In focus from foreground to background

HoppyUK said:
If you shoot with a short focal length lens, and then move back and take an identically framed shot with a longer lens (without changing format), the depth of field will be the same in both images,
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This is quite interesting, but I dont understand this part Hoppy.

Firstly, surely it's impossible to frame a shot identically if you move the camera.
Or at least, the perspective would change.

And according to the dofmaster site, and my own thoughts.
If i take a portrait with a 10mm lens from about 3 feet away at f8, my dof is massive, 2ft to infinite.

If i try to frame the whole shot with my 200mm lens the same, by say, standing 50 feet further back. my dof becomes just a few feet.

Nowhere near like your statement.

It says id have to be 760 feet away to get focus on subject to infinity.
Now i cant demonstrate, but i think that the framing of a 10mm portrait from 3 feet, is not the same as framing 200mm at 760 feet.

Im not trying to argue, but i cant understand your statement. :thumbs:


just to add, i need to go out and shoot something at 3 feet with 10mm, then again at 760 feet at 200mm before i should of said the above, but it just dont sound right in my head :)
 
idigitize said:
This is quite interesting, but I dont understand this part Hoppy.

Firstly, surely it's impossible to frame a shot identically if you move the camera.
Or at least, the perspective would change.

And according to the dofmaster site, and my own thoughts.
If i take a portrait with a 10mm lens from about 3 feet away at f8, my dof is massive, 2ft to infinite.

If i try to frame the whole shot with my 200mm lens the same, by say, standing 50 feet further back. my dof becomes just a few feet.

Nowhere near like your statement.

It says id have to be 760 feet away to get focus on subject to infinity.
Now i cant demonstrate, but i think that the framing of a 10mm portrait from 3 feet, is not the same as framing 200mm at 760 feet.

Im not trying to argue, but i cant understand your statement. :thumbs:


just to add, i need to go out and shoot something at 3 feet with 10mm, then again at 760 feet at 200mm before i should of said the above, but it just dont sound right in my head :)
Click to expand...

Haha you've caught me out there! :) But only a bit. To be strictly accurate, depth of field does change when you alter the distance but maintain framing by also changing the focal length to match, but only very slightly. For practial purposes you can ignore it, especially considering how many other things change a great deal when you alter the distance - as you have mentioned, perspective changes a lot and so does the field of view.

But when you take the extremes of focal length as you have done, yes you will get more depth of field with the shorter lens even if it's not massively more relative to all the other big changes. In practice though, you don't really shoot something at 10mm and then change to 200mm, it's just not the same shot at all. However, you might well have a go at 50mm and then think, maybe for perspective or field of view reasons, that it would look better at 200mm and in that instance depth of field is effectively identical.

For example, with a 50mm lens on a crop format Canon 40D, set at f/8 and 10ft distance, DoF is 3.77ft. Change that to a 100mm lens at 20ft and DoF is 3.68ft, and at 200mm and 40ft it is 3.65ft. So, over a range from 50-200mm, and 10-40ft, DoF has only shifted by a bit over an inch or 3%.

Extending the range a bit more just for example, if you go even longer and fit a 400mm lens and shoot at 80ft, DoF stays at 3.65ft. But going the other way into the wide angle area, with a 25mm lens at 5ft DoF increases a bit more to 4.2ft.

I'm trying to keep things on a practical level. If you go into the detail of DoF it gets horrendously complicated, which would be worthwhile if DoF itself was a concept that was actually that accurate in practise, or significant. What I mean there is that while DoF appears to be a very exact science, it's actually based on a whole raft of subjective assumptions and approximations as to what is sharp and what is not (CoC size), in a print of a certain size and a certain viewing distance.

Standard reference there is a 10in print viewed at one foot, or thereabouts. This takes no account of, for example, the very high levels of sharpness you can get with digital these days which I think means the standard CoC is not tight enough now, and many people only view on screen these days and pixel peep when they're doing so. When you also throw into the equation that everything changes if you crop the image (essentially a format change) then it becomes clear that DoF measurements are only an approximate guide, not to mention (again) the very big changes in perspective and field of view that also go on when you change distance and focal length.
 
nikonuser said:
WOW!! did not expect my question to gather such furor, but some very interesting replies for which I thank you all.
Just found a HFD chart on the Nikonians site so will print off and have it to hand
http://www.nikonians.org/html/resources/guides/dof/index.html

Thanks again, everyone
Dave
Click to expand...

