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Crope mode Vs Crop in post affecting Background blur?
  
 
patonima
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p.1 #1 · p.1 #1 · Crope mode Vs Crop in post affecting Background blur?


OK I know this have been explained many times and I read and watched tone of videos about it. I still can't fully grasp the exact logic behind it. I completely get the concept though. My Full frame 55mm f1.8 lens would give me more or less the same field of view of a 83mm f2.8 used on an APSC sensor.
I use my Sony a7RII in both modes. And I can see clearly that by switching to APSC mode less light is gathered on the sensor therefore higher ISO is necessary. However what about how blurry the background gets (is it really a f2.8 APSC now)?
From what I understood, switching to APSC does the exact same thing as cropping later in post (But does it?). The only advantage should be to frame better and have more visual reach.
Now if I think about it, the amount of background blur in Full frame mode won't change if I crop later. My lens is and stays f1.8 cropped. So when I use it in cropped APSC mode directly on my A7RII does it slow down it's aperture to give less background blur to a f2.8 (or only limits the light as of a f2.8 without affecting its 1.8 blurriness). If that's the case then cropping in post is not equivalent to cropping in camera right?
Also, without thinking about these considerations, by switching to APSC mode in camera, does that affect my position in space to the extent that it changes the background blur behind my subject more than I would later in crop and therefore giving better/worst results?
Has anyone tried to completely understand that to know better when to crop in post when to do it in post? The Sony A7RII gives so much pixels that both possibilities seem to be working well.
How much background blur (shallow depth of field) do I loose to not shoot with a proper 85mm f1.8 apart from Pixel counts and higher ISO for same exposition?
Please don't be afraid to push on the math that are happening here as it looks more complex than anyone dares to say.
Thanks 



Sep 13, 2017 at 05:15 PM
p.sage
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p.1 #2 · p.1 #2 · Crope mode Vs Crop in post affecting Background blur?


Cropping in camera and cropping in post does exactly the same thing. The reason you get "less blur" on apsc is because you have to be further away to get the same framing at a shorter focal length. For example, if you were to shoot a 35mm f/1.8 on a crop camera, and a 35mm f/1.8 on a full frame camera of the same thing at the same distance, the amount of in focus area would be the same across both photos. You would just have a wider field of view at the same dof with the full frame camera. To get the same field of view with the apsc camera, you would either need a wider lens (larger dof) or to back up (larger dof and altered perspective).


Sep 13, 2017 at 06:38 PM
AmbientMike
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p.1 #3 · p.1 #3 · Crope mode Vs Crop in post affecting Background blur?


A 1.8 lens is a 1.8 lens. If you shoot it on aps, or crop in post, it's the same field of view as a 1.5x longer focal length lens on ff.

But it is still a 1.8 lens, for exposure.

If you used an 83mm lens, at 1.8, on ff, you'd have less dof than a 55mm at 1.8 on aps. But both would give about the same view.

You might have to stop an 83mm down to 2.8, to approximate the dof with a 55mm at 1.8, used as aps.

This might require a higher iso, f/2.8 vs. 1.8. But you should get the same ss at 1.8 using the 55/1.8 on crop or ff. So if the ss is high enough, no need to turn up iso.

But since 55mm a bit longer on crop, more tele, I might prefer a higher ss, and turn up iso for that reason.


Edited on Sep 13, 2017 at 06:54 PM · View previous versions



Sep 13, 2017 at 06:51 PM
patonima
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p.1 #4 · p.1 #4 · Crope mode Vs Crop in post affecting Background blur?


p.sage wrote:
Cropping in camera and cropping in post does exactly the same thing. The reason you get "less blur" on apsc is because you have to be further away to get the same framing at a shorter focal length. For example, if you were to shoot a 35mm f/1.8 on a crop camera, and a 35mm f/1.8 on a full frame camera of the same thing at the same distance, the amount of in focus area would be the same across both photos. You would just have a wider field of view at the same dof with the full frame camera.
...Show more

That's what I thought, but then I believe that f/1.8 is supposed to become f/2.8 on APSC mode which would also be a factor that reduces blur. However I have the feeling it's not true. f/1.8 doesn't become a real f/2.8 equivalent on APSC. It's equivalent in light gathered but not in "blurred background" effect. What do you think?
If that's the case, that means that my 55mm f/1.8 would give the exact same results in APSC mode as a 85mm f/1.8 in FF mode (same Depth of field) except it would need higher ISO in Low Light.
Can anyone confirm that?
Thanks

f/1.8



Sep 13, 2017 at 06:52 PM
p.sage
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p.1 #5 · p.1 #5 · Crope mode Vs Crop in post affecting Background blur?


