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Wide open lens focused to infinity. General photography question.

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This is a general photography question.

When a fast lens such as the DJI 15mm f/1.7 has the aperture wide open but the focus is set to infinity what is the intended outcome of this? My brain is just having trouble wrapping my brain around this.

Since the aperture is wide open it would lend itself to shallow depth of field but with the focus set to infinity everything is suppose to be in focus.

When I try to take a photo with the DJI 15mm f/1.7 with the lens wide open and a focus subject close to the camera it produces a pretty clear image with the focus subject acceptably sharp with pleasing bokeh. However, if the focus is set to infinity on a subject that is far away the whole image seems to be mostly out of focus. I am not sure if that is the expected outcome with a fast lens set to infinity or not.

Any insight would be helpful. Thanks
 
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When the subject is far away, and the lens is focused at infinity, all far subjects will seem to be sharp, at any aperture. The shallow-focus look (lens wide open) is only evident when the subject is rather near, and other parts of the image are at different distances. Basic depth-of-field principles are discussed in most basic photo blogs. Make some tests, with subject at different distances, to compare.
 
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This is a general photography question.

When a fast lens such as the DJI 15mm f/1.7 has the aperture wide open but the focus is set to infinity what is the intended outcome of this? My brain is just having trouble wrapping my brain around this.

Since the aperture is wide open it would lend itself to shallow depth of field but with the focus set to infinity everything is suppose to be in focus.

When I try to take a photo with the DJI 15mm f/1.7 with the lens wide open and a focus subject close to the camera it produces a pretty clear image with the focus subject acceptably sharp with pleasing bokeh. However, if the focus is set to infinity on a subject that is far away the whole image seems to be mostly out of focus. I am not sure if that is the expected outcome with a fast lens set to infinity or not.

Any insight would be helpful. Thanks

You have to calibrate your camera so ∞ actually equals infinity. Without that calibration, ∞ will actually equal < or > infinity. Try calibrating your camera.

Oh...almost forgot...

I audit the focus before every shoot. Even then, I honestly don't trust ∞ 100%. I use the focus assist (red line things) to insure sharp focus on the subjects I want in focus. As you eluded to, I generally focus to ∞ 99% of the time. It's very rare I fly 10' away from something that I want in focus.

D
 
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You have to calibrate your camera so ∞ actually equals infinity. Without that calibration, ∞ will actually equal < or > infinity. Try calibrating your camera.

Oh...almost forgot...

I audit the focus before every shoot. Even then, I honestly don't trust ∞ 100%. I use the focus assist (red line things) to insure sharp focus on the subjects I want in focus. As you eluded to, I generally focus to ∞ 99% of the time. It's very rare I fly 10' away from something that I want in focus.

D
What I am really inquiring about is how depth of field works when the lens is properly set to infinity?

What is the difference between f/1.7 at infinity focus and f/8 at infinity focus besides shutter speed? Does this have an affect on depth of field or sharpness?
 
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When the subject is far away, and the lens is focused at infinity, all far subjects will seem to be sharp, at any aperture. The shallow-focus look (lens wide open) is only evident when the subject is rather near, and other parts of the image are at different distances. Basic depth-of-field principles are discussed in most basic photo blogs. Make some tests, with subject at different distances, to compare.
Thanks this is the gist of my question. So is there any reason why you wouldn’t always use the widest aperture possible when the subject is far enough away that the focus is set to infinity?

After doing some tests it seems the photos taken at f/5.6 are considerably sharper than photos taken at f/1.7 even though the focus was not changed and set to infinity. This would seem contradictory to the basic principal that a faster shutter should produce sharper images in the in focus regions of the photo.

Is this because the lens is just less sharp at f/1.7 due to the construction of the lens or is infinity focus just generally less sharp at wider apertures?
 
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Most lenses are NOT at their best when wide open. Best performance is usually 2 or 3 stops down. When you get to f11 or smaller, other optical problems will degrade performance. The consensus seems to be that f4 to f5.6 is best for many of the DJI lenses. Shutter speed is usually not an issue, when shorter that 1/60th of a sec.
 
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What I am really inquiring about is how depth of field works when the lens is properly set to infinity?

I gotcha. For the purpose of this tutorial, I think it's more accurate to use the term "focal depth" or "focus depth" as opposed to "depth of field." "Focal depth" isn't a real photography term, but just as it suggests, I'm using this term to describe the depth of the focus field. The more wide open the aperture, the longer or "deeper" the focal depth.



What is the difference between f/1.7 at infinity focus and f/8 at infinity focus besides shutter speed?

The focal DEPTH changes, but it's position doesn't. I'll use some arbitrary numbers to illustrate my point.

Say f/1.7 nets a deep focal depth of 10 feet. Anything in that 10 foot range will be in focus. If you close the aperture to f/8, you will narrow the focal depth to say 2 feet. Less focal depth gives you more "depth of field." Keep in mind that in reality, focal depth can be much deeper - say 50'. A super deep focal depth can leave you with the illusion that EVERYTHING is in focus. But rest assured, it's not. If set focus to ∞ and you hold your finger 5 inches from the lens, it will be out of focus.