It's a good thread! I've enjoyed it. This is generally a very friendly place, but don't post anything about shooting HDR at a wedding :eek:

On HFD scales, it's easy to scribble a few numbers on the back of a business card for easy reference. HFD is really only relevant at short focal lengths, say 16-50mm or so on full frame, so if you write down HFD for f/4, 5.6, 8, 11, 16 and 22 and at maybe 16mm, 20, 25, 30, 40, and 50mm, that's a table of only 36 numbers to sort you out.

Bear in mind that DoF tables measure from the front of the lens while the focus scale measures from the sensor, so take that into account at distances below about 3ft or so.

Another very useful bit of knowledge is that the HFD is always exactly double the nearest point of sharp focus, so you can work this like: Frame up the scene and decide that you want those pebbles 2.5ft away to be sharp; that means the hyperfocal distance setting will be double that, ie 5ft, regardless of focal length or format or anything else. Look up 5ft on your scale against the focal length you've chosen, and that's your f/number.

Or there's another more rough and ready way that I use. If I want those pebbles sharp then I know HFD is 5ft, I then set the highest f/number that I dare, bearing in mind camera shake and diffraction etc, and I then know that however it turns out the pebbles will be as sharp as I can get them even if they're not perfect.

Another trick, if you look at your scale and find that you can't get enough DoF for whatever reason, if you zoom back to a shorter focal length, this makes the image smaller, which increases DoF (lower magnification). You can then restore the framing you want by enlarging in post porcessing. Of course you will sacrifice some image quality in this way, but you might get the shot you want.

The difference is related to the crop factor again here, but boils down to this. If you enlarge just 50% of the frame area, you will gain one stop of DoF - that's equivalent to a 1.4x crop factor very similar to Nikon/Canon crop sensor cameras. You can take it even further if you dare and crop down to 25% of the image area, which is basically the same as 4/3rds format, and gain two stops of DoF. I've done that with macro, where DoF gets extremely shallow, but you have to be pretty desperate.
 
HoppyUK said:
Or there's another more rough and ready way that I use. If I want those pebbles sharp then I know HFD is 5ft, I then set the highest f/number that I dare, bearing in mind camera shake and diffraction etc, and I then know that however it turns out the pebbles will be as sharp as I can get them even if they're not perfect.
Click to expand...

I like that tip.... I'll try and remember it next time I'm out! :thumbs:
 
jontucker said:
I like that tip.... I'll try and remember it next time I'm out! :thumbs:
Click to expand...

There is another little problem with it though, which is that if you get the f/number wrong and say you use f/8 when you really need f/11, then not only will the foreground not be quite as sharp as you want, but neither will the extreme far distance be at infinity - that won't be as sharp as it should be either!
 
HoppyUK said:
There is another little problem with it though, which is that if you get the f/number wrong and say you use f/8 when you really need f/11, then not only will the foreground not be quite as sharp as you want, but neither will the extreme far distance be at infinity - that won't be as sharp as it should be either!
Click to expand...

Sure, I understand that but as you said, you can go as high as you dare I guess, taking into account diffraction and the fact that you still have to expose properly. If you actually can't achieve it then knowing the actual HFD won't really help you anyway. :shrug:
 
this is all good reading...

Sorry for picking stupid numbers, just find its helpful cos it shows things in extreme more, therefore, generally easier to see the pattern of how things are working.

For a laugh today, i went down to the local playing fields, took a pic of a footy goal, 3 ft away from the line at 10mm, then walked 2 football pitches away and took the same shot at 200mm.
Well, it wasnt the same shot at all lol, so i dont know what i proved. I found out it was cold, thats about it.
I will try it in a more real life situation.
 
Enjoyed reading through all that folks and some good stuff from Hoppy.
 
Yes, I agree, thanks Hoppy for the invaluable information.
Great tips
Dave
 
Another thanks here as well for some great reading, some great information here on DOF which is no doubt my weakest part
 
Was posted from my phone and to be fair such a minor typo that I don't think it really matters whether it was picked up or not but each to own, espcially in the way you did anyway..

