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10-02-2008, 09:02 AM   #16
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I completely agree with Marc's "stop down for more, open up for less" statement. I know there probably is some math formula to calculate the exact depth of field for a lens but I would never remember it much less have time figure it out on the fly. An example, I know I have stop down when shooting my daughters crew team to get all 8 rowers in focus. On my 70-300 Sigma, the auto focus wants to re-adjust for each person in the boat so I have found that I get much better results with a manual focus and f/11 or f/16. Also, to get faces you have to be on an angle where the boat is moving away so the distance is changing. The smaller point and shoot cameras with smaller sensors and lenses seem to have much more depth of field than our SLR's so I think that causes problems with new users moving up.

10-02-2008, 09:16 AM   #17
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QuoteOriginally posted by brownargus Quote
Depth of field is directly related to the reproduction ratio, i.e. the relation of the image and its size on the sensor/film. It is not related to the focal length of the lens - two different focal length lenses at the same f no. will give the same DOF for the same reproduction ratio. The best way is to use the DOF button which allows one to have some indication of the DOF before shooting.
so, is that a yes or a no??
10-02-2008, 11:08 AM   #18
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QuoteOriginally posted by debbie Quote
ok, but if I use, lets say a 35mm on a aps-c camera to make a picture and make the same one on a FF with a 50mm (lets asume that would give the same object size in the image) and print them out in the same size (and use the same f-stop)
Would the DOF be the same??
The answer is no. The 35mm lens on APS-C will have greater DOF since it provides significantly less magnification.

There are two basic variables:
  • Actual subject image size at the detector/film plane (magnification)
  • Lens aperture

There is actually a third variable (circle of confusion), that is a function of the detector size, but magnification and aperture have far more weight for this comparison.

Here is a calculator that factors in the circle of confusion:

DOF Calculator

Steve

Last edited by stevebrot; 10-02-2008 at 05:32 PM. Reason: Edited for accuracy and to suggest a better calculator
10-03-2008, 12:44 AM   #19
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QuoteOriginally posted by sewebster Quote
So if you take a picture of something with a 50mm lens, and also with a 100mm lens where you stepped backwards to get the same framing, the DOF will be the same.
I think I'm getting the point. Since DOF is a range of distances from of the lens, DOF is wildly different in these two cases. Same objects will appear with approximately the same sharpness in this case.
In other words: the range of acceptable sharpness around the focussed object is approximately the same. This will only hold when the focussed distance of both lenses is small in relation to their respective hyperfocal distance.
Also perpective is different.

10-03-2008, 12:53 AM   #20
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QuoteOriginally posted by debbie Quote
ok, but if I use, lets say a 35mm on a aps-c camera to make a picture and make the same one on a FF with a 50mm (lets asume that would give the same object size in the image) and print them out in the same size (and use the same f-stop)
Would the DOF be the same??
Even if the same objects appear to have the same sharpness and the same size in both images, DOF is different because DOF is a range of distances from your camera.

Realise that with this setup you change about every variable that influences DOF:
focal length, focussed distance and magnification from captured to displayed image. About the only thing that remains the same is f/stop (relative aperture)
10-03-2008, 01:51 AM   #21
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QuoteOriginally posted by debbie Quote
so, is that a yes or a no??
I would have thought that the DOF will be the same simply because the APS sensor is just taking a crop from what would be a FF image. Circle of confusion is simply a means of specifying whether part of the image is in focus or not.

W. S. Bomback in Manual of Colour Photography 1964, says: "the depth of field can be calculated by the following formula in which m is the ratio of the object to image (reproduction ratio) and d is the diameter of the disc (circle) of confusion. Total depth of field = m x (1+m) x 2d x f/no. Thus if the object is 10 times as big as the image, d = 1/500 in. and the f/number is 4, the total depth of field is 1.75 in. …It may be deduced from the above formula that depth of field is the same for any given scale of image on the film (sensor) irrespective of the focal length of the lens."

