What causes the increased DOF on FF?

CyBeRkId2002

CyBeRkId2002

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Hi all,

I have been considering a FF camera for a while now, having looked at the kind of pictures I mainly take, considered my lens choices (my favourite lens on DX is the 35mm f2 so I think a 50mm f1.4 on FF would suit me great) and am just trying to get my head around why FF cameras have a greater DOF? I understand how and why the other benefits exist (greater high ISO, bigger, brighter viewfinder etc.) but just cannot understand this one?

The only reason I can think of is to do with focusing distances. For example if I got a portrait at or close to it's maximum focusing distance on my 35mm lens on DX it would fill the frame (and then some). If I did this on FX I could get the lens closer, whilst still including the surroundings. Therefore lens to person is closer creating a greater DOF? I may not have explained it very clearly but my thought process is complicated (:cuckoo:) and I am just trying to get my head around it.

Obviously none of this affect the camera and whether I should buy it etc., I just like to understand how things work.

Cheers,
Michael
 
try to imagine looking at the scene through a window... then looking at the same scene though a bigger window.

The DOF is the same, it just you see more of the image if that makes sense?

Imagine getting an image and cutting a smaller square out of the middle... tis the crop factor
 
You get an apparant decreased Depth of Field on a FF camera compared to a crop for the same focal length.

decreased Depth of Field = less in focus,
increased Depth of Field = more in focus.
 
Hi Michael,

If I'm reading you correctly then you're totally right.

Technically there is no change in DoF from one format to another. If you take a 50mm lens from a crop camera and place it on a full frame camera, keep all the settings and positions the same, the DoF will be exactly the same.

Where people talk about the changing DoF is when you look at producing the same shot from two different formats. If you want to replicate a shot from a crop camera with FF equipment you need to use a longer lens or move closer and either of these changes will reduce the DoF.
 
Gaz81 said:
try to imagine looking at the scene through a window... then looking at the same scene though a bigger window.

The DOF is the same, it just you see more of the image if that makes sense?

Imagine getting an image and cutting a smaller square out of the middle... tis the crop factor
Click to expand...

Cheers for the reply... I do understand the crop factor but this alone why there appears to be a decreased DOF in shots
 
EdBray said:
You get an apparant decreased Depth of Field on a FF camera compared to a crop for the same focal length.

decreased Depth of Field = less in focus,
increased Depth of Field = more in focus.
Click to expand...

your quite right... got the wording the wrong way round in the original post
 
dazzajl said:
Hi Michael,

If I'm reading you correctly then you're totally right.

Technically there is no change in DoF from one format to another. If you take a 50mm lens from a crop camera and place it on a full frame camera, keep all the settings and positions the same, the DoF will be exactly the same.

Where people talk about the changing DoF is when you look at producing the same shot from two different formats. If you want to replicate a shot from a crop camera with FF equipment you need to use a longer lens or move closer and either of these changes will reduce the DoF.
Click to expand...

So assuming the same lenses and replicating the shot I was right in that it is distance to the subject that is creating the effect. Thanks for the confirmation.
 
Gaz81 said:
Have a look at this

http://www.have-camera-will-travel.com/field_reports/full_frame_vs_crop_sensor_-.html
Click to expand...

Very interesting argument and I agree and disagree with it at the same time! What is being said is quite right and it is the crop factor which is being affected but it still causes an apparent decrease in DOF. For example, if the shot on the right was recreated with a crop camera (ie. moving the lens back so both shots had the same FOV) I am almost certain that the DOF would be far greater than that of the 5D. Whilst I am not disagreeing that this is to do with the crop factor I am still sure that it has an affect on DOF when shooting two identical scences.
 
Depth of field is controlled by three things - regardless of camera:
1 distance from object to image - i.e. subject to sensor
2 focal length of lens
3 aperture
 
Thanks for all the responses. I do understand the crop-factor but my question was more related to differences with the same FOV in DX and FX. The answer, as I suspected, was all to do with the the distance from the camera lens to the subject being closer on full-frame to get the same FOV, therefor descreasing the DOF. Thanks all for the response, just wanted to clarify it in my head.
 
CyBeRkId2002 said:
Thanks for all the responses. I do understand the crop-factor but my question was more related to differences with the same FOV in DX and FX. The answer, as I suspected, was all to do with the the distance from the camera lens to the subject being closer on full-frame to get the same FOV, therefor descreasing the DOF. Thanks all for the response, just wanted to clarify it in my head.
Click to expand...

The defining factor in depth of field is sensor size. Working that back, it is basic physics related to the concept of depth of field and an agreed standard of sharpness.

That agreed standard is expressed in terms of the size of the circle of confusion. When you put that into a DoF formula, considering all the various magnification factors involved in forming an image of a certain size and field of view, and then creating the finished 'standard' size print which is viewed from a 'standard' distance, the mathematics comes out with larger imaging formats delivering less depth of field.

That's all there is to it. If you use full frame as a starting point, the crop factor is a handy way of calculating the change in DoF - multiply the f/number by the crop factor. Therefore with Canon 1.6x crop, DoF at f/4 is equal to f/6.4 on full frame, Nikon 1.5x would be f/6, 4/3rds format 2.0x would be f/8. Compacts have massive DoF with a crop factor usually around 5x, so f/20 in this comparison.

