Long lens DOF. Please explain.

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I was reading through the critique forum when I came across the advice “I would have used a longer lens to get better DOF”. The comment was aimed at a portrait of a woman which was in focus (as well as the majority of the background).

I’m sure I could Google this, but I’d like a simple uncomplicated answer at this time of night. How does lens length affect DOF. E.g How would using a 70-300 to zoom in on a subject from a distance using a large aperture compare to getting close with a 50mm prime? I’m sure I am missing something because I’ve heard so many comments such as “They’ve used a long lens there otherwise you wouldn’t have got such DOF”.

Sorry if that doesn’t make too much sense, but hopefully someone will fathom my gibberish!

TeaVR
 
DoF depends on f-stop and focal length, so longer length gives a shorter DoF at the same framing and f-stop
 
That's becuase the perspective of the focal lenghts is different. The physical length of the DoF is going to be the same.
 
Sorry by focal length is a part of the maths behind it - have a look at the formulas: http://en.wikipedia.org/wiki/Depth_of_field#Foreground_and_background_blur

From the wikipedia article which is actually well worth a read,

The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number (relative aperture)...For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF.
 
8utters said:
Focal length doesn't effect depth of field.
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So you'd advise using a 10mm fisheye for a nice creamy bokeh then?

You've given a possibly technically correct, but at the same time almost practically useless answer there!
 
and thats roughly what flash in the pan said...(decreasing magnification increases DOF)
thats how i understand it
 
this is a little trick i got taught at the eye doctors when getting my specs fitted, with ref to DOF

try this... clench both fist and strech both arms out full length making sure both fist are together, close 1 eye and focus your open eye on the fist on the same side (left eye, left fist), then slowly bring the fist towards you. as you do so watch the DOF of the fist still being held out stright. try not to take you eye off the fist in focus, if you do this correctly the you will start to see the DOF produced. you can also slightly detect the fist which is out of focus shrink (get compressed) it does work.
 
So much rubbish is talked on this subject
The DOF is effected by focal length, f-number, circle of confusion and image format (the diagonal measurement)

The focal length F-stop and circle of confusion PLUS the size of film or sensor all make a difference so for the same lens at the same F-stop you can get different DOF

Another point that is often wrong is A longer focal length compress elements in the background of the image IT will not

Take a photo from one spot with two lenses one wide and one long crop the wide one to the same field as the long lens and you will see the same
The only difference can me made is your position if you take two photos with a wide and long lens and have both taken the same in shot you will then see a difference BUT not take from the same spot
Its all about perspective
 
sorry i read this wrong.

Diffraction is ignored. DOF formulas imply that any arbitrary DOF can be achieved by using a sufficiently large f-number. Because of diffraction, however, this isn't quite true. Once a lens is stopped down to where most aberrations are well corrected, stopping down further will decrease sharpness in the center of the field. At the DOF limits, however, further stopping down "decreases the size of the defocus blur spot", and the overall sharpness may increase. Consequently, choosing an f-number sometimes involves a tradeoff between center and edge sharpness, although viewers typically prefer uniform sharpness to slightly greater center sharpness. The choice, of course, is subjective, and may depend upon the particular image. Eventually, the defocus blur spot becomes negligibly "small," and further stopping down serves only to decrease sharpness even at DOF limits. Typically, diffraction at DOF limits becomes significant only at fairly large f-numbers; because large f-numbers typically require long exposure times, motion blur often causes greater loss of sharpness than does diffraction. Combined defocus and diffraction is discussed in Hansma

so if it decreases the blur spot, is that not compression?? i am cofussed.com
 
Longer lens = compressed perspective = better percieved background blur (or 'bokeh').

Granted, this is the Sesame Street version and will have many pulling their hair out, but essentially this is what is meant when people refer to 'better dof' from longer lenses

Cheers,
James
 
Woz! said:
So you'd advise using a 10mm fisheye for a nice creamy bokeh then?

