DOF on Full Frame...?

Desert Rat

Desert Rat

Suspended / Banned
Messages
78
Name
Steve
Edit My Images
No
Is the DOF narrower on FF compared to a crop sensor?
 
No. Not intrinsically.

But a FF sensor will require you to get closer to a subject for like-for-like composition, which decreases depth of field.
 
Last edited:
ghoti said:
No. Not intrinsically.

But a FF sensor will require you to get closer to a subject for like-for-like composition, which decreases depth of field.
Click to expand...

Yup. With FF cameras you may end up either closer to the subject or using longer lenses, both of which can make the DoF look thinner.
 
Also, if you stand in exactly the same position, using exactly the same focal length, a FF camera will produce an image with smaller *apparent* dof than a crop sensor. That is, the zone of sharp focus will occupy a proportionately smaller part of the image. However, this is just because your field of view is larger. The real dof is actually exactly the same. The same objects will fall within the zone of sharp focus with each camera.
 
ghoti said:
Also, if you stand in exactly the same position, using exactly the same focal length, a FF camera will produce an image with smaller *apparent* dof than a crop sensor. That is, the zone of sharp focus will occupy a proportionately smaller part of the image. However, this is just because your field of view is larger. The real dof is actually exactly the same. The same objects will fall within the zone of sharp focus with each camera.
Click to expand...

That explains it perfectly. Thanks :thumbs:
 
ghoti said:
Also, if you stand in exactly the same position, using exactly the same focal length, a FF camera will produce an image with smaller *apparent* dof than a crop sensor. That is, the zone of sharp focus will occupy a proportionately smaller part of the image. However, this is just because your field of view is larger. The real dof is actually exactly the same. The same objects will fall within the zone of sharp focus with each camera.
Click to expand...

There is no zone of sharp focus. There's a plane of focus, and a zone of acceptable focus which depends on magnification and viewing distance.

It's the magnification difference between FF and crop which gives the apparently different DoF.
 
Phil V said:
It's the magnification difference between FF and crop which gives the apparently different DoF.
Click to expand...

Whilst it's true that magnification alters how you perceive the DoF I've always thought that camera to subject distance and focal length have a much more visible real world effect given that with a larger format you will probably be altering your camera to subject distance and/or using longer lenses regardless of how large you print or at what distance you view the image.

Therefore the format size can dictate the camera to subject distance and/or the focal length and therefore what the image will look like at any given image size or viewing distance.

PS. The best way to get it all straight in your head, IMVHO, is to theorise less as it only gives you brain ache and to instead shoot with different formats and view the results.
 
Last edited:
Phil V said:
There is no zone of sharp focus. There's a plane of focus, and a zone of acceptable focus which depends on magnification and viewing distance.

It's the magnification difference between FF and crop which gives the apparently different DoF.
Click to expand...
It's not true magnification though, is it?
Your field of view has just been expanded (all other things being equal) with the FF camera. So the "zone of acceptable focus", if you want to call it that (I don't care, we know what we're talking about), is a smaller part of the image. Proportionately.
The fact you've moved to a FF camera (again, keeping subject distance and focal length equal) has changed precisely nothing about the physics of the dof. A given focal length, aperture, and focus point will still produce the same *objective* zone of acceptable focus in terms of distance from the camera.
 
Last edited:
BUT.

For an equivalent field of view the DOF is shallower, is it not?

The difference between my Canon S90 and Nikon D700 is huge at the f/2.0 that is the widest the S90 goes.

Difference being that the Nikon is at 24mm and the Canon at 6mm or something. Yes, if you have a 6mm lens on the Nikon the DoF will be equivalent but a 6mm lens on the D700 would be silly, as is this argument.
 
Nawty said:
BUT.

For an equivalent field of view the DOF is shallower, is it not?

The difference between my Canon S90 and Nikon D700 is huge at the f/2.0 that is the widest the S90 goes.

Difference being that the Nikon is at 24mm and the Canon at 6mm or something. Yes, if you have a 6mm lens on the Nikon the DoF will be equivalent but a 6mm lens on the D700 would be silly, as is this argument.
Click to expand...
For an equivalent field of view the dof is shallower on FF because you have to be closer to the subject.
It's not really anything intrinsic to the FF camera.
 
ghoti said:
For an equivalent field of view the dof is shallower on FF because you have to be closer to the subject.
It's not really anything intrinsic to the FF camera.
Click to expand...