Since, in the case of an APS sensor, for a given lens focal length and f no., the image will be the same size whether it projects a FF image or a cropped image onto the film plane, the reproduction ratio will be the same therefore the DOF will be the same.

John
10-03-2008, 09:29 AM   #22
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QuoteOriginally posted by brownargus Quote
Since, in the case of an APS sensor, for a given lens focal length and f no., the image will be the same size whether it projects a FF image or a cropped image onto the film plane, the reproduction ratio will be the same therefore the DOF will be the same.
The image projected by the lens on the sensor plane will be the same of course, but since, all other things being equal, you will be enlarging the image captured by the APS-C sensor more to get to your "usual" viewing size (eg. 4x6 print, 19" monitor, whatever), you will be enlarging the "blurryness" of the OOF areas more, and therefore the depth of field will seem/be smaller.
10-03-2008, 10:48 AM   #23
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QuoteOriginally posted by brownargus Quote
I would have thought that the DOF will be the same simply because the APS sensor is just taking a crop from what would be a FF image.
Sure, but she was comparing images with different reproduction ratios. A 35mm lens on APS-C might provide the same angle of view as a 50mm lens on FF, but the actual projected image is smaller. Meaning the reproduction ratio is smaller.

If we were comparing DOF on two different sensors with the *same* reproduction ratio, and the only difference was the cropping, then indeed, DOF would be closer to the same (although not totally the same in practice, because the smaller sensor image would need to be magnified more to view it the same size at that from the larger sensor, which changes the circle of confusion).

QuoteQuote:
Circle of confusion is simply a means of specifying whether part of the image is in focus or not.
Right, and as such it is very much dependent on how much magnification you apply to the image when viewing it. Some DOF calculations pretend it is a constant, but it isn't. The more magnification you apply, the less DOF you see, because what looks in focus at a small viewing size doesn't necessarily look so in focus a a larger viewing size.

QuoteQuote:
Since, in the case of an APS sensor, for a given lens focal length and f no., the image will be the same size whether it projects a FF image or a cropped image onto the film plane, the reproduction ratio will be the same therefore the DOF will be the same.
Correct, but again, she wasn't asking about situations that yielded the same image size (reproduction ratio) - she was asking abut two situations with very different image sizes. Standing in the same place, 35mm lens on APS-C, 50mm on FF. Totally different reproduction ratios. The framing of the images is identical, but the APS-C image is just physically smaller.

10-04-2008, 05:51 AM   #24
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QuoteOriginally posted by sewebster Quote
The image projected by the lens on the sensor plane will be the same of course, but since, all other things being equal, you will be enlarging the image captured by the APS-C sensor more to get to your "usual" viewing size (eg. 4x6 print, 19" monitor, whatever), you will be enlarging the "blurryness" of the OOF areas more, and therefore the depth of field will seem/be smaller.
A bit of fuzzy logic there. Enlarging the image for printing, etc., will make no difference to the actual DOF since that is fundamental to the original shot. The image is the same on both so you are simply cropping the full size image on the APS sensor, hence the DOF cannot be anything other than the same. If, however, the camera is moved back to produce the same view on the APS as on the FF, then the DOF on the APS shot will obviously be greater since the reproduction ratio is smaller.

Marc, I think that you are conufusing two things. The circle of confusion is a facet of the original image and is used in the calculation of DOF. The fact that you enlarge an APS image more when you print it for the same size print just makes it look worse, just like any blemish would/

John

Last edited by brownargus; 10-04-2008 at 06:30 AM. Reason: To make an addition
10-04-2008, 07:07 AM   #25
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Vieiwng images

QuoteQuote:
Enlarging the image for printing, etc., will make no difference to the actual DOF since that is fundamental to the original shot.
Sorry John, but it's back to the drawing bord

Magnification, viewing distance, visual acuity, lighting conditions etc. all influence DOF. Remember DOF is about the portion of the viewed image that is ACCEPTABLY sharp. Just putting on your glasses when viewing an image does change DOF, because you'll find less of the image to be acceptably sharp than without glasses.
10-04-2008, 01:14 PM   #26
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Yes, I agree with Baw here, but I think Brownargus just has a different sort of definition for DOF in his head... sounds like we are understanding the principles the same.

