Depth of Field, Depth of Focus, and all Circles of Confusion

[erich6]

I've seen this topic discussed in different venues and I have yet to find a source that can clearly show how these things are related without being inconsistent. I see a major issue with how books, articles, and professional photographers refer to these terms almost interchangeably. I think this is part of the reason this is such a confusing (no pun intended) topic.

Now, I will be the first to admit that, being an amateur photgrapher, I lack the experience and intuitive feel for photography that many of the experts discussing this topic have. However, I do have a strong technical background in the area and I think I may have broken the code on this issue.

I'd like to share with you what I understand of this problem in the hope that many of you will provide additional insight and maybe we can come up with terminology that can consistently and clearly articulate this concept.

The best article I've seen on this subject is by Mr. Michael Reichmann (see http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml). But, even in this article I believe Mr. Reichmann makes the error that causes all the problems as I see it. Depth-of-field is NOT depth-of-focus (I believe Mr. Reichmann knows this and has actually demonstrated it in his examples but the terminology is still too vague). I've seen others state this as well but they don't take it far enough. Depth-of-focus is NOT the focal length at which light comes to focus. Depth-of-focus is the range that the image plane can move in and out of best focus and still be *in* focus. Again, this is not depth-of-field and I will explain why I think it is not. What qualifies as "in-focus"? This is the "circle-of-confusion" which is how blurry a spot can be before we say "aha! it isn't a spot anymore!".

When we talk about depth-of-focus, we are talking about an effect that occurs around the image plane. There are only three factors that affect depth-of-focus (assuming a perfect lens without any aberrations so that the only thing we're dealing with is blur from defocus): 1) aperture stop; 2) focal length; and 3) the wavelength of light. Depth-of-focus increase with a large f-number (small aperture, long focal length) and long wavelength. Indeed, it is proportional to the product of the f-number and the wavelength.

So why do long-lenses give poor "depth-of-field"? If I have a wider range of distances that I can be in focus then I should, by definition, have a wider range of depth....right? Well, the problem is with the word "field". All of the sudden, the angle-of-view of the lens becomes an important parameter. The wider the lens, the wider the perspective and the more of the background gets collected by the lens system and imaged. What happens now is that the various elements making up the background get squeezed into the film format or digital sensor. In other words, subjects that are actually larger than something closer to the camer will actually look smaller. Since they are smaller, they *appear* to be in focus. This is the same effect you get from looking at a thumbnail of an out-of-focus image that looks OK until you see it on the computer screen or on a larger print and you go "ugh!". As a side note, it is interesting to me to note that a wide-angle lens is by necessity a short focal length lens. Again, it seems counterintuitive (again please bear with my techie vs. photographic background) that a short focal length lens has more depth-of-field since it actually has less depth-of-focus.

The reason wider lenses give better depth-of-field is because of the perspective effect I just mentioned. Objects in the foreground and background really stand out from each other because of the relative sizes and how the geometry projects unto the image plane. Coversely, a long focal-length (telephoto) lens will only image objects that are relatively close to each other in depth. Therefore the shallow depth-of-field. In some books I've seen this described as a "compression" of the field. It is true, but only in the direction of the camera to the subject. In the lateral dimension the field is compressed by a wide-angle lens!

Now I see this topic further complicated by the difference in formats between 35 mm film and digital SLR's. We tend to hear about "effective" focal length due to the format factors. It is true that you will get a different field-of-view from a small digital sensor format than a 35 mm film format. [EDIT: This changes the crop for sure and may "force" a different perspective depending on the focal length of the lens]. But it does NOT affect depth-of-focus nor the actual focal length of the lens. Some have said that because of this it doesn't affect depth-of-field. That's wrong. Since depth-of-field depends on perspective, as I have explained above, then digital cameras with a format factor smaller than 35 mm will also have [EDIT: more] depth-of-field.

I'm sorry if I'm rambling on here or if I'm beating a dead horse. I'm really thinking this stuff out as I write. I hope several of you can shed more insight into this or maybe tell me I've got it all wrong and I should stick to book-worm material instead.... Otherwise, I'd like to know how this topic got so convoluted in the photographic literature.

Thanks,

Erich :scratch

[erich6]

Was I way off here? I'm wondering how come nobody has replied.... Please let me know if I made a mistake.

Erich

[wxwax]

Erich, I suspect that you haven't had many responses because it's a long post and very technical. We have a few wizards here who are technically inclined who might eventually see this thread and respond.