The only thing affected by using the lens on apsc is the blur. An exposure requiring 1.8 and 1/50th of a second on apsc would require 1.8 and 1/50th on full frame and 1.8 and 1/50th on 645.

Exposure remains constant across formats, only depth of field is affected.



Sep 13, 2017 at 06:55 PM
patonima
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p.1 #6 · p.1 #6 · Crope mode Vs Crop in post affecting Background blur?


p.sage wrote:
The only thing affected by using the lens on apsc is the blur. An exposure requiring 1.8 and 1/50th of a second on apsc would require 1.8 and 1/50th on full frame and 1.8 and 1/50th on 645.

Exposure remains constant across formats, only depth of field is affected.


Hmm that's weird because if I use Auto ISO, I can see clearly the ISO bumping up to get the right exposure when I switch to APSC... Which makes sense, the sensor gets less light




Sep 13, 2017 at 07:01 PM
p.sage
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p.1 #7 · p.1 #7 · Crope mode Vs Crop in post affecting Background blur?


I imagine that at the same time, your shutter speed is increasing though. It's a simple test to do, put your camera in full manual, no automatic features at all, set your exposure to 1/50th, iso 100, and f/2.8, and take the same shot with crop mode turned on and off


you'll see that the brightness is the same



Sep 13, 2017 at 07:02 PM
patonima
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p.1 #8 · p.1 #8 · Crope mode Vs Crop in post affecting Background blur?


p.sage wrote:
I imagine that at the same time, your shutter speed is increasing though. It's a simple test to do, put your camera in full manual, no automatic features at all, set your exposure to 1/50th, iso 100, and f/2.8, and take the same shot with crop mode turned on and off

you'll see that the brightness is the same


You're right that's just what I did. Now I'm lost, I thought that Full Frame was gathering more light and therefore needed less ISO to get the same exposure in low light. Also I thought that the effective size of the sensor was affecting my Background blur too.
To simplify things, would a real 85mm f1.8 show any difference with my 55mm 1.8 in Crop?




Sep 13, 2017 at 07:11 PM
p.sage
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p.1 #9 · p.1 #9 · Crope mode Vs Crop in post affecting Background blur?


Full frame sensors receive more light in total, because it has a larger surface area. This allows full frame to have a better signal to noise ratio. This is what makes full frame better in terms of image noise. However, the two sensor sizes are receiving the same light intensity per area, so the exposure remains the same. So in summation, yes, you will be using the same exposure settings for your 55 1.8 on crop and 85 1.8 on full frame, but the 85 on full frame will be showing a thinner depth of field.


Sep 13, 2017 at 07:33 PM
patonima
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p.1 #10 · p.1 #10 · Crope mode Vs Crop in post affecting Background blur?


p.sage wrote:
Full frame sensors receive more light in total, because it has a larger surface area. This allows full frame to have a better signal to noise ratio. This is what makes full frame better in terms of image noise. However, the two sensor sizes are receiving the same light intensity per area, so the exposure remains the same. So in summation, yes, you will be using the same exposure settings for your 55 1.8 on crop and 85 1.8 on full frame, but the 85 on full frame will be showing a thinner depth of field.


OK I think I'm starting to get it, thanks for the info. From what I understand then the thinner depth of field is just inherent to the longer 85mm Focal length which combined with F1.8 can't be beaten by a 55mm depth of field. Even if I switch to APSC mode I don't have a true 85mm but truly a zoom into the 55mm with an 55mm depth of field right? At least that means I'm keeping my f1.8 depth of field I always thought it was going to be f2.8 on APSC.
Thanks a lot for the explanation!!




Sep 13, 2017 at 07:40 PM
 

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patonima
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p.1 #11 · p.1 #11 · Crope mode Vs Crop in post affecting Background blur?


This video is juts making me more confused:
I guess it might be a mistake




Sep 13, 2017 at 07:55 PM
p.sage
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p.1 #12 · p.1 #12 · Crope mode Vs Crop in post affecting Background blur?


What specifically is confusing you? I'd be happy to elaborate any points in the video.


Sep 13, 2017 at 08:10 PM
patonima
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p.1 #13 · p.1 #13 · Crope mode Vs Crop in post affecting Background blur?