The DOF calculators I use definately ask how good your vision is, how big you'll print, how close you will stand when looking etc.

The calculators that don't ask those things assume an old standard for viewing conditions.

Understanding Depth of Field in Photography

This link covers the viewing conditions stuff. There is also a link to a calculator that allows you to include more details on the page (next to the simple calculator).
10-04-2008, 01:54 PM   #27
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QuoteOriginally posted by brownargus Quote
Marc, I think that you are conufusing two things. The circle of confusion is a facet of the original image and is used in the calculation of DOF. The fact that you enlarge an APS image more when you print it for the same size print just makes it look worse, just like any blemish would/
No, that's just not true. The "circle of confusion" is not simply an attribute of the image, but rather, of the viewing size and your distance from it when viewing.

See, for instance,

Circle of confusion - Wikipedia, the free encyclopedia

and read the section entitled, "Circle of confusion diameter limit in photography". It lists the factors that influence this, and even gives a precise formula for computing it.

Look at it this way: the image on the sensor itself has a fixed CoC, and cropping doesn't change it, to be sure. But the more you enlarge the image, the more you enlarge the CoC. And since a smaller image needs to be enlarged more than a larger one to get a print at the same size, the CoC of a 8x10" (say) print made from an APS-C image will differ from one made from a 35mm image.

It's actually quite easy to see how this works in practice: view an image at 100% and make a determination of what seems to be in focus and what doesn't. Now view the same image as a thumbnail. That line you drew between in-focus and out-of-focus at 100% no longer looks meaningful - plenty of stuff that appear OOF at 100% looks just fine in the thumbnail. That's because you changed the enlargement ratio but kept viewing distance the same. Of course, in practice, it's also because the thumbnail had to throw away a lot of data, but it would remain true even if you had super-duper high-res screens capable of displaying 10MP of detail in a thumbnail. After all, it was true of contact sheets with film.

All of this is really a fancy way of saying: something might look in focus when viewed from a distance or in a small reproduction, but look out of focus in a larger reproduction. We all know instinctively that this is quite true, and it's why we all want big viewfinders when focusing manually, or why people use magnifying loupes when viewing negatives or contact sheets. Size matters when judging focus.

The relevance of that here is that a small image needs to be enlarged more to get to a certain size than a big image. And it isn't just the size of the image that influences the circle of confusion, but rather the extent to which it was enlarged.
10-05-2008, 12:33 AM   #28
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Back to the basics.

This thread seems to raise more questions than it answers. I'll try to summarise my take on DOF.

First the definition: DOF is the range of objectdistances from the camera in your scene , within which objects will be judged as acceptably sharp when viewed in the resulting image.
See this page for some similar defintions.
As an example take the image with the domino stones on that page. I find the stones 4,5 and 6 (with 3 black dots) acceptably sharp.
Now back to the scene and measure the distance from your lens to stone 4 and stone 6, and you have the DOF.
That's basically what DOF is all about.

To give this a more theoretical framework the following:


The optical part.

Our lenses can't project everything in a scene equally sharp. To quantify this the Circle of Confusion (CoC) has been introduced. Points in the scene that are not at the focussed distance will be projected by the lens on the imaging plane behind it as circles (CoC's). The size of these circles depends on focal length, f/stop (relative aperture) and focussed distance of the lens plus the position of the point in the scene.
See this page for a nice illustration.

The imaging/viewing part.

We define DOF by choosing a maximum size for all those CoC's, the CoC limit.
The CoC limit depends on a lot of factors in the imaging/viewing process, ao the sensor resolution, magnification, viewing distance, visual acuity, lighting conditions etc. etc.
You should decide on a CoC limit for your particular imaging /viewing situation.

DOF.

