DOF FF vs Crop...

[Kernuak]

The main reason that a larger sensor means shallower DoF is that for the same framing with a smaller sensor, you need to either be further from the subject, or use a longer focal length, either of which means deeper DoF.

shorter

Maybe I'm just reading it wrong after a long day, but if you're referring to a full frame sensor, shouldn't it be closer to the subject or longer focal length?

[neuroanatomist]

Maybe I'm just reading it wrong after a long day, but if you're referring to a full frame sensor, shouldn't it be closer to the subject or longer focal length?

I think it's correct as revised:

The main reason that a larger sensor means shallower DoF is that for the same framing with a smaller sensor, you need to either be further from the subject, or use a longer shorter focal length...

For the same framing using a smaller (APS-C) sensor, you need to be further from the subject, or use a shorter lens.

[Kernuak]

For the same framing using a smaller (APS-C) sensor, you need to be further from the subject, or use a shorter lens.

That matches my understanding, I was just reading the original sentence as further away or longer for full frame, which is of course contradictory.

[JonJT]

I'm going to try posting this again.  Canon Rumors seems to be deleting my posts. 

Anyway, FF sensors will have a shallower DOF for a given perspective and aperture.  But, considering how small the DOF is with my crop camera and the fast lenses I have, I have no need for the even smaller DOF a FF sensor would provide me.  I don't really see FF as having an advantage in this regard.

[shinyknights]

I'm going to try posting this again.  Canon Rumors seems to be deleting my posts. 

Anyway, FF sensors will have a shallower DOF for a given perspective and aperture.  But, considering how small the DOF is with my crop camera and the fast lenses I have, I have no need for the even smaller DOF a FF sensor would provide me.  I don't really see FF as having an advantage in this regard.

I think the DOF on crop sensors work perfectly fine.  You can still achieve creamy bokeh   FF mainly wins in low light situation.  They also win in their ability to collect more packets of photons with their 2x surface area.

[neuroanatomist]

But, considering how small the DOF is with my crop camera and the fast lenses I have, I have no need for the even smaller DOF a FF sensor would provide me.  I don't really see FF as having an advantage in this regard.

I do find the shallower DoF with FF to be a benefit, with no real downside at all.  I see your point that the DoF of an f/1.2 or 1.4 lens on FF is often just too thin to be useful, and you'd frequently stop down to f/1.6 or f/2 anyway (that's often the case when I use my 85L II).  But...I find the DoF of f/2.8 on FF to be excellent for portraits.  Now, I can achieve that same DoF with an 85mm prime (even the 85/1.8 woiuld work), but that means giving up some flexibility - my 70-200mm f/2.8L IS II is a a great portrait lens, but there's no such thing as an f/1.8 zoom lens, is there?  If I want even deeper DoF, I can just stop the f/2.8 lens down to f/4.5 on FF, and bump up the ISO to compensate for the lost light, and still have a slightly less noisy image.  So...more flexibility, and no downside, except cost.

[JonJT]

I'm going to try posting this again.  Canon Rumors seems to be deleting my posts. 

Anyway, FF sensors will have a shallower DOF for a given perspective and aperture.  But, considering how small the DOF is with my crop camera and the fast lenses I have, I have no need for the even smaller DOF a FF sensor would provide me.  I don't really see FF as having an advantage in this regard.

I think the DOF on crop sensors work perfectly fine.  You can still achieve creamy bokeh   FF mainly wins in low light situation.  They also win in their ability to collect more packets of photons with their 2x surface area.

Yes, this is where I think FF really makes the extra money worthwhile.  High ISO operation and tonal range.  Hopefully the new generation of crop sensors will close that gap.

[JonJT]

But, considering how small the DOF is with my crop camera and the fast lenses I have, I have no need for the even smaller DOF a FF sensor would provide me.  I don't really see FF as having an advantage in this regard.

I do find the shallower DoF with FF to be a benefit, with no real downside at all.  I see your point that the DoF of an f/1.2 or 1.4 lens on FF is often just too thin to be useful, and you'd frequently stop down to f/1.6 or f/2 anyway (that's often the case when I use my 85L II).  But...I find the DoF of f/2.8 on FF to be excellent for portraits.  Now, I can achieve that same DoF with an 85mm prime (even the 85/1.8 woiuld work), but that means giving up some flexibility - my 70-200mm f/2.8L IS II is a a great portrait lens, but there's no such thing as an f/1.8 zoom lens, is there?  If I want even deeper DoF, I can just stop the f/2.8 lens down to f/4.5 on FF, and bump up the ISO to compensate for the lost light, and still have a slightly less noisy image.  So...more flexibility, and no downside, except cost.

