View Full Version : aperture at night or low light?

Ms Monny
21-02-2011, 10:19pm

I am new to night shots and did a few night sky shots the other night but when I saw a few threads from some other people, I noticed that they had f2 (indoors ice hockey) and f4 (night sky and landscape). I was wondering how this would work as I always thought that a larger aperature would result in a smaller depth of focus - these images looked like they had good dof.

Has it got anything to do with the further you are away from the subject?

I think I do get a bit confused with with aperature to use in which situation. To me larger f/stops are for a shorter dof and smaller f/stops are for longer dof (f22 = infinity).

Hope you can help.

22-02-2011, 12:22am
Yes, you're basically correct but there are other considerations too. When shooting sport you need a fast shutter speed to freeze the action and this normally forces the use of a fast or large aperture. On the other hand shooting a stationary subject at night is often done on a tripod where the shutter speed is much less important so a 'better' aperture can be used. I say better because using a medium aperture, such as 5.6-8, will maximise image quality in those circumstances (as there are issues of flare to consider and lenses are better at that when stopped down a couple of stops). Shooting at wide open apertures is fine too but flare is often a major issue as soon as there are any very bright light sources in the image, or even near the edge of it.


22-02-2011, 12:38am
Hey Ms Monny, I know exactly what you are talking about, lol. It took me a while to work aperture out and I thought to myself, I am never going to get this. In a nut shell, the larger the f/stop (f22 for example) the smaller the aperture and larger depth of field bringing everything (foreground and background) in to focus. The smaller the f/stop (f2.8 or smaller) the larger the aperture and shallow depth of field bringing only the subject into focus and blurring the background. Indoor sports shooters that do not use flash will always shoot at f2.8 or below because the aperture is larger and allows more light in to freeze motion. Don't know about f4 for night sky and landscapes. F4 still has pretty shallow depth of field. From what I have read and put into practice myself is that f8 or higher is more ideal for night and day landscapes, provided you use a tripod for the longer exposures.

Okay, I guess the rule of thumb that I found easy to remember is - The smaller the f/stop number like f2.8 the larger the aperture (more light comes in and you get shallow depth of field creating blurry backgrounds)
- The larger the f/stop number like f22 the smaller the aperture (less light comes in and you get a deeper depth of field creating an in focus foreground and background)
- For night or day landscape shots I find using F8 and above works well, just remember to use a tripod to allow for the longer exposures, especially at night.
- Whenever you are using your camera at night or in a low light situation hand held, a fast lens like f2.8 or below is a must or every shot will blur.

I know it's a handful, but you will get the hang of it:)

P.S. Oh and what jjphoto said as well;-)

Dylan & Marianne
22-02-2011, 7:59am
Ms Monny, alot of the night shots that are shot at F2.8 or largish apertures are so that the exposure time doesn't get too long.
If you get any longer than 30 seconds, you start to notice star movement and I'm guessing the ones you've seen may have been with frozen stars rather than trails?

Ms Monny
22-02-2011, 12:49pm
thanks for your answers.....

I guess what I am saying is that I understand that in low light f2 is better as it lets in more light BUT doesn't that also affect the dof? Wouldn't you have problems with lots of things being out of focus and in a landscape you need more in focus than not (depending on what you want to convey)? The photos I have seen were shot with f2 because of the low light but there was sooo much that was in focus! This was the confusing part.

Roosta has put me onto an area that will hopefully help this query too. Ive got a lot of reading to do! :)

James Axford
22-02-2011, 1:47pm
Are you talking about my photos ms monny?
If so, it's really hard to see what's in focus and not in some of them in the foreground.
As ice would be the foreground and really never has that much detail.
When you're talking about shooting landscapes in low light you need a tripod so you can get the dof. Theres no other way of going about it.

The hockey photos I took didn't need a lot of dof.

I'm sorry if they were not the photos in question.

22-02-2011, 2:30pm
Hi Monny
Just about all has been mentioned except one really critical bit:

DOF is also hugely influenced by the lens' focal length.
see http://www.dofmaster.com/dofjs.html (pay attention also to the "hyperfocal distance")

You may find that a lot of landscape shots are taken with a 10 to 20mm lens. (e.g. http://www.flickr.com/photos/46454728@N07/5424155455/meta/in/photostream/)
When I take a landscape shot with my wideangle lens at f/11 or say f/13 (at the 10 mm setting) all I need to do is set the focal distance to about 1m (hyperfocal distance) and I know that everything from 40cm to infinity is in focus.
So to sum up:
increases with lower focal length (eg. DOF greater at 10mm than at 20 mm)
Increases with higher f/stop number (e.g. DOF greater at f/13 than f/2.8)
and increases when focussing further away. (eg. DOF greater when focussing at 10m than 0.4m)

often people utilise the "sweet spot" on their lens with sharpest pictures achieved at a f - stop number generally 2 to 3 stops above the smallest f - stop number. Furthermore, zoom lenses (lower quality ones at least) are often sharper in the mid range of their zoom.

