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Gear Talk => EOS Bodies - For Stills => Topic started by: kdw75 on February 13, 2012, 09:41:21 AM

Title: DOF FF vs Crop...
Post by: kdw75 on February 13, 2012, 09:41:21 AM
The way I understand it FF gives you a smaller DOF at the same focal length. So if I zoom to 17mm on my crop body and to 28mm on my FF camera, giving them effectively the same field of view, the FF will have a shallower DOF?
Title: Re: DOF FF vs Crop...
Post by: Tijn on February 13, 2012, 10:20:14 AM
Terminology wise: they're different REAL focal lengths, but with the crop factor included they result in the same EFFECTIVE fullframe-equivalent focal length; i.e. the same framing.

17mm f/2.8 on your crop body vs. 28mm f/2.8 on a full frame camera means the same aperture, same subject distance and the same effective focal length. Real focal lengths are different. Blur is the same for both in this situation - as I understand it.

But 28mm on a crop body vs 28mm on a fullframe body means that to get the same framing, you will physically move forward. When you then have the same framing, aperture is the same, focal length is (effectively) different, and subject distance is closer. Resulting in more background blur on the fullframe camera.

A FF camera will give more background blur at the same (real) focal lengths if you have the same framing, because a FF camera will have moved closer to the subject. If you zoom in (increase focal length) on a fullframe camera, and compare it to a shorter focal length with the same framing on a crop, at the same distance to the subject, I believe there's no difference in DOF.
Title: Re: DOF FF vs Crop...
Post by: bycostello on February 13, 2012, 10:26:08 AM
The way I understand it FF gives you a smaller DOF at the same focal length. So if I zoom to 17mm on my crop body and to 28mm on my FF camera, giving them effectively the same field of view, the FF will have a shallower DOF?

yes and no...  for the same composition, yes as u stand closer with the FF for different compositions then it would be the same for your example.
Title: Re: DOF FF vs Crop...
Post by: ghosh9691 on February 13, 2012, 10:45:05 AM
Let's say that you have a 5D2 and a 7D camera. Now if you put a 50mm f/2 lens on the 5D2 and stand such that you are 10ft away from your subject, you will have a DOF of about 1.45ft. In order to get that same DOF with your 7D, you will need to use a 31.4mm lens at f/2 and stand about 8ft away from your subject. But your framing will now be different because even though your lens has the same angle of view as the 50mm, your subject distance is different. If you were to maintain the same framing, i.e. stand at 10ft from your subject, your DOF would be higher and will be about 2.36ft.

With a Canon APS-C camera, the "crop" factor is 1.6x. Which means that the 31.4mm lens above will give you the same field of view (angle of view) as a 50mm lens on a full frame. The term "focal length equivalent" is a simplification but it causes a lot of confusion. The correct term would be "angle of view equivalent", but that is a lot harder for a lay person to understand. Focal length is the focal length - it does not change. If you take the EF 50mm lens and put it on a 7D, it does not become 80mm focal length...it's angle of view is now the equivalent of what you would get with an 80mm lens on a 5D.

Hope this helps some...

Edit: You can go here and try different calculations out to see the difference :) http://www.dofmaster.com/dofjs.html (http://www.dofmaster.com/dofjs.html)
Title: Re: DOF FF vs Crop...
Post by: funkboy on February 13, 2012, 10:45:32 AM
This tutorial (http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml) should clear up the subject for you.  Don't think of it as "smaller DOF at the same focal length", but more like "subject size given the desired depth of field" (http://www.luminous-landscape.com/tutorials/dof2.shtml).

