The 500 Rule of Astrophotography

When shooting stars and the Milky Way in particular, you can use the so called 500 Rule to calculate the longest exposure time (shutter speed) that still allows you to avoid star trails.

Baia das Caldeirinhas, Faial, Azores, Portugal
(Sony A77ii, Sigma 18-35mm F1.8 DC HSM Art, 18mm, 18s, F/1.8, ISO 2000)

Since our planet spins around its axis, we move relative to the stars. As a consequence, keeping the shutter open for too long will result in stars trailing in your pictures, as opposed to looking like dots. 

While star trails are sometimes a desirable feature, more often than not photographers try to avoid them and prefer to capture well-defined, sharp stars. 

If stars were visible during the day, trailing would not be an issue, as we could always use a fast enough shutter speed to avoid them. However, we can only photograph stars at night, making long exposures a necessity if we are to avoid underexposing our images or, worst case scenario, even ending up with a completely black shot.

This is when the 500 Rule comes in handy, as it lets us calculate the maximum number of seconds we can keep the shutter open before star trails will begin to appear.

In case you are new to night photography and wondering what star trails actually look like, I took for you an almost 4-minute long exposure of Mount Pico at night, which you can see below. 

Mount Pico (2351 metres), Pico Island, Azores, Portugal
225 seconds exposure (almost 4 minutes)

Here is how the 500 Rule works.

500 Rule (Expression 1):

Max. shutter speed = 500 / focal length

Example 1 -
Imagine you are shooting at a focal length of 16mm => Max. shutter speed = 500 / 16 = 31.25 seconds.
In other words, you can shoot a 31 second exposure and still have sharp stars. Any longer than that and star trails will begin to appear.

Easy, right? Well, I wish it was that easy. Things are slightly more complicated, mainly because there are 3 types of photographers with respect to this rule:

  1. Type 1: those who never heard about the 500 Rule (well, some more do now!).
  2. Type 2: those who know about the 500 Rule, but they do not know how to apply it correctly (believe me, there are still lots of them out there).
  3. Type 3: those who know exactly how to apply the 500 rule.

If you are a Type 3 photographer, I guess there is nothing here for you to learn, unless you think you know everything but, in fact, you do not. Keep reading to find it out!

If you are a Type 1 photographer and you have a full frame camera, Expression 1 (see above) is all you need.

If you are an APS-C shooter, like I am, or a Micro 4/3 shooter, you are more likely to be a Type 2 photographer, because Expression 1 is what you are likely to find in most websites and texts, except it does not work as is for crop sensor cameras.

No big deal. The expression simply needs to be corrected to account for crop factor of your camera, which leads us to the more general Expression 2:

500 Rule (Expression 2):

Max. shutter speed = 500 / (focal length x crop factor)

The crop factor for APS-C sensors is 1.5, with the notable exception of Canon, for which the crop factor is 1.6 (i.e. Canon makes  slightly smaller APS-C sensors).

The crop factor for Micro 4/3 is 2.

For full frame cameras, the crop factor is 1, which means the sensor is not cropped. I guess this is why we call it FULL frame!

As a further example, I will show you how I calculated the shutter speed for my Milky Way shot of the Bay of Caldeirinhas (Faial Island, Azores).
My camera is a Sony A77ii (ILCA-77M2) and has a 1.5 crop factor. My lens of choice was a Sigma 18-35mm F1.8 DC HSM Art and I took the shot at 18mm.
The important fact here is that lens manufacturers always indicate focal length as a full frame equivalent value.
Therefore, 18mm on my Sony APS-C camera is not as wide as it would be on a full frame camera and corresponds to 18mm x 1.5 = 27mm.
It is absolutely crucial to take this into account when applying the 500 Rule to crop sensor cameras.

Example 2 -
Shooting at 18mm on a Sony APS-C camera => Max. shutter speed = 500 / (18 x 1.5) = 18.52 seconds

To be conservative, I rounded the shutter speed to 18 seconds rather than 19.

Failing to take the crop factor into account would have led me to think I could shoot a 27 second exposure and still get no star trails (500 / 18 = 27.7, rounded to 27 to be conservative). No way!

Please keep in mind that the 500 Rule is a rule of thumb. That said, it does work very well.

