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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 03 Dec 2014 (Wednesday) 14:27
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Does changing aperture size affect the focal point?

 
trale
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Dec 03, 2014 14:27 |  #1

Something that I've been trying to sort out lately...

Take the following scenario for example. I'm shooting a flower in a bush. I manual-focus on the flower at f/1.8, and take the shot. Now what if I want to get more of the bush in focus, so I step down to f/5.6, but don't touch the manual focus ring. Can I safely assume that the flower is still the focal point / focal distance?

Or the other way around. I start at f/5.6, focus, and then widen aperture to f/1.8. Do I need to re-focus?

Please note that I'm not talking about DOF. I know that DOF changes with aperture size. I'm just referring specifically to the focal point/plane.

In the past, I've been a bit paranoid and recheck the focus and fudged around with the focus ring after changing the aperture to ensure that my original focus subject is still in critically in-focus. But do I need to do this? Theoretically speaking, if my focus (on some object or part of an object) was spot-on initially, I shouldn't need to re-focus after simply changing the aperture right?

Knowing this definitively would help me speed up my work-flow in the field when I'm experimenting with bracketed shots.

Thanks!




  
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rent
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Dec 03, 2014 14:41 |  #2

Which lens are you using? Focus shift is usually more pronounced with fast aperture lenses (the 50mm f/1.0L comes to mind).

That said, AF (phase detect, not live-view) is always done at lens' maximum aperture. So it's always possible to have focus shift if you are not taking the shot at maximum aperture.

However, stopping down to something like f/5.6 (or even 2.8) would effectively negate any minute focus shift due to the increased DOF. You can also focus in live view, which is always done with the aperture set to whatever setting you choose.

-alex


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ejenner
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Dec 03, 2014 15:15 |  #3

Except for select lenses such as the 50L you don't need to worry about . So you will actually more accurately position the focus wide open than stopped down were it can be difficult, even in LV, to exactly position the fous point.


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trale
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Dec 03, 2014 20:55 |  #4

rent wrote in post #17309058 (external link)
Which lens are you using? Focus shift is usually more pronounced with fast aperture lenses (the 50mm f/1.0L comes to mind).

That said, AF (phase detect, not live-view) is always done at lens' maximum aperture. So it's always possible to have focus shift if you are not taking the shot at maximum aperture.

However, stopping down to something like f/5.6 (or even 2.8) would effectively negate any minute focus shift due to the increased DOF. You can also focus in live view, which is always done with the aperture set to whatever setting you choose.

-alex

I have a 17-40mm F/4 L.

But it would be nice if whatever the answer is to my question that it applies to all lenses.




  
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WhyFi
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Dec 03, 2014 21:09 |  #5

trale wrote in post #17309995 (external link)
I have a 17-40mm F/4 L.

But it would be nice if whatever the answer is to my question that it applies to all lenses.

You're going to have to settle for it applying to most lenses.

Focus shifting is going to be most apparent with fast lenses and in situations where you're stopped down from the max aperture, but the DoF is still fairly thin (relatively close distances, relatively large apertures). If you're having problems with your 17-40L, it's very doubtful that it's related to focus shift (relatively slow max aperture and pretty short focal lengths).


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rent
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Dec 03, 2014 21:55 |  #6

trale wrote in post #17309995 (external link)
I have a 17-40mm F/4 L.

But it would be nice if whatever the answer is to my question that it applies to all lenses.

I wouldn't worry about it on this lens. Again, it might be something to consider on a lens with very large aperture. F/4's DOF is more than sufficient to cover any tiny shift it might have.

-alex


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chauncey
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Dec 07, 2014 18:05 |  #7

Based upon my tethered, tripod mounted shooting utilizing Canon Utility Software and controlling the camera settings from the computer...
changing nothing but the f/stop will affect the focus point not at all, even when examined @ 200% in that software. ;-)a


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20droger
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Dec 07, 2014 18:29 |  #8

All modern autofocus lenses, and most manual focus lenses, focus with the aperture wide open. Even in the days of all manual focus and manual aperture rings, serious photographers focused with the lens wide open: greater aperture = more light = better focus.

Focus shift is the result of a position shift of one or more elements within the lens. Changing the aperture affects the elements not at all, especially on modern autofocus lenses when there are no fingers involved.

That said, changing aperture on lenses with significant chromatic aberration will change the amount of aberration, which can sometimes appear to be a focus shift, as the image can become blurrier the further from the center you look.




  
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SkipD
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Dec 07, 2014 19:01 |  #9

trale wrote in post #17309020 (external link)
Take the following scenario for example. I'm shooting a flower in a bush. I manual-focus on the flower at f/1.8, and take the shot. Now what if I want to get more of the bush in focus, so I step down to f/5.6, but don't touch the manual focus ring. Can I safely assume that the flower is still the focal point / focal distance?

Or the other way around. I start at f/5.6, focus, and then widen aperture to f/1.8. Do I need to re-focus?

Because you're using a lens which has its iris controlled by the camera (as opposed to a fully MANUAL lens), you're actually focusing with the lens wide open all the time. The lens' iris is only closed down during the process of making an image (after pressing the shutter release button, that is).

Cameras from the mid 1960s such as my two Nikon F cameras and MANY more (Pentax, Miranda, Canon, and lots of others) mechanically kept the iris open wide for focusing and never let it close to the user-set value until the shutter release was pressed.

When using a lens with a purely manual aperture control (such as my old Nikon lenses on my modern Canon camera), I open the lens wide manually for focusing and then close it down to the aperture I want to use to make the image.


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trale
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Dec 08, 2014 22:43 |  #10

Thanks everyone. Seems like this won't be a worthwhile concern for me in the field. This should speed up my workflow and alleviate my OCD with re-checking focus quite a bit.




  
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Does changing aperture size affect the focal point?
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