Bokeh at 135 f2 cropped to 200 mm vs native 200 mm f2.8

Assuming 'megapixels'/image size is no big deal, which would produce better background blur and subject isolation?

• Photo taken at 135 mm f2 cropped down to 200 mm equivalent
  
• Photo taken at 200 mm f2.8
  

Cropping doenst change DOF so you could compare them running scenarios on a calculator like http://dofmaster.com/d​ofjs.html I assume you mean shooting from the same distance.

i won't answer your question directly. But provided you have the working distance, if you take 2 shots with the same framing (meaning you have to stand further back with the 200mm), I'd take the 200mm @ f2.8 anyday. More dof on the subject and just as good background separation if not better.

EOS Man wrote in post #15151565
Assuming 'megapixels'/image size is no big deal, which would produce better background blur and subject isolation?

• Photo taken at 135 mm f2 cropped down to 200 mm equivalent
  
• Photo taken at 200 mm f2.8
  

So you stay at the same range to the victim and to the background? If yes:

You'll simply get more blur from the 135. Stepping down the 135mm to f/2.8 would get the same amount of blur (again same distances and then crop).

However, the quality of the blur might still differ. Canon's 200mm f/2.8 prime is pretty much identical to the 135L, but the zooms will have quite a bit of difference.

I don't think I agree with uOpt's analysis. Assuming FF camera

• 135mm f/2 at 30' has DOF zone of 1.48' and sees frame area of 5.25' x 7.88'
  
• 200mm f/2.8 at 44.44' has DOF zone of 1.48' and sees frame area of 5.25' x 7.88'
  
• The 135mm f/2 aperture is 63mm in diameter, the 200mm f/2.8 aperture is 71.4mm in diameter. So the near background objects will be more out of focus with the larger aperture, the 200mm.
  
• And far field background will be more blurred with the longer FL, the 200mm again.
  

Wilt wrote in post #15152264
I don't think I agree with uOpt's analysis. Assuming FF camera

• 135mm f/2 at 30' has DOF zone of 1.48' and sees frame area of 5.25' x 7.88'
  
• 200mm f/2.8 at 44.44' has DOF zone of 1.48' and sees frame area of 5.25' x 7.88'
  
• The 135mm f/2 aperture is 63mm in diameter, the 200mm f/2.8 aperture is 71.4mm in diameter. So the near background objects will be more out of focus with the larger aperture, the 200mm.
  
• And far field background will be more blurred with the longer FL, the 200mm again.
  

OTOH if the camera position is identical with both FL, and we crop the photo during post processing, I am uncertain about the effect of the increased magnification of far field blur, in blowing up the smaller image area to the same final print size. compared to the 200mm. Therein lies my uncertainty about disagreeing (or not) with uOpt!

I said same distance to both subject and background.

EOS Man wrote in post #15151565
Assuming 'megapixels'/image size is no big deal, which would produce better background blur and subject isolation?

• Photo taken at 135 mm f2 cropped down to 200 mm equivalent
  
• Photo taken at 200 mm f2.8

I think you will pretty much see the answer to your question by reading our "sticky" (found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance?.

uOpt wrote in post #15153490
I said same distance to both subject and background.

Since I couldn't predict what to expect under the circumstance (because of cropping in post processing, demonstrated in shot 30), I set up a tripod and mounted 5D and 40D. I used 70-200mm f/4L IS, at 81mm FL setting for both cameras, then at 127mm setting for the 5D.

We have the point of focus, near background, and far background items all in view. I focused on the coffee mug (at the left) which was 8' away from the focal plane; the light (at the right) is back at 10' from the plane of focus, the apple is at 30' from the plane of focus. In the below comparison, this is the sequence of shots

29. the 81mm 40D shot was at f/4 (shot 29)
    
30. the 81mm 5D shot was at f/5.6 and was cropped (shot 30) to same FOV as the 40D shot
    
31. the 127mm 5D shot was at f/4 (shot 31)
    
32. the 127mm 5D shot f/4 and f/5.6 (shot 32)
    

• the 40D 81mm f/4 has DOF zone of 4.3"
  
• the 5D 81mm f/5.6 has DOF zone of 6.1" when cropped to APS-C frame size
  
• the 5D 127mm f/4 shot had DOF zone of 2.7"
  
• the 5D 127mm f/5.6 shot has DOF zone of 3.9"
  

• the longer FL (in shots 31, 32) wins the greater far field blur even when the wider FL is cropped in post processing to same FOV (shot 30), and
  
• the choice of aperture did NOT have a major effect on far field blur (shot 31 f/4 vs. shot 32 f/5.6)
  
• proving that DOF and f/stop are NOT necessarily altering the amount of far field blur as much as FL does.
  

I did the math at some point for the circle of confusion equation as a function of the aperture size, FL, distances to the subject and the background. (CoC is what is directly related to the OP question; DoF is a less direct measure).

What I recall is that for a fixed framing and remote background, the amount of blur is simply proportional to the effective aperture (that is, FL / f-number), and doesn't depend on the sensor size, FL etc.

So 200mm f/2.8 should produce slightly more blur than a cropped 135mm f/2.0 ("cropped" is equivalent to using a smaller size sensor) simply because its aperture is bigger (71.4 vs. 67.5 mm).

pulsar123 wrote in post #15154873
What I recall is that for a fixed framing and remote background, the amount of blur is simply proportional to the effective aperture (that is, FL / f-number), and doesn't depend on the sensor size, FL etc..

Your statement applies to the amount of near field background blur, but not to the amount of far field background blur

Both points are illustrated in my post 8, from an hour ago.

Wilt wrote in post #15154929
Your statement applies to the amount of near field background blur, but not to the amount of far field background blur

Both points are illustrated in my post from an hour ago.

No, it does specifically apply to remote background blur. (That's how I derived it). I'll look up my original derivation - it's here somewhere...

...

Google is your friend - I found it:

https://photography-on-the.net …p?p=12702175&po​stcount=10

It is possible that we are just making different assumptions. In my derivations, I assumed that the foreground framing is fixed.

pulsar123 wrote in post #15154973
No, it does specifically apply to remote background blur. (That's how I derived it). I'll look up my original derivation - it's here somewhere...

Forget the numbers for a second...look at examples 1&2 vs. examples 3&4...the longer FL has the more blurry far background.
If aperture controlled the relationship of far field background, example 1 and 4 should have similar blur quantity, as example 1 has 20mm aperture and example 4 has 22.6mm aperture.

And in all four examples, the foreground framing is identical.

Could it simply be because your background wasn't remote enough?

I have trust in math, so if there is an actual big discrepancy with the real life, I like to know why. (Did I make mistake? Did wikipedia? etc.)

pulsar123 wrote in post #15155030
Could it simply be because your background wasn't remote enough?

I have trust in math, so if there is an actual big discrepancy with the real life, I like to know why. (Did I make mistake? Did wikipedia? etc.)

I'll have to reshoot the series some time when the weather is clear and not varying the clarity of the far hillside with visible moisture, to really settle the question.

Sounds like a good plan. Thanks!