[4labs]

I am far from an expert but there is a book "Photography"
by Barbara London and John Upton that could explain things better than I ever could..You can get it on Amazon.

[Mitch]

Ok I am going to shoot from the hip, but as I remember....

Depth of Field - The area beyond and in front of the object/subject focused on that is in acceptable focus.

Depth of Focus - Inside the camera. The amount that the recording medium can be off plain and still yeild acceptable focus.

Circle-of-Confusion - Your's is as good an answer as any. "which is how blurry a spot can be before we say "aha! it isn't a spot anymore."

I beleive this is correct.:D

Mitch

[DoctorIt]

Now I see this topic further complicated by the difference in formats between 35 mm film and digital SLR's. We tend to hear about "effective" focal length due to the format factors. It is true that you will get a different field-of-view from a small digital sensor format than a 35 mm film format. This changes the perspective for sure. But it does NOT affect depth-of-focus nor the actual focal length of the lens. Some have said that because of this it doesn't affect depth-of-field. That's wrong. Since depth-of-field depends on perspective, as I have explained above, then digital cameras with a format factor smaller than 35 mm will also have less depth-of-field.I think you are right on target, and this is a very good point that I hadn't thought of.

I'm guessing you are still going after this depth of field vs. focus argument. I think circle of confusion is pretty well defined and used correctly, but you are totally right in that these two are interchanged very often.

Lets get down and dirty then, we need some variables and equations. I'm not an optics expert, but I am an engineer. And I'm going to get my optics book right now and see what I can find...

this'll be fun :D

[bkriete]

Don't smaller sensors have greater depth of field? P&S cameras tend to have everything in focus to a greater extent than SLRs...or is this because they usually have smaller max apertures?

[DoctorIt]

It seems Reichmann directly contradicts your view on digital (you said digital has less depth-of-field):

What About Digital?

A common complaint about digital cameras is that when using one it's not possible to get nice out-of-focus backgrounds. Why therefore do digital cameras have greater Depth of Field? The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?

This is one of the unspoken drawbacks of low-end digital cameras. Only expensive SLRs like the Nikon D1x, Canon 1D and their ilk have chips close to the size of a 35mm frame, and therefore offer enough DOF to allow creative control over out-of-focus backgrounds.

[erich6]

It seems Reichmann directly contradicts your view on digital (you said digital has less depth-of-field):

What About Digital?

A common complaint about digital cameras is that when using one it's not possible to get nice out-of-focus backgrounds. Why therefore do digital cameras have greater Depth of Field? The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?



This is one of the unspoken drawbacks of low-end digital cameras. Only expensive SLRs like the Nikon D1x, Canon 1D and their ilk have chips close to the size of a 35mm frame, and therefore offer enough DOF to allow creative control over out-of-focus backgrounds.





He actually hasn't contradicted my statements. Mr. Reichmann says *low-end* digital cameras have greater DOF but not because they're digital but because the lens is so [EDIT: short]:



The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity.









If those small-format cameras had a long lens like those used in a 35 mm SLR they would end up with less DOF.


Erich

[wxwax]

Pssst, I think he said it's because the chip is so small. :evil

[Antonio Correia]

Lattely I have been reading about these subjects and here I send you an excellent link on this matter.
http://www.dofmaster.com/
I am "building" a Depth of Field Calculator for my two lenses. IN PAPER !!!...http://www.dgrin.com/images/icons/Laughing.gif
I will show it to you all, as soon as it is finished. I think I'll remember to do so.

Meanwhile here is a site which gives differents formulas from the one above:
http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html

I think here they are wrong because I tried the formulas in Excel. With dofmaster Things worked fine.

[Shay Stephens]

I studied the topic of circles of confusion and related info but have found very little practical application for this knowledge and see it as a needless complication in the photographic process. The fact that you can go your whole life without having to know this info and yet succeed in photography tells me all I need to know ;-)

Some of the calculator types might need this for camera design and such, but the average Joe doesn't. It can all be boiled down to a few rules of thumb that are easy to remember and implement.

Shay (putting my slide rule away now) Stephens

[pathfinder]

I think the Single lens Reflex design, as well as view camera view finders generally make this kind of information unnecessary complications for most SLR shooters. The old range finder cameras could use this information more.

