DOF and crop factor


Feb 16, 2008
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the blue planet
#1
hi,
I was confused the other day when I was asked by a fren about crop factor in relation to DOF :confused:

Since he is using Nikon D90, a 50mm prime will give him a FOV of 75mm equivalent on an FX (a D700 for e.g), right?

but what about the DOF?
at a given subject distance, the DOF of 50mm f1.4 on an FX is equivalent to 75mm f4.8 on a DX?
I was trying to explain him using the DOFMaster's DOF calculator and it became more confusing to me :embrass:

So, if a 50mm f1.4 prime is used at f1.4 on FX and DX cameras(for a fixed subject distance), what will be the DOFs that are simply given by that lens? Let's forget about the print size (viewing distance).

thanks
 

ZerocoolAstra

Senior Member
Mar 13, 2008
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rainy Singapore
#2
if you have the same lens at the same aperture on both bodies, and focusing to the same distance, the DOF should be exactly the same.

BUT... the field of view will be different. The DX camera will be seeing much less.
If you pull back the DX camera so that it sees the same view, then your focusing distance changes, which affects the DOF.

hope that helps.
 

ovaltinemilo

Senior Member
Sep 12, 2009
2,819
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Sin jia Po lah
#4
I am not sure if calculating a number for DOF is really meaningful. Anyway, the 50mm lens is still 50mm. DOF should be unchanged unless you want same FOV on both FF and APSC. The so-called "changed in focal length" due to crop factor should not be used in your formula.
 

ziploc

New Member
Jan 17, 2002
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Snoopyland
#5
I am not sure if calculating a number for DOF is really meaningful. Anyway, the 50mm lens is still 50mm. DOF should be unchanged unless you want same FOV on both FF and APSC. The so-called "changed in focal length" due to crop factor should not be used in your formula.
This is true as long as the image maintains it's original size when presented on a media (e.g. screen or prints), e.g. the full frame is printed as 35mm size while the cropped frame is printed as APCS size. If the cropped frame is enlarged to 35mm size then the DOF will change, due to the enlargement of the circle of confusion. There are 2 factors to be considered: (1) enlargement of the cropped frame to print as the same size of the full frame pic, and (2) adjustment of camera-to-subject distance to obtain the same FOV. In (1) the DOF would reduce, but in (2) the DOF would increase. But the effect of (2) is a lot more than (1), so the nett effect is that you get deeper DOF with DSLR that has cropped factor. :)
 

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ovaltinemilo

Senior Member
Sep 12, 2009
2,819
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Sin jia Po lah
#6
This is true as long as the image maintains it's original size when presented on a media (e.g. screen or prints), e.g. the full frame is printed as 35mm size while the cropped frame is printed as APCS size. If the cropped frame is enlarged to 35mm size then the DOF will change, due to the enlargement of the circle of confusion. There are 2 factors to be considered: (1) enlargement of the cropped frame to print as the same size of the full frame pic, and (2) adjustment of camera-to-subject distance to obtain the same FOV. In (1) the DOF would reduce, but in (2) the DOF would increase. But the effect of (2) is a lot more than (1), so the nett effect is that you get deeper DOF with DSLR that has cropped factor. :)
wow, detailed.:thumbsup:
 

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torak

New Member
Sep 4, 2009
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#7
Lay man terms. let's assume u r using 1.5 crop aps-c and ur friend using ff. U r both standing at same spot taking portrait shot of a model. In order to frame a half body shot u had to use 50mm, while for ur friend he can use 75mm to frame his shot and still maintain the same field of view as u (the picture composition coming out similar). However he will have 50% shallower depth of view (and much better isolation of subject and bokeh).

Therefore if u wish to take pictures with very shallow dof, to isolate subject from background and create nice bokeh (this depends on lens too, but shallower dof makes nicer looking bokeh imo), then the ff will be better for this purpose, cos if distance and fov remains the same, the ff can use 1.5 times the focal length of the aps-c and thus will always hv 50% shallower dof.
 

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ziploc

New Member
Jan 17, 2002
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Snoopyland
#8
Lay man terms. let's assume u r using 1.5 crop aps-c and ur friend using ff. U r both standing at same spot taking portrait shot of a model. In order to frame a half body shot u had to use 50mm, while for ur friend he can use 75mm to frame his shot and still maintain the same field of view as u (the picture composition coming out similar). However he will have 50% shallower depth of view (and much better isolation of subject and bokeh).

