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Thread started 21 Aug 2012 (Tuesday) 08:54
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Cropping into a 85mm f1.8 = poor man 190mm f1.8?

 
Earwax69
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Aug 21, 2012 08:54 |  #1

Hello... that might be a stupid question but I have been cropping a lot into my pics lately, thanks to the higher definition of my 15-85mm lens and my 18mp sensor.

I was salivating over the ef200mm f2.8 prime. However maybe I should just crop into my 85mm to get somewhat similar results.. ??

My new t3i have 2x more resolution than my old 20D... I could use that at my advantage I guess.

I go on a trip tomorow (not bringing the 85mm sadly) for 2 days and will look the answer on my iphone if I can. Dont be insulted if I dont answer right away!

thanks


Canon 6D | S35mm f1.4 | 135mm f2 The rest: T3i, 20D, 15mm f2.8, 15-85mm, 24mm f2.8, 50mm f1.8, 85mm f1.8, 90mm f2.8 macro, 55-250mm.
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Maverique
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Aug 21, 2012 08:57 |  #2

Not that close to 200mm. It will have the same field of view as a 135mm, approximately, but the optics are those of an 85mm. Always remember that. It might have the field of view of X or Y, but the optics are always "the original's".


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Earwax69
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Aug 21, 2012 09:35 as a reply to  @ Maverique's post |  #3

I am not talking about a crop sensor but croping the photo in photoshop or lightroom. And what do you main the optics stay the same ?? F1.8 will stay f1.8?


Canon 6D | S35mm f1.4 | 135mm f2 The rest: T3i, 20D, 15mm f2.8, 15-85mm, 24mm f2.8, 50mm f1.8, 85mm f1.8, 90mm f2.8 macro, 55-250mm.
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thestone11
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Aug 21, 2012 09:38 |  #4

Whenever you crop, you got less resolution~!


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Aug 21, 2012 09:39 |  #5

Yes you can do this as long as 1) you like the quality of the cropped image (many don't for whatever reason) and 2) you are left with enough resolution to match that of the 20D, since that seems to be your baseline.

However you won't be able to realize #2, I don't believe. In order to crop a 5184 to a 20D resolution, you need 3502 pixels on the long side, which is a 32% reduction on one dimension. I think this really means your cropped 85mm shot really will have the view of a 112mm, not a 190mm, if I did this correctly. I doubt I did this correctly though, I always seem to mess it up, and others have to come to my rescue. :)


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Maverique
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Aug 21, 2012 09:42 |  #6

Yeah, I meant that if you're cropping to emulate a 200mm on an 85, you're not going far, they're tele lenses, but the 85 is just barely one and its optical compression is very different from a 200mm lens as well.


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Echo ­ Johnson
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Aug 21, 2012 09:43 |  #7

The aperture and light-gathering abilities of a lens won't be affect by cropping, obviously; f/1.8 when you take the picture will always result in a picture taken at f/1.8, no matter what you do to it afterwards. However, the depth of field from your 85mm f/1.8 will not change to the (shallower) DoF of a 190mm f/1.8 just because you crop an image.


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Earwax69
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Aug 21, 2012 10:00 as a reply to  @ Echo Johnson's post |  #8

Ok i understand what you mean. Double the "zoom" double the dof. It make sens. However i believe the compression effect will be there. You loose the side of the picture.

Let say for the theorie that i take a 5000x5000 image a crop to 2500x2500 in the middle.

I will shoot street stuff, low light. Sharpness is not the priority if its good enough for the web. However big bokeh is a must.


Canon 6D | S35mm f1.4 | 135mm f2 The rest: T3i, 20D, 15mm f2.8, 15-85mm, 24mm f2.8, 50mm f1.8, 85mm f1.8, 90mm f2.8 macro, 55-250mm.
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dannybres
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Aug 21, 2012 10:12 as a reply to  @ Earwax69's post |  #9

big bokeh comes with wide aderture and long focal length......

you will get much better bokeh at 200mm than at 85mm....


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Aug 21, 2012 10:14 |  #10

dannybres wrote in post #14886787 (external link)
big bokeh comes with wide aderture and long focal length......

you will get much better bokeh at 200mm than at 85mm....

Your DOF will be thinner with a 200mm at f2.8 than an 85mm at f1.8, but the bokeh may not be necessarily better. ;)


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dannybres
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Aug 21, 2012 10:16 as a reply to  @ TeamSpeed's post |  #11

true...

does that depend on lens contruction and number of blades etc?


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Aug 21, 2012 10:19 |  #12

dannybres wrote in post #14886809 (external link)
true...

does that depend on lens contruction and number of blades etc?

Yes, that and the type of subject material in the OOF areas. You may notice that a lens generates great looking light highlights bokeh, but grass/branches generate nasty bokeh. Some lenses just do a wonderful job of smoothing out all items in the OOF areas into creamy blur.


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Earwax69
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Aug 21, 2012 10:32 |  #13

The bokeh size will double with the crop however, if the subject is far, i might not get a lot of bokeh at all. For that a 200mm would be best. Am i right?


Canon 6D | S35mm f1.4 | 135mm f2 The rest: T3i, 20D, 15mm f2.8, 15-85mm, 24mm f2.8, 50mm f1.8, 85mm f1.8, 90mm f2.8 macro, 55-250mm.
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Aug 21, 2012 14:30 |  #14

Multiple factors at play, here.

The lens has a resolution of so many line-pairs per millimeter, and that is a fixed amount. If you cut an image (or piece of film) in half, there are half as many line-pairs left...the resultant cropped image has HALF of the resolution as the original image.

Then you double the mag factor to achieve a certain print size...if you magnified 22.3mm APS-C frame to 16x24", that is about 27x magnification; from 11.15mm recropped image, that would take almost 55x magnifcation to make the same 16x24" print! So your print quality reflects half as much resolution as well.

As for DOF, standard DOF calculations assume 8x10" print viewed from 12" away. When you double the magnification to make the print (above paragraph), the sharp details as well as the blurred details (Circle of Confusion) are all magnified, so your perception of 8x12" print made from your 11.15mm wide recropped image would have half the DOF as an 8x12" print made from 22.3mm APS-C frame!

As for bokeh, the quality remains the same. But your amount of blur is twice as much (magnified by twice as much, 2x the size of CofC blur circles). If it is pleasing bokeh that the lens provides, you have twice as much of it; but if the bokeh sucks (think about donuts from mirror lenses) the blur circles (or donuts) are twice as apparent, too.


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Earwax69
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Aug 21, 2012 17:15 |  #15

Thanks wilt, i think you covered it all. It also why fullframe camera make sharper pictures, because they use more of the lens resolution.


Canon 6D | S35mm f1.4 | 135mm f2 The rest: T3i, 20D, 15mm f2.8, 15-85mm, 24mm f2.8, 50mm f1.8, 85mm f1.8, 90mm f2.8 macro, 55-250mm.
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Cropping into a 85mm f1.8 = poor man 190mm f1.8?
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