Struggling mightily with focal length and effect it has on dof

[Michael Fryd]

Ok, I'm sure this is a dumb question...why would the bird be slightly out of focus with a smaller aperture setting. And yes I was wanting the whole image to be in focus. Wouldn't f8 be enough to bring the whole thing into focus? Obviously not, but this is where I begin to think underwater basket weaving with pasta would be easier to understand.

The depth of field you get is dependent on many factors, not just f/stop.

In some situations, f/8 will give generous depth of field, and in others f/8 will yield shallow depth of field.

Consider a 215mm lens on an EOS R7 (that's a 1.6X crop body, so you get the same angle of view as 346mm on a full frame). Perhaps you are shooting a bird sitting on a branch that's 80 feet away.

According to this Depth of Field calculator, your depth of field ranges from about 74.4' to 86.5'. That's about a 12' deep depth of field. Don't expect anything more than 6.5' behind the subject to be in focus.

But that's assuming that the camera has focused on the bird, and not the background. If the background is 10' past the bird (90' from you), and the camera focuses on the background, we would expect the leaves in the background to be sharp, and the bird to be a little bit out of focus. To me, it looks like that's what has happened in your image.

Are you using manual focus or auto-focus? If auto-focus, are you selecting the focus point, or are you allowing the camera to select it? If you are allowing the camera to select it, what has it chosen to focus on?

 

[chrisfisheye]

I don't think that is consistently true either. And doesn't it depend on how you double the subject size?

For instance, if I start with a 6m subject distance and a 60mm focal length, and I change the focal length to 30mm, doesn't that double the subject size? (FOV changes from 2.4m x 3.6m to 4.8m x 7.2m). But the DOF changes from 2.47m to 26.1m, which is an increase of 10.57 times, not the 4 times you suggested.

I could have said that it is an approximation, I agree.

That said, when we read your post, it gives the feeling that this is only a rough approximation..

It depends of course but in practice, even if this is a simplification, there are 2 use cases:

1. the approximation formula applies when the dof is small compared to focus distance
2. hyperfocal

In wikipedia, the formula for case 1 is given here!

Loading…

en.wikipedia.org

Very strangely, they do not even mention when the approximation applies !! More precisely this is when hyperfocal is much larger than focus distance but then this means that dof is much smaller than twice the focus distance.

In practice, we apply case 1 or case 2 very naturally. In your example , 30mm FF lens with a subject at 6 meters, we usually do not really care about dof unless we want focus until infinity in which case we apply hyperfocal.

On the opposite, if we shoot a portrait we do not really care about hyperfocal...

I am not saying it works for all use cases, it is easy to find counter examples, but honestly, in practice, we generally have 2 distinct use cases.

In fact, I find the approximation very accurate for my use cases. The biggest change is the dof behind / in front of the subject, this is in fact where there is a major difference..

 

[FingerPainter]

Hello,

Instead of taking into account focal length and subject distance, it is better to only consider the subject size, or more precisely how large is the visible focus plane. This is called the field of view, but this terminology is a bit ambiguous

So for a given framing (and sensor size), the dof depends only on f#.

So if you keep the same framing, you can either shoot from a larger distance but with a longer focal length or get closer with wider focal length, the dof will be the same provided the f# is the same.

There will be a difference though with the background blur which will be more important with the longer focal. The background blur depends on aperture size, not f#.

The dof is highly dependaInt on subject size (field of view), it varies with the square of FOV. If you double the subject size, the dof is mulitplied by 4.

Also this simplifies how to estimate dof. For instance, you can consider that you can shoot close portraits at f/4 (for instance) with FF, this is all you need to remember.

I thought the f # was the aperture. I'm really not following what you're trying to explain. sorry.

"Aperture" is an ambiguous term. when it comes to numbers and measurement. Some people use it to mean aperture diameter, and others use it to refer to the f-number. I find it avoids confusion if one doesn't refer to "aperture" alone as a measurable attribute, but instead to "aperture diameter" or "f-number".

When chrisfisheye refers to "aperture size" he apparently means aperture diameter. He is correct that the size of blur circle for objects at infinity will be proportional to aperture diameter, while DOF depends on f-number. So (as long as the subject distance is long enough) a 200mm lens at f/4 will give the same DOF as a 50mm lens at f/4 and 1/4 the subject distance but the blur circles for objects at infinity on the image from the 200mm lens will have four times the diameter of the blur citcles for those objects on the 50mm lens' image.

 

[Mark S Abeln]

There’s no need to include the camera settings in a different image. The image’s Exif metadata—originally embedded by the camera—is preserved in the image and can be viewed.

The focus appears to be behind the bird, which is a tiny portion of the image and so it’s plausible that the camera’s focus mechanism missed the bird altogether.


--

The Refracted Light

A website about the Art and Science of Photography

therefractedlight.blogspot.com

 

[chutestrate]

Wow not sure how you are spotting that, but maybe. I think I can remember the eye being tracked, buuut that was a few weeks ago. You guys are good.

 

[chutestrate]

Thank you, but that went right over my head.

 

[NikonNature]

info in next image.