Back on topic

HoppyUK said:
Another trick, if you look at your scale and find that you can't get enough DoF for whatever reason, if you zoom back to a shorter focal length, this makes the image smaller, which increases DoF (lower magnification). You can then restore the framing you want by enlarging in post porcessing. Of course you will sacrifice some image quality in this way, but you might get the shot you want.

The difference is related to the crop factor again here, but boils down to this. If you enlarge just 50% of the frame area, you will gain one stop of DoF - that's equivalent to a 1.4x crop factor very similar to Nikon/Canon crop sensor cameras. You can take it even further if you dare and crop down to 25% of the image area, which is basically the same as 4/3rds format, and gain two stops of DoF. I've done that with macro, where DoF gets extremely shallow, but you have to be pretty desperate.
Click to expand...

That is another very handy tip and one I have just re-read through now and got my head around correctly. In fact, reading back through your posts on this subject has been extremely handy. I had seen the DOF master site before but your explanations and examples have made it a lot clearer for me, will go back to the site tonight for a further read
 
PaulyD said:
<snip>

That is another very handy tip and one I have just re-read through now and got my head around correctly. In fact, reading back through your posts on this subject has been extremely handy. I had seen the DOF master site before but your explanations and examples have made it a lot clearer for me, will go back to the site tonight for a further read
Click to expand...

Well I'm really quite chuffed at the feedback. Cheers :) It doesn't always happen... :eek:

The key to understanding depth of field is getting hold of the idea that it is the image size that matters - the magnification - and of course the f/number. Those are the two ultimate drivers.

Lens focal length is just another way of changing the magnification, same as moving closer is, or further away. It's the physical size of the image finally planted on to the sensor that makes the difference.

Part of the confusion is historical, when everybody used 35mm film pretty much universally. Then, since the format never changed, referring only to focal length could be used as a bit of a shortcut as it didn't need to be qualified by what sensor was being used. Even then it was only a half-truth but today it is meaningless in isolation.
 
HoppyUK said:
snip...
Part of the confusion is historical, when everybody used 35mm film pretty much universally. Then, since the format never changed, referring only to focal length could be used as a bit of a shortcut as it didn't need to be qualified by what sensor was being used. Even then it was only a half-truth but today it is meaningless in isolation.
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I knew there was going to be some advantage in my being an old giffer who started on film :lol: Seems everywhere I go on this forum you're having to explain sensor size/DOF effects at the moment!
 
TheBigYin said:
I knew there was going to be some advantage in my being an old giffer who started on film :lol: Seems everywhere I go on this forum you're having to explain sensor size/DOF effects at the moment!
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:)
 
HoppyUK said:
Well I'm really quite chuffed at the feedback. Cheers :) It doesn't always happen... :eek:

The key to understanding depth of field is getting hold of the idea that it is the image size that matters - the magnification - and of course the f/number. Those are the two ultimate drivers.

Lens focal length is just another way of changing the magnification, same as moving closer is, or further away. It's the physical size of the image finally planted on to the sensor that makes the difference.

Part of the confusion is historical, when everybody used 35mm film pretty much universally. Then, since the format never changed, referring only to focal length could be used as a bit of a shortcut as it didn't need to be qualified by what sensor was being used. Even then it was only a half-truth but today it is meaningless in isolation.
Click to expand...

Can we get your avatar description changed to 'Obi-Wan'? ;)
 
jontucker said:
Can we get your avatar description changed to 'Obi-Wan'? ;)
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Haha Jon!

I would, but don't want to rip off any copyrighted avatar images ;)
 
I thought the following would provide good visual support to the good explanations provided in this thread; 'pictures painting a thousand words' if you like, lol.

This set of four 'quicky shots' were taken at the same focal length with the apeture changed in each one to demonstrate changing DoF. To keep things relatively constant, all were taken with no flash, just using illumination from my lightbox stood on its end...

1. f/4.5

DSC_0159.jpg



2. f/9

DSC_0158.jpg



3. f/18

DSC_0157.jpg



4. f/29

DSC_0156.jpg
 
So let me try to understand this as I to have asked myself the same question. If I set up my shot on say f16 or f22 with say an ISO of 200 using a standard kit lens at 20mm, on a tripod for a possible longer exposure to remove hand shake, take the shot, letting the camera determine shutter speed, the shot should have focus throughout ?? :thinking:
 
with a 20mm lens using f16, focussing greater than 1.6 meters you will be ok up to infinity.
 
