Professor Tips

Choosing the Right f-stop for Night Shots with Long Exposure

By using long exposure times for exposing water in movement, its possible to achieve a very nice silky effect. Typically an exposure time of 15-60 sec is enough to turn a moving water surface into a flat silky looking surface.  Depending on the conditions you may need to expose up to a couple of minutes.

To achieve this long exposure time for a night shot, simply by selecting a smaller aperture like f22 and f32, may not be the best solution. Lens diffraction at these apertures has a significant negative influence on the contrast and the sharpness of the final image.

Lower f-stop

You will achieve a much sharper image, if you use a lower f-stop in the range f8  f11. To ensure the right exposure time, try using high quality Neutral Density (ND) filters instead.

The image above shows a zoom into the central part of the image without any image adjustments added.  The same image is shot at both f=8 and f=22. It is obvious using f=22 decrease the sharpness and the local contrast significantly. You can hardly read the Gulf logo.

Tripod and manuel focus

Sometimes, it’s a good idea to make an aperture sweep to learn more about the behavior of a specific lens. It’s necessary to use a very sturdy tripod and manual focus in order to not be influenced by variations in the autofocus system.

The 4 images above are 200% zooms into to central part of an image.  With this zoom level we can see how the lens diffraction effect gradually decreases the sharpness of the image.  At f11 we still have great sharpness, but at f16 an obvious loss of sharpness appears. At f22 we really lose a significant amount of sharpness and local contrast.

Lets take a look at the surface of the water and observe the effect of using long exposure time for the shoot.

In this case, the 15 sec exposure gives the wanted silky look of the water surface. For the best result, I should use a neutral density filter giving a 15 sec exposure at f8 or f11. The image is shot with a wide-angle lens, so there’s be no problem with depth of field when I use f8 or even f5.6.

Clarity Tool

If I don’t have my ND filters with me, and I still want the silky water look from the f22 shot, I use the Clarity tool in Capture One 7. This will help me improve the lost local and also improve quite a bit on the loss of sharpness.

A 100% zoom into the central part of the image.  The Clarity tool from Capture One 7 has been used in the image to the right.  I use Clarity=40 and Structure=20 for this image.  These settings bring back the loss of local contrast and improve the perceived sharpness of the image.

With the Clarity tool we have been able to dramatically improve the loss of quality due to lens diffraction at f22. A better solution to achieve the 15sec exposure time would be to use a high quality ND filter and an aperture between f8 and f11.

All the best,


The Image Quality Professor
The Image Quality Professor

The digital pioneer, Niels V. Knudsen, is Phase One’s Image Quality Professor and founder of the IQP blog. Moreover, he is responsible for breakthrough advancements in image quality both in Phase One’s medium format camera systems and in Capture One Pro.

Comments (19)

This is one of the things you learn in college, and take for granted! In the film days it was almost impossible to see this, but with digital it is obvious and can be viewed instantly in Capture One. I would urge every photographer to test this for themselves – you will be as amazed as I was when I photographed a ruler with lens tilt at various apertures – do test it!

I have a basic Canon EOS50D and Kit Zoom lens. If my lens gave crappy results at F/22 I would be looking to change my lens. I photograph many items where I need the extra depth of field and do not want to deal with focus stacking. I made many comparison images at a four stop range and the difference is barely perceptable. The lens you are using IMHO is not a very good one.!

Very late response but I am looking through old posts and thought I would add this…

I hear that lenses affect when diffraction is visible. I believe it is also a thing that happens anyway and so isn’t just that some lenses show it yet some lenses don’t…

I will make up a unit of blur and say the lens and camera show 12 units at f/11 as diffraction starts to become visible. The exact aperture when diffraction becomes visible at a pixel level varies by pixel size.. With good technique, a really good lens might show only 4 units of blur at f/5.6. An average lens might be at its best at f/8 with 10 units of blur.

Looking at the average lens only, I might think it is a better lens because I will see little difference between its sharpest point at f/8 (10 units) and when diffraction becomes noticeable (12 units). The lens appears to show no real decrease in quality but this is because the lens, at its best, was already providing a level of quality similar to that when diffraction first occurs… A poor lens may, at its best, be so not sharp that diffraction effect is masked until very small apertures…

The good lens, on the other hand, will appear to suffer horribly. There will be a noticeable loss. The loss stands out because the good lens was a lot better at f/5.6… By time diffraction arrives at f/11, both the good and not so good lens will show similar levels of sharpness…

There are things other than sharpness and the good lens may still offer some benefit even as diffraction becomes visible.

