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still wrapping head around FF DOF and cropped DOF
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PostPosted: Tue Mar 02, 2010 10:52 am    Post subject: still wrapping head around FF DOF and cropped DOF Reply with quote
I've read far too many articles about this comparison to make sense of it any more. I think I've finally figured it out, but would like to know if people could maybe confirm my thoughts, so that I no longer am agonizing over trying to explain this.

You have a 50mm f/1.4 Canon lens.

This lens will provide the exact same DOF on a cropped and FF sensor, nothing should change since the lens is still projecting the same image, it is merely cropped on the crop sensor.

The issue becomes prevalent because 50mm appears like 80mm on a cropped sensor.

So this means that since 50mm is "equivalent" to 80mm on full frame, the dof comparison would be between a 50mm lens on cropped and an 80mm lens on full frame, which would give the full frame shallower depth of focus?


PostPosted: Tue Mar 02, 2010 11:54 am    Post subject: Reply with quote
You got it the other way around Smile The DOF will remain the same on all types of sensors if you are using a) the same lens, and b) shooting an object at the same distance. It's the distance that matters.

OK, here's an example. Let's take your 50/1.4 lens and take a portrait shot from 1.5m (this is crucial: the distance remains the same). FF body: you'll take a half body length shot. APS-C body: head and shoulders shot. However, the DOF will be the same for both shots.

Let us say, you want a half body length shot taken with an APS-C body. You have two options: a) using a 35mm lens AND shooting from 1.5m, or b) using the same 50/1.4 lens and moving further from your subject, to about 2.25m. In case a) you'll get the same perspective as with a full-frame body, but deeper DOF (less blur) because you used a shorter focal length. In case b) you'll get a DIFFERENT perspective (proportions of your subject will be slightly different, e.g. the nose will appear smaller and the face a bit more flat), and the DOF will be deeper (less blur) because you are further away from your subject.


PostPosted: Tue Mar 02, 2010 12:10 pm    Post subject: Reply with quote
if you are closer to the subject (with same fstop and mm), there's much more DOF at the background. Fullframe makes you just that, you can be closer to subject, while having the same photographic crop and therefore more dof.


PostPosted: Tue Mar 02, 2010 12:31 pm    Post subject: Reply with quote
combination that give the same field of view and depth of field
30:1.4 crop = 50:2.8 FF
50:1 crop = 85:2 FF
85:1.4 crop = 135:2.8 FF
someone who like bokeh have to pay more money on crop for the same results
to match a FF 50:1.4, a crop 30:0.7 would be needed


PostPosted: Tue Mar 02, 2010 12:40 pm    Post subject: Reply with quote
egidio wrote:
if you are closer to the subject (with same fstop and mm), there's much more DOF at the background. Fullframe makes you just that, you can be closer to subject, while having the same photographic crop and therefore more dof.


Of course, now it makes sense perfectly.

Thank you all for the explanations. While I had an idea of the concept behind what was going on, the online explanations really went into far too much technical detail regarding this subject. It ended up simply confusing me even more, so thanks for making it easier to understand.


PostPosted: Tue Mar 02, 2010 5:31 pm    Post subject: Reply with quote
close focus is actually very important if you are bokeh-lover, you can get minimal DOF with longer lens like 135mm f2.8, but it can be problematic with portraits, when you cannot be that close to subject.
That's why 85mm lenses (f2, or larger) are that popular among bokeh fans, they mostly have ability to focus closer than 2m


PostPosted: Tue Mar 02, 2010 5:35 pm    Post subject: Reply with quote
I choosed the easy way to avoid maths : i tooked a FF camera Laughing


PostPosted: Tue Mar 02, 2010 6:44 pm    Post subject: Reply with quote
hexi wrote:
I choosed the easy way to avoid maths : i tooked a FF camera Laughing


That doesn't avoid maths. It avoids "I know what I will get on FF camera but have no idea what I get on this new crop camera" maths. Which, as time progresses, is not the situation that many people find themselves in. Nowadays many people came to photography with a crop DSLR, so when they get a full frame, they still have to do "what will this look like on the camera I am less familiar with" calculations.

And staying with a single form factor does not avoid "what aperture will i use to get most of my subject in focus and is it even possible" calculations.


PostPosted: Tue Mar 02, 2010 7:25 pm    Post subject: Reply with quote
ChrisLilley wrote:
hexi wrote:
I choosed the easy way to avoid maths : i tooked a FF camera Laughing


That doesn't avoid maths. It avoids "I know what I will get on FF camera but have no idea what I get on this new crop camera" maths. Which, as time progresses, is not the situation that many people find themselves in. Nowadays many people came to photography with a crop DSLR, so when they get a full frame, they still have to do "what will this look like on the camera I am less familiar with" calculations.

