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DOF on different size sensors
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PostPosted: Thu Dec 09, 2010 4:27 am    Post subject: DOF on different size sensors Reply with quote
I know that 35mm/FF sensors have greater DOF than ASP-C, 16mm and the like, but I don't understand why that is. Can anyone here explain it to me?


PostPosted: Thu Dec 09, 2010 4:45 am    Post subject: Reply with quote
If you take a picture with the same composition on both APS-C and full frame, you either need to step up closer or use a longer focal length on full frame. This leads to higher magnification (on sensor) which leads to shallower depth of field at a given aperture.

However if you use the same focal length and distance, then obviously the centre part of the full frame image will be identical to APS-C. If you then enlarge both images to a given output size, e.g. to fill your screen or to make a certain size of print, the APS-C image is enlarged more, meaning that the circles of confusion (“blur circles”) are also enlarged more, leading the APS-C image to have shallower depth of field…

So it's actually what you do, not what the sensor does, that determines depth of field. But in practice you will probably tend to shoot for the same compositions and not think about the sensor size, and so larger sensors have less depth of field at the same settings. Of course you can stop down more if you want more depth of field, so larger sensors give you better control over DoF.


PostPosted: Thu Dec 09, 2010 4:47 am    Post subject: Reply with quote
This thread might help (especially if you ignore my early posts in the thread, which contain errors Embarassed)

edit: Now that I see it, Arkku's nice summary is more accessible imho.


PostPosted: Thu Dec 09, 2010 5:08 am    Post subject: Reply with quote
visualopsins wrote:
This thread might help (especially if you ignore my early posts in the thread, which contain errors Embarassed)

edit: Now that I see it, Arkku's nice summary is more accessible imho.


TY for link.

Now I need to get a FF someday--hope they make 'em a bit smaller and lighter soon.


PostPosted: Thu Dec 09, 2010 5:19 am    Post subject: Reply with quote
This is also a good resource: http://photo.net/learn/optics/dofdigital/.


PostPosted: Thu Dec 09, 2010 5:55 am    Post subject: Reply with quote
Arkku wrote:
If you take a picture with the same composition on both APS-C and full frame, you either need to step up closer or use a longer focal length on full frame. This leads to higher magnification (on sensor) which leads to shallower depth of field at a given aperture.

However if you use the same focal length and distance, then obviously the centre part of the full frame image will be identical to APS-C. If you then enlarge both images to a given output size, e.g. to fill your screen or to make a certain size of print, the APS-C image is enlarged more, meaning that the circles of confusion (“blur circles”) are also enlarged more, leading the APS-C image to have shallower depth of field…

So it's actually what you do, not what the sensor does, that determines depth of field. But in practice you will probably tend to shoot for the same compositions and not think about the sensor size, and so larger sensors have less depth of field at the same settings. Of course you can stop down more if you want more depth of field, so larger sensors give you better control over DoF.


ah HA! that's what i thought! (particularly the part i highlighted in red). and this makes sense to me, it really does. I'm going to try and take this a step further, and someone please correct me if I'm wrong or confused.

The reason consumer-grade (and even some professional grade) video cameras have such a large DOF is because in order to achieve a comparable Field of View to larger-format cameras, the lenses have to be extremely wide, which reduces the depth of field.
...right?

Cristofor and Visualopsins: thank you for your links, they were very helpful indeed Very Happy


PostPosted: Thu Dec 09, 2010 10:15 am    Post subject: Reply with quote
gearsNcogs wrote:
The reason consumer-grade (and even some professional grade) video cameras have such a large DOF is because in order to achieve a comparable Field of View to larger-format cameras, the lenses have to be extremely wide, which reduces the depth of field.
...right?


Right, except that it's not correct to say that the lenses have to be extremely wide; the focal length is just very short (a few mm). This is indeed a good example to explain why small sensor camera's have deeper DOF in practice.


PostPosted: Thu Dec 09, 2010 11:50 am    Post subject: Reply with quote
Arkku put it correctly: the meter of comparison is the framing. When you frame two images in the same way, one with a small format the other with a larger format, the camera with a larger format will need a longer focal lenght to obtain the same framing - hence the shallower depth of field.

This difference is obviously more perceptible in the normal and wide ranges, because in the tele ranges the dof is already thin also for small formats.


PostPosted: Thu Dec 09, 2010 11:52 am    Post subject: Reply with quote
Arkku wrote:
If you take a picture with the same composition on both APS-C and full frame, you either need to step up closer or use a longer focal length on full frame. This leads to higher magnification (on sensor) which leads to shallower depth of field at a given aperture.