On a crop-frame camera, if you want a very shallow depth of field then the lens that will provide that DoF on a FF camera will require you to be further away from your subject in order for the subject to be framed identically.  And of course as you move further away from your subject (to compensate for the smaller sensor only "seeing" the center of the image rendered by the lens) the depth of field increases as you increase the focus distance...
Title: Re: DOF FF vs Crop...
Post by: PaperTiger on February 13, 2012, 10:57:17 AM
Just an aside as well that I think is interesting (something I spent some time investigating empirically and mathematically) is that depth of field is identical (for the same sensor) between lenses if the framing and aperture is identical. E.g. if you take a 135 mm at f2 at 3 m distance from the subject and an 85 mm at f2 at 2 m (or whatever the distance is to make the framing the same), the depth of field will be the same. Same identical picture (aside from perspective compression). Just an interesting occurrence to note.

This doesn't apply between different sensors, however. The framing could be identical between different sized sensors, but depth of field would be different.
Title: Re: DOF FF vs Crop...
Post by: marinien on February 13, 2012, 11:12:43 AM
The way I understand it FF gives you a smaller DOF at the same focal length. So if I zoom to 17mm on my crop body and to 28mm on my FF camera, giving them effectively the same field of view, the FF will have a shallower DOF?

If you use a 17mm at f/4.0 on a crop body (crop factor 1.6), in terms of field of view and depth of field, it is similar to a 27mm at f/6.4 mount on a FF body.
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 11:48:22 AM
17mm f/2.8 on your crop body vs. 28mm f/2.8 on a full frame camera means the same aperture, same subject distance and the same effective focal length. Real focal lengths are different. Blur is the same for both in this situation - as I understand it.
Sorry, that's not correct.  Effective focal length doesn't matter - real focal length matters.  In the situation you describe, the FF sensor would give shallower DoF.

The way I understand it FF gives you a smaller DOF at the same focal length. So if I zoom to 17mm on my crop body and to 28mm on my FF camera, giving them effectively the same field of view, the FF will have a shallower DOF?
Basically, there are four factors that determine DoF:


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, either of which means deeper DoF.

For example, say you put a 70-200/2.8 lens on an APS-C camera, set it to 125mm, and take a picture at f/2.8 of a model's head.  Now you put that lens on a FF camera, and if you don't change anything else, you'd be taking a shot of the torso and head.  To re-create the framing of the head shot, you have two options - stay in the same place and zoom the lens to 200mm, or leave the lens at 125mm and walk closer to the subject.  Either way, you get a shallower DoF, based on points 2 and 3 above.  To get the same DoF as you had with the APS-C shot, you'd have to stop down to f/4.5 on the FF camera (in addition to moving or zooming). 

Just an aside as well that I think is interesting (something I spent some time investigating empirically and mathematically) is that depth of field is identical (for the same sensor) between lenses if the framing and aperture is identical.

Exactly, for the same reasons as above.  Focal length and subject distance have equal and opposite effects, so if you're talking about same framing and same sensor, DoF is determined only by the aperture.



Read on if you have some tolerance for a detailed technical point that's not usually relevant in practical terms.


You may have noticed that I left this one out of the disucssion - the reason is that, as the term implies, it's confusing.  In reality, if you hold the other three factors constant - same aperture, same distance, same focal length - and change only the sensor size, the APS-C sensor will actually produce a shallower DoF than FF (CoC is smaller for APS-C).  Obviously, this is contrary to the conventional wisdom that FF means shallower DoF, but nevertheless, it's true (spend a little time with DoFMaster (http://www.dofmaster.com/dofjs.html) to convince yourself).  The reason it's practically irrelevant is that you'd have a completely different framing, i.e. to expand the example above, if you frame the headshot on FF, then switched to APS-C and didn't move or zoom or change aperture, your DoF would be shallower - but the crop factor would give you an eyes-and-nose portrait instead of a head shot, and as soon as you zoomed out or backed up, your DoF would get deeper.  The only place you really notice this is when testing a lens, e.g. for AF microadjustment.  In that scenario, you shoot with the lens at a given distance independent of sensor size, and when you do that, you notice that DoF really is thinner with APS-C. 
Title: Re: DOF FF vs Crop...
Post by: bainsybike on February 13, 2012, 12:11:12 PM
Quote
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
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 12:14:04 PM
shorter

