I recently wrote a blog article about choosing the right f-stop. I feel like I’ve got a decent grasp on the subject now, but I’d any feedback or corrections anyone might have.
The gist of the article is to consider using a larger f-number, especially for landscape photography. This helps avoid things being out of focus. It sounds simple, but it’s a mistake I make much more often than I’d like! I have a bad habit of going for wide apertures when it doesn’t suit the situation. Am I the only one with this problem?
With respect I find your use of the terminology large and small confusing.
“for portrait or macro photography, you’d want to choose a smaller f-stop so that background and foreground details don’t distract from your subject.”
When I talk about f-stops I refer to the size of the opening so a large f-stop to me would be a large opening such as f-2 and conversely a small f-stop would refer to a small opening such as f16.
You clearly understand what you are talking about. I wonder if everyone else agrees with you that f16 is a large f-stop or would agree with my take that it is a small f-stop. I know it is just semantics but it could be a source of confusion. I am happy to be corrected by everyone and told I am wrong if I am wrong.
But here is a sample image found through a google search
The blog post makes no mention of sensor size (the images I checked were full frame, shot on a Sony, but I had to extract that info using exiftool). Crop sensors have comparatively larger DoF, while medium format has less.
Circle of confusion in mm can be used for calculations, but is pretty hard to interpret for most people. A fraction of the diagonal (eg 0.1%) is usually a better unit to work with in calculators.
Bokeh quality/type/kind matters a lot for results. A lens that has onion ring bokeh has narrower “perceptual” DoF than one with “smooth” circles of confusion.
A lot of modern cameras have a preview mode with aperture set, you can check what the image looks like before exposing.
I will echo @Terry and say that, in my experience, photographers seem to use “f-stop” as a stand in for aperature. So a large f-stop equates to a large aperature. If you changed the wording in that rules of thumb section to “large f-number” it would still make sense to newer people (and avoid the wrath of older hands).
I might go into more detail regarding the “focus 1/3 of the way into the scene”. Since that assumes you are focused at the hyperfocal distance, and maximizes the amount of foreground that is in “acceptable focus”. Not all images will have the focal plane at the hyperfocal (even though grand landscape usually will). For example, portraits usually focus on the eyes and use DoF to get the lips and maybe ears into focus.
I am glad you did not make the mistake of saying “sensor size” matters to DoF.
The f-number and depth of field are proportional to each other, so as the f-number goes up, the depth of field gets wider.
The first half of that sentence is not accurate. As one rises, so does the other, but they are not proportional.
The next time I try photographing landscapes with wildlife, I’ll start with a larger f-stop, something closer to f/8 or higher.
“Larger f-stop” is ambiguous and confusing. Instead, I suggest “larger f-number” or “smaller aperture”.
In your calculator at Depth of Field Calculator | Randy Reflects , changing the f-number doesn’t change the four calculated numbers. To see the effect of changing the f-number, we also need to change something else (eg the focal length) and change it back.
A common rule of thumb is to focus 1/3 of the way into your scene.
That is a poor rule of thumb. A better rule, if your lens has a manual focusing ring, is to focus on the nearest point, twist the ring to focus on the far point, then move the ring to the centre of these two settings.
Depth of field changes linearly with f-number and circle of confusion, but changes in proportion to the square of the distance to the subject and inversely in proportion to the square of the focal length.
Wow thank you all for your comments! This stuff is gold for my learning. It looks like I’ve got some more research and editing to do. I’ll focus on tightening up the language around f-stops, f-numbers, and apertures to improve the article’s accuracy and clarity. I truly do appreciate everyone’s feedback; if you’ve got any more, leave me a comment!
Hmm, I definitely see what you mean. This is one thing that was a little confusing to me at first, that the number gets bigger as the aperture gets smaller. I believe I understand it fairly well, now, but It seems I’m using the terminology in a way that most photographers don’t, so thank you for pointing that out.
I wasn’t aware that this was the case re: sensor size. As for the image metadata, I wasn’t aware that it survived the image optimization applied during the website build. That’s good to know!
I actually debated whether or not to use the term at all, since I don’t really understand the term bokeh all that well, but I needed a sentence to end my paragraph, so… At any rate, thanks for the info—I’ll look more into it. It seems interesting.
Thank you for the confirmation. I will make some updates to the article. I would like to clarify one thing about the terms f-stop and f-number. My Wikipedia readings suggested they refer to the same thing, but I’ve never heard people use f-number in videos and articles—they always use f-stop. Are they really synonymous?
If I understand your suggestion about the wording, photographers don’t use “a large f-number” to mean the same thing as “a large f-stop;” the former more clearly refers to the number itself, while the latter typically refers to the aperture. @snibgo’s comment below seems to echo this understanding.
If that’s the case, then I’m thinking I’ll edit the article to use the term “f-number” exclusively and use the term “aperture” when referring to aperture size. I’ll need to do some more research and choose my wording carefully.