Just LOOK through the viewfinder. You can also evaluate DOF after you press the DOF preview button to stop your lens down if needed. But I bet the pros rarely use this feature, just like Shay said.

[mercphoto]

But I bet the pros rarely use this feature, just like Shay said.
I was at a class held by one of the local wedding/portrait guys in town (great guy, unreal photography) and he showed an indoor portrait he did that had, in my opinion, just the right amount of background blur. I asked him if he, after all this time, simply knew what aperture setting to use to get the desired background blur in that situation and he said yes. He's done shots like that so often on that equipment that he doesn't need preview or bracketing to get the effect he wants.

[David_S85]

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

Also has software available from the menu on the left side of that page that will print out DOF charts for your own camera and lenses you use.

Good tutorial on hyperfocal distance calculations, charts, etc.

[Antonio Correia]

I studied the topic of circles of confusion and related info but have found very little practical application for this knowledge ... The fact that you can go your whole life without having to know this info and yet succeed in photography tells me all I need to know ;-)Shay.
I agree with you but if you study a little of this it is no harm at all... It is knowledge. :):
Finally, it gives you another way to look at the picture. It gives, shall I say culture ? and that is good.
Let me join a little example.

The photo you will see at http://www.antoniocorreia.com/album/album50/Brasil_Iguasu_2005_01?full=1, was shot in Brasil Iguasu, a place to visit.
The lens - no question - is a supperb L USM 16 35 mm f/2.8 Canon.The body is a 20D what does not matter here.
Shutter Speed 1/320; Normal Program f/9 ISO 100 Focal lenght 16 mm.

It was focused to the water. Because the depth of field was not enought my wife and I are a little bit out of focus.
This can only be seen when you blow up the photo.
We are not so sharp as we shoul be if I had rotated a little bit the focus ring after having switched off the auto-focus.

This picture made me to search for information I knew the existence of.
I'm sure you have seen on some old - say 30 years - zoom lens, the graphics on the cylinder. In blue for the f/5.6 as a can remember.
This was quite usefull (for the connoisseur).
Thank you for your point of view and forgive ye bad english for I have studied it only for 3 years. :):

[Sam]

Depth of field: This refers to how deep the field is. You can grow different crops based on this.

Depth of focus: This has nothing to do with depth of field. This has to do with ones ability to see past the surface and focus more deeply to find the burried meaning. If say you drink a glass of fine wine, and all you can think of is "a couple more of these and I can get a good buzz on" you have a very limited depth of focus....Or a very bad first date.

Circle of confusion: Liberals trying to discuss facts.

:D :D :D :D

Sam

[TOF guy]

I studied the topic of circles of confusion and related info but have found very little practical application for this knowledge and see it as a needless complication in the photographic process. The fact that you can go your whole life without having to know this info and yet succeed in photography tells me all I need to know ;-)
It may not be very useful to know what a circle of confusion is, but personally I like the fact of knowing the principle behind the concept of depth of field :scratch .

It can all be boiled down to a few rules of thumb that are easy to remember and implement.

Please share these rules with us :wave !

Thierry

[Shay Stephens]

It may not be very useful to know what a circle of confusion is...Exactly :wink


Please share these rules with us :wave !You already know them:


Wide aperture = small DOF...............Small aperture = wide DOF
Telephoto = small DOF.....................Wide angle = wide DOF
Close focus on subject = small DOF....Far away focus = wide DOF
Do you want really wide DOF? Use a small aperture (e.g. f/32), wide angle lens (e.g. 28mm), and focus far away from the lens (e.g. try infinity)

Want crazy small DOF instead? Use wide aperture (e.g. f/2.8), telephoto lens (e.g. 200mm), and focus up close to the lens (e.g. closest focus distance).

Want something in between? Modify one or more of the above variables to be somewhere between the extremes. You can add salt and pepper to taste, bake for 20 minutes, and enjoy :wink

[Antonio Correia]

Exactly :wink


You already know them:

Wide aperture = small DOF...............Small aperture = wide DOF
Telephoto = small DOF.....................Wide angle = wide DOF
Close focus on subject = small DOF....Far away focus = wide DOF
Do you want really wide DOF? Use a small aperture (e.g. f/32), wide angle lens (e.g. 28mm), and focus far away from the lens (e.g. try infinity)

Want crazy small DOF instead? Use wide aperture (e.g. f/2.8), telephoto lens (e.g. 200mm), and focus up close to the lens (e.g. closest focus distance).