Well. Yes it is. It's exactly intrinsic to the size of the sensor.
 
Nawty said:
Well. Yes it is. It's exactly intrinsic to the size of the sensor.
Click to expand...
I disagree. Although we may be talking at cross purposes.
What I'm saying is that there's nothing about the FF camera *per se* that alters a dof for a given focal length, aperture and subject distance. It's simply the fact that to get an equivalent field of view you need to move closer to a subject. Moving closer to a subject decreases dof, everything else being equal. This is true of any camera.
 
Last edited:
It's not theorising it's physics. The only 2 variables that actually affect DoF at the film plane* are focal length and subject distance.

This is where it twists though, because the focal length effectively alters the general subject distancefield of view dependant on sensor size. So a 6mm lens on a FF sensor will have exactly the same DoF as it does on the compact. But it's impractical and more likely to be 24mm lens for the same shot, which then has a smaller DoF.

The other 'real world' twist is the real magnification that happens when an image is printed, because the size of print and viewing distance affect the apparent circle of confusion (what we see as sharp or blurred). Things which appear to be within the acceptable DoF on a small print will look OoF if the print is enlarged and viewed close.
 
Phil V said:
The other 'real world' twist is the real magnification that happens when an image is printed, because the size of print and viewing distance affect the apparent circle of confusion (what we see as sharp or blurred). Things which appear to be within the acceptable DoF on a small print will look OoF if the print is enlarged and viewed close.
Click to expand...

So the DOF is directly proportional to the DPI :thinking:









(tongue so far in cheek it came out of my ear)
 
Phil V said:
It's not theorising it's physics. The only 2 variables that actually affect DoF at the film plane* are focal length and subject distance.
...
Click to expand...
And will it always be the 'film plane'?:thinking:

I still 'tape' stuff on the telly that I want to watch later, despite the fact it's been on a hard drive for about 5 years?:D
 
Desert Rat said:
Is the DOF narrower on FF compared to a crop sensor?
Click to expand...

The short answer is yes.

Longer answer, DoF is ALL about magnification - the relative size of all contributing factors. Sensor size, lens focal length, the diameter of the aperture, subject distance, circle of confusion, print size and viewing distance. Change any one, and perceived DoF alters.

Importantly, DoF cannot be measured or compared unless the final output size and viewing distance is specified, and the international standard for that is a 10in print, viewed from a distance equal to the diagonal, ie 12in, with a 0.2mm circle of confusion.

The reasoning behind the short answer relates to the way we work, ie the sensor size is fixed by the camera and we adjust all other considerations to suit that (focal length, distance, aperture). Therefore, sensor sizes drives everything.
 
I think I found a table somewhere which had a rough guide what fstop would look like at at different formats
 
Wow! I'm possibly more confuddled now...

The reason I asked is because I was playing with a lens on different bodies. One FF and the other cropped. At the same focal length and aperture, the crop sensor seemed to have a wider DOF. To acheive the same DOF on the FF I had to go f/16 or above from f/10......
 
Desert Rat said:
Wow! I'm possibly more confuddled now...

The reason I asked is because I was playing with a lens on different bodies. One FF and the other cropped. At the same focal length and aperture, the crop sensor seemed to have a wider DOF. To acheive the same DOF on the FF I had to go f/16 or above from f/10......
Click to expand...

The reality which has been sort of said already is with a crop sensor and 50mm lens, you will need (approximately) an 85mm lens on full frame to get the same field of view... It will have the same things framed in it. The fact that you will be using an 85mm lens instead of a 50mm lens decreases the depth of field.

Alternatively, keep the 50mm lens on the full frame camera and move closer to frame it the same. This will also lead to a shallower depth of field due to being closer to the subject.

So usually if you move to full frame you would have longer lenses to get the similar framing results and this leads to shallower depth of field. That's the way I did it anyway.
 
Basically what changes with sensor size is focal length needed to get the same field of view and as you increase focal length DOF at the same aperture will decrease.

Ghoti's argument might give people the impression that had you not "cropped" the image on a smaller sensor using a FF lens the DOF would be larger, this isn't the case, what would have changed is that you'd have used a smaller focal length on a smaller sensor to get the same field of view and that would have increased DOF.
 
Last edited:
Moreorless said:
Basically what changes with sensor size is focal length needed to get the same field of view and as you increase focal length DOF at the same aperture will decrease.