In my research, I actually hadn’t come across anyone saying that. It wasn’t mentioned in the mathematical explanations that I read in The Manual of Photography (2011, Google books link). Would sensor size’s relevance (or irrelevance) be worth mentioning in an update to the article? I don’t know if it’s a common idea. As with other articles I’ve published on photography, I’m typically writing for a beginner audience, since that’s more or less my level of skill and understanding. With this audience in mind, clarity and simplicity are usually the most important elements.
Thanks for finding a bug! I’m not a programmer, so things like this break when I try to write them, but I can confirm the bug and I’ll fix it as best I can.
I think I had landscape photography in mind when I wrote that, but I may not have made that clear. Can I ask what you mean by the far point? In landscape photography, the far point would be anything past the hyperfocal distance, if I understand correctly. I will do some more research and update the article accordingly.
Sensor size comes up a lot on forum and youtube discussions of DoF. Especially those whose information is simplified for newer photographers.
It stems from the fact that to maintain a certain composition, a crop sensor camera will have to move further away (relative to FF with the same lens). As we should expect, given the equations used to describe the relevant physics, this changes the DoF because the distance to the focal plane changes.
Edit: For grand landscape this is often of little consequence. Short focal lengths (wide angle), large distances, and sticking to the hyperfocal (often) renders the differences in FF and crop sensors meaningless in terms of DoF. It matters a lot more as the scene becomes more “intimate”.
I would, OTOH, dispense with the term “f-number” altogether. I don’t think it is commonly used by photographers (and I’ve been into all this since the late-70s). “Aperture” is always the proper term. And “f-stop” is usually just called “a stop”. It is variable in nature, e.g., f/2.8 to f/5.6 is one stop, f/5.6 to f/11 is one stop, and f/11 to f/22 is one stop. One stop is specifically a halving or doubling of the amount of light that is let in.
Stops also apply to shutter speed. At a given aperture, the change from 1/250 sec to 1/500 sec is one stop, as is the change from 1/125 sec to 1/250 sec, etc.
I was being pedantic about proportionality. The common approximation for Depth of Field D ≈ 2NCU^2 / f^2 is only an approximation, so D is only approximately proportional to N. If D was actually proportional to N, then we couldn’t get a Depth of Field from, say, 3 metres to infinity at f/16.
Can I ask what you mean by the far point?
Suppose two people are in front of me, at different distances, and I want them equally out of focus. I adjust the focusing ring until one person is in focus. Then I turn the ring until the other person is in focus. Finally, I turn the ring back so it is halfway between these two settings.
Um, yes, one stop means halving or doubling the light. So f/2.8 to f/5.6 is two stops.
My mistake. You are correct. It is not multiplying or dividing by 2, but by the square root of 2 (approximately 1.41). Thanks
And this leads to why the “f-numbers” are what they are. If there was an f/1 lens, half the light would be at f/1.4, then f/2, f2.8, f/4, f/5.6, and so forth.
I agree with this, and I would go one further to avoid the words “large” and “small”. I would just talk about wide or narrow apertures and not muddy the waters. Also, because of sensor size equivalence, the DoF at specific f-numbers will not be the same from the same position, so it might not be useful to talk about “F/2.8”, for example, for a specific depth of field.
Thanks again, everyone, for your feedback. I’ve updated the article with the following changes:
Clearly indicate that my use of the term “f-number” in the article refers to the number itself, rather than to the aperture
Avoid the terms “large” and “small,” favoring high/low for “f-number” and wide/narrow for “aperture”
Add more detail to the ⅓ rule by indicating the general conditions under which it applies
I’ve also fixed the bug in the depth of field calculator, so changes in all form fields update in real time. If I had more time, I’d probably attempt a full rewrite to further improve the article, but I think the changes based on your suggestions have already made it a lot more coherent. Thank you!
I try to get my writing more polished before I share it here, but I’ve switched careers recently and have had less time for writing than before. I’ll make sure future articles do a better job than this one!
I wish cameras had a feature where I pick two points (touchscreen, joystick) and it would calculate the widest aperture where both would be “acceptably sharp” (for a threshold I set in the menu), and focus accordingly.
That’s one of those solutions that makes you wonder why it’s not become a standard feature in every camera. I vaguely remember my old EOS Rebel T3i had something similar.
My Fuji has something called the Distance Indicator, which shows you depth of field against some units you choose, such as feet or metres, but I personally find it quite hard to visualize distances beyond 10 metres away, and I think I turned it off. Other than that, I think most modern cameras have a DoF preview feature, which is often mapped to a button.
But neither of those solutions are particularly useful for helping you choose the right aperture for a given hyperfocal distance.
With all of OM System’s computational chops, you would think they would have thought of something more useful by now. I wonder if there’s a reason features like this haven’t caught on. Is it because they aren’t considered professional enough?