However if you use the same focal length and distance, then obviously the centre part of the full frame image will be identical to APS-C. If you then enlarge both images to a given output size, e.g. to fill your screen or to make a certain size of print, the APS-C image is enlarged more, meaning that the circles of confusion (“blur circles”) are also enlarged more, leading the APS-C image to have shallower depth of field…

So it's actually what you do, not what the sensor does, that determines depth of field. But in practice you will probably tend to shoot for the same compositions and not think about the sensor size, and so larger sensors have less depth of field at the same settings. Of course you can stop down more if you want more depth of field, so larger sensors give you better control over DoF.


Well said! A tricky concept to get one's head around, very nicely and succinctly summed up.


PostPosted: Thu Dec 09, 2010 5:38 pm    Post subject: Reply with quote
awesome. thanks so much everyone! that's been bugging me for a while, thanks for all your help!


PostPosted: Thu Dec 09, 2010 7:15 pm    Post subject: Reply with quote
Arkku wrote:

However if you use the same focal length and distance, then obviously the centre part of the full frame image will be identical to APS-C.


This is so true. The sensor size forces/makes you change the distance (to capture an equivalent frame.) And that would change the DoF. The big part of the confusion came from the typical bogus claims (usually seen on other forums, *cough* DPR *cough*.): "Even if the same lens was used, the sensor would (some how magically) change the DoF... The proof, just look at the pics from P&S, the DoFs are ocean-deep!"

wait a minute, how do you mount a Planar 1.4/50 on an Exilim again?


PostPosted: Thu Dec 09, 2010 8:39 pm    Post subject: Re: DOF on different size sensors Reply with quote
gearsNcogs wrote:
I know that 35mm/FF sensors have greater DOF than ASP-C, 16mm and the like, but I don't understand why that is. Can anyone here explain it to me?


Actually, its the other way round!
The smaller the sensor (or film medium), the deeper the DOF and visa versa.
That is why it is almost impossible to get a shallow DOF on P&S/small sensor digital cameras.
If you want really shallow DOF you need to go for medium format or better still, large format cameras.


PostPosted: Fri Dec 10, 2010 1:12 am    Post subject: Re: DOF on different size sensors Reply with quote
DSG wrote:
gearsNcogs wrote:
I know that 35mm/FF sensors have greater DOF than ASP-C, 16mm and the like, but I don't understand why that is. Can anyone here explain it to me?


Actually, its the other way round!
The smaller the sensor (or film medium), the deeper the DOF and visa versa.
That is why it is almost impossible to get a shallow DOF on P&S/small sensor digital cameras.
If you want really shallow DOF you need to go for medium format or better still, large format cameras.


by "greater DOF" i meant shallower, but i mis-wrote. but yeah, i know exactly what you mean.


PostPosted: Fri Dec 10, 2010 1:45 am    Post subject: Reply with quote
It's very easy really, same lens from same distance on both ff and asp-c and m 4/3 would look the same. Asp-c and m 4/3 would just capture a smaller area or a crop of the ff result! Wink

That's why you get more tele on them and that's why the dof is greater.


PostPosted: Fri Dec 10, 2010 1:47 am    Post subject: Re: DOF on different size sensors Reply with quote
DSG wrote:

That is why it is almost impossible to get a shallow DOF on P&S/small sensor digital cameras.


If you got closed enough... like macro distance. Found those by googling, they are not mine:





PostPosted: Fri Dec 10, 2010 1:58 am    Post subject: Reply with quote
Nordentro wrote:
It's very easy really, same lens from same distance on both ff and asp-c and m 4/3 would look the same. Asp-c and m 4/3 would just capture a smaller area or a crop of the ff result! Wink

That's why you get more tele on them and that's why the dof is greater.


"greater" = deeper or "greater" = shallower?

It you stayed at the same distance then you would frame a different picture. And if printed out at the same size as the picture from the FF cam, you would get shallower DoF. because it's the same as if the central portion of the FF pic was magnified.

If you stepped back to frame an equivalent picture then you would get deeper DoF because you're further from the subject.


PostPosted: Fri Dec 10, 2010 2:12 am    Post subject: Reply with quote
bogolisk wrote:
Nordentro wrote:
It's very easy really, same lens from same distance on both ff and asp-c and m 4/3 would look the same. Asp-c and m 4/3 would just capture a smaller area or a crop of the ff result! Wink

That's why you get more tele on them and that's why the dof is greater.