Therefore if u wish to take pictures with very shallow dof, to isolate subject from background and create nice bokeh (this depends on lens too, but shallower dof makes nicer looking bokeh imo), then the ff will be better for this purpose, cos if distance and fov remains the same, ff will always hv 50% shallower dof.
You won't get exactly 50% shallower DOF, due to the 2 factors mentioned above, plus the DOF graph is a curve rather then linear. But it is close, so you can use 50% as a rough guage.

For example, let's use the DOF Mater online DOF calculator, and assuming aperture of f/2.8 and subject-to-camera distance of 5m:

Full frame (e.g. D3): f=75mm, f/2.8, d=5m => DOF = 4.65 to 5.4m, total 0.75m
Cropped frame (e.g. D90): f=50mm, f/2.8, d=5m => DOF = 4.5 to 5.63m, total 1.13m

And 0.75 x 1.5 = 1.125 != 1.13 but close. :)
 

Feb 16, 2008
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the blue planet
#9
You won't get exactly 50% shallower DOF, due to the 2 factors mentioned above, plus the DOF graph is a curve rather then linear. But it is close, so you can use 50% as a rough guage.

For example, let's use the DOF Mater online DOF calculator, and assuming aperture of f/2.8 and subject-to-camera distance of 5m:

Full frame (e.g. D3): f=75mm, f/2.8, d=5m => DOF = 4.65 to 5.4m, total 0.75m
Cropped frame (e.g. D90): f=50mm, f/2.8, d=5m => DOF = 4.5 to 5.63m, total 1.13m

And 0.75 x 1.5 = 1.125 != 1.13 but close. :)
now I see the catch..
I calculated with f=50mm for full frame and f=75mm for cropped frame :bsmilie:
thatz why the result(total figure of DOF) was impossibly huge..
thank you for the posts..
 

Feb 16, 2008
610
0
0
the blue planet
#10
Lay man terms. let's assume u r using 1.5 crop aps-c and ur friend using ff. U r both standing at same spot taking portrait shot of a model. In order to frame a half body shot u had to use 50mm, while for ur friend he can use 75mm to frame his shot and still maintain the same field of view as u (the picture composition coming out similar). However he will have 50% shallower depth of view (and much better isolation of subject and bokeh).

Therefore if u wish to take pictures with very shallow dof, to isolate subject from background and create nice bokeh (this depends on lens too, but shallower dof makes nicer looking bokeh imo), then the ff will be better for this purpose, cos if distance and fov remains the same, the ff can use 1.5 times the focal length of the aps-c and thus will always hv 50% shallower dof.
thanks. that's we've been discussing about..
I told my fren that FF will give shallower DOF than DX when same FL lens is put on and taking the shot from same distance and using same aperture..
then I calculated with FF at 50mm and DX at 75mm (I applied 1.5 to the FL) then the figures of DOF calculator gave me ridiculous results :bsmilie:
 

Feb 16, 2008
610
0
0
the blue planet
#11
This is true as long as the image maintains it's original size when presented on a media (e.g. screen or prints), e.g. the full frame is printed as 35mm size while the cropped frame is printed as APCS size. If the cropped frame is enlarged to 35mm size then the DOF will change, due to the enlargement of the circle of confusion...
that's clear. thanks.

...
...There are 2 factors to be considered: (1) enlargement of the cropped frame to print as the same size of the full frame pic, and (2) adjustment of camera-to-subject distance to obtain the same FOV. In (1) the DOF would reduce, but in (2) the DOF would increase. But the effect of (2) is a lot more than (1), so the nett effect is that you get deeper DOF with DSLR that has cropped factor. :)
as described by torak, if I'm framing a subject with a 50mm lens on APSC camera and a fren with an FF using the same lens, he needs to get closer to the subject in order to obtain the same FOV as mine.. rite? DOF also changes accordingly, in this case, shallower than mine.. is that correct?
 

ziploc

New Member
Jan 17, 2002
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Snoopyland
#12
as described by torak, if I'm framing a subject with a 50mm lens on APSC camera and a fren with an FF using the same lens, he needs to get closer to the subject in order to obtain the same FOV as mine.. rite? DOF also changes accordingly, in this case, shallower than mine.. is that correct?
Yes, and yes.
 

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