Just messing around with a DOF calculator, and guessing the subject is 50 feet away, I see that f/8 gives you about 5 feet of DOF. If you go to f/16 you'd get about 10 feet, so still not enough to get everything sharp.

By comparison, if you zoomed out to 100mm and stayed at f/8, you'd have about 25 feet of DOF. That might be enough, but of course the subject would be much smaller.

As a side observation, most bird and wildlife photographers use even longer focal lengths. Rather than trying to keep the whole scene in focus, they do the opposite. They focus on the subject, and use a wider aperture (e.g.: f/4). As a result, the background has a pleasing blur to it, and the subject really stands out. This may or may not be what you want.

--
Jeff

Jeff Loomis

Explore Jeff Loomis’s 1,058 photos on Flickr!

www.flickr.com

 

[lehill]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

Most of us just use an app on our phones (or on websites) to calculate DOF.

The reason we use apps is the interaction between distance, f-number, and focal length is not obvious when it comes to depth of field. The actual equation* looks like this:


N = f-number, f = focal length, C = circle of confusion, D = distance to focused subject

The complexity comes from transforming "What look fuzzy on my camera sensor" (circle of confusion in image space) into distances in the real world (object space). That darn lens makes everything complicated.

A pinhole lens has a very large DOF but they're not really sharp.

*The entire derivation is available here in PDF format. The document also includes special cases ("same image size" case, "hyperfocal distance" case, macro case). You may notice the whole thing is based on the thin lens equation so it's an approximation from the very start. [Credit: Frédo Durand, Bill Freeman, MIT - EECS]

--
Lance H

 

[chrisfisheye]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

Most of us just use an app on our phones (or on websites) to calculate DOF.

I think that most of us do not use dof calculators to take pictures.

Maybe we have one installed, which we have used 2 or 3 times and that's it.

This is a guess but I almost never see somebody using one !

 

[Michael Fryd]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

Most of us just use an app on our phones (or on websites) to calculate DOF.

I think that most of us do not use dof calculators to take pictures.

Maybe we have one installed, which we have used 2 or 3 times and that's it.

This is a guess but I almost never see somebody using one !

An online Depth of Field calculator can be a useful tool for getting a feel as to what Depth of Field to expect for various shooting conditions.

For instance, if one often shoots birds at a distance of 50 feet, one could run a few scenarios to see what to expect with various f/stops on their camera with their lens.

I would expect that once a photographer gets a feel for a particular situation, he doesn't need to constantly refer to a Depth of Field calculator.

 

[Mark S Abeln]

Wow not sure how you are spotting that, but maybe. I think I can remember the eye being tracked, buuut that was a few weeks ago.

I zoomed way into your photo.

The bird's eye is probably way too tiny for good focus. Even the bird itself is fairly small and might not be detected.

You guys are good.

True.

--
http://therefractedlight.blogspot.com

 

[FrancoD]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

Most of us just use an app on our phones (or on websites) to calculate DOF.

I think that most of us do not use dof calculators to take pictures.

Maybe we have one installed, which we have used 2 or 3 times and that's it.

This is a guess but I almost never see somebody using one !

I have never seen any photographer using a DOF calculator when taking photos ( and I don't mean as they press the shutter button).

here comes the bride ... hang on I am looking at the DOF app!

but it would be usefull for the OP to play around with one to translate the hints given here.

BTW, I think that the first reply, by Steve, was more that good enough for the OP to work with.

Beginners don't need to be confused with too many details .

 

[Michael Fryd]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

Most of us just use an app on our phones (or on websites) to calculate DOF.

The reason we use apps is the interaction between distance, f-number, and focal length is not obvious when it comes to depth of field. The actual equation* looks like this:


N = f-number, f = focal length, C = circle of confusion, D = distance to focused subject

The complexity comes from transforming "What look fuzzy on my camera sensor" (circle of confusion in image space) into distances in the real world (object space). That darn lens makes everything complicated.

A pinhole lens has a very large DOF but they're not really sharp.

*The entire derivation is available here in PDF format. The document also includes special cases ("same image size" case, "hyperfocal distance" case, macro case). You may notice the whole thing is based on the thin lens equation so it's an approximation from the very start. [Credit: Frédo Durand, Bill Freeman, MIT - EECS]

The problems with these formula is that they tend to obfuscate the underlying concepts. They tend to be based on intermediate values that vary with the situation.

For instance, a critical concept is the Circle of Confusion (CoC). Roughly speaking, this is the size of the blur. If the CoC is small, the eye doesn't see it, and we think that it's in focus. Conceptually, what we care about is the size of the CoC in the final print. Larger prints tend to be viewed from a greater distance, and therefore we can tolerate a larger CoC in the larger print. It turns out that what we really care about is the size of the CoC compared to the size of the image. It turns out a commonly used criteria for CoC is 1/1500 of the diagonal of the image. If the CoC is greater than that we see blurriness. This diagonal/1500 criteria is true independent of sensor size, focal length, etc. However, most formula's hide this by giving the formula in terms of the CoC size on the sensor. This confuses the issue as one needs to use different sizes of CoC depending on the sensor size. That makes it appear that the sensor size plays a big role in Depth of Field.



