from Hacker News

Silly Camera Numbers

by mkchoi212 on 7/31/22, 11:09 AM with 50 comments

by Animats on 8/1/22, 5:46 PM
For film cameras.
For film cameras, holding exposure time times f-stop constant does not produce the same image density. Exposure time for film is not straight photon integration, like it is for a CCD array. You can only trigger each film grain once, but new photons can repeatedly add electrons to a single CCD pixel. So there's what's called "reciprocity failure".[1]
[1] https://en.wikipedia.org/wiki/Reciprocity_(photography)
by buildbot on 8/1/22, 7:15 PM
One little nit is the acceptable shutter speed for a given lens focal length depends _heavily_ on the camera and shutter type being used!
In the past, 1/focal length was perfect advice. You can adjust this down very quite far if you have a lens or body with image stabilization, typically down to as far as 1/30 or even lower.
Also bodies get higher resolution, the amount of blur you can see goes up as well as you start sampling higher frequency movements of the camera body. You probably need 1/2 x focal length above 24MP, unless you have stabilization.
There are also different types of shutters, for example a leaf shutter in the lens vs. a focal plane shutter. Focal plane shutters can inject some shutter shock at lower speeds, where as leaf shutters don't have this effect and can be handheld to much lower speeds, I have gotten very sharp shots as low as 1/8th.
Finally, if your camera has a mirror or not also effects this, as the mirror swing imparts vibration as well.
Knowing exactly how the camera reacts at different speeds will let you tweak this rule a lot!
by starky on 8/1/22, 4:17 PM
I don't find the explanation here to be clear. This suffers from the classic issue where they explain the basics and then proceed to give practically no details on how to start applying the knowledge.
For anyone wanting to learn this more in detail I highly recommend the book "Understanding Exposure" by: Bryan Peterson.
My personal approach to taking handheld photos in manual mode using natural light is as follows:
1. Look at the scene and determine what the most important factor is. For example: * Do I need to freeze motion? Then I need to set a short shutter speed. * Is the image being taken in low light? Then I probably want to open the aperture and set my shutter speed as long as is suitable for what I can hold. * Do I need a large depth of field or to maximize sharpness? Then I need to set the aperture to a suitable value to achieve that.
2. After I consider the most important factor and set either shutter speed or aperture to a starting value, set the second most important factor using the same thought process.
3. Adjust the ISO so the image is exposed properly. Knowing your camera, is the ISO going to result in an acceptable image? If not, then go back to the first step and adjust your shutter speed and aperture to bring the ISO down to an acceptable value.
4. Take the photo and review.
Of course, this doesn't apply to every scenario but it generally is a good approach for the types of photos where the exposure settings are critical. If I'm shooting a landscape outdoors, then I'm probably starting off by setting ISO to 100 first and then the aperture to the sharpest value for the particular lens I'm using as I'm sure that the shutter speed will be acceptable regardless of what it ends up having to be or will be what drives me to use a smaller aperture.
by mgdlbp on 8/1/22, 7:38 PM
Technical minutia, but I found that I felt a lot more confident once I understood this: The reason f-number is written like so is because it's not a measure of actual aperture diameter but is instead the ratio of aperture diameter to focal length. Specifically, (f-number) = (focal length)/(aperture diameter), or:
```
  N = f/A
```
equivalent to,
```
  A = f/N
```
Hence the format f/N, since you (theoretically) use f-number to calculate actual aperture diameter when given a specific focal length. That aperture is in the denominator also explains why smaller f-numbers are brighter.
Why those weird numbers in particular? As in the article, a 'stop' is a doubling or halving of the brightness (technically, 'illuminance') of the image produced by the lens. Since the amount of light that passes through an aperture is proportional to its area, which is in turn proportional to the square of its diameter, one stop corresponds to a sqrt(2) ~ 1.4 ratio between two aperture diameters. Hence, the standard scale of full f-stops is the powers of two and their geometric means:
```
  f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64
```
The full stop scale in the article has some errors: f/1.8 instead of f/1.4, a typo for f/4/0, and a lack of f/45 (though f/1.8 is the stop below f/2 in the finer 1/3- and 1/4-stop scales):
```
  f/1.8, f/2.8, f/4/0, f/5.6, f/8.0, f/11, f/16, f/22, f/32, f/64
```