Want something in between? Modify one or more of the above variables to be somewhere between the extremes. You can add salt and pepper to taste, bake for 20 minutes, and enjoy :winkPlain and simple.
Good point Shay ! :rofl

[Antonio Correia]

It may not be very useful to know what a circle of confusion is, but personally I like the fact of knowing the principle behind the concept of depth of field :scratch .
ThierryI like things here.
People are interesting !
Good point Thiery :rofl

[TOF guy]

link for a DOF calculator.
http://www.dofmaster.com/

Meanwhile here is a site which gives differents formulas from the one above:
http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html

I think here they are wrong because I tried the formulas in Excel.
These two sites use the same formula but ... different definitions for the circle of confusion. Confusing, isn't it :rofl ?. Nikonians.org also lets you enter the value of the CoC, and if you enter the same number as the one found on the other web site for a specific media you should get the same results.

But who is right :dunno ?

The latter (nikonians.org, shared for us by J. Ramón Palacios - who I don't know, but he deserves to be acknowledged for his efforts) uses a value that is close to Carl Zeiss's recommendation (1/1730th of the length of the "negative" diagonal ). The former is based on a more "generous" value that is the one used by Nikon for the markings on its lenses - when there used to be markings :cry (funny, you'd think it would be the other way, with the Nikon CoC definition used by nikonians.org :scratch rather than the Zeiss value. Kudos, Ramón :thumb !). Nikon was long criticized for this: all photographers who wants a very precise DOF for a specific picture will tell you that the value proposed by Zeiss is the one to start from (sometimes a smaller value is used, adjusted for a specific enlargement).

Thierry

[TOF guy]

You already know them:

But few people would have explained these rules as well as you did. Thanks, Shay :clap !

Thierry

[Antonio Correia]

These two sites use the same formula but ... different definitions for the circle of confusion. Confusing, isn't it :rofl ?. Nikonians.org also lets ... ...ThierryGood work Thierry.
Thank you.

[erich6]

Hi everyone. Glad to see a revival of this topic.

Shay, I agree with you on the practical application of these concepts. That's why I added the caveat that I was trying to reconcile my techie perspective with a practical one.

Since I posted this I've learned a lot from just shooting. I find myself worrying less and less on the specific settings I'm applying and more and more on naturally using the rules of thumb Shay laid out.

That said, there is a venue for working through these technical details. I can see it in terms of comparing lenses in a quantitative way for the purposes of design evaluation, etc. Useful for the lens makers, maybe not so much for the shooter. On the other hand, having insight into both should give you a, shall we say, "more complete picture"?

One item I didn't mention in my original post was the effect of distance to subject. What happens here is that if you are close to the subject, it doesnt take much distance in depth for the image to be out of focus on the focal plane. On the other hand, if you are far way, the same depth doesn't span as large a range at the image plane and therefore is in focus (within the circle of confusion).

Erich

[chet79]

I have a question for the DOF-masters out there. I am quite new to the technicalities of photography, and don't have a dslr to practise on. I'm just keen to learn as much as possible before I do get one.

Anyway, my question is this:

I understand how to get a shallow DOF, by focusing in close to the end of the lens. But what if I want a shallow DOF, but in the middle of my shot? i.e. I want a very narrow band, in the middle of my shot, to be in focus (and everything before and after it, out of focus).

like this (OOF = out of focus):
http://img.photobucket.com/albums/v312/chetau/DOF.jpg

[devbobo]

I have a question for the DOF-masters out there. I am quite new to the technicalities of photography, and don't have a dslr to practise on. I'm just keen to learn as much as possible before I do get one.

Anyway, my question is this:

I understand how to get a shallow DOF, by focusing in close to the end of the lens. But what if I want a shallow DOF, but in the middle of my shot? i.e. I want a very narrow band, in the middle of my shot, to be in focus (and everything before and after it, out of focus).

like this (OOF = out of focus):
http://img.photobucket.com/albums/v312/chetau/DOF.jpg

Andrew,

The problem here is, the technique that you would need to adopt to do this in a point and shoot camera is unneccessary with a dSLR.

With a P&S, say you want a certain shot with a narrow DOF at about 1-2m from a subject. You would need to adjust the zoom and move backwards away from the subject until you achieved the desired framing and narrow enough DOF. This is due to the small sensor size, that even at large apertures everything is in focus.