You've given a possibly technically correct, but at the same time almost practically useless answer there!
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He's correct, it's just that because the angles and distances involved, it makes the depth field seem smaller.
 
ROFL2K9 said:
Epic Fail chaps :lol:
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Oh dear. But the Wiki quote is actually correct.

Depth of field is affected by just two things: image size, and f/number. A bigger image, either by moving closer or by using a longer focal length lens, decreases depth of field by an equal amount. A lower f/number decreases depth of field. That's it.

So to answer the OP, if you take a picture, maybe a portrait, with a 70-300mm lens, then move forward with a 50mm lens and fill the frame with the subject to the same size, then depth of field will also be the same (at the same f/number). You can change focal length and distance as much as you like, but if the subject stays at the same image size, depth of field is unchanged.

Those are the facts of the matter as far as depth of field is concerned, but there's more to it. The reason why longer lenses appear to to give less depth of field and make the subject stand out from the background more prominently, is due to the narrower field of view.

If you stand back and shoot with a longer lens, there is far less background visible behind the subject. This more simplified, less cluttered background is what people are meaning when they say longer lenses give less depth of field. It's technically wrong, but it certainly looks that way.

Only one caveat, and that is the format size (image sensor) must remain the same. If you change format, it's different, eg full frame delivers less depth of field than crop format, and compacts much more.
 
8utters said:
Focal length doesn't effect depth of field.
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Now you are talking silly. the hyperfocal is the basic part of calculation for all DOF. the greater the hyperfocal the smaller the DOF and vice versa. :bonk:
 
HoppyUK said:
Only one caveat, and that is the format size (image sensor) must remain the same. If you change format, it's different, eg full frame delivers less depth of field than crop format, and compacts much more.
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Can you explain why? is that the focussing distance between the lens and the sensor? I try for blurred backgrounds with my compact but with limited success.

I get the whole DoF thing ( I think) with regard to the aperture. My optician says that the reason I don't need glasses is because I have such tiny pupils so I can focus over such a long range of my vision.
 
I wish that I had a pound for the number of times this discussion pops up on forums. Next thing it will be daggers drawn.
 
it is perception that matters. Long focal length = less (percieved) DOF. The science of it matters not in this case.
 
AndyB said:
it is perception that matters. Long focal length = less (percieved) DOF. The science of it matters not in this case.
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I disagree Andy.

The myth that longer focal length lenses, by themselves, inherantly deliver less depth of field is confusingly unhelpful. It's the way that you tend to use them that makes them different and it's important to understand that. Some people think that simply by fitting a tele lens they are going to get less in focus. I've been hearing that for as long as I can remember, and it's still propagated today - like on these boards most weeks. It's wrong; a misleading half-truth at best.

Here's a problem: some people buy a short focal length macro lens because very shallow depth of field is a major difficulty with the huge image size (magnification) you get with ultra close up. So they mistakenly avoid 100/150/180mm macros and get a 50mm, which actually makes life more difficult in some ways.
 
Don't forget the OP said "better" dof.

to one person that would mean in a portrait photo the background is blurred and to another it may mean more in focus.... deeper dof.

so maybe the quote he was given was a bit confusing to start with, as he doesnt really know what it is he is supposed to be trying to achieve.
 
trapper501 said:
Can you explain why? [why different sensor formats give different depth of field] is that the focussing distance between the lens and the sensor? I try for blurred backgrounds with my compact but with limited success.

I get the whole DoF thing ( I think) with regard to the aperture. My optician says that the reason I don't need glasses is because I have such tiny pupils so I can focus over such a long range of my vision.
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You will struggle to get blurred backgrounds with a compact. The way to do it is by getting the image size as big as possible, by moving close and/or using the lens on max zoom, and the lowest f/number. But some compacts focus closest at the short end of the lens where depth of field will be least (which is the reverse of the 'long lenses give less depth of field' myth). And unless you've got manual exposure control, the camera might not set its lowest f/number.