Absolutely true. 

[arad85]

Teehee..

I've just registered to reply to this thread. In ALL the discussions above, no one has mentioned print size. Why is that important? Because DoF is ONLY relevant when you actually render the image so you can see it. Depending on how big you render the image and how closely you view it, the DoF will change.

Huh? But surely DoFmaster gives precise figures - yes it does, but it does based on assumptions of how big you are printing and how far away you are viewing it. The figures aren't there directly, but they are captured by the CoC value. If you look up how CoC is derived, it is obtained from a standard size print from a standard distance and the thing that matters is how much you are magnifying the sensor image to the final print size. Change the assumptions and the DoF changes.

Don't believe me? Take a look here: http://en.wikipedia.org/wiki/Circle_of_confusion How is CoC defined:

CoC (mm) = viewing distance (cm) / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement / 25

Given an output print size, a desired viewing distance and a resolvability measure, the ONLY thing that dictates CoC is the enlargement. Bigger sensor for a given photo -> less enlargement->larger CoC. That's what defines DoF.

Try this. Print an image from a FF camera at 12x8. Now crop it so that it is the same sensor image area as a crop camera and print the resulting image at 12x8. The depth of field will change because you have altered the zoom factor. You are seeing more of the image magnified. Same photo - different DoF because you are magnifying it differently.

[neuroanatomist]

Wait, wait...you mean we can print these digital thingies?   

I've gone deeper into the CoC concept in other threads, which often just descend into circular arguments that are confusing.

But you're absolutely correct - since DoF is defined by what's sharp vs. what isn't, print size and viewing distance matter.

[arad85]

I've gone deeper into the CoC concept in other threads, which often just descend into circular arguments that are confusing.

Why would they be confusing?

The only really confusing thing is why people never refer to rendering the image when they discuss depth of field.... The heated arguments are always about what DoFmaster says but the calculations there rely on assumptions about how big you are going to magnify the resulting image

[arad85]

But you're absolutely correct - since DoF is defined by what's sharp vs. what isn't, print size and viewing distance matter.

Teehee.. you added that as I replied

[neuroanatomist]

Sorry about that.

Not sure - for some reason, people are ok with sensor size affecting DoF, but have a hard time grasping that cropping an image in post changes DoF, too.

[arad85]

I've noticed that too. I'm convinced it's because people see DoF as something that is frozen when the photo is taken, whereas in reality, it's only evident when the image is rendered so we can actually see it.

[JonJT]

Teehee..

I've just registered to reply to this thread. In ALL the discussions above, no one has mentioned print size. Why is that important? Because DoF is ONLY relevant when you actually render the image so you can see it. Depending on how big you render the image and how closely you view it, the DoF will change.

Huh? But surely DoFmaster gives precise figures - yes it does, but it does based on assumptions of how big you are printing and how far away you are viewing it. The figures aren't there directly, but they are captured by the CoC value. If you look up how CoC is derived, it is obtained from a standard size print from a standard distance and the thing that matters is how much you are magnifying the sensor image to the final print size. Change the assumptions and the DoF changes.

Don't believe me? Take a look here: http://en.wikipedia.org/wiki/Circle_of_confusion How is CoC defined:

CoC (mm) = viewing distance (cm) / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement / 25

Given an output print size, a desired viewing distance and a resolvability measure, the ONLY thing that dictates CoC is the enlargement. Bigger sensor for a given photo -> less enlargement->larger CoC. That's what defines DoF.

Try this. Print an image from a FF camera at 12x8. Now crop it so that it is the same sensor image area as a crop camera and print the resulting image at 12x8. The depth of field will change because you have altered the zoom factor. You are seeing more of the image magnified. Same photo - different DoF because you are magnifying it differently.

Indeed.  But, for argument sake, sensor size would be irrelevant if the enlargement factor were the same AND the pixel count was the same, as well, right?