See here www.dofmaster.com and note the input criterior. Circle of confusion, format, focal length, focusing distance, and finally f/number. You can lump all of the first four factors into one and just call them collectively magnification. So in a nutshell, there are only two factors determining depth of field - magnification and f/number.
 
Are you still on with your theory. GAZ 81 is correct in saying that the DOF is the same thru different size windows. The defining factor is the lens. Your theory about having to walk back to equal the same size image, therefore the further back you go there will be a slight increase in DOF. This is true of any lens. I will not bother any more with your theory.
 
awp said:
Depth of field is controlled by three things - regardless of camera:
1 distance from object to image - i.e. subject to sensor
2 focal length of lens
3 aperture
Click to expand...

Actually, it's just a factor of one thing. Magnification.


Steve.
 
HoppyUK said:
The defining factor in depth of field is sensor size....If you use full frame as a starting point, the crop factor is a handy way of calculating the change in DoF - multiply the f/number by the crop factor...with 1.6x crop, DoF at f/4 is equal to f/6.4 on full frame, Nikon 1.5x would be f/6, 4/3rds format 2.0x would be f/8. Compacts have massive DoF with a crop factor usually around 5x, so f/20 in this comparison.
Click to expand...

Thought this thread might get a reply from Hoppy :wave::thumbs:

Hoppy's spot on. Think of it in the limit - DoF on a phone camera with a tiny sensor is basically everything in focus. DoF on medium format is rediculously small (F/64 being a 'normal' setting for group shots IIRC).
 
pearce_jj said:
Thought this thread might get a reply from Hoppy :wave::thumbs:

Hoppy's spot on. Think of it in the limit - DoF on a phone camera with a tiny sensor is basically everything in focus. DoF on medium format is rediculously small (F/64 being a 'normal' setting for group shots IIRC).
Click to expand...
I think you should get a few more years under your belt before you judge. Bet you cannot lift and use a Canon 5D with a 300mm zoom on. ;)
 
You do get a greater DOF with larger image sensors/film it's all down to Circles of Confusion, you can have a read here about it http://en.wikipedia.org/wiki/Circle_of_confusion

It's a very visible effect in video where by if you have a video camera that only has 1/3 or 1/2" ccds then the depth of field is totally different to that of 3/4" and larger ccds it's nearly impossible to have nice depth of field effects on the smaller sensor cameras
 
pearce_jj said:
Thought this thread might get a reply from Hoppy :wave::thumbs:

Hoppy's spot on. Think of it in the limit - DoF on a phone camera with a tiny sensor is basically everything in focus. DoF on medium format is rediculously small (F/64 being a 'normal' setting for group shots IIRC).
Click to expand...

LOL It seems to need repeating on a regular basis ;)

I don't understand the disagreement. There appear to be two schools of thought here, and both are true.

One school says if you don't change the focal length (or distance and f/number) then format has no effect on depth of field - true, but rather unhelpful. This is view put forward in the link post #8 by Gaz81.

The other school of thought (ie me and others) says that you must change the focal length when you change format in order to restore the field of view and subject framing you need. For this reason, depth of field changes.

It is blindingly obvious that field of view is the most important and relevant consideration when choosing lens focal length, and it is therefore rather pointless to talk about what happens if you don't change focal length. Field of view dictates choice of focal length, and this is fundamentally driven by the size of the sensor.

Good link on depth of field here, clearly written http://toothwalker.org/optics/dof.html
 
classcams said:
I think you should get a few more years under your belt before you judge. Bet you cannot lift and use a Canon 5D with a 300mm zoom on. ;)
Click to expand...
Hi, in case you did not receive my reply, I always judge a person by his avatar. :)
 
hahahah...I love threads like this: everyone getting all bent out of shape and slanging each other off...it doesn't matter what anyone says or what opinions you have about the subject here...

There's what's right and what's wrong - none of this is subject to interpretation...

As Scotty says: "Ye canna change the Laws of Physics, Cap'n..."
 
The next person who brings up D O F will be barred from this forum.
 
HoppyUK said:
There appear to be two schools of thought here, and both are true.

One school says if you don't change the focal length (or distance and f/number) then format has no effect on depth of field - true, but rather unhelpful. This is view put forward in the link post #8 by Gaz81.

The other school of thought (ie me and others) says that you must change the focal length when you change format in order to restore the field of view and subject framing you need. For this reason, depth of field changes.

It is blindingly obvious that field of view is the most important and relevant consideration when choosing lens focal length, and it is therefore rather pointless to talk about what happens if you don't change focal length. Field of view dictates choice of focal length, and this is fundamentally driven by the size of the sensor.
Click to expand...
I think this is well put and explains why DOF causes so many arguments.
 
classcams said:
The next person who brings up D O F will be barred from this forum.
Click to expand...

actually the next person to be barred from the forum will be you, a couple of your posts have been reported, i suggest winding your neck in
 
Hoppy's spot on. Think of it in the limit - DoF on a phone camera with a tiny sensor is basically everything in focus. DoF on medium format is rediculously small (F/64 being a 'normal' setting for group shots IIRC).
Click to expand...

Well for many years BD (Before Digital) I used to shoot on 2 1/4 sq and never used f64 - in fact I don't think my camera lenses (Mamiya C2) actually had such a setting.

I used to shoot wedding groups at whatever the weather dictated - usually between F5.6 - f16, and never had any problems with a "ridiculous" DOF - and neither did anyone else I knew.

And when I changed to 35mm I also never worried about DOF on groups or anything else for that matter - I just took the pics.
 
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