Struggling mightily with focal length and effect it has on dof

Can you explain to me how it is that somehow when you switch the TV on an image appears ?

How does it work ?

I don't have a clue, all I know is that it does, but that does not stop me from watching TV.

If you think about it, most things are like that.

chutestrate said:

FrancoD said:

This is how Adobe explains the two :

"The f-stop explained.

F-stop is the term used to denote aperture measurements on your camera. The aperture controls the amount of light that enters the camera lens and it’s measured in f-stops. Along with shutter speed and ISO (sensitivity to light), aperture is the third fundamental component that makes up the exposure triangle in photography.

Not only does your f-stop setting or f-number, help you to get a proper exposure, it also helps establish the look and feel of your photo by determining the depth of field. “Unless you’re working with a whole lot of light or in very low light, your f-stop is usually more about style and how you want the photo to look than about necessity,” says photographer Nicole Morrison.

All things aperture.

The “f” in f-stop stands for thefocal length of the lens. While focal length itself refers to the field of view of a lens, f-stop is about how much light you allow to hit the sensor via the aperture opening. The aperture is the hole in the middle of the lens, made up of rotating blades that open to let in light when you press the shutter release. The diameter of the aperture determines how much light gets through and thus how bright your exposure will be."

Keep in mind that some here will disagree because they know better...

Note that your camera probably has an A (for aperture) setting but that of course is to set the f stop on the lens you are using.

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I ran across that explanation as well, and I just thought that Adobe explained aperture in a very poor way. Guess I was wrong, but I still see fstop as a way to measure aperture. You can't see the hole to say, " yeah that looks about right".

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Exactly right. The problem is that the term 'aperture' is used to name multiple things, as described in this Wikipedia article. F-stop describes the important aspect of the aperture: the intensity of the light reaching the sensor or film.

chutestrate said:

Ok, I finally have a specific question and if it's been covered already I apologize. Aperture/fstop of 2.8 on 2 different lenses is still 2.8 right?

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Correct

chutestrate said:

Will it let in the same amount of light in regardless of the focal length.

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No, with a longer lens, more light will be let in, but that light will decrease in intensity as it travels to the sensor, with the square of the distance. The intensity of the light reaching the sensor will be the same in each case. This is the reason why we use f-stop, the ratio of aperture to focal length, instead of the physical size of the aperture. f2.8 on a 50mm lens gives you the same light intensity as f2.8 on a 200mm lens, even though the aperture of the latter is four times greater.

chutestrate said:

And if I am wandering away from my original question I'll stop. I'm trying to assemble this big jigsaw puzzle that all the responses have given me.

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Good point

chutestrate said:

Michael Fryd said:

chutestrate said:

Michael Fryd said:

chutestrate said:

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How is this remotely possible? It's f/2 on both lenses. The hole would be the same size.

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No. "f/2" does not tell you how big the hole is. It tells you how the size of the hole relates to the focal length.

This seems to be a source of your confusion. "F/2" is not a particular size of aperture. "F/2" will, in fact, be different sizes of aperture on lenses of different focal lengths.

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Consider a situation where you are told that two pots are both half full of water. This doesn't tell you how much water is in each pot, nor does it tell you that both pots have the same amount of water (unless the pots happen to be the same size).

On pot might have two cups of water, and the other 4 cups, yet both pots are half full.

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fPhotographers typically talk in f/stops because that was useful when shooting film. Film shooting is generally an "exposure centric" workflow. Step one, is to determine a target exposure, and load matching film into the camera. Everything from that point on is about hitting that target exposure.

"Exposure" is the light per unit area reaching the sensor while the shutter is open. This depends both on how big the hole is, and how far it is from the sensor. Roughly speaking, the "focal length" of a lens is how far away it is from the sensor when it is focused on things that re very far away.

If you move the hole further from the film, the light spreads out to a wider area. If you move it twice as far, you need a hole twice as big. Thus if you have the right exposure with a 50mm lens and a 25mm aperture (a ratio of 50/25 or "f/2") you will also have the right exposure with a 100mm lens and a 50mm aperture (a ratio of 100/50 or "f/2").

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Digital does not require an exposure centric workflow. You can set your camera to Auto-ISO, and it will adapt to whatever thee exposure happens to be. With film, if you change the f/stop, you need to change the shutter speed in order to maintain that same exposure. With digital, you can change the f/stop and not change the shutter.

If photography had started with digital rather than film, it's quite possible that lenses would be labeled with aperture diameter, not f/stop.

chutestrate said:

Michael Fryd said:

chutestrate said:

I'm sure you're right, but then I'm back to where I began of random guessing and being disappointed in my results. Oh well. Think i'll hang up the camera for a while and try to take some lessons. Personally the guessing is not teaching me anything because I can't track what I'm doing and learn from it.

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If you want to play with guessing use a depth of field calculator. Estimate the focal length you would use, and the subject distance. Then try various f/stops and see what depth of field the calculators predict.