Oops - good catch, thanks!
Title: Re: DOF FF vs Crop...
Post by: Tijn on February 13, 2012, 01:04:57 PM
17mm f/2.8 on your crop body vs. 28mm f/2.8 on a full frame camera means the same aperture, same subject distance and the same effective focal length. Real focal lengths are different. Blur is the same for both in this situation - as I understand it.
Sorry, that's not correct.  Effective focal length doesn't matter - real focal length matters.  In the situation you describe, the FF sensor would give shallower DoF.
Thanks for the explanation. I did put in the "as I understand it" because I wasn't totally sure. :-)
Title: Re: DOF FF vs Crop...
Post by: Cosk on February 13, 2012, 01:38:15 PM
This is how I think about it:

The rule of thumb is to multiply the aperture and focal length by 1.6x for equivalence.

In your example, a 17mm f/2.8 on a crop body will have the same depth of field as a 28mm f/4.5 on a full frame body.

(f/2.8 x 1.6 = f/4.5)

Since your 28mm opens wider than f/4.5, you can achieve a shallower DOF on your FF.

Conversely, if you opened your 28mm to f/2.8 on a FF camera, to achieve an equivalent DOF on a crop body, you would need a 17mm at f/1.8 - which doesn't exist in Canon's lineup...




Title: Re: DOF FF vs Crop...
Post by: kdw75 on February 13, 2012, 01:52:05 PM
Say I have two camera bodies that are 18 megapixel and both are using a 50mm lens. The only difference between the two is that one is FF and the other APS-C. I realize the borders would be cropped on the C but would the zoom be the same, or would things appear larger on the FF when viewed at 1:1 pixel size?
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 01:57:27 PM
Say I have two camera bodies that are 18 megapixel and both are using a 50mm lens. The only difference between the two is that one is FF and the other APS-C. I realize the borders would be cropped on the C but would the zoom be the same, or would things appear larger on the FF when viewed at 1:1 pixel size?

Assuming you did not change location between shots, the APS-C image would be a sub-sample of the FF image, and things would appear larger on the APS-C, since the entire 18 MP would be covering a smaller portion of the image.  But crop factor doesn't increase apparent magnification by itself - if you did that comparison with the 21 MP 5DII and the 8 MP 20D (which have the same pixel pitch), the subject would look the same size at 1:1 on the screen, but the FF image would have more background around the subject.  The apparent increase in magnification comes from the more densely packed pixels on APS-C.
Title: Re: DOF FF vs Crop...
Post by: shinyknights on February 13, 2012, 02:13:26 PM
17mm f/2.8 on your crop body vs. 28mm f/2.8 on a full frame camera means the same aperture, same subject distance and the same effective focal length. Real focal lengths are different. Blur is the same for both in this situation - as I understand it.
Sorry, that's not correct.  Effective focal length doesn't matter - real focal length matters.  In the situation you describe, the FF sensor would give shallower DoF.

The way I understand it FF gives you a smaller DOF at the same focal length. So if I zoom to 17mm on my crop body and to 28mm on my FF camera, giving them effectively the same field of view, the FF will have a shallower DOF?
Basically, there are four factors that determine DoF:

  • Aperture - wider means shallower DoF
  • Subject distance - closer means shallower DoF
  • Focal length - longer means shallower DoF
  • Circle of confusion - smaller means shallower DoF

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, either of which means deeper DoF.

For example, say you put a 70-200/2.8 lens on an APS-C camera, set it to 125mm, and take a picture at f/2.8 of a model's head.  Now you put that lens on a FF camera, and if you don't change anything else, you'd be taking a shot of the torso and head.  To re-create the framing of the head shot, you have two options - stay in the same place and zoom the lens to 200mm, or leave the lens at 125mm and walk closer to the subject.  Either way, you get a shallower DoF, based on points 2 and 3 above.  To get the same DoF as you had with the APS-C shot, you'd have to stop down to f/4.5 on the FF camera (in addition to moving or zooming). 

