Struggling mightily with focal length and effect it has on dof

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.

Sounds like your blurred images are more due to camera shake than anything else. Try doing your dof testing using a camera on a tripod.

But in simple terms for a given camera

1. The longer the focal length the smaller the dof will be.

2. The larger the aperture (smaller the f-number) the smaller the dof will be.

3. The larger the camera to subject distance, the larger the dof will be.

To see the effect any one of those 3 parameters has on dof, keep 2 constant and vary the third.

chutestrate said:

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.

Click to expand...

This would be a good opportunity for you to post some pictures illustrating examples of your concern. Be sure that the pictures retain the exif information.

Dave

chutestrate said:

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.

Click to expand...

Firstly, I fully agree with what the other two responders said - that you have some noticeable motion blur while using your telephoto lens. Although there IS a correlation between f-stop and focal length, that may not be the problem you're having.

Again, as a first step, please post up some EXIF-intact examples.

atom14.

chutestrate said:

...I want everything sharp in my picture and choose the right fstop and/or distance.

Click to expand...

What you focus your camera on will be sharp (barring camera shake) and your depth of field will dictate what else is acceptably sharp to you.

Search depth of field calculators. It will then calculate depth of field for your camera, lens and fstop combinations.

From some of the things you've written, you may have some misconceptions. Some (sometimes simplified) explanations below.

chutestrate said:

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.

Click to expand...

Your camera has a plane of focus, not a single focal point. Think of it as all the points in the field of view that are at the same distance as the point on which you are focussing (not exactly true, because you sensor is not curved and because of lens design constraints, but it will do).

chutestrate said:

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?

Click to expand...

Sadly no. As you increase the f-stop number, diffraction plays an increasing role and blurs your image. At f11, f16, f22 you may begin to notice this.

chutestrate said:

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

Click to expand...

Apart from diffraction, there are other factors: camera movement during the exposure (use a tripod to eliminate, learn to improve your hand-holding technique), subject movement (e.g. foliage moving with the wind, people not standing still - shorter exposure times counter this), atmospheric haze, especially heat haze, which has a more noticeable effect at longer camera to subject distances.

chutestrate said:

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?

Click to expand...

As suggested by steve_z

chutestrate said:

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

Click to expand...

Yes, diffraction effects.

chutestrate said:

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.

Click to expand...

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 dependant 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.

chrisfisheye said:

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.

Click to expand...

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

chutestrate said:

chrisfisheye said:

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.

Click to expand...

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

Click to expand...

The f-stop is a ratio: focal length / size of aperture. The linked article explains it better than I can.

First off, I agree that some examples would help us answer in more specific detail. Don't worry if the quality isn't great. That's how we learn.

Next, as someone suggested, play around with a DoF calculator (free online tool like PhotoPills or DOFMaster). You will quickly see the affect that focus distance and aperture have.

Testing... place something in your backyard - stuffed animal, chair, trashcan - doesn't matter. Keep your shutter up around 1/250 so camera shake doesn't interfere. Set your lens to 18mm and f/8, then frame the shot so the 'subject' is on the right third, leaving room to see the background. Focus on the 'subject' and take the shot.

Next, move back and zoom in to, say, 100mm. Without changing any settings try to get the same framing and take another shot. Obviously, there will be less background, but try to keep the subject the same size. You can repeat this at f/4 and f/11 or f/16.

Keep in mind that the distance from the subject to the background also plays a major roll. If the subject is 30 feet from the camera and background is 40 feet, it will be easy to get them both in focus. However, if the background is 200' away it will be harder. Again, here's where some sample images would allow us to analyze the specific scenario.

chutestrate said:

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.

Click to expand...

Heres a long winded explanation of depth of field that may help you understand what's happening.

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A scientist might tell you that there is only one exact distance where things are in focus. Even a fraction of an inch before or after that distance, and you are out of focus. An engineer might point out that there are a range of distances where the focus is close enough, and to the unaided human eye, it looks to be sharp. This range is the depth of field.

Focus is not the only thing that affects image sharpness. Pretty much anything that affects sharpness also affects depth of field.

Often, when people talk about depth of field, they are referring to the range that would look sharp if focus was the only factor. I use the term "apparent depth of field" to include all the factors. It is not uncommon for the apparent depth of field to be shallower than the depth predicted by focus issues.

There are a number of online calculators that will show you how much depth of field to expect with various settings of your camera. A web search will give you many choices, https://www.dofmaster.com/dofjs.html Is a reasonable one.

Sensor size, focal length, aperture, and subject distance are going to be the key factors.

While sensor size is a constant for your camera, different cameras will have different sensor sizes. Therefore you can't assume that the results you get for your camera will match the results someone else gets for their camera.

For instance, on a full frame camera, f/2.8 might yield a shallow depth of field, and on a smartphone f/2.8 might give you a generous depth of field.

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f/stops are the "relative aperture". They are a mathematical formula for the aperture diameter. "F/4" means the aperture diameter is the focal length divided by 4. On a 100mm lens, f/4 is a 25mm aperture diameter. On a 50mm lens, f/4 is a 12.5 mm aperture diameter.

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On a practical basis, and seldom accounted for in online depth of field calculators, anything that reduces sharpness, will reduce the depth of field.

One factor is diffraction. This is a blur in the image caused by light scattering off the edges of the aperture. When the aperture is big, these scatterings are insignificant compared to the unscattered light. Thus, with large aperture diameters, we tend to ignore diffraction. However, as aperture diameters get smaller, the scattered light becomes significant compared to the unscattered light. At some point the scattered light becomes significant enough that nothing is sharp, and it appears that nothing is in focus. At this point I would say the apparent depth of field is zero.