Hope this helps,

David

[chet79]

David, I had assumed it would be near-impossible with a p&s actually. I guess with experimentation, I could pull it off.

I was actually wondering how it would be done with a dslr :)

[devbobo]

I was actually wondering how it would be done with a dslr :)

set a large aperture (eg f2) then starting with the closest focal point slowly adjust the focus and the field of focus will slowly away from you... like in your diagram.

[Antonio Correia]

I found this.
Hope to be usefull.
http://www.night-ray.com/photores/canon16_dof_10-50mm.png
the previous:
http://www.night-ray.com/photography.php?id=6
:thumb

[erich6]

David, I had assumed it would be near-impossible with a p&s actually. I guess with experimentation, I could pull it off.

I was actually wondering how it would be done with a dslr :)

The lens focus ring allows you to adjust what object distance gets to be at "best focus". Some of the better lenses will actually show you what distance you are focused at. If you use a large aperture you will limit the range that is in focus. This affects both the objects before and after the location you focused at.

The longer the focal length the shallower this in-focus region will be. So, if you want to have a very specific and limited range in-focus you should be using a telephoto lens. Shorter focal length lenses will give your more DOF which means more will be in-focus and it's harder to limit the range where objects look sharp.

Erich

[Antonio Correia]

I made this instrument with the use of the information available here.
The 1.st picture is one side
http://antoniocorreia.smugmug.com/photos/92004422-M.jpg
The 2.ed picture is from the other side.
http://antoniocorreia.smugmug.com/photos/92005013-M.jpg
The 3.ed picture is the demonstration of how to use it.
http://antoniocorreia.smugmug.com/photos/92005542-M.jpg
I have seen something similar somewhere in the net.
But it was so small I could not see it properlly.
How do you like this ?
It should/can be used in conjunction with this table
http://antoniocorreia.smugmug.com/photos/86066459-L.jpg
These "instruments" are for my lenses.
You can see the originals at:
http://antoniocorreia.smugmug.com/gallery/1708250/1/86066459
Thank for watching and commenting.

[athos]

digital cameras with a format smaller than 35mm, i.e. aps-c or 1.6x crop - will have MORE depth of field than a larger sensor. which is why the 5d can give you less DOF at the same aperture as a 20d or 30d or nikon dxx.

[athos]

He actually hasn't contradicted my statements. Mr. Reichmann says *low-end* digital cameras have greater DOF but not because they're digital but because the lens is so small:





If those small-format cameras had a long lens like those used in a 35 mm SLR they would end up with less DOF.[/left]


Erich

that is simply not the case. it has nothing to do with the lens. it has to do with the sensor size.

of course a longer lens will give less DOF but we are talking about sensor size here with the equivalent lens/f stop.

this is definitely one of the reasons i want a 5d or FF camera.

[Antonio Correia]

Ny tables take that in account because they are based on CoC = 0,019

[Antonio Correia]

Erich, I hope you don't mind that I keep posting here about DOF. Obrigado.:wink
I made today the chart herewith.
You may ask: what for ? There's no use for it.
And may be you are right.
But let me explain what I had in mind.

The depth of field tables are usefull but are difficult to work with when the distances are too big.
I have written about this problem on Nikolai's assignment (http://www.dgrin.com/showthread.php?t=42757&page=2) and my difficulties...
The table is to show the photographer, using 16 mm to 35 mm lens, where the DF is when focusing on the hyperfocal distance.
The look at this table gives a quick glance of the DOF under different apertures.
It's far easiest to estimate that the object is 4 or 6 meters away than 200 m and the quick look to the table tell us that, using an aperture under 8, we will have all in focus.
Am I messing up and making things more complicated that they are ?:dunno

http://antoniocorreia.smugmug.com/photos/96969174-M.jpg

[andymillson]

I posted about this in the Gadgets forum but it bears montioning here as well

Check out http://www.expodisc.com/products/product_detail.php?prodid=4&productname=ExpoAperture_Depth_of_Field_Guide

The expoaperture disc will help you calculate DOF etc for different lenses etc in a single gadget

Seems to be pretty good so far in the tests I have carried out. The manual is available online so you can see how it works

Andy

[Antonio Correia]

Andy,
Have a look at my post here (http://www.dgrin.com/showthread.php?t=11015&page=7) .
I made the expo disk myself ! :D
It was agood ideia to post that link here..
:thumb

[andymillson]

Andy,
Have a look at my post here (http://www.dgrin.com/showthread.php?t=11015&page=7) .
I made the expo disk myself ! :D
It was agood ideia to post that link here..
:thumb

Kudos for doing all that work!