Gazbag said:
So let me try to understand this as I to have asked myself the same question. If I set up my shot on say f16 or f22 with say an ISO of 200 using a standard kit lens at 20mm, on a tripod for a possible longer exposure to remove hand shake, take the shot, letting the camera determine shutter speed, the shot should have focus throughout ?? :thinking:
Click to expand...

Yes, pretty much. Using a crop format 450D and 20mm lens, you must set the focus to the hyperfocal distance, which at f/16 is 1.34m. You will then have sharp focus from 0.67m to infinity.

Remember that DoF distances measure from the front surface of the lens, whereas the focusing scale marked on the lens is taken from the sensor plane.

Avoid f/22 if you can - it kills critical sharpness on a crop format camera due to diffraction. Diffraction is an unavoidable optical aberration, not a lens fault.

Check it out with the DoF calculator here http://www.dofmaster.com/dofjs.html
 
HoppyUK said:
Yes, pretty much. Using a crop format 450D and 20mm lens, you must set the focus to the hyperfocal distance, which at f/16 is 1.34m. You will then have sharp focus from 0.67m to infinity.

Remember that DoF distances measure from the front surface of the lens, whereas the focusing scale marked on the lens is taken from the sensor plane.

Avoid f/22 if you can - it kills critical sharpness on a crop format camera due to diffraction. Diffraction is an unavoidable optical aberration, not a lens fault.

Check it out with the DoF calculator here http://www.dofmaster.com/dofjs.html
Click to expand...

Errm, Do you have shares in the dofmaster company ? You mention it in about 80% of your posts.;) While we are on the subject of DOF, I have been trying to get a spread sheet for meters and feet that I wrote. Cut it down to the minimum, even deleted the feet section, but to no avail. Always the same, I get the message "to wide". It`s the TP loss. :bang:
 
classcams said:
Errm, Do you have shares in the dofmaster company ? You mention it in about 80% of your posts.;) While we are on the subject of DOF, I have been trying to get a spread sheet for meters and feet that I wrote. Cut it down to the minimum, even deleted the feet section, but to no avail. Always the same, I get the message "to wide". It`s the TP loss. :bang:
Click to expand...

There are lots of on-line DoF calculators about. I reference DoFmaster, as most people do, because it allows you to input all the relevant data to see changes easiliy and uses international standards for CoC.

It is also very comprehensive and things like the ability to print out customised DoF scales and tables is extremely useful. For example, you can print out a rotary DoF scale here http://www.dofmaster.com/custom.html just like the old DoF scales we used to get on some lenses, but vastly better and up to three focal lengths on each one :thumbs:
 
Say with this following shot then, if i focussed a third up the way into the picture, it'll come out all in focus, but the DOF is smaller due to the larger aperture? Whereas if it were are aperture of 11, there would be a greater DOF and thus easier to get the focussing spot in the area?

MScotts said:
1. f/4.5

DSC_0159.jpg
Click to expand...
 
drb5 said:
Say with this following shot then, if i focussed a third up the way into the picture, it'll come out all in focus, but the DOF is smaller due to the larger aperture? Whereas if it were are aperture of 11, there would be a greater DOF and thus easier to get the focussing spot in the area?
Click to expand...

I think you've got the gist of it, but be wary of the 'focus a third in' rule. It's not a universal truth at all, more of a 'if in doubt, focus a third in and you probably won't be a million miles out.'

The only absolute truth is that there is always more DoF behind the point of sharp focus than in front of it and 33% is not a bad average, but it typically varies from 10% at long distance to nearly 50% in a close up shot like this.

Look at the front blades to see how the higher f/number is pulling closer things more in focus.
 
Ok.

Thing tha bother me, if the FL is the same for each shot, but aperture is changed, why does the optimum focal distance change?

I know using a wide angle lens is better for front to back sharpness, but as the FL goes up, i struggle more to understand where to focus, if you have no distance scale on the lens. Getting a tape measure out is hardly do-able all the time. :D
 
Suvv said:
Yes there are - proofread or even proof-read never proof read :D
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I have skimmed over this thread some useful info in here and have jotted down all the pints useful to myself :).

I thought I would chip in with

"pwned" lol why does he come back to a boring forum just leave already.
 
As your name is Lee, you are not a foreigner. So you must be grammatically inferior. Live and let live old boy. Mines a pint of bed raddel
 
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