All the best, Frank

Ms Monny
22-02-2011, 3:06pm
Yes, James, your ice hockey ones intrigued me as you had an aperture of f2 or there abouts but I now understand that it is a 'fast' aperature and more light in the lens means quicker speed PLUS your dof was shallow, now that I look back on them. (great photos too!)

There was a night shot from someone else who used f4 but I could see the building in the distance was sharp but so was some of the foreground. That confused me.

I should have said whos photos as this would probably help my questioning....they were good photos too.

I shall be back!

Ms Monny
22-02-2011, 3:11pm
Thanks Frank - I have just been reading about the hyperfocal distance. These are the photos that stumped me.....maybe it is just me but f4 with these images, esp the one with the building, I can't understand. Maybe I need to read a hell of a lot more, and re-read to make it all sink in...or just accept the fact!!


thanks to jasnat for these photos too!! I hope he doesn't mind me using these :o

22-02-2011, 3:24pm
The amount of DOF you get is a combination of focal length, distance from the subject, sensor size and of course, aperture.

It's my understanding that DOF will be the same no matter what lens your using if the focal length, aperture and sensor size are the same. Although the quality of the Bokeh (background blur) will change from lens to lens.

Ms Monny
22-02-2011, 3:32pm
Yes, adrian, I read that somewhere today. Michael, on another site, did a test with different lens' and if the image that is in focus stays the same size, dof is the same with a 200mm all the way down to a 10mm lens....the background may change with perspective but the dof is the same!!

amazing really.

22-02-2011, 7:17pm
Have a look at Depth of Field Calculator (http://www.dofmaster.com/dofjs.html)

An interesting site which will answer a few questions for you, I feel.

For example, with a Canon 60D and a 55mm lens set to f2.8, if you focus on something 100 m away, as you might shooting a landscape scene, the area in focus is from 36m to infinity!

Why shoot at f8 or smaller? At f4, it extends from 28.5m to infinity!

Ofcourse, some lenses are at their sharpest around f8, so to obtain optimum sharpness, shooting around f8 may be the go. But it may not make much difference either - depends on the lens.

22-02-2011, 8:26pm
G'day Ms Monney

Others above have given valuable comments -
Regarding star trails & star pix ... infinity focus on ANY camera lens is taken as being "1000 times the focal length" ... and as the stars are a 'just a tad' further away than that, just set your lens to infinity and forget about DoF

The main reason that anyone stops down a lens while taking stars / star trails is to overcome any weakness in the camera lens with loss of optical sharpness towards the edges while shooting at max aperture. Thus, while you may have an f2,8 lens [for example] you might find that f4 or f5,6 will give better edge/corner sharpness ... it's not a DoF issue

Hope this helps
Regards, Phil

Ms Monny
22-02-2011, 10:27pm
Thanks Zolaxi and Phil and everyone else!! I am actually understanding all this now. It now seems so simple but I just couldn't get my head around it!

I have been playing around with the dof calculator with some peoples exif's and yep, there it is....they focused far far away (maybe horizon?) and when they used a wide angle lens that made the dof from approx. 4 to infinity!

YAY! I can now sleep tonight knowing this now. I tend to want to know the 'why' and 'how' instead of the 'just because'!!

:th3: :th3: :th3:

23-02-2011, 1:03am
... infinity focus on ANY camera lens is taken as being "1000 times the focal length" ...

Phil, do you have any reference for this? I've heard similar things said previously but with different figures. I've never really seen it written anywhere by an expert source such as a lens maker/designer etc so I don’t really believe it. I don’t really know if this kind of figure is true or if it has become a kind of ‘urban legend’, although a very lame one as urban legends go. I'm not being argumentative (not that that's a bad thing anyway) just curious.

For example, in the case of a 28mm lens, this would put focus at infinity at about 28 metres (28mm x 1000). I can say with absolute certainty that a 28mm F2.8 lens focused 28 metres away will only ever be sharpest at 28 metres and sharpness will fall off further away from that point until infinity. I have tested this several times, because it could be argued that I have a fetish for 28mm lenses so I have plenty of them, and a lens that does not reach infinity focus is noticeably soft, even if it focuses about 100metres away. Even a 28mm lens must be focused at infinity, not 1000 x focal length to render a sharp image at infinity.