And staying with a single form factor does not avoid "what aperture will i use to get most of my subject in focus and is it even possible" calculations.

I think it was meant in jest.


PostPosted: Tue Mar 02, 2010 7:33 pm    Post subject: Reply with quote
It was indeed. I won't do this again in this thread Very Happy


PostPosted: Tue Mar 02, 2010 7:33 pm    Post subject: Reply with quote
This site may help:
http://www.dofmaster.com/dofjs.html

As you will see, all things being equal (aperture, distance & focal length) but changing cameras will make a difference to DOF.


PostPosted: Tue Mar 02, 2010 8:02 pm    Post subject: Reply with quote
Just when people were happy with the explanations... changing cameras? Unless this is a phenomenon I don't understand, depth of field is strictly a function of focal length and aperture. Forget about perceived focal length in the viewfinder, a 50mm lens is 50mm regardless of sensor size. We have just gotten so used to 35mm film photography that it became the basis for comparison. That is, those of us old enough to have used 35mm. We are quickly approaching a time when APS-C might be a person's perception of what 50mm should look like.

That aside, and as I said a moment ago, 50mm is 50mm and it coupled with aperture will determine depth of field at any given distance. There lies one of the keys - distance. When you stand ten feet away from a subject your depth of field is going to be x regardless of format. It is moving closer or farther away to compensate for framing differences (composition) that changes the DOF. Likewise, the compressed image depth provided by long telephoto lenses does not change either.


PostPosted: Wed Mar 03, 2010 12:00 am    Post subject: Reply with quote
martinsmith99 wrote:

I think it was meant in jest.


I know. I thought the point being made was worth responding to in any event, since it raised an interesting question.


PostPosted: Wed Mar 03, 2010 12:00 am    Post subject: Reply with quote
hexi wrote:
It was indeed. I won't do this again in this thread Very Happy


Oh. My apologies for finding what you wrote worthy of comment. Rolling Eyes


PostPosted: Wed Mar 03, 2010 7:09 am    Post subject: Reply with quote
woodrim wrote:
Just when people were happy with the explanations... changing cameras? Unless this is a phenomenon I don't understand, depth of field is strictly a function of focal length and aperture. Forget about perceived focal length in the viewfinder, a 50mm lens is 50mm regardless of sensor size. We have just gotten so used to 35mm film photography that it became the basis for comparison. That is, those of us old enough to have used 35mm. We are quickly approaching a time when APS-C might be a person's perception of what 50mm should look like.

Here is an article which will explain it better than I can: http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm


PostPosted: Wed Mar 03, 2010 8:53 am    Post subject: Reply with quote
martinsmith99 wrote:
This site may help:
http://www.dofmaster.com/dofjs.html


As a warning about this calculator; it does not work for practical hyperfocal distance calculations, because it ignores diffraction and therefore computes the hyperfocal distance with an unreachable definition of what is “in focus” (which is quite arbitrary in the first place) whenever the aperture is stopped down over the diffraction limit of the chosen format. (Likewise the depth of field shown will be incorrect; in this situation the depth of field would be zero because by the calculator's definition of “in focus” nothing would be focused well enough…)



In general one should remember that the notion of depth of field always depends on what one considers to be “in focus”; a pixel peeper might define depth of field based on the pixel pitch of the camera, but typically it is chosen based on the assumption that the final images would be viewed from a distance equal to the diagonal of the (enlarged) image by a person with normal vision. This assumption may be as good a generalisation as any for prints (i.e. a poster will typically be viewed from farther away than a 4×6" print, and a billboard from even farther away than the poster), but it may not hold very well for digital photos viewed on a computer screen.

Last edited by Arkku on Wed Mar 03, 2010 10:25 am; edited 1 time in total


PostPosted: Wed Mar 03, 2010 9:04 am    Post subject: Reply with quote
woodrim wrote:
Just when people were happy with the explanations... changing cameras?

Unless this is a phenomenon I don't understand, depth of field is strictly a function of focal length and aperture.


If you include the camera subject distance in your list then some people would share your understanding as shown by earlier posts in this thread. However there is a different point of view.

I'm not sure that I understand this view. However FWIW...

It is important to remember that Depth of Field is not a physical quantity which can be meeasured in the same way as focal length, or aperture, for example. Some degree of subjectivity is involved.