It turns out that the aperture diameter is important, but they hide that. Instead of using aperture diameter, they hide things by using both f/stop and focal length.



It tuns out that once you understand the concepts behind depth of field, you can calculate it without knowing either the sensor size or f/stop. The only camera parameters you need to know are angle of view and aperture diameter. At the same subject distance, aperture diameter, and angle of view, you get the same Depth of Field. You don't need to know the particulars of focal length or sensor size.

 

[chutestrate]

Thats the point I would love to get too.

 

[Mark S Abeln]

Thank you, but that went right over my head.

https://en.wikipedia.org/wiki/Aperture

https://en.wikipedia.org/wiki/F-number

Depth of field - Wikipedia

en.wikipedia.org

 

[BackToNature1]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

What is good method of trying to walk through a progressive set of steps to see the effects that distance and fstop, and focal length have on dof? Or is there such an exercise?

Is it possible that as the fstop is decreased (smaller hole) that the clear focus point shrinks?

I've tried a couple of simulations, when I see it say ok I got it, practice it, and the results just seem to be different.

My goal is to eventually able to say I want everything sharp in my picture and choose the right fstop and/or distance. Or go the opposite and blur my picture deliberately.

For instance with the Kit Lens from Sony.

Sony E 55-210mm f/4.5-6.3 OSS Lens

Minimum Focus Distance

3.28' / 1 m

Which is pretty much Poor Min Focusing distance for any Lens. So I can't get really close up to anything. I don't know if your 18 150mm Canon lens is similar. Anything within that distance, I literally can't get focus on. So when you are outside the issue of Min Focus distance, with a Stellar AF system like which the R7 has, I would like to see some of your blurred images issues. Samples.

However, even at F stops like F16 and similar, one can still isolate the subject. For example. 3 Snapshots.......

 

[Mark S Abeln]

I think it is fair to say that this is a focus problem and not a depth of field problem.

Instead of assuming that the camera does animal focus well, it might be better placing a focus point on the bird.

 

[Martin_99]

I have been taking pictures and practicing. I just cannot make the correlation of focal length or distance to dof. I'm using my kit lens 18-150 with an canon r7. I have been trying various fstops, and In my pictures i am getting blurred images around my center of focus. I'm expecting everything to be in focus by using fstops around 11 and up. Is it only fstop that can control the blurry edges?

How do i determine if my blurred images are blurred because of distance or fstop?

Most of us just use an app on our phones (or on websites) to calculate DOF.

I think that most of us do not use dof calculators to take pictures.

Maybe we have one installed, which we have used 2 or 3 times and that's it.

This is a guess but I almost never see somebody using one !

Of course not, you will guess, or make several photos at various apertures.

Specially on wildlife shooting, selection is pretty limited. OP have a lens with maximum aperture f8 at long end. Due to diffraction and available light is only other reasonable option f11, which make pretty small difference anyway. So in reality you will shoot f8 all the time to get better ISO or shutter speed.

 

[Bill Ferris]

info in next image.

Here's a crop from the online Depth of Field Calculator app (https://www.dofmaster.com/dofjs.html)



As you can see, the depth of field in a photo made with an APS-C camera at 215mm, f/8 with focus set 75 feet from the camera is about 11 feet centered on the focus distance. If the bird was nearer (50 feet), DOF would be half. If focus was more distant (100 feet) DOF would be about double.

In no situation would a single exposure with your camera at the settings you chose produce a version of this image in complete focus at every distance.

In this situation, your best approach will be to keep the lens wide open and use a slightly slower shutter speed (1/500) to maximize exposure within the constraints of your objectives for the finished photo. Make an exposure with focus on the bird. Then, without changing the composition, make an exposure with focus on the nearest large fallen tree in the water.

Blend the two photos in post with the top 40% of the image being the first exposure and the bottom 60% being the second exposure.

Good luck.

--
Bill Ferris Photography
Flagstaff, AZ

http://www.billferris.photoshelter.com

 

[chutestrate]

Bill,



Thank you very much for the advice. I'll try to put it into practice. As far as bleending in post...maybe in my next life. I'm struggling with the basics

info in next image.

Here's a crop from the online Depth of Field Calculator app (https://www.dofmaster.com/dofjs.html)



As you can see, the depth of field in a photo made with an APS-C camera at 215mm, f/8 with focus set 75 feet from the camera is about 11 feet centered on the focus distance. If the bird was nearer (50 feet), DOF would be half. If focus was more distant (100 feet) DOF would be about double.

In no situation would a single exposure with your camera at the settings you chose produce a version of this image in complete focus at every distance.

In this situation, your best approach will be to keep the lens wide open and use a slightly slower shutter speed (1/500) to maximize exposure within the constraints of your objectives for the finished photo. Make an exposure with focus on the bird. Then, without changing the composition, make an exposure with focus on the nearest large fallen tree in the water.

Blend the two photos in post with the top 40% of the image being the first exposure and the bottom 60% being the second exposure.

Good luck.