DOF FF vs Crop...

[ghosh9691]

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"? 

[TexPhoto]

But wait!, is light particles or waves?

Yup, FF menas shallower DOF vs cropped.  Yup.

And I go with Wave–particle duality btw.

[Jamesy]

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.

[Cosk]

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)

[Jamesy]

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?

[Cosk]

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/

[skitron]

...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.

[Meh]

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.