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Author Topic: Focal length change from close to infinity and how that affects...  (Read 930 times)

sanfairyanne

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I believe I'm right in camera lenses do not hold exact focal length throughout their range from close to infinity. If this is correct does it affect focus stacking. If so how do photographers correct for this?

Many thanks in advance.
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BobShaw

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Re: Focal length change from close to infinity and how that affects...
« Reply #1 on: April 22, 2022, 07:06:06 pm »

Yes and yes. Each image is a different magnification so ideally avoid it with a good lens and small aperture but most software like Photoshop or Helicon are pretty good at fixing.
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sanfairyanne

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Re: Focal length change from close to infinity and how that affects...
« Reply #2 on: April 23, 2022, 01:56:13 am »

Yes and yes. Each image is a different magnification so ideally avoid it with a good lens and small aperture but most software like Photoshop or Helicon are pretty good at fixing.
Thanks Bob, from my experiences Helicon and Zerene Stacker have been ok, but rarely perfect. I've learnt how to improve a Photoshop stack using layers by brushing in imperfections, then stamping the layer.  I've yet to see a tutorial for focus stacking that actually explains the different magnifications from close focus to infinity. As you say a small aperture would help but who wants to shoot at f16, that sort of defeats the purpose. Personally I wonder why no lens manufacturer has developed a slow lens i.e. one where the widest aperture was for example f8 or even f11. Imagine having a lens where the sharpest f stop was f16. One would assume the lens would also be much lighter. It's for this reason I'm happy with the kit f4 lenses on my Z7.
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PeterAit

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Re: Focal length change from close to infinity and how that affects...
« Reply #3 on: April 23, 2022, 10:51:38 am »

This is one reason that most folks seem to prefer moving the camera rather than changing the focus.
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kers

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Re: Focal length change from close to infinity and how that affects...
« Reply #4 on: April 23, 2022, 11:44:54 am »

... As you say a small aperture would help but who wants to shoot at f16, that sort of defeats the purpose. Personally I wonder why no lens manufacturer has developed a slow lens i.e. one where the widest aperture was for example f8 or even f11. Imagine having a lens where the sharpest f stop was f16. One would assume the lens would also be much lighter. It's for this reason I'm happy with the kit f4 lenses on my Z7.

f16 will always be less sharp than f8, independent of type of lens, because of the physical effect of diffraction.
some explantion here: https://photographylife.com/what-is-diffraction-in-photography.

btw; The interest in making slow lenses has grown lately due to the stabilisation built into camera's.
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BobShaw

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Re: Focal length change from close to infinity and how that affects...
« Reply #5 on: April 23, 2022, 07:43:55 pm »

f16 will always be less sharp than f8, independent of type of lens, because of the physical effect of diffraction.
Possibly, but product photographers shoot at f16,18,20 all the time.
The product has to be in focus front to back and with possibly hundreds of shots a day you don't have the time or money to take multiple images of each and then combine them.
If you use high quality lenses that go to f32 or f45 it's no problem.
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kers

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Re: Focal length change from close to infinity and how that affects...
« Reply #6 on: April 24, 2022, 07:05:12 am »

Possibly, but product photographers shoot at f16,18,20 all the time.
The product has to be in focus front to back and with possibly hundreds of shots a day you don't have the time or money to take multiple images of each and then combine them.
If you use high quality lenses that go to f32 or f45 it's no problem.

it is a relative figure... f16 is less sharp ( less microdetail/crisp) but with some extra sharpening it is very good even at pixel level.
and when made smaller for the internet it is perfect.
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Pieter Kers
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TechTalk

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Re: Focal length change from close to infinity and how that affects...
« Reply #7 on: April 24, 2022, 03:40:30 pm »

This is one reason that most folks seem to prefer moving the camera rather than changing the focus.

Moving the camera is changing the focus... and image magnification.
« Last Edit: May 02, 2022, 05:31:46 pm by TechTalk »
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TechTalk

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Re: Focal length change from close to infinity and how that affects...
« Reply #8 on: May 02, 2022, 03:14:34 pm »

I believe I'm right in camera lenses do not hold exact focal length throughout their range from close to infinity.

The overwhelming majority of lenses change in focal length as you change the focus distance on the lens. How much and in what direction is dependent on the lens design.

This is referred to as focus breathing or lens breathing. In the cinema/video world this can be distracting to the viewer. There are cinema lenses designed to counteract this and which retain a consistent focal length and field of view. Here's a video which discusses focusing breathing and cine lenses.
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TechTalk

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Re: Focal length change from close to infinity and how that affects...
« Reply #9 on: May 02, 2022, 03:25:24 pm »

If this is correct does it affect focus stacking.

Yes, it alters image size/magnification and angle of view. Depending on the focusing method it may also change perspective. It's a much easier task for stacking programs to correct for changes in magnification than in perspective.

If you change the lens to subject distance, you will change perspective. Perspective is determined by lens to subject distance not the focal length of the lens.
« Last Edit: May 03, 2022, 12:25:18 am by TechTalk »
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TechTalk

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Re: Focal length change from close to infinity and how that affects...
« Reply #10 on: May 02, 2022, 03:41:19 pm »

If so how do photographers correct for this?

The first thing you may want to consider is to avoid using a focusing method that alters the distance of the lens to the subject. Keeping lens position and distance fixed will avoid changes in perspective as you change focus. Of course, there may be practical limitations on how this can be accomplished depending on what you're photographing and the equipment you have available.

Differences in subject matter and magnification will likely require different methods of focusing for best results. This document from Zerene Stacker may be helpful in explaining the issues and best methods for different scenarios.
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