1. "greater" = deeper or "greater" = shallower?

2. It you stayed at the same distance then you would frame a different picture. And if printed out at the same size as the picture from the FF cam, you would get shallower DoF. because it's the same as if the central portion of the FF pic was magnified.

3. If you stepped back to frame an equivalent picture then you would get deeper DoF because you're further from the subject.


1. Greater like deeper or more dept of field yes Wink

2. Dof would be the same from the same distance, but you would have to crop ff result to get the same field of view.

3. And yes, that's what I ment between the lines Smile


PostPosted: Fri Dec 10, 2010 3:03 am    Post subject: Reply with quote
Nordentro wrote:
It's very easy really, same lens from same distance on both ff and asp-c and m 4/3 would look the same.


The common (centre) part of the image on the sensor would look (and be) the same. However, in practice the images will almost certainly be viewed at a size chosen without consideration for the sensor size, e.g. the image will fill a computer screen or a given piece of paper whether it was shot on FF, APS-C, 4/3, or compact camera… Now, for a given output size the smaller the sensor, the more the image needs to be enlarged. So it will look different—smaller sensors will have shallower DoF with the same focal length and same distance.

(But of course the composition would also be different, and so this is not a realistic scenario.)


PostPosted: Fri Dec 10, 2010 4:06 am    Post subject: Reply with quote
Ah yes, print size. Printing the COC larger DOF becomes less than in viewfinder, printing COC smaller DOF becomes more than in viewfinder. Same with changes in viewing distance. My question is for given print size what is viewing distance for same DOF seen in viewfinder? Or same question re-phrased, given viewing distance how to calculate print size to produce same DOF as in viewfinder? I know some calculation methods, however those need distance from eye to image in viewfinder, or dimensions of viewfinder image eye sees, how to determine that?


PostPosted: Fri Dec 10, 2010 5:32 am    Post subject: Reply with quote
how would projection (like a movie) factor into the COC and print size and viewing distance and the like?


PostPosted: Fri Dec 10, 2010 6:24 am    Post subject: Re: DOF on different size sensors Reply with quote
gearsNcogs wrote:
by "greater DOF" i meant shallower, but i mis-wrote. but yeah, i know exactly what you mean.


'Greater' and 'lesser' can be ambiguous when referring to exposure parameters. That's why I prefer 'thick' and 'thin' for DOF, and 'open' and 'closed' for aperture, and 'faster' and 'slower' for shutter speeds. I don't know how well these terms communicate through computerized translators. But in English, I try to be a clear as possible. Unless I want otherwise, of course.


PostPosted: Fri Dec 10, 2010 10:04 am    Post subject: Reply with quote
Here's a good example of ff compared to m 4/3:


Bokeh and dept of field will be the same from the same distance with the same lens, just cropped Wink

To frame the same picture you obviously need twice as fast lens on m 4/3 for the bokeh and half the focal length.


PostPosted: Fri Dec 10, 2010 11:41 am    Post subject: Reply with quote
Thank you Arkku for your clear explanations. I always look at it the other way round - I guess it's because I'm so used to film cameras, and using a smaller size sensor still seems foreign. After all, all my lenses were designed for full frame! Smile

In my mind it is easier to understand that to achieve the same composition on a APS-C camera, I would either have to use a shorter FL lens or move away from the subject and the depth of field would therefore increase, giving me less control.


PostPosted: Fri Dec 10, 2010 12:24 pm    Post subject: Re: DOF on different size sensors Reply with quote
gearsNcogs wrote:
by "greater DOF" i meant shallower, but i mis-wrote. but yeah, i know exactly what you mean.


Did you think DOF means background blurring capacity or something like that? Greater Depth of Field = deeper DOF.


PostPosted: Fri Dec 10, 2010 6:48 pm    Post subject: Re: DOF on different size sensors Reply with quote
RioRico wrote:
gearsNcogs wrote:
by "greater DOF" i meant shallower, but i mis-wrote. but yeah, i know exactly what you mean.


'Greater' and 'lesser' can be ambiguous when referring to exposure parameters. That's why I prefer 'thick' and 'thin' for DOF, and 'open' and 'closed' for aperture, and 'faster' and 'slower' for shutter speeds. I don't know how well these terms communicate through computerized translators. But in English, I try to be a clear as possible. Unless I want otherwise, of course.


yeah, i'm going to adopt your system, it's just easier that way Smile
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