For each point in the scene the CoC diameter is known.
CoC diameter > CoC limit: outside DOF
CoC diameter < CoC limit: inside DOF

Comparing different situations.

Discussions around DOF usually are about the effect of a certain parameter.
The only way to show this effect is by keeping all other parameters the same.
To show the effect of different lens settings, keep the imaging/display part the same by eg. viewing all images on the same monitor at full screen. Now change only the parameter you are interested in.
You'll find that focal length, f/stop and focussed distance all influence DOF.
By changing 2 or more parameters, you can create images that look (or even are) the same, but you can't conclude that this is due to the changing of a single parameter.

Some examples

Sensor size.
For different sensor sizes you end up with different sized images, having the same DOF.
Now if you compare different sensor sizes at the same image size, you'll see different DOF's, but this is due to different magnification to arrive at the same image size.

Constant magnification ratio.
You'll find claims that DOF is independant of focal length when keeping the magn. ratio the same.
This is usually shown with two images shot at different focal lengths showing the same scene.
What happens here is that for the different focal lengths to give the same magn. ratio you need to change the object distance and thus the focussed distance as well. You change TWO of the DOF parameters.
Also perspective is different.
At macro distances the comparison won't hold, same for distances where the focussed distance isn't small in relation to the hyperfocal distance. Lastly the distribution of the DOF in front and behind the focussed distance isn't constant.

Last example.
You can create equal images with exact same DOF for different focal lengths by chosing your settings carefully.
eg. a 25mm lens at f/2 and a 100mm at f/8. Focus at eg. 10 meter.
When cropping the 25mm image to have the same scene as the 100mm one and then magnifying it 4 times, you'll have two identical images, even the same perspective, but you had two change 3 parameters to create this result.

Finally.
In the above I didn't mention several assumptions/simplifications to keep the text readable.
eg. diffraction, CoC's looking like cat's eyes or having the shape of the diaphragm blades, sensors having enough pixels for large magnification etc.

Last edited by baw; 10-05-2008 at 03:08 AM.
10-05-2008, 09:43 AM   #29
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QuoteOriginally posted by baw Quote
Last example.
You can create equal images with exact same DOF for different focal lengths by chosing your settings carefully.
eg. a 25mm lens at f/2 and a 100mm at f/8. Focus at eg. 10 meter.
When cropping the 25mm image to have the same scene as the 100mm one and then magnifying it 4 times, you'll have two identical images, even the same perspective, but you had two change 3 parameters to create this result.
I find this statement fascinating (truly). Could you indulge us by providing an example image? Something with a meter stick and markers at various points.

In general, my belief has been that post-exposure enlargement does not affect the final image DOF. Having said this, I have been wrong, very wrong before...


Steve
10-05-2008, 10:46 AM   #30
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Just a brief note regarding my post above. I really am interested in seeing an example image. The reason being is that a full calculation of CoC (and by extension, DOF) involves the final image size (enlargement) and viewing distance for the final observer. The CoC assigned to either APS-C (0.020mm) or 35mm (0.025mm) assumes a standard final image size and viewing distance (about 11"x14" at 1 meter).

If you want to play with some numbers:
Julian's Lens Calculator
The interface is not the easiest to use, but once you figure it out, the results will definitely raise the eyebrows!

The important thing to take away from all of this is that the numeric DOF is based on a perceptual standard (CoC) applied to optical physics. The final determination of "acceptable focus" includes the visual acuity of the final observer, the enlargement of the final image, the magnification of the taking lens, and the aperture of the taking lens.

So, you might ask...how can they put a DOF scale on a lens?

The convention among lens manufacturers has been to standardize the visual acuity of the final observer and the final image enlargement*/viewing distance leaving two factors:
  • Magnification (reproduction ratio) at the focal plane
  • Taking aperture

Steve

* The final image enlargement is related to both the final image dimensions and the size of the detector/film media. That is why APS-C has a smaller CoC than 35mm film.

Last edited by stevebrot; 10-05-2008 at 10:54 AM.
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