Again, it is the small image size that delivers greater depth of field with compacts - it's not just focal length or the back focus you're referring to. You can recreate the same effect with a DSLR by either moving back or zooming back, so that the image is small in the frame, and then enlarging it in post processing. You will get more depth of field that way, and some people use this technique, but you can't push it too far as the image obviously deteriorates with too much enlargement in post.

The reverse is true with full frame compared to crop format digital. The bigger image size of full frame inherantly delivers less depth of field, a bit over one stop in the case of Nikon/Canon. The exact answer is to multiply the f/number by the crop factor, so for example f/4 on a Canon crop camera is f/4 x 1.6 = f/6 on full frame - that's 1.28 stops.
 
I do have manual control on my latest compact (ixus 980is) but I haven't explored much with that yet. I generally use the half pressure to focus and then recompose the picture with the focus moved out of centre.

I shall try a bit of aperture control and see how I get on.
 
Could you please explain to me your last paragraph stating that FF shots have less DOF. DOF is a condition of the lens alone. Whether it be a full frame or a crop camera (5D or 10D) the image on the focal plane is the same size on both cameras. Admitted the cropped shot does not have the periphery of the shot with its slight fall off. I can see the glint of cold steel showing under coats now.
 
classcams said:
Could you please explain to me your last paragraph stating that FF shots have less DOF. DOF is a condition of the lens alone. Whether it be a full frame or a crop camera (5D or 10D) the image on the focal plane is the same size on both cameras. Admitted the cropped shot does not have the periphery of the shot with its slight fall off. I can see the glint of cold steel showing under coats now.
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If you're jangling the cold steel, best get your facts in order ;) Only joking, I'm so used to this particular battle :lol:

Depth of field is only partly dependent on lens focal length. It's the size of the image that matters, and you can change that by either moving forwards/back, or by fitting a different focal length lens. The effect purely on depth of field is the same.

I don't know why people are surprised by statements like that. It is absolutely true that compacts give loads of depth of field, crop cameras less, full frame less, medium format less than that, and large format even less still. The key factor that is changing is the size of the image on the sensor/film.

Just a guess, but I wonder if we're talking at cross purposes here when you say "Whether it be a full frame or a crop camera (5D or 10D) the image on the focal plane is the same size on both cameras." Of course that's true, but you would not get the same framing with the two cameras; you would have to move back with the crop camera to get the same framing, or switch from say a 50mm lens to a 30mm lens (actually 31mm on a Canon crop). In both case, the image size will then get smaller and the depth of field will increase. I'm sure you're familiar with this depth of field calculator www.dofmaster.com and will note how you have to input the sensor format to get a result.
 
classcams said:
Could you please explain to me your last paragraph stating that FF shots have less DOF. DOF is a condition of the lens alone.
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At the same framing this is, i.e. the exact same shot would be possible, from the same point, with 135mm F2.8 on FF as 85mm F1.8 on 1.6x crop.

The crop would have a faster shutter though.
 
pearce_jj said:
At the same framing this is, i.e. the exact same shot would be possible, from the same point, with 135mm F2.8 on FF as 85mm F1.8 on 1.6x crop.

The crop would have a faster shutter though.
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Correct :thumbs:
 
:clap:Ah Ah
I never mentioned framing or moving back to get the same image. As a matter of interest ,if you move back slightly you actually improve the DOF. So let us drop the subject.:wave:
 
Nice semantics there.

For added credit explain how this is affected by the fact that you have to enlarge a smaller image more to have the same size print.

And don't say "improve the DOF". Improve means to make better, depth is a measurement. Increase or decrease would make sense.
 
Blapto said:
Nice semantics there.

For added credit explain how this is affected by the fact that you have to enlarge a smaller image more to have the same size print.

And don't say "improve the DOF". Improve means to make better, depth is a measurement. Increase or decrease would make sense.
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I explained that earlier. Post #24, second para.
 
HoppyUK said:
I explained that earlier. Post #24, second para.
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Don't worry, I understand this already.