You probably noticed by now that the maximum shutter speed calculated with the 500 Rule is solely a function of the focal length. In particular, it is not affected by the aperture and the ISO.
While this is absolutely true, not all combinations of apertures and ISO will produce an acceptable Milky Way image.

In most cases, your camera will struggle to gather enough light and you will find yourself cranking the ISO more than you wanted to, along with shooting wide open (lowest possible F-stop for our lens).
Remember: you cannot increase the shutter speed any further or you will get star trails. Hence, aperture and ISO are the only two variables you can play with in the attempt to produce a correctly exposed image.

This is the main reason why I purchased the Sigma 18-35mm F1.8, which is arguably the best lens ever made for APS-C cameras. It lets me shoot at a maximum aperture of... 2.7!
Why 2.7 and not 1.8? Confused?? You are not alone.
This is indeed the single, most misunderstood truth about APS-C and Micro 4/3 camera: with crop sensors, you have to multiply both the focal length AND the aperture by the crop factor
In this case: 1.8 x 1.5 = 2.7.

To learn more about it and understand why shooting the Milky Way with no star trails is one of the few fields of photography where my gear (as an APS-c shooter) is actually holding me back, you can read my post on why I do not need a full frame camera

That is pretty much it, ragazzi!

If you enjoyed reading, please pick a time to like and share.


Want lower noise? Bump the ISO!

It sounds like nonsense, but it is not.
What I am talking about is action shots in low-light conditions, where the subject must be in focus and low shutter speeds are just not an option. Wildlife photography is a good example.

We all know that the higher the ISO, the more noisy the picture. This is why bumping the ISO for no good reason is a mistake, as it will produce an image that could have been much cleaner if captured at lower ISO.

That being said, sometimes there are very good reasons to bump the ISO well above our comfort zone
Some photographers freak out at the idea of shooting at ISO 400. For many, shooting at ISO 800 strikes fear of noise into their hearts. Certainly, most photographers (though not all) avoid ISO 1600 or higher unless they are shooting the Milky Way.

As for me, in low-light conditions, I am very happy to push the ISO to whatever value it takes to avoid underexposing the shadows.

Look at the picture below:

Sunset on mid-Atlantic crossroad
1/1250s, F/4.5, ISO 1000

Fin whale heading offshore, while a Cory's shearwater flies back to its nest on Pico Island

I shot this scene in shutter speed priority (1/1250) with my Sony A77 Mark 2, an APS-C camera that features a translucent mirror (its full-frame sister is the Sony A99 Mark 2). The mirror is fixed and reflects some light up to a phase-detection autofocus sensor, thus reducing the amount the light reaching the camera sensor by about 1/2 stop.

To make things apparently more challenging, my lens was a Tamron 70-300mm F4.5-5.6 Di VC USD.
With a maximum variable aperture of 4.5-5.6, we can all agree it is not a fast lens.

(as an aside, if you fear that relatively inexpensive gear will never take you very far in photography, you may find it encouraging that I used exactly this kit to photograph my Portuguese Man o' War)

Now the key part: I decided to let my camera choose the ISO in the range 100-1600 and that resulted in a value of 1000. Do you find my image grainy? Do you see any significant loss of details due to noise reduction applied in post? Honestly, I don't.

Here is the trick: I knew that, in such light conditions and with the settings indicated above, my camera was going to expose to the right - i.e. it would expose the highlights correctly.
As you probably know (but just in case you did not), the highlights populate the right side of the histogram, hence the expression exposing to the right ;-)

I knew I was exposing to the right because I checked the histogram before the Fin whale surfaced next to the boat. From the histogram, I also knew in advance that no part of the image was going to be underexposed.
If your camera features an electronic viewfinder, like mine does, things are even easier because what you see through the viewfinder is basically what you get.

At lower ISO, the image would have been significantly darker. And you know what? If you drastically open the shadows of a dark picture in post, the amount of noise introduced will often exceed the noise produced by shooting the same scene at a higher ISO value.

Below is an image I took at ISO 1600 in Postojna Cave, Slovenia. There is no action going on here but, since tripods are not allowed in the cave, I handheld my camera and managed to get a pretty sharp image at 1/20s (Light Bless In-Camera Stabilisation!).

Faces everywhere (Postojna Cave, Slovenia)
1/20s, F/3.2, ISO 1600, handheld

 While the trick works in general very well, sometimes conditions are just too extreme and it becomes impossible to produce a usable shot (although how usable a shot is depends on what we want to do with it!). 