Ghoti's argument might give people the impression that had you not "cropped" the image on a smaller sensor using a FF lens the DOF would be larger, this isn't the case, what would have changed is that you'd have used a smaller focal length on a smaller sensor to get the same field of view and that would have increased DOF.
Click to expand...

I get it.:thumbs:
 
Desert Rat said:
Wow! I'm possibly more confuddled now...

The reason I asked is because I was playing with a lens on different bodies. One FF and the other cropped. At the same focal length and aperture, the crop sensor seemed to have a wider DOF. To acheive the same DOF on the FF I had to go f/16 or above from f/10......
Click to expand...

When comparing DoF with different formats, the difference is f/number x crop factor - with subject framed the same (by changing focal length or shooting distance), same aperture, everything else the same. Basically, it's just over one stop difference, and if the crop factor was 1.4x (square root of 2), it would be exactly one stop difference. Check it out with this DoF calculator http://www.dofmaster.com/dofjs.html

Going back to the magnification thing, DoF is an illusion - there is only ever one plane of perfect focus, but also a zone of acceptable sharpness either side of that point, defined by the circle of confusion standard. When the image is made larger - by any means, at any stage in the process - then that reveals more finer detail, which then looks less sharp, and so the DoF zone of acceptable sharpness is reduced.
 
Desert Rat said:
I get it.:thumbs:
Click to expand...

Looking back I actually got a bit mixed up in the second part, what I ment to say was that cropping a FF lens doesn't increase the DOF, it merely reduces the field of view.

Basically you take a shot with a 50mm lens at F2.8 on any camera system with the subject and the camera in the same positions DOF will be the same, what will change is the field of view.

One other thing that can change is that DOF is a product of resolution, theres no "in focus" and "out of focus" black and white situation, focus is actually a sliding scale and how large an area in focus appears to be depends on how much resolution your viewing as you can see bluring earlier. Since larger sensor(or film format) cameras tend to have higher resolution that's used for larger prints or more cropping DOF will appear to be smaller in this respect aswell.
 
Last edited:
Moreorless said:
Looking back I actually got a bit mixed up in the second part, what I ment to say was that cropping a FF lens doesn't increase the DOF, it merely reduces the field of view.

Basically you take a shot with a 50mm lens at F2.8 on any camera system with the subject and the camera in the same positions DOF will be the same, what will change is the field of view.
Click to expand...
That's exactly what I was saying.

Moreorless said:
One other thing that can change is that DOF is a product of resolution, theres no "in focus" and "out of focus" black and white situation, focus is actually a sliding scale and how large an area in focus appears to be depends on how much resolution your viewing as you can see bluring earlier. Since larger sensor(or film format) cameras tend to have higher resolution that's used for larger prints or more cropping DOF will appear to be smaller in this respect aswell.
Click to expand...
Though this is an aspect that I hadn't fully considered. However, display medium, size and resolution will affect perceived dof, but they're not necessarily intrinsic to the FF format in and of itself. The best one can say (as you imply) is that certain things that can decrease perceived dof - like higher resolution - tend to correlate with FF cameras.
 
Last edited:
Moreorless said:
Looking back I actually got a bit mixed up in the second part, what I ment to say was that cropping a FF lens doesn't increase the DOF, it merely reduces the field of view.
Click to expand...

That changes the image completely. To make any valid comparison, the pictures must be the same - same framing (adjust focal length) and same viewpoint (to maintain perspective).

If you simply crop a FF image (same lens, same position) you are effectively using a crop format camera.

Basically you take a shot with a 50mm lens at F2.8 on any camera system with the subject and the camera in the same positions DOF will be the same, what will change is the field of view.
Click to expand...

When you change any aspect, DoF changes. One thing affects the others and you have to re-run the calcs.

One other thing that can change is that DOF is a product of resolution, theres no "in focus" and "out of focus" black and white situation, focus is actually a sliding scale and how large an area in focus appears to be depends on how much resolution your viewing as you can see bluring earlier. Since larger sensor(or film format) cameras tend to have higher resolution that's used for larger prints or more cropping DOF will appear to be smaller in this respect aswell.
Click to expand...

Sensor resolution is insignificant. If you calculate back from the circle of confusion, ie 0.03mm on FF, then that equates to less than 1m pixels total resolution. That may be a fraction of the camera's maximum potential, but it's not far off what you're usually looking at with an on-screen monitor image, and rather blows the popular myth that more pixels equals more sharpness ;)
 
You must log in or register to reply here.
Back
Top