But why measure image brightness using f-number instead of aperture diameter directly? Because brightness also depends on focal length: Doubling the focal length of a lens doubles the size of the image of an object seen through it. Since the light from the object is spread out over four times the area, the image brightness is quartered. This means lenses with equal aperture diameters and different focal lengths will not have the same exposure. (below edited, I confused myself at first!) But photographers would rather not have to consider focal length when choosing an aperture for proper exposure. Designing lenses such that their aperture controls are labelled with f-number enables this, since actual aperture diameter then scales automatically with focal length to cancel out its effects on exposure.
by frizkie on 8/1/22, 4:18 PM
I've always been interested in photography but really got into the weeds over the pandemic. Learning the exposure triangle (how aperture, shutter speed, and ISO are related/not related) has completely transformed how I think about cameras and taking photos. There is definitely some inherent mathematical/practical beauty in the meaning and relationships between these different camera settings.
I wouldn't want this comment to be interpreted as "shooting in manual is superior", in fact I would suggest against it in almost all cases - the deeper understanding will help you use nearly any camera in existence.
by ChuckMcM on 8/1/22, 3:39 PM
This is a good basic discussion, and it works well for digital cameras as well as film cameras. The difference is that you can take 100 digital photos, see the differences, and it didn't cost you anything to develop them :-).
I encourage new people to photography to try to be disciplined about learning how their camera changes as different settings. Get out of the tyranny of "auto" mode and try all the things. You may find it quite fun.
by scrumbledober on 8/1/22, 4:23 PM
i was hoping for cameras with silly numbers i.e. very very long focal lengths (x000mm) or incredibly small apertures (f/9001)
by kibibyte on 8/1/22, 7:56 PM
Something worth adding here for those who want to complicate their lives even more, these equivalences get complicated when you need to compare across different sensor formats (full-frame vs crop vs micro-4/3rds). Something I learned recently is that f/5.6 on a 300mm lens on a crop sensor is more similar to something like an f/8 or f/11 on a 450mm on a full frame. Including ISO into this makes that comparison more complicated as well.
More on that subject https://photographylife.com/equivalence-also-includes-apertu...
by joshe on 8/1/22, 5:43 PM
I miss this old school straightforward writing style.
I'd love to have a search engine that just searches writing like this.
by themadturk on 8/1/22, 10:19 PM
There's an old exposure rule that is probably next to useless now, but worked pretty well for film cameras when you didn't have a light meter: exposure of a subject in sunny bright conditions is an aperture of f/16 and shutter speed set to the film's ISO. So a starting place for a bright, sunny day when using Kodachrome 64 film would be f/16 at 1/60th of a second (or an equivalent aperture/shutter speed combination). From there an experienced photographer could estimate exposure for other lighting conditions. Of course, negative film (either color or black and white) is more forgiving of exposure errors than slide film. These days if your exposure meter isn't working, it means your whole camera isn't working...
by dm319 on 8/1/22, 6:22 PM
Funny, I was just comparing two photos from some holiday snaps I have taken. One on a Google pixel phone and another on an old 100mm lens produced in the late 70s paired with a DSLR. The differences are stark, and remind me that focal length, dynamic range, shutter/aperture settings all have big impacts on how the final image turns out. Particularly important is how dynamic range and tones of the scene are translated to the recorded medium, as decisions always need to be made with a focus on specific tones. HDR goes some way to help with this these days, but artistic decision-making is still a thing.
[1] https://photos.app.goo.gl/aeCJPnp5BSZ9aBxT6
by fastaguy88 on 8/2/22, 4:59 PM
On modern cameras, you rarely have to worry about the numbers, unless you a deliberately doing something fancy (like continuous waterfalls).
The biggest mistake most people make is not filling the frame, because they are focused on the center. Checking the borders of the picture is a simple habit that can dramatically improve composition.
by xtiansimon on 8/2/22, 1:04 PM
Silly… I’d like to read the article which takes to task digital cameras for carting over mechanical camera exposure concepts.
by Lammy on 8/1/22, 5:16 PM
Realtalk: focal distance is the only number a beginner needs to learn about. Just set your camera to “Auto” and it will take great photos. This is why we invented computers lol