When a camera lens is placed at some distance in front of a sensor or film plane only one plane (or maybe a curved surface) in front of the lens is in focus. This plane is very thin. Everything in front of the plane and everything behind it are out of focus. However it may be that near to the plane, the degree of "being out of focus" or blur is acceptable to the viewer. The further we move away from the plane the worse the blur becomes and at some stage everything is out of focus. However this point is to some degree arbitrary or subjective.

The degree of blur is quantified by defining the so-called Circle of Confusion (COC). This is discussed here ...

http://en.wikipedia.org/wiki/Circle_of_confusion

The COC determines the hyperfocal distance, which determines the DOF, as described here...

http://en.wikipedia.org/wiki/Depth_of_field

So what values should we assign to the COC ? If we are viewing an an image at the size of the sensor it is appropriate to use a constant value (which will depend upon the degree of blur which is acceptable to us) which does not depend upon sensor size. However images are usually enlarged. And if we take two images, one from an APS-C sensor, and one from a FF sensor, one will need to be magnified more than the other in order to reach the same final image size. This means that any blur in the smaller sensor will be magnified more than the blur in the larger sensor. So in order to achieve the same degree of blur in the two magnified images the the starting image from the smaller sensor must start with a smaller degree of blur than that from the larger sensor. This is reflected in the use of different COC values, for cameras with different sensor sizes, in the various DOF calculator's such as the one in the link given in Martin's post.

And the use of different COCs leads to different DOFs as shown in the links.


PostPosted: Wed Mar 03, 2010 9:33 am    Post subject: Reply with quote
It is really interesting this psychological DOF.
Which one do you think has thinner DOF of these two pictures?












Answer: it is the same picture, just cropped differently.


PostPosted: Wed Mar 03, 2010 9:45 am    Post subject: Reply with quote
Now I'm confused again...


PostPosted: Wed Mar 03, 2010 9:54 am    Post subject: Reply with quote
dave_t wrote:
Now I'm confused again...


Don't worry, that's why it is called circle of confusion. Smile

Here is my attempt to understand it:
http://forum.mflenses.com/what-focal-lenght-aperture-would-you-use-in-low-light-t21355,highlight,circle+confusion.html


PostPosted: Wed Mar 03, 2010 10:36 am    Post subject: Reply with quote
kansalliskala wrote:
Answer: it is the same picture, just cropped differently.


Not just cropped, but the crop has been enlarged more to make it display at the same size, i.e. same difference as between full frame and APS-C when using the same lens at the same distance: the uncropped shot shows more depth of field because it has been enlarged less to reach the same output size. This is not subjective as it coincides with the calculated DoF when the acceptable circle of confusion is defined as a fraction of the image size (which, in turn, coincides with human perception when the images are viewed at equal size and equal distance, and therefore is the typical way used to generate DoF scales printed on lenses etc).


(Of course, it's not realistic for the photographer to decide beforehand the focal length, aperture, and distance regardless of the format being used, so in practical use it would be more typical to obtain the same composition either by stepping back on the smaller format, or changing to a shorter focal length, in which case the comparison of depth of field would play out differently.)


PostPosted: Wed Mar 03, 2010 7:41 pm    Post subject: Reply with quote
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.

sichko: Injecting the size of an image to the argument also further confuses the issue. The image - or print size does not change the depth of fi3eld, it just changes one's perception of focus. This also isn't new. Again, our making comparisons to old 35mm, or what we now call full frame, was just to help orient 35mm based photographers. Whether it be large format vs. medium format; full frame 35mm vs. APS-c, or any other format comparison, any lens focal length will have all the same characteristics except for field of view. Neither the film plane nor sensor will have any impact on depth of field the lens delivers. ONLY field of view will be varied.


PostPosted: Wed Mar 03, 2010 9:27 pm    Post subject: Reply with quote
woodrim wrote:
sichko: Injecting the size of an image to the argument also further confuses the issue. The image - or print size does not change the depth of fi3eld...


Perhaps this contibution from Carl Zeiss AG might help...

http://www.zeiss.com/C12567A8003B58B9/Contents-Frame/71E8DD8F96A9CF14C125697700546F2F


PostPosted: Wed Mar 03, 2010 11:44 pm    Post subject: Reply with quote
Good grief, sichko, that gave me a headache. It was a 1997 article that basically spoke of a film's ability to resolve. Still, what the lens delivers is unaffected by what the lens is delivering it to. And besides, this really does murk up and overly complicate the initial issue of this thread. What the article really gets at is how we determine what is in focus vs. the areas just outside or inside that range. 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.

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


PostPosted: Wed Mar 03, 2010 11:54 pm    Post subject: Reply with quote
BobbyR wrote:

GIVE the man a CIGAR-- winner-winner chicken-dinner.


Thanks. Can you make that FIVE cigars - for me and the boys?
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