What influence does aperture have on infinity focus?

Question

I recently bought a fast aperture lens which can be used only in manual mode. I bought this lens specifically for low light landscape photography.

However, that has got me thinking about how does focus set to infinity get impacted by wide open aperture (f/2.8) vs closed aperture (f/8 or f/11)?

In other words, if I am taking a landscape photo in low light conditions with infinity focus, is it better to take picture with

1. open aperture, short shutter speed, acceptably high ISO (not grainy)
2. closed aperture, long shutter speed, low ISO

Both approaches will use a tripod. It is my understanding that with focus set to infinity, everything from min. focussing distance should be in focus.

Is lens aperture a factor with focus set to infinity? Would you be able to get razor sharp photos with all apertures (f/2.8 - f/22) regardless? Or it depends on best performing aperture for the specific lens?

Does 2nd approach result in softer images because of long shutter speed? (thus 1st approach is better)?

I understand a very important factor is what is the landscape. But taking out that factor, which is better?

Answer 1

If you need to have as much as possible including infinity sharp, it's better to focus at the hyperfocal distance instead of infinity. Then everything from half the hyperfocal distance to infinity is acceptably sharp. There are websites and smartphone apps to calculate that distance.

I'm not sure what you mean by "It is my understanding that with manual focus set to infinity, everything from min. focussing distance should be in focus.". What do you mean by minimum focussing distance?

Lens aperture is a factor when focussing at infinity, or focusing at the hyperfocal distance, or focusing at any distance: smaller aperture results in larger depth-of-field, and larger aperture results in smaller depth-of-field. That is always true, regardless of focusing distance. Note that the hyperfocal distance changes when you change the aperture: since the depth-of-field is smaller when using a larger aperture, the hyperfocal distance needs to be larger (closer to infinity) in order to have sharpness up to infinity.

Answer 2

For landscape photography where the scene is at infinity (or where the depth of field is not an issue, e.g. you don't want the grass in the field to be in focus, only the far away mountain range matters), you should set the aperture to that value for which your particular lens used by your camera, is the sharpest. This can be as large as f/4 but more typically it will be something like f/7. A larger aperture, say f/2.8 will cause unsharpness (assuming optimal focus) due to lens imperfections while a smaller aperture, say f/11, will produce less sharp images due to diffraction.

Focusing accurately is important, autofocus may not yield good results. Manual focus by using maximum magnification to check the focus often yields better results. Then the ISO should be chosen as small as possible. You must check if you can a take pictures at the required exposure time without unsharpness due to objects moving in the scene. E.g. on a windy day tree branches may move, even your tripod may shake in the wind, requiring you to use a higher ISO to reduce the exposure time.

Then to reduce the noise further, you can use image stacking. The best results are obtained by processing your raw pictures without any noise reduction at all, and then to use image stacking to average out the noise. That way the noise is reduced without compromising on image quality. The problem with using even the best noise reduction algorithms is that they will degrade the very small scale details in your image. Fundamentally, there is no way to tell from a single picture whether some fluctuation in the gray value of a pixel is noise or whether it is a real signal.

You should use long exposure noise reduction, the camera then performs a dark frame subtraction for each shot. This is necessary to remove the hot pixels which would otherwise affect the final image, as these pixels will affect every image at the same point, so they don't get removed by averaging.

Answer 3

there is no best

Best is what suits your photo. For landscape that is often low iso, relatively low shutter speed and a slow aperture. But more important the aperture your lens performs best at and fits your picture.

the fast and short answer

I don't know what your level of knowledge is so I'll do a quick guess here and answer in short that I believe you should look for sharpness in landscape photography and it's important to know that your lens performs a lot better if the aperture is closed 1 or more stops.

For that reason alone I wouldn't use it wide open. Also in landscape photography you often want the maximum depth of field you'll get closer to by closing the aperture ring.

more details

iso

With low or high ISO you just get more or less noise. Noise can be something you're looking for in certain situations. Often not for landscape though.

shutter

With a higher shutter speed you'll freeze motion. But you have to ask yourself if anything is moving at all in your frame and at what distance. General rule is 1/"focal length" of lens so for a 28mm (thinking landscape again) you could shoot out of your hand at 1/30.

However if you take landscape seriously you'll want a tripod and you'll be able to go longer. Again as long as you don't have anything moving in your frame.

aperture

And finally the lens aperture. I'm guessing you understand you get more depth of field with a slower aperture. There are cases where you don't want this but than you'll often also need a non wide angle lens since all wide angle lenses already have a lot of depth.

focus point

Also of relevance is the fact that your sharpness reaches 1/3 before and 2/3 after your focus point.