Sorry to the OP for getting so far off topic!


23-02-2011, 12:40pm
Phil, do you have any reference for this? For example, in the case of a 28mm lens, this would put focus at infinity at about 28 metres (28mm x 1000). I can say with absolute certainty that a 28mm F2.8 lens focused 28 metres away will only ever be sharpest at 28 metres and sharpness will fall off further away from that point until infinitySorry to the OP for getting so far off topic!

I do think the 1000xfocal lengths might be indeed an urban legend.

Here is an exif info of another AP members's shot that I used yesterday to reitterate on DOF (http://www.flickr.com/photos/jasnat93/5460437061/meta/ - hope you don't mind James).
In this exif data the subject distance is given at 4294967295 m at a focal length of 24mm. That is about 4.29 million km. (but maybe that is just a random value for infinity put there by Canon). I checked EXIF of another shot by James and it gives the same value for infinity.

Thanks, Frank

Also sorry to Ms Monny for Thread Highjacking. But hey, we are all learning.

Ms Monny
23-02-2011, 4:21pm
ha!! no need to say sorry for hijacking!! I am also intrigued by this.....anyway, I hijacked a thread from jasnat!! What comes around.....:lol:

23-02-2011, 8:19pm
..... That is about 4.29 million km. (but maybe that is just a random value for infinity put there by Canon). I checked EXIF of another shot by James and it gives the same value for infinity.


Frank, that subject distance is most likely a problem with Lightroom rather than a technical analysis of how DOF works in unison with focal length.(or any randomness on the part of Canon)

I've never seen any comments or issues raised on the topic, but it seems that any image edited in LR3(at least for me), gets this 4million klm figure as subject distance... even macros! :D

Then again, the fact that they(Adobe) can't keep their grubby hands of your metadata and replace it with their own garbage is probably some master plan of theirs to rule the world!(and now I'm off into wild blue OT territory too! :p)

Ms Monny;

your comment
To me larger f/stops are for a shorter dof and smaller f/stops are for longer dof (f22 = infinity) indicates that you may have a basic grasp of aperture and DOF.

larger apertures(small f/numbers) give you a shallower DOF for sure, but this is important to note that only for when there is a larger distance between near and far subject matter in the scene.

That is, you can still shoot a landscape at f/1.4 if you like, but you need to focus very far out into the deeper part of the scene, not close up. If you focus on something close up then the background is more blurry.
If you focus on something out in the far distance, then everything close in will be blurry.

eg. if everything in the scene is at a distance of 5 meters, and you shoot at f/0.75(yep 0.75, not 7.5!!) and you've focused at that 5meter distance, then everything is in focus, there is no sense of a shallow DOF.

Apart from your comment that a smaller aperture gives you a deeper DOF, other reasons for using a smaller aperture other than those given already, may be for a longer shutter speed, or better image quality all across the frame.

many lenses my give their best ultimate sharpness result at say f/5.6 or f/8, but the sharpness of the details at the corner of the frame may still be low. So, even tho we may know that the lenss is best at f/8, we may set it to f/11 because at f/11 the corners may produce better image quality than at f/8.

shooting at f/22 doesn't instantly mean that you get a DOF that extends to infinity either! it may give you an impression that it does when using a short (say 10mm) lens, but using a longer focal length doesn't necessarily give you this result if you've focused in very close. Also think of macro, and how magnification works there. Even a short focal length lens can give you very little DOF at f/22, because you may need to focus in very close.
So again, the near distance is more important for determining DOF, than just about any other variable.

Also, someone made the comment that sensor size has an impact on DOF.(this is not true). It only has a bearing, because for some reason, we humans need to alter another variable called Field of View(which is similar to changing sensor size, but not the same thing).
So when going from a smaller sensor to a larger one, there seems to be this need to maintain a particular FOV. This is a human failing that often gets confused as a variable in the determination of DOF.
There's absolutely no reason for the need to move closer in, or further out from a scene to match a focal length to a sensor size.

Ms Monny
23-02-2011, 9:49pm
Thanks for that info Arthur. The part on LR using the 4mill km focal subject is sooo true...Earlier I checked the EXIF of one of my shots and I KNOW I didn't focus on the far horizon, but on the trees about 500m away and it showed up with that spectacular number!

I am slowly getting the hang of this. I stuffed up in another thread with a silly comment and now think I need to do alot of practice and theory to get this to sink into my head!