If your focus is set to ∞, your 10-foot focal depth STARTS maybe 30'-50' away, and continues forever. Change your focal point to 10' away, now all subjects from 10' to 20' will be in focus. Change your aperture to f/8, now all subjects 10' to 12' will be in focus. And so on.

So...your focus doesn't really effect your focal *depth* for anything => ∞. Same goes for any focus < ∞. The difference is that < ∞ the focal depth will always be 10' deep. Set focus to infinity and the focal depth will extend out past the normal 10' depth to forever or "infinity." Make sense? Again, these numbers are completely arbitrary.

Note: Technically I know there's nothing greater than infinity, but I think you know what I'm saying.


Does this have an affect on depth of field or sharpness?

Your aperture DOES effect depth of field and CAN have an effect on sharpness. Depending on the lens type and the quality, the photographer will experience different artifacts and aberrations at different aperture settings. This may manifest itself as softness around the edges or some color casting around the edges. In most situations, most photographers agree that for most lenses the aperture's "sweet spot" with the fewest imperfections is usually around f/5.

The best thing to do is take several photos of the same subject from a static position using different settings so that you can compare them. You may find your aperture "sweet spot" closer to f/4 or f/6. Once you start really scrutinizing the edges, focus and color casting, you will start to see how settings effect the image.

D
 
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Thanks this is the gist of my question. So is there any reason why you wouldn’t always use the widest aperture possible when the subject is far enough away that the focus is set to infinity?

Read the other replies. This is all well explained.



After doing some tests it seems the photos taken at f/5.6 are considerably sharper than photos taken at f/1.7...."

Yep. Though your profound results aren't typical (results are usually more subtle), they're not unexpected. If you're seeing VERY profound focus differences, I would suspect possible lens problems.



"...even though the focus was not changed and set to infinity. This would seem contradictory to the basic principal that a faster shutter should produce sharper images in the in focus regions of the photo."

You're mixing up functions. A faster shutter produces sharper focus for a MOVING subject (called "motion blur"). All other settings being equal, shutter speed matters very little on a static subject.




Is this because the lens is just less sharp at f/1.7 due to the construction of the lens..."

Yes and no. You should experience subtle changes between F/8 and F/1.7. If you're seeing PROFOUND focus difference, you may have a bad lens.




or is infinity focus just generally less sharp at wider apertures?

It really depends on the lens. While fully-open apertures generally cause some outer-edge issues, you shouldn't see overall profound changes in sharpness. You might have something wrong with your lens.

D
 
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I gotcha. More accurate would be to use the term "focal length" as opposed to "depth of field."

"Focal length" is just as it suggests. The more wide open the aperture, the longer or "deeper" the focal length.





The focal LENGTH changes, but it's position doesn't. I'll use some arbitrary numbers to illustrate my point.

Say f/1.7 nets a wide focal length of 10 feet. Anything in that 10 foot range will be in focus. Close the aperture to f/8, and you narrow the focal length to say 2 feet. Less focal length gives you more "depth of field." Keep in mind that focal length can be much wider - say 50'. This leaves you with the illusion that EVERYTHING is in focus. Rest assured, if you hold your finger 5 inches from the lens, it will be out of focus.

If your focus is set to ∞, your 10-foot focal length STARTS maybe 50' away, and continues forever. Change your focal point to 10' away, now all subjects from 10' to 20' will be in focus. Change your aperture to f/8, now all subjects 10' to 12' will be in focus. And so on.

So...your focus doesn't really effect your focal *length* for anything => ∞. Same goes for any focus < ∞. The difference is that < ∞ the focal length will always be 10' wide. Greater than infinity and the focal length extends out past the normal 10' width. Make sense? Again, these numbers are completely arbitrary.

Note: Technically I know there's nothing greater than infinity, but I think you know what I'm saying.




Your aperture CAN have an effect on the image. Depending on the lens type and the quality, the photographer will experience different artifacts and aberrations at different aperture settings. This may manifest itself as softness around the edges or some color casting around the edges. In most situations, most photographers agree that for most lenses the aperture's "sweet spot" with the fewest imperfections is usually around f/5.

The best thing to do is take several photos of the same subject from a static position using different settings so that you can compare them. You may find your aperture "sweet spot" closer to f/4 or f/6. Once you start really scrutinizing the edges, focus and color casting, you will start to see how settings effect the image.

D
 
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"Focal Length" is not what you are suggesting. Rather, it is a basic defining characteristic of any lens. Please look it up.
 
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"Focal Length" is not what you are suggesting. Rather, it is a basic defining characteristic of any lens. Please look it up.

You are correct. I meant to write "focus DEPTH" as a tool for illustrating my point. I have gone back and corrected my syntax. Thanx for the heads up.

D
 
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One quick example that illustrates the relationship between aperture size and depth of field is the simple pinhole camera. Very little light coming in means they have long exposure times, but the depth of field, or focus depth ( a great term, Donnie Frank) is to infinity and most closer objects are in pretty good focus too.
 

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