I have made similar calculators to the one you showed here, using the DOFMater program, but I like the Expo one more as it is simple to use for pretty much all focal lengths

Admittedly there is some tweaking required to use with a non full frame DSLR but they tell you how to do it in the manual and it is pretty easy to do in the field

[TOF guy]

that is simply not the case. it has nothing to do with the lens. it has to do with the sensor size.



Mr. Reichmann statement is correct :

Cameras with smaller sensor do not have long lenses

But maybe it could be worded a little better: a long lens (meaning lens with long focal length) yield impractical fields of view in combination with a small sensor: too small for a camera sold to a wide audience. Therefore manufacturers use short lenses in their P&S cameras. When these lenses are used at typical FOV equivalent to 50 mm on a 35 mm camera, they are actually using 15 mm (say, size of sensors in P&S cameras are not all the same) focal length (that's wide-angle!).

The effect of the sensor size itself goes the other way: smaller sensor needs means smaller circle of confusion which implies smaller (not larger) DOF for same focal length. However the impact is proportional to sensor size. By contrast the impact of focal length on DOF is inversely proportional to the square (for most situations) of focal length. As a result for a same field of view captured with sensor of different sizes, the effect coming from the shorter focal length "wins", hence camera with the smaller sensor will have wider DOF.

The difference between FF and APS-C is small enough that it can be handled with a small aperture adjustement in most shooting situations. Most people who buy a 5D would be better served buying a 30D, save the money for better glass (which will also opens wider), and invest in a $50 book on the basics of photography. One has just to peruse the EXIF data of the 5D pics posted in dpreview forum to get an idea of what I'm talking about.

Thierry

[erich6]

that is simply not the case. it has nothing to do with the lens. it has to do with the sensor size.

of course a longer lens will give less DOF but we are talking about sensor size here with the equivalent lens/f stop.

this is definitely one of the reasons i want a 5d or FF camera.

You are partly correct.

I used "small lens" incorrectly here. I meant "small" in terms of focal length, not aperture (this was early on when I was still sorting through all this...the right terminology is "short").

It is the focal length which determines perspective. The size of the focal plane just determines FOV and what crop you get. That said, for an equivalent FOV, a smaller focal plane requires a shorter focal length than a larger focal plane. Therefore, small focal planes tend to be paired up with *short* focal length lens. That's what creates the increased DOF.

If you take the same lens at the same aperture and put it in front of a 5D you'll just get a bigger FOV than with a 20D. The DOF will be the same all else being equal. But not all else is equal because I think the 5D's pixels are larger than the 20D's. I think larger pixels give you more "depth-of-focus" because you are not as sensitive to changes in the optical focus.

Erich

[erich6]

Erich, I hope you don't mind that I keep posting here about DOF. Obrigado.:wink


I don't mind at all! IT's entirely appropriate. This is not *my* thread after all. It's the forum!

Cheers,

Erich

[erich6]

Mr. Reichmann statement is correct :

Cameras with smaller sensor do not have long lenses

But maybe it could be worded a little better: a long lens (meaning lens with long focal length) yield impractical fields of view in combination with a small sensor: too small for a camera sold to a wide audience. Therefore manufacturers use short lenses in their P&S cameras. When these lenses are used at typical FOV equivalent to 50 mm on a 35 mm camera, they are actually using 15 mm (say, size of sensors in P&S cameras are not all the same) focal length (that's wide-angle!).

The effect of the sensor size itself goes the other way: smaller sensor needs means smaller circle of confusion which implies smaller (not larger) DOF for same focal length. However the impact is proportional to sensor size. By contrast the impact of focal length on DOF is inversely proportional to the square (for most situations) of focal length. As a result for a same field of view captured with sensor of different sizes, the effect coming from the shorter focal length "wins", hence camera with the smaller sensor will have wider DOF.

The difference between FF and APS-C is small enough that it can be handled with a small aperture adjustement in most shooting situations. Most people who buy a 5D would be better served buying a 30D, save the money for better glass (which will also opens wider), and invest in a $50 book on the basics of photography. One has just to peruse the EXIF data of the 5D pics posted in dpreview forum to get an idea of what I'm talking about.

Thierry

:agree Nicely stated Thierry.