Image Sharpness: DOF -vs- Aperature -vs- Shutter Speed

[BobvdMeij]

Dear all,

We are currently in the process of defining ideal camera settings for our DJI Zenmuse X5 that we use for our mapping projects. More specifically, how we can achieve the optimal sharpness, lowest image blur and the best identifiability of features in the scene. I have read and studied a lot on the matter in recents weeks, but having no background in camera technology I am easily humbled by the vast array of seemingly complex jargon that is thrown around. I am hoping there's people out here that may enlighten me on certain matters in our quest to reach the best results.

Image sharpness/ focus, from what I have understood, eventually comes down to the Depth of Field of the image capture. Or the minimum and maximum distance from the camera outwards that are (reasonly) sharp. Furthermore, DOF may be influenced by various optical camera parameters such as the aperture (f-number). Focal length may also influence image sharpness but as that is a constant in the X5 I will keep it out of the equation from now. A third parameter that influences the image sharpness is shutter speed. Although this does not follow from it influencing the DOF (it does not) but rather through motion blur, since the images are captured from a moving platform.

I have used an online calculator to find that using a f2 aperture (at 50m AGL) should result in a DOF from approximately 18m to infinity. F5.6 would give 8m-infinity and f22 an even larger DOF of 2.5m-infinity. Now this makes me wonder, in all instances the DOF should be sufficiently large to get all of the terrain in focus; even using a large f2 aperture the DOF starts over 30m 'in front' of the terrain. Still, is there an added value to increase the aperture, does it make images even sharper? Why would I increase the f-number if in all instances the terrain seemingly fits well within the DOF?

The reason why I ask this is the following. The smaller the aperture (i.e. the higher the f-number) the larger the DOF, or the larger the range that is in focus. On the other hand, smaller apertures result in less light hitting the sensor, therefore requiring to lower the shutter speed for compensation. A lower shutter speed, however, may result in increased image blur and therefore less sharp images. Therefore, considering that even the lowest possible aperture of f2 seems to produce a sufficiently large DOF, I would think that keeping the aperture larger is recommended to keep the shutter speed as low as possible.

Or, am I missing somethere there? Could there still be a reason to increase the f-number by a few stops at the cost of shutter speed and therefore motion blur?

[Packy_GIS]

Hi - based on my experience:
1. use the F-number in the middle of the designed range (8 - 9). Then the image is sharp all the way to the edges
2. the shutter speed should be at least 1/500 so your images are not blurry

I am shooting with Nikon D810 - I use the above described setting - F=8 and shutter speed 1/1000 (fixed) and Auto ISO in case the light conditions are changing during the flight. The D810 is not making noise in high ISO - so consider what type of camera you are using.

Jan

[Yoann Courtois]

Hi - based on mine:

1. First of all the ISO: The best is to keep ISO at 100 the whole time. But as Jan said, if you work with high quality camera, I would say you can let the it go until 400-800 in very dark conditions, depending on your camera. But the less your ISO value, the less noisy your pictures !
2. Aperture: The higher the value (small aperture), the sharper your pictures, the more accurate your measurement. Indeed, you can find many only calculator to get your DoF depending on your camera parameters. But be carreful: At the beginning of the calculation, there is a tacit value of "when is a pixel becoming blurred ?" (Usually D/1730 or D/3000). This value is valid for usual photography, not photogrammetry. I would invite you to go more deeply on this calculation and use personal value.
As an example, using my DJI Phantom 4Pro (set at F/2): Online calculator give me a DoF from 8m to the infinity, whereas my calculation give me from 16m to infinity (blurred at 1 pixel size) or even 32m to infinity from high accuracy (1/2 pixel size). You see a flight planed at 30m can quickly be a bit blurred with grey weather.
3. Shutter: I usually remember the value of 1/100 sec for freehand pictures. With that value pictures are visually clear. But in the same way as aperture, the eye is not enough sharp. Using a UAV, a good gimbal and a higher value will guarantee you sharp pictures ! If you can respect the 1/500 value of Jan, go ahead !

In a word, everything is a delicate balance: the smallest ISO value, the smallest aperture (high value) and the longest shutter, while keeping the most luminosity of your pictures

[Packy_GIS]

But you know what? It is the best to learn from your own mistakes ...

Jan

[Jeremiah_ROWE]

Stopping down the aperture a bit will help overall sharpness, especially in the corners of the frame. Keeping it wide open allows faster shutter speeds to help limit motion blur, however the lends attached will determine the best potential sharpness if you shoot wide open.

Most consumer lenses tend to be sharpest in the f/3.5-5.6 range. Depending on flight speed and wind, I would keep shutter speed to 1/500 or faster.

All of this will be relative to the sensor and lens setup you are using. A full-frame sensor with a nice prime lens will allow you to using wider apertures, higher shutter speeds, and even to bump up the ISO as needed. I would not use the X5 camera at f/8,  the m4/3 sensor size will start to show diffraction issues and soften the image.

