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still wrapping head around FF DOF and cropped DOF
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PostPosted: Thu Mar 04, 2010 5:16 am    Post subject: Reply with quote
woodrim wrote:
Good grief, sichko, that gave me a headache. It was a 1997 article that basically spoke of a film's ability to resolve.


The relevant part was where it confirmed that the “standard” definition of “in focus” is dependant on the size of the film format, namely 1/1500th of the diagonal (at least for small formats). This definition is still used by many DoF calculators and for printing the DoF scales on most lenses that have them anymore. The parts about films ability to resolve were probably intended as an excuse why “modern” (1997) pro users may be unsatisfied with the results, e.g. remember that focusing at the hyperfocal distance places infinity just barely “in focus”, so having a suitably stringent definition is necessary to obtain the desired effect.

woodrim wrote:

Still, what the lens delivers is unaffected by what the lens is delivering it to.


Obviously true, but I haven't seen anyone contesting this? The result is not, however, unaffected by enlargement or viewing distance, as you agree:

woodrim wrote:

A simple way of looking at this is to take any picture that has both in focus and out of focus portions and keep moving it away from the viewer. At some point the OOF and IF will be indistinguishable and the photo will look move evenly focused. Conversely, take any crisp image and keep enlarging it and soon it will all look OOF.


Now, since you agree that enlarging has an effect on how “in focus” it looks, it should be clear that the original size of the image also affects it; if we enlarge to a given size (e.g. to fill a computer screen, to an 8×10" print, etc), the enlargement ratio is greater for smaller formats. Hence, small formats will appear less in focus at a given output size with all other things being equal, i.e. larger formats have more depth of field with the same focal length, distance, aperture, output size, and viewing distance.

This is also why the definition of “in focus” is, as the Zeiss article affirms, given as a fraction of the diagonal of the frame—this results in a more stringent definition for smaller formats, as is required to give the same apparent DoF at a given output size. Of course, such a definition is by necessity a generalisation, and more demanding users/situations call for other definitions… For example, the definition based on the image size assumes that the viewing distance grows with enlargement, as viewing an image up close has the same effect as enlarging it more. (Typically the assumption with photos of “usual” proportions is that the viewing distance is equal to the diagonal of the image.)


woodrim wrote:

So where does this leave us? With the same answer. Depth of field of a lens is the same regardless of camera format using it.


This is true only in the type of situation where the viewing size (consisting of enlargement + viewing distance) of the final image depends on the size of the format. As an example, if you print a full frame image, you can crop it by cutting it with scissors to make it the exact equivalent of an APS-C image. Now, if you view this cropped print at the same distance as you viewed the full frame image, obviously it is unchanged. However, if you bring this smaller image closer to you, it will look less “in focus” (as you agreed above). Likewise, if you scan this crop and print it at the size of the original full frame print, you have enlarged it more (because to make that size of print by cropping with scissors the full frame print would have been larger)—again, it will look less in focus due to increased enlargement (as you agreed above).

So, in a practical situation where the output size and viewing distance is decided independently of the original size, the smaller format has less depth of field due to greater enlargement required to reach a given viewing size. Makes perfect sense, too; that's why you zoom in on (= enlarge) digital images to check critical focus…


However, in practice, using the smaller format is not strictly equivalent to cropping like this, but rather the original image itself is composed for the desired effect. This leads to using shorter focal length and/or greater distances on smaller formats, which leads to more depth of field than in a full frame image with the same composition (as opposed to the same magnification on film/sensor, as discussed above). But, this you already said yourself in an earlier post.



woodrim wrote:
What "IS" depth of field? We'll cover that after we solve the meaning of life.


There is no universal definition for “in focus” (which determines “depth of field”). With digital, however, it is possible to use a pixel-based definition of in focus, and similarly on a given film one could base it on its resolving capability. These definitions would have the advantage of being “good enough” for any use up to the limits of the film/sensor, and indeed some online DoF calculators use this type of definition for digital, but the problem is that in most uses the definition will be stricter than necessary. So, a practical definition is based on assumptions about how the image will be viewed…


PostPosted: Fri Mar 05, 2010 9:07 pm    Post subject: Reply with quote
The way i see it
at the same aperture :
All lenses from ultra wide wide to the longest ones have exactly the same dof for the same picture taken
The only factors that can change dof are the relative size of the photographed subject against the projected size of that image on the sensor the coc and the selected aperture
The size of projection are changed by the distance from the subject or the view angle of the lens
in other words if you frame a 1 square meter box to fill your viewfinder with a 25mm wide or you fill it with 500mm tele from a larger distance the dof will be the same only perspective will change

if you take a full frame cam and you only fill an apsc sized space on the sensor with the above mentioned box you will get the same dof as an aps cam (given the same pixel/coc/pitch) of the sensors are the same

every single composition or photograph taken have only one doffor the specific sensor used no matter anything else

Shocked Very Happy Very Happy


PostPosted: Fri Mar 05, 2010 9:36 pm    Post subject: Reply with quote
Quote:
in other words if you frame a 1 square meter box to fill your viewfinder with a 25mm wide or you fill it with 500mm tele from a larger distance the dof will be the same


actually this is false, DOF changes a lot with distance of the focused subject (at same fstop).