Dear readers, trick to learning this is not to listen to anybody and go off and do the maths yourselves. People will post "xyz is true" when they have never worked it out for themselves and are just copying someone else's error with some blind faith that it must be right. If you are going to listen to someone else, trust it only if it's backed up with actual science.

See:
http://www.largeformatphotography.info/fstop.html

He quotes his references and explains everything pretty thoroughly.
 
Blapto said:
Don't worry, I understand this already.

Dear readers, trick to learning this is not to listen to anybody and go off and do the maths yourselves. People will post "xyz is true" when they have never worked it out for themselves and are just copying someone else's error with some blind faith that it must be right. If you are going to listen to someone else, trust it only if it's backed up with actual science.

See:
http://www.largeformatphotography.info/fstop.html

He quotes his references and explains everything pretty thoroughly.
Click to expand...

To be honest Blapto, I don't think that link will be very helpful to the OP, or most other people reading this. Here is the first line of a mammoth article (which concentrates on large format :thinking: ) and starts like this - you have been warned: "Summary: a systematic method to find the f-stop which as a function of the displacement of the standards between the far and near points, measured on the camera with a scale (the focus spread)."

This original question was quite simple, and so is the answer. There is no need to go anywhere near focus spread and diffraction. The correct answer was actually given in post #6 with the quote from Wikipedia, and I attempted to make it more clear in post #16.

Most of the other posts have either been wrong, or have misunderstood what's been said, or are just being argumentative ;) I really like to keep things simple. Photography is not that complicated, even though some people seem to enjoy making it that way.
 
classcams said:
LOL See what I mean. Slash slash jab jab
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LOL But no slashing or jabbing from me. Well I certainly don't mean to :lol:

It's quite possible to have a decent debate, even a full-on disagreement, without any of that sillyness you are attempting to promote ;)
 
classcams said:
LOL See what I mean. Slash slash jab jab
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Why aren't you contributing to the discussion? All you've done so far is try to start arguments in an otherwise civil thread.

HoppyUK, the reason I recommend that people work it out from themselves (and really, it's not that hard with a little effort and not expecting the answer on a plate) is because there's so much misinformation out there. For example, the quote in post 6 says that depth of field is determined by the focal length of the lens (amongst other things).

Yes, that's true, but it's only true in edge cases, like using wide angle lenses near the hyperfocal distance.

I know and you know that a 200mm lens focused at 10m will give the exact same depth of field as a 400mm lens focused at 20m (giving the same subject magnification). Have a look here. The difference is one of composition, with a wide angle lens there is more likely to be more in the frame that is within the depth of field. With a telephoto lens this is less likely. It's also true that if a wide angle photograph is printed to the same size as a normal photograph then more will appear in focus from the same viewing distance, because people's eyes aren't perfect.

Now, I think that's useful information for any photographer who is interested in depth of field. It's not information that you'd get normally though, because of all the misinformation without proof that gets slung about. As such, people should do their own research and not have lies ingrained into how they work.
 
Ok joke over, let us be serious now.It is an interesting theory, but it is something you cannot carry around when you are on a shooting trip. Everything is changing for every shot.
 
I can't disagree with any of that Blapto, but where I perhaps differ is where it is best to draw the line with technical explanations, and I guess that is a subjective thing.

I take the view that simple is best, so long as it is correct. But if you have to leave bits out of an explanation, or assume certain things, then you have failed. But equally, if you throw everything in, then people are just not going to digest it, and nor can they prioitise important stuff - can't sort the wheat from the chaff. It is then perhaps better to break it up into edible chunks. Tricky balance ;)

Then there is the ambiguous terminology, such as saying large aperture when you mean high f/number, or big aperture when you mean low f/number, not to mention wide apertures and wide lenses, or fast apertures, or fast shutter speeds, or high shutter speed when you mean short shutter speed... :lol:

And getting back to depth if field, it is a very inexact science based purely on a set of assumed parameters. It has to be made exact in order to make measurable sense of what's going on, but at the end of the day, what's an f/stop between friends?
 
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