Just an aside as well that I think is interesting (something I spent some time investigating empirically and mathematically) is that depth of field is identical (for the same sensor) between lenses if the framing and aperture is identical.

Exactly, for the same reasons as above.  Focal length and subject distance have equal and opposite effects, so if you're talking about same framing and same sensor, DoF is determined only by the aperture.



Read on if you have some tolerance for a detailed technical point that's not usually relevant in practical terms.

  • Circle of confusion - smaller means shallower DoF

You may have noticed that I left this one out of the disucssion - the reason is that, as the term implies, it's confusing.  In reality, if you hold the other three factors constant - same aperture, same distance, same focal length - and change only the sensor size, the APS-C sensor will actually produce a shallower DoF than FF (CoC is smaller for APS-C).  Obviously, this is contrary to the conventional wisdom that FF means shallower DoF, but nevertheless, it's true (spend a little time with DoFMaster (http://www.dofmaster.com/dofjs.html) to convince yourself).  The reason it's practically irrelevant is that you'd have a completely different framing, i.e. to expand the example above, if you frame the headshot on FF, then switched to APS-C and didn't move or zoom or change aperture, your DoF would be shallower - but the crop factor would give you an eyes-and-nose portrait instead of a head shot, and as soon as you zoomed out or backed up, your DoF would get deeper.  The only place you really notice this is when testing a lens, e.g. for AF microadjustment.  In that scenario, you shoot with the lens at a given distance independent of sensor size, and when you do that, you notice that DoF really is thinner with APS-C.



Great Stuff!  And so true!  In layman's term, a FF gives you a shallower depth of field!!!  If take pictures with both APS-C and Full Frame, and ALL the composition in the picture is the same, (subject + background), the FF will yield a thinner Depth of Field!!

Neuroanatomist's last paragraph can be confusing, but it does make sense, only because the image composition changes, because you are now zoomed closer with the APS-C.
Title: Re: DOF FF vs Crop...
Post by: Kernuak on February 13, 2012, 03:02:32 PM
Quote
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?
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 03:25:55 PM
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.
Title: Re: DOF FF vs Crop...
Post by: Kernuak on February 13, 2012, 06:00:59 PM

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.
Title: Re: DOF FF vs Crop...
Post by: JonJT on February 13, 2012, 06:24:20 PM
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.
Title: Re: DOF FF vs Crop...
Post by: shinyknights on February 13, 2012, 07:28:19 PM
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.
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 07:39:39 PM
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.
Title: Re: DOF FF vs Crop...
Post by: JonJT on February 13, 2012, 07:40:15 PM
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.
Title: Re: DOF FF vs Crop...
Post by: JonJT on February 13, 2012, 07:41:31 PM
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. 
Title: Re: DOF FF vs Crop...
Post by: arad85 on February 13, 2012, 08:34:46 PM
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 (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.
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 08:47:41 PM
Wait, wait...you mean we can print these digital thingies?   :P

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.
Title: Re: DOF FF vs Crop...
Post by: arad85 on February 13, 2012, 08:54:05 PM
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 ;)
Title: Re: DOF FF vs Crop...
Post by: arad85 on February 13, 2012, 08:55:08 PM
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 ;)
Title: Re: DOF FF vs Crop...
Post by: neuroanatomist on February 13, 2012, 09:04:43 PM
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.
Title: Re: DOF FF vs Crop...
Post by: arad85 on February 13, 2012, 09:10:34 PM
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.
Title: Re: DOF FF vs Crop...
Post by: JonJT on February 13, 2012, 09:33:04 PM
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 (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?
Title: Re: DOF FF vs Crop...
Post by: ghosh9691 on February 13, 2012, 09:54:20 PM
I've gone deeper into the CoC concept in other threads, which often just descend into circular arguments that are confusing.

Isn't that why it's called "Circle of Confusion"?  ;D
Title: Re: DOF FF vs Crop...
Post by: TexPhoto on February 13, 2012, 10:10:33 PM
But wait!, is light particles or waves?