PostPosted: Sat Mar 06, 2010 3:03 pm    Post subject: Reply with quote
Aarku: I won't argue with your explanation, but only say that we have drifted way off the original poster's issue. I believe we have provided the answer for him, but have taken this far beyond what is reasonable thought for practical purposes. I can only imagine the same discussions going on with the people that determined the scales.

Here is a chart that will help with understanding depth in macro applications. Apparently, depth of focus is not changed by focal length, but by image magnification on film/sensor. Furthermore, the field of view for any given magnification will be constant regardless of focal length. Likewise, depth of field will be constant for a magnification ratio regardless of focal length. This chart will pretty well explain why we have such a hard time getting the desired subject point in focus.



PostPosted: Sat Mar 06, 2010 3:13 pm    Post subject: Reply with quote
I should also note that it's interesting that they specified "35mm Cameras". Also, I'm not sure I agree with the footnote of depth of field being equally divided by plane of focus. I've always believed that depth was great beyond the plane.


PostPosted: Sat Mar 06, 2010 4:39 pm    Post subject: Reply with quote
woodrim wrote:
Apparently, depth of focus is not changed by focal length, but by image magnification on film/sensor. Furthermore, the field of view for any given magnification will be constant regardless of focal length.


As magnification depends on the subject distance and focal length, it is just another way of saying that DoF depends on distance and focal length. However, regarding the latter statement of field of view, one should remember that the depth of field itself encompasses more than one distance, and therefore more than one magnification, while the magnification is typically calculated only for the actual plane of focus. Hence it makes more sense in non-macro use to speak of distance and focal length rather than magnification, as magnification is not very well suited for long-distance photography (e.g. remember that at infinity magnification is zero, but even though a lens may be focused at infinity, the actual objects seen in the picture are at finite distances).

woodrim wrote:
I should also note that it's interesting that they specified "35mm Cameras".


Yes, because the definition of “in focus” used for creating the chart is based on this format and assumptions about enlargement and viewing distance. The chart is not transferable to different formats unless one also assumes that the enlargement ratio of the final image is kept constant, which it isn't in practice because people enlarge to their desired viewing size (e.g. to fill a paper or a screen).

Likewise the composition of the image would be different if the magnification was kept constant on film/sensor. Generally people are more interested in magnification as a proportion of the image size, so larger formats require more magnification to fill the same amount of the frame. So, again, we come to the same conclusion that in any practical scenario format affects depth of field. =)

Last edited by Arkku on Sat Mar 06, 2010 7:59 pm; edited 1 time in total


PostPosted: Sat Mar 06, 2010 4:41 pm    Post subject: Reply with quote
woodrim wrote:
Also, I'm not sure I agree with the footnote of depth of field being equally divided by plane of focus. I've always believed that depth was great beyond the plane.


Farther away from the camera there's less magnification, so depth of field is indeed greater “on the other side”—even “infinite” at the hyperfocal distance and beyond.

Last edited by Arkku on Sun Mar 07, 2010 9:28 am; edited 2 times in total


PostPosted: Sat Mar 06, 2010 5:10 pm    Post subject: Reply with quote
woodrim wrote:
martinsmith99: I believe you are misunderstanding what was explained. And I don't agree with the point being made, not that it isn't correct, but it's irrelevant and just serves to prolong the confusion. The gent stated: "larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame", which is just again making an image size comparison in the viewfinder. We already know that different focal lengths need to be used to get equal field of view in different size sensors. This isn't new to digital cameras, this has been the case since the beginning of photography with different format cameras. Medium format cameras used 50mm as wide angle, whereas they were considered to be normal on 35mm cameras.

No misunderstanding whatsoever. I am not sure why you think you need to explain that to me.

Arrku wrote:
Likewise the composition of the image would be the different if the magnification was kept constant on film/sensor. Generally people are more interested in magnification as a proportion of the image size, so larger formats require more magnification to fill the same amount of the frame. So, again, we come to the same conclusion that in any practical scenario format affects depth of field. =)


Precisely!

Whichever way you look at it DOF is down to aperture, distance from subject, focal length and the format you are recording it on. if I had 2 very different formats to record an image on, framing it the same would mean that one of the other factors would have to change, therefore different format affect DOF.


PostPosted: Sat Mar 06, 2010 6:10 pm    Post subject: Reply with quote
woodrim wrote:

Here is a chart that will help with understanding depth in macro applications. Apparently, depth of focus is not changed by focal length...



Can you tell us where the chart comes from ?

Are you using depth of focus synonymously with depth of field ?


PostPosted: Sat Mar 06, 2010 10:19 pm    Post subject: Reply with quote
No, don't know where it came from. Was shared by someone else on another forum.

Yes, I was using the term depth of focus synonymously with depth of field. It's a term I like better, especially when speaking to others who aren't familiar with photographic language.
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