If you want to track what you are doing, just look at your results. Your camera embeds into the image the f/stop and focal length used. Some cameras even embed focus distance. You can look at the results in the image, and the settings used, to see if you like the results.

If you are getting shallower depth of field than you want, try stopping down a little bit (compensate by turning up the ISO and/or a longer shutter speed).

It turns out that photography is not completely trivial. Once you start taking more control over noise, depth of field, motion blur, color balance, tone curves, etc., you will find that you need to pay attention to how you balance the various factors.

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I can do this in the camera while I'm on a location?

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I am a very clinical person, to my detriment. At times I have used a laser tape measure to get a fairly accurate distance measurement to my subject. Then using a website or app, plug in the available info and get DoF. It's very rare that I am this pedantic.

There are several rules of thumb that I find useful in many situations. I describe them below in case you, or others, may find them useful too.

The way that most photographers calculate the depth of field for a particular situation is to use a depth of field calculator such as dofmaster.com. You feed in the type of camera, the focal length, the aperture (f-number) and the subject distance and it gives you the depth of field and the hyperfocal distance. However, it becomes very tedious if you have to recalculate the depth of field every time you change the aperture or zoom the lens or move the camera a bit closer or a bit further away. That is where these simple rules of thumb can be very helpful.

When taking a portrait, suppose you decide you want to use a longer focal length to get a different perspective. You move back to keep your subject the same size in the frame. In these circumstances Rule 1 tells you that the depth of field remains approximately the same.

Rule 1
If the subject remains the same size in the frame, then the depth of field remains approximately the same as you change the focal length and subject distance, but keep the f-number unchanged.

Rule 2
If you keep the focal length and the subject distance fixed and only change the aperture (f-number), then the depth of field is approximately proportional to the f-number.

For example, in going from f/4 to f/8, the depth of field approximately doubles.

Rule 3
If you keep the focal length and the aperture (f-number) fixed and only change the subject distance, then the depth of field changes approximately in proportion to the square of the distance.

For example, for a subject 2 metres from the camera there will be approximately 4 times as much depth of field as for a subject 1 metre from the camera.

Limitations
These rules work for any type of camera, including phone cameras. In other words, they work for any size of sensor, but you must keep to one sensor size. The rules do not work when you switch from one sensor size to another.

In most circumstances these rules work very well. However, they break down if the the subject distance gets close to the hyperfocal distance or greater (when the depth of field is infinite). In practice, the rules work best for subject distances that are less than one third of the hyperfocal distance.

They may also break down if the subject is very close to the camera (for macro photography).

OOOOOOH now that makes sense to me. It really confused me trying to think of the concept as just a formula. Ok, thanks. 2 dots are actually connecting.

Very good explanation. Thank you very much

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?

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I am not going to speak much to Distance other than to say Fstops and apertures behave quite different when we are using Far Longer Focal Lengths then what some might expect.

I would say it's Far easier to Blow out the Background around one's subject using Fast Glass as opposed the opposite by comparison. Now how much the quality of Glass plays a Role, I really can't say just for testing. However I do test a ton, sometimes with my Kit Lens and sometimes with my Sony FE 90mm f/2.8 Macro G OSS Lens. That's always on my FF camera.

However on my cropped Camera, I love to play around with shot concepts with my 55 210mm since it is cheap and light. Good enough for testing and snapshots. For instance this Morning. I couldn't have gotten much closer, so it's not just about getting close to one subjects.

It's clearly also about having the right settings.

However as I was testing F stops, I was eventually at F20, then a Hummingbird Flew into the scene. I was at 1/30 and F20. So the resulting shot wasn't not going to be good of and unexpected fast moving subject. Any movement, only worsen it.

But it wasn't about the bird, but the shot before. Which was at F6.3, then I switched. Which why I was shooting at F20, not just to show you, but myself as well. The testing and trying out things really never ends. Straight out Camera Jpeg, no Edit.

I actually meant to post this shot with that of the one with the Hummingbird since both were both at F20. The Distance from Flower/Bird, maybe 10 ft.

This is why I am always thinking in terms of FF these days. More ability to push boundaries when it come to certain shots. It wouldn't have matter as much with the bird since I was at such a low F stop and didn't really expect it.

However as poster Franco has mentioned about getting closer to the subject. That's why I planted a new Flower Bed this year. That can clearly bring your subjects closer. Same techniques for other Birds, Bugs, Butterflies or whatever. I don't do Feeders or the like.



Unedited Jpeg, straight out of Camera.

I'm having trouble with this explanation. I should not have thought about it.

No, with a longer lens, more light will be let in, but that light will decrease in intensity as it travels to the sensor, with the square of the distance

If the more light is admitted to the sensor, what is meant by decreasing intensity? In my mind more light = more intensity

The intensity of the light reaching the sensor will be the same in each case.

This is just my reading comprehension, but I'm not following the explanation. This seems to negate the statement above.