Yup, FF menas shallower DOF vs cropped.  Yup.

And I go with Wave–particle duality btw.
Title: Re: DOF FF vs Crop...
Post by: Jamesy on February 13, 2012, 10:32:28 PM
I have posted these once before here - they are far from perfect 'lab' shots but they show the rough difference in bokeh between a 5Dc/85_1.8 and my 40D/50_1.8, both at 1/3200, ISO100, F2. Clearly the 5D has way more bokeh.

Title: Re: DOF FF vs Crop...
Post by: Cosk on February 14, 2012, 09:37:50 AM
I'll add to Jamesy's test...  Here are two 'equivalent' images:

#1: 135mm f/4 on a 5D
#2: 85mm f/2.5 still on a 5D, but I cropped the center in post. 

The camera-to-subject distance was fixed. 

The key thing to note is that the dof/blur appears identical between the two focal lengths. 

#3 is the 135mm FF at f/2...

Note that a 135 f2 has far more blur than an 85mm f2.5 cropped.

(Unfortunately I didn't do an f2<>f2 comparison, and I ate the tomato)
Title: Re: DOF FF vs Crop...
Post by: Jamesy on February 14, 2012, 09:48:05 AM
I'll add to Jamesy's test...  Here are two 'equivalent' images:

#1: 135mm f/4 on a 5D
#2: 85mm f/2.5 still on a 5D, but I cropped the center in post. 

The camera-to-subject distance was fixed. 

The key thing to note is that the Bokeh appears identical between the two focal lengths. 

#3 is the 135mm FF at f/2...

Note that a 135 f2 has far more bokeh than an 85mm f2.5 cropped.

(Unfortunately I didn't do an f2<>f2 comparison, and I ate the tomato)
Interesting - what made you select 85_F2.5 vs 135_F4 - did you just chimp until the bokeh looked similar or did you bust out the calculator?
Title: Re: DOF FF vs Crop...
Post by: Cosk on February 14, 2012, 04:56:56 PM
Interesting - what made you select 85_F2.5 vs 135_F4 - did you just chimp until the bokeh looked similar or did you bust out the calculator?

I busted out the calculator.  I didn't really believe the math at first, so I did the test and the math checks out - perfectly.  Even a 1/3 of a stop on either side has visibly different bokeh.

85mm x 1.6 = 136mm (I had those primes)
f/2.5 x 1.6 = f/4 (only a few f/stop combos are 1.6x apart)

I also did the test comparing 50(crop) vs 85(ff), and 35(crop) vs 50mm(ff)... which aren't as closely matched as the 85/135... but it was the same conclusion. 

Once again, Math wins. 


Here is the article on the math (I aspire to one day reading the whole thing...)
http://www.josephjamesphotography.com/equivalence/ (http://www.josephjamesphotography.com/equivalence/)
Title: Re: DOF FF vs Crop...
Post by: skitron on February 15, 2012, 01:36:45 PM
...but have a hard time grasping that cropping an image in post changes DoF, too.

I think this is because it comes across as a ruse. Anyone who has ever taken an image and cropped it knows the "out of focus-ness" of the background doesn't magically change just because the photo has been cropped, any more than it changes when the photo is shrunk or blown up. It's the math to describe the result of those manipulations that changes, not the pixels themselves, hence the perception of a ruse. JMO of course.

Title: Re: DOF FF vs Crop...
Post by: Meh on February 15, 2012, 02:54:56 PM
I have posted these once before here - they are far from perfect 'lab' shots but they show the rough difference in bokeh between a 5Dc/85_1.8 and my 40D/50_1.8, both at 1/3200, ISO100, F2. Clearly the 5D has way more bokeh.

You can't really have more bokeh.   The OOF areas can be more OOF but that is not bokeh.

Bokeh refers to the quality of the OOF parts of an image... how smooth, creamy, uniform, etc. but there is no formal definition.  Bokeh is generally (some may debate) thought to be a function of the lens, for example the shape and number of aperture blades contribute to the look of the OOF areas.