As a general rule, apparent depth of field is shallow when the lens is wide open. it gets deeper as you stop down, until diffraction becomes significant. At that point, the apparent depth of field (what looks to be in focus) actually starts to get smaller again as you stop down.

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Let's bring this back to the OP's issue.

There are a number of factors that can reduce sharpness, and one or more of these can be affecting his apparent depth of field.

First of all are focus issues. As previously mentioned, online depth of field calculators will tell him what he should expect from this.

Motion blur, either of the camera or the subject can be a factor.

Diffraction blur from a small aperture can be a factor.

The camera focusing on the wrong subject can play a role.

There is also quality questions of the lens. If the lens is defective, or there is noticeable cr*p on the lens, then this can be an issue.

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Sorry, about being long winded, but hopefully there is something here helpful.

There are a bunch of things that can be happening.

Examine your existing bad photos closely, and don’t just look at the subject you are trying to photograph, but the entire image. Are there parts that actually are acceptably sharp? That would indicate misfocus. Look for any consistency of focus in the photos, like if the background is always sharp.

First thing to check is if autofocus is actually turned on. Looking through the viewfinder, see if the camera definitely changes focus between distant and close subjects. If not, find the autofocus switch or setting and turn it on. If it turns out that you are actually doing manual focus on purpose, then I suppose you need to refine your technique or just use autofocus.

Second thing to test is if autofocus is working correctly. Find a large, flat, fairly distant subject, like a brick wall maybe ten paces away. Be sure you are facing the wall head-on with the direction of the camera being perpendicular with the wall, put the camera in full auto mode, and try to take pictures at various focal lengths on your zoom. Examine each photo for sharpness.



Look up the minimum focus distance for your lens, which is likely different for each focal length. You won’t be able to focus on anything closer to that. There are items you can purchase (not necessarily expensive) that can let you focus closer. Some zoom lenses at minimum focus distance actually do have strange optical defects.



Check your focus area settings. There are a multitude of settings in modern cameras, but one to watch out for is single point focus: usually that point is in the center of the image, but it’s probably easy to move it off to the side, which is very likely to cause misfocus if you aren’t paying attention. If your subject is only a tiny part of the image, it’s possible that the autofocus doesn’t see it at all and is focusing beyond it. As they say, “fill the frame with your subject”.

chrisfisheye said:

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#.

Click to expand...

That is only an approximation and it breaks down badly at shorter distances and focal lengths.

I think you would agree that 10mm focal length and 1m subject distance gives the same framing as 20mm and 2m, 30mm and 3m, 40mm and 4 m, 50mm and 5m, 60mm and 6m,... etc.

But the DOF's for those combinations on a FF sensor are, respectively, infinite, 3.68m, 2.82m, 2.61m, 2.52m and 2.47m, I believe the progression is asymptotic to 2.4m.

chrisfisheye said:

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.

Click to expand...

Only if the distances and focal lengths are long enough.

chrisfisheye said:

There will be a difference though with the background blur which will be more important

Click to expand...

"Will be more imortant"?

chrisfisheye said:

with the longer focal. The background blur depends on aperture size, not f#.

Click to expand...

Yes.

chrisfisheye said:

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

Click to expand...

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.

If I start with a 6m subject distance and a 60mm focal length, and I change the subject distance to 12 mm, doesn't that also double the subject size? (FOV goes from 2.4m x 3.6m to 4.8m x 7.2m). In this case DOF changes from 2.47m to 11.3m, which is still more than the four times you claim, but a lot closer to four times.

chrisfisheye said:

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.

Click to expand...

Except that the idea of a constant DOF for a constant f-number and FOV breaks down at the subject distances at which I'd use f/4 for a portrait. And for a head shot from 3m distance, I'd probably not have enough DOF to keep everything from the eye back to the ear lobe adequately sharp at f/16, let alone f/4.

chutestrate said:

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.

Click to expand...

You need to realise, what parameters or aspects have influence on DoF. I will make a list from practical point of view, so hopefully optical experts will not make angry responces.

- aperture (faster f-stop = thinner DoF)

- focal length (longer = thinner DoF)

- focus distance (closer = thinner DoF)

So for blurry portrait shots you will probably choose 50 or 85mm lens with fast aperture at least f2.8. Focus is clear here - on eyes .

For landscapes with deep Dof you usually choose wider focal lengths (but nice are also compressed distant sceneries with longer FL) and smaller apertures. On your apsc are most common f8 or f11. If your landscape photo have interesting objects in both foreground and background, the best is usually to focus in first third of the scene to get the most of the photo in focus.

Thank you everyone. I'm trying to upload an image as an example. I was shown how to upload a jpeg with camera data but I can't remember how to do it. Can someone refresh my memory?

info in next image.

figured it out

chutestrate said:

info in next image.

Click to expand...

You have no description here, but I assume, that you want to have the whole photo in focus. This will not really happen, but you can get closer, by focusing on some of the branches in foreground with eg.f11. Bird will be less in focus, but the whole image will look sharper/in focus.

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.

chutestrate said:

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.

Click to expand...

At 214mm (350mm FF equivalent) is DoF simply too thin. Effect of change from f8 to f11 is relatively small in comparison. So moving the focus point from the bird to closer branch will make the bird out of focus. But this may not be apparent at first glance in the photo. So depends on your targets.

I would personally try to wait for bird to get on closer branch, focus on him and get nicely blurred background with fastest available aperture.