He multiplies the aperture by the crop factor therefore my 55 f1.8 would become about a 85 f2.8
Which if true would mean I would get less light with f2.8 by switching in APSC mode with a field of view close to a 85mm. I get that in crop mod to get equivalent Depth of field one need to multiply the f stop by the crop factor. But from what you explained, it doesn't change the exposure so it's really only a 2.8 for Depth of field not for the amount of light gathered. the aperture should still be f1.8. Or am I lost again?



Sep 13, 2017 at 08:29 PM
p.sage
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p.1 #14 · p.1 #14 · Crope mode Vs Crop in post affecting Background blur?


The aperture being multiplied by crop factor only applies to depth of field. Because the smaller sensor is receiving the same intensity of light, just on a smaller area, exposure remains the same. So, your 55 1.8 on crop is equal to an 85 2.8 on full frame in terms of depth of field only.


Sep 13, 2017 at 08:42 PM
patonima
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p.1 #15 · p.1 #15 · Crope mode Vs Crop in post affecting Background blur?


Great that's what I needed to know
Thanks



Sep 13, 2017 at 08:59 PM
Snopchenko
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p.1 #16 · p.1 #16 · Crope mode Vs Crop in post affecting Background blur?


That's why the whole equivalence thing is really snake oil.


Sep 14, 2017 at 09:47 PM
patonima
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p.1 #17 · p.1 #17 · Crope mode Vs Crop in post affecting Background blur?


Snopchenko wrote:
That's why the whole equivalence thing is really snake oil.

Yeah but I don't see any other way, manufacturer can't advertise a 55mm 1.8 as a 85mm 2.8 on APSC like Tony&Chelsee are proposing. Things are more complex. From what I understood, the 55mm becomes close to a 85mm as for field of view. But it's just a zoom in the 55mm field of view, therefore you don't have the same compression, but the effect is close enough.
For the Fstop on the other side, 1.8 stays 1.8, same exposure, you get a comparable 85mm 1.8 but with a Depth of field equivalent to 2.8. That's really difficult to grasp and difficult to advertise, I guess manufacturer are just simplifying stuffs without especially trying to rip off the consumer about the apertures equivalent.




Sep 15, 2017 at 01:06 AM
Snopchenko
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p.1 #18 · p.1 #18 · Crope mode Vs Crop in post affecting Background blur?


patonima wrote:
Yeah but I don't see any other way, manufacturer can't advertise a 55mm 1.8 as a 85mm 2.8 on APSC like Tony&Chelsee are proposing. Things are more complex. From what I understood, the 55mm becomes close to a 85mm as for field of view. But it's just a zoom in the 55mm field of view, therefore you don't have the same compression, but the effect is close enough.
For the Fstop on the other side, 1.8 stays 1.8, same exposure, you get a comparable 85mm 1.8 but with a Depth of field equivalent to 2.8. That's really difficult to grasp
...Show more

Yeah, and thank God they don't. 55mm is 55mm and f/1.8 is f/1.8 whatever the format is, it's the perception that changes (and if I'm not using 35mm "FF" - I'm really not using it - why should it matter to me how my lenses are correlating to that format. Moreover, precise focal lengths and apertures don't matter to me either - my lenses could have been labelled "DA WIDE", "DA LONG", "DA FAST/WIDE" and "DA SUPERWIDE" for all I know.



Sep 15, 2017 at 01:50 PM
patonima
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p.1 #19 · p.1 #19 · Crope mode Vs Crop in post affecting Background blur?


HAHA True!!! I have also DA ONE I LIKE and DA ONE NOT SO MUCH too


Sep 15, 2017 at 02:09 PM
arduluth
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p.1 #20 · p.1 #20 · Crope mode Vs Crop in post affecting Background blur?


Seriously, equivalence isn't just "snake oil." It's a way of relating relative performance in terms of FOV and DOF when you want to put things in terms of another format. Usually that format is 135 film/FF digital sensor. It's a way of comparing lenses and some aspects of performance/behavior between systems. There's nothing wrong with that, and it can be very useful.

Sometimes, I'll hear of a lens in MF or LF and I'll apply the "crop factor" so that I can visualize what a shot from that lens on a given format would look like. This makes sense for me, most of my experience is with 135/FF, which means I can easily visualize the FOV and DOF for a given focal length an aperture. It's a handy shortcut.

It's another story when folks like Northrup and other bombastic know-it-all types decide to make it their hobby horse and make wild claims. It's so, so much simpler than people like that let on. If only they just explained the simple technical aspects and moved on.



Sep 15, 2017 at 03:06 PM







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