But you know what? When that happens to me, at least I know I would not have got a better exposure shooting at lower ISO.

If you enjoyed reading, please pick a time to like and share.


Making great shots out of snapshots

Nothing to do with over-processing a single photograph. Rather, I am referring to a technique known as exposure blending, where two or more exposures of the same composition are used to create the final image. I use it mainly for interior design, exteriors and commercial spaces, although it can also be very useful for landscape photography.

Every winter I spend some time in the alps and I love to shoot buildings that I find particularly inspiring.
As an example, here you see my exterior shot of Villa Kofler Wonderland Resort, a beautiful hotel in Campitello di Fassa (Trento, Italy).
The shoot was also an opportunity to meet the owner Mr. Ivo, who made history as a climber in the area and showed me some absolutely stunning pictures of frozen waterfalls he took with his phone the day before!

Back to the main topic, I blended 10 exposures to compose the final image below.

Before I show you those 10 exposures, please have a look at a comparison between an average shot I took that day and the final image I pulled off from my entire shoot.

 Move the vertical slider left or right too see a complete 'Before' or 'After'. 

To me, the image produced by blending several exposures is far more compelling than the average one.

Exposure blending comes in handy in quite a few scenarios. For one thing, no matter how great the dynamic range of your camera is, there will always be scenes that exceed it, so that you end up clipping either the highlights or the shadows, or both. Clipping means that parts of your image become completely white or completely black, so that no information can be recovered from them.

When this happens, you are left with only two options:

  1. Take a single shot, with a combination of aperture, shutter speed and ISO that produces a good compromise.
  2. Take several shots (e.g. normal exposure, + 2 stops, - 2 stops) and blend them together in some way.

To get good results using the first method, you need to have a camera with an amazing dynamic range.
It goes without saying that such cameras tend to be expensive and, no matter how good, the result will always be some sort of compromise.

The second method has the potential to lead to perfection and is all about producing a single shot by combining two or more exposures. It also allows for much cheaper gear, as your camera does not need to have such a great dynamic range.
Several tools are available and HDR software is one of them (HDR stands for High Dynamic Range). These programs are usually easy to use, but there is a tradeoff as they do not provide full control over the final result.

If you do want to have full control (guess what? I do!), you are better off using exposure blending. 
The tradeoff here is that exposure blending requires a fair bit of post-processing. To tell you the truth, it sometimes involves a lot of post-processing, especially if you are blending a lot of images.

What you need to do is brush in and out parts of different exposures to compose a blended image in Adobe Photoshop or similar software, using masks to control what parts of each shot are going to be revealed in the final image.

There is actually more to exposure blending than simply being able to produce high dynamic range images with full control. For example, I love to take several exposures of the same composition at different times of the day. For my final image of Villa Kofler Wonderland Resort, I kept shooting the hotel for 1.5 hours, as natural light and artificial lights changed their intensity and quality from before sunset until it was completely dark.

While this is not intended to be a tutorial on exposure blending, here is a tip: if you are planning to get a final image as a blend of several exposures, it is important to shoot those exposures with that precise idea in mind.
What I mean by this is that you should expose for a single feature at a time, ignoring entirely how under or overexposed all the other elements of your composition are going to be in that particular shot, since you will expose them correctly in a different shot.

Now have a look at the bunch of snapshot-looking images I took to produce the final image of this hotel in the Italian Dolomites: 

Exposed for the mountain and nothing else.

Façade (pretty dark) + some artificial lights.

Façade, + 2 stops over (nice white!).

First artificial lights appearing on balconies.

Balcony lights + Christmas decorations.
Artificial lights again.
Terrace and artificially lit façade.
Best exposure for the sky.
Lights/shadows (electric blue sky not good).
Restaurant lights (sky pitch black, not good).


If you consider each shot in isolation, they all look pretty average at best. Nonetheless, each exposure had a precise purpose and was key to the final result.

I always stop shooting exteriors once the sky gets completely dark. Also, I am not at all a fan of electric blue skies. With no exposure blending planned, I would not have taken the last two shots in the sequence above.

Here is the final image again:

I reckon the result I achieved with exposure blending is well worth the extra time spent shooting and post-processing the images.