I believe this to be generally accurate.

Robert Ezergailis

Rather than neutral density filters, change the ISO setting on the camera to a lower ISO to get the f stop and exposure duration that is optimal. Neutral density filters always reduce the optical quality of a lens.

Unfortunately the sharpness in the buildings is counterproductive to the “silky” appearance of the water. If the best appearance for the water is (in your example) f/22 at 15 sec, then it will probably need an even longer exposure than 15 sec at f/8 – f/11in order to counteract the increased sharpness in the water. This doesn’t apply any more after the exposure time is long enough to thoroughly merge the water movement. With my particular lenses I am happy with my results up to f/16 but avoid going for a smaller aperture.

As to the comment about lowering the ISO setting on the camera: If you are trying to get maximum sharpness then I assume you are already shooting at the lowest ISO possible. So ND filters are the only alternative.

This is clearly very pure lens as well. Buy yourself a decent lens and you will not see such a loss of quality.

How did you know that I use an extremely good lens? Ah, I guess you actually meant the opposite of what you said.

“Pure”, according to the dictionary is (among other things): a (1) : unmixed with any other matter (2) : free from dust, dirt, or taint (3) : spotless, stainless
b : free from harshness or roughness and being in tune —used of a musical tone.
In other words: “the best”. Your command of the language seems to be on a par with your “critical eye”.

I have no quarrel with your being happy with what you get from your camera but there are some guys (and gals) out there, myself included, that aim for something better. The loss of quality at both ends of the aperture range (large and small) is well documented and is the result of “diffraction” (law of physics). It applies to all lenses, although it is of course less noticeable with better lenses.

Last, not least, assuming that the Image Quality Professor and everybody else are using inferior lenses (your original and your last post) is quite presumptuous; especially considering the lens that you use (a “Kit Zoom lens”, is it?).

Oops, made a mistake.

The first post with reference to an inferior lens was by Ian, not by Dragan Mikki. So, my apology to Dragan Mikki for my last paragraph: the “Image Quality Professor” and the reference to the “Kit zoom lens” should be excluded from it (the rest is still applicable). I guess my last paragraph can still stand for Ian though (“with everybody else” excluded) along with my third paragraph.

I better stop before I stumble over myself.

Thank you for pointing out at my obvious mistake. Worth noting that you have made a career of stating obvious! Isn’t that why you call yourself a Professor?
Instead of vesting your time why don’t you try and help integrate Wacom Pen and Tablet with Windows 8 OS? Instead of skirting this issue it is about time you do something about it. Windows 8 OS have been around for a year!

You think that I am the Image Quality Professor? A great honor, but I can’t claim that title. You are right though that I am vesting my time in a hopeless discussion. I hope that you will find your peace.

Deep stuff Professor! Please give us more.
Interestingly, you appear to be undermining the legitimacy of Phase One as well! Who are you? Please don’t tell us you are Pippa Middleton disguising as an Image Quality Professor.
And you won’t waste my time if you tell me that one day Capture One will work with Wacom in Windows 8.

Very good demonstration of sharpness effects of different apertures. Importantly this applies to day time photography also.

francesco marchetti

where is that building located?

Hi Francesco,

This building is located just outside Copenhagen in a harbour called Tuborg Harbour. It is a bank called Saxo Bank.

All the best,

One thing to consider for night pictures are stars formed by light sources.

In the samples, the stars are more visible at smaller apertures. A decision can be made to use an aperture for maximum sharpness or use a smaller aperture for more star effect.

How stars look vary by lens. Some lenses don’t need to be stopped down as much for stars to form. Some lenses have well defined stars while others have less sharp stars and some have stars with more points/arms. Number of arms varies with aperture blades.

I think I remember odd number of blades are good but that would have to be confirmed if anyone is interested in lens star characteristics…

Alessandro Guida

I can’t see the images of this article

Camilla Kristensen

Dear Alessandro,

Thank you for your comment.

You’re right, something has happened here. We’ll try to find them in the archives ASAP! Hopefully, they’ll be back soon.

Best regards,
Camilla, Phase One

Camilla Kristensen

Hi again Alessandro,

The images are now back. Enjoy!

Best regards,
Camilla, Phase One