I would run a series of tests at different settings. Don't be afraid to bump up the ISO setting a bit to allow smaller apertures and faster shutter speeds. The X5 sensor should remain relatively noise-free until 400 or even 800 ISO.

It does seem like you have a good understanding of the three-way relationship of f-stop, shutter, and ISO!

[BobvdMeij]

Thanks all for the swift and explanatory replies. I hope the weather will be fine to do a few test runs with different settings next week, will post my findings here when they're in!

To summarize all comments I reckon I should keep the following things in mind:
- Aperture: f5.6 should do (according to Jeremiah)
- ISO: 400 at most
- Shutter speed: Minimum of 1/500, but higher is better

I am somewhat reluctant, however, to set all these parameters as fixed as this might result in poor lightning conditions. One would rather keep at least 1 parameter flexible to accomodate lightning conditions during and between different flights. However, as far as I know I cannot define two parameters and keep one flexible. I can chose between the following options:

Auto-mode
The shutter speed and aperture are set automatically to onbtain the correct exposure.

Shutter Priority-mode
Set your desired shutter speed, while the camera chooses the aperture automatically. This mode is ideal for freezing action, creating motion blur or low-light shots.

Aperture Priority-mode
Set your desired aperture, while the camera chooses the shutter speed automatically. This mode provides a wider depth of field and can be used to blur out backgrounds.

Manual Exposure-mode
In general, increase the ISO for low ligh environments, and decrease the ISO if the surrounding is very bright.

In short then I can only i) set ALL parameters as fixed (Manual), ii) keep ALL parameters flexible (Auto), iii) fix Shutter Speed (S-mode) and keep Aperture+ISO flexible or iv) fix Aperture (A-mode) and keep Shutter Speed+ISO flexible.

Out of these, what would be the best mode? I would say chosing A-mode and setting f5.6 should do, provided that it keeps the ISO low (100 by default if I am correct) and the resultant shutter speed is 1/500 or shorter?

[cadm8]

My small addition to all the above great posts is to set the white balance and not use auto, so you don't get pictures with different colors

[Yoann Courtois]

I would say it depends on how your camera handles flexible parameters.
Indeed, you can't define for example: "Well, aperture flexible but never less than 5".

So I would also suggest you to do some test on how it is performing with flexible parameters.
According to my background, I always give the priority to ISO because noise has a really bad influence on matching.
So I set a fixed value of 100 for ISO and let shutter speed and aperture flexible.

I've made an outdoor survey (with good weather I have to admit) with a flight planned 30m above the ground:
(UAV used: DJI Phantom 4 Pro)
-1- Longest shutter speed was 1/240 (all pictures between 1/240 and 1/500)
-2- Largest aperture F/5 (all pictures between F/5 and F/5.6)

-1- About shutter speed:
As I said before, multi-copter UAVs are really well stabilized and gimbals prevent most of motion blurred. As an example I've made another survey inside and got pictures with 1/30 sec shutter speed. No motion blurred at all and I got sub-millimeter accuracy, so no need to keep 1/500 with multi-copter !

-2- About aperture:
According to my DoF calculation table, F/5 aperture while focusing to infinity give me:
- Artistic point of view: sharp from 5m to infinity
- Photogrammetric: sharp from 10m to infinity
- Sub-pixel accuracy: sharp from 20m to infinity
So, with a flight height of 30m, everything's fine !

As Jan said, try to do your own test, we learn more from our test than our readings ^^
But I'm pretty sure you shouldn't be so strict on shutter speed and keep ISO value as small as possible. Even you have a really good camera, ISO value greater than 100 is just a multiplier of data acquired, so you obviously lose contrast.

Regards

P.S. About cadm8 post: How do you set white balance so ?

[Jeremiah_ROWE]

I wouldn't say that 5.6 is a set value to use for f-stop. I'd start at 2.8 or 3.5 and work tests from there upwards.

cadm8 - White-balance is a very good point!

Yoann - My references to shutter speed were in regards to flight motion/speed, not the gimbal or stabilization. Faster flights require faster shutter speed. An indoor flight should not be used to compare collection of large amounts of field data outdoors. Also, I am aware that increasing ISO brings about some compromise, however if you have a good knowledge of how ISO/shutter/aperture are linked together you will see that using IS0 of 200 vs 100 would allow you to cut your shutterspeed in half for any given f-stop setting. For general hobby photography (non-UAV) I keep my ISO low and just bump up the shadows in post-processing as needed. For UAV work, images need to be usable without editing.

[cadm8]

P.S. About cadm8 post: How do you set white balance so ?

Depends on the software you're using... With Ultimate flight there is a setting within the manual parameters, same is for DJI GO

[Kasimga8]

Sounds interesting.