots and just experiment until you get it right
Click to expand...

As Hoppy says, go out & experiment.

This Set was shot for the sole purpose of getting my head round DoF. I've also used beer bottles on a table, just find a subject and play....

Steve
 
topcat07 said:
If I got a lens with a focal length of 300mm could I stand quite far back and get all of the subject, use f5.6 and get a very blurry background?
Click to expand...

Not with the shot John posted no, as you'd probably end up falling into Poole harbour :D
 
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