KAP image resolution

What makes a good KAP camera? In the quest for better image resolution for KAP and with my D5100 out of action for its 2nd sensor failure in 5 months (it’s down to Nikon-their fault, not mine!) I’m looking at cameras again. There are no cameras specifically designed for KAP (yet) so it’s a case of looking at what there is and trying to figure out what will work best at 60m AGL pointing at the ground.

There are some difficult choices to make. The compromise between /weight /width of view/sensor size is a tricky one. I have a small collection of Nikkor lenses and a D70 body for reference. I find the D70 heavy to work with but I have stripped down its rig to 1,200g and found the resolution to be pretty good even at 6MP. The ff30 will loft it comfortably in Bft3 on a 250DaN line.

The performance of a camera is revealed by the resolution of the photo it produces. This is determined by 2 factors:

  1. Photo-scale =  how many dots per Metre are recorded in the image
  2. Resolving power of the camera =  how clearly are the dots captured

The first factor is largely controlled by the height of the camera and the second is a fixed property of the camera.

In getting a DSLR to work as a KAP camera I have been chasing better image resolution (I want this stuff to print BIG)  I think I  have got to the limit possible by the compact cameras I have available, and given my flying height is constrained by law and the days I can fly are in the hands of the weather gods I’m keen to push KAP image quality as far as I can on the rare days of kind wind and sun as are allowed to me.

The DSLR opens up the possibility of using lenses with better light loss characteristics and better resolving power than those supplied for the smaller format cameras. A lens which lets in more light will capture more at a higher shutter speed /smaller aperture. DSLRs are appearing now weighing around 600g.So how do I know which camera is going to give the best resolution? The simple answer is the one with the biggest sensor and longest focal length but the biggest variable for KAP resolution is the photoscale. It’s possible to calculate the photoscales possible for a given height: let’s assume we are able to fly at a fixed height of 60m.

Photoscale is the ratio of the size of things on the ground and the size they are captured on the image, assuming both camera and the ground is level The camera’s focal length (f)  is the distance between the lens centre (c) and the image (or ‘film’ in the old money) plane. Note that ‘f’ is made up of 2 parts – the fixed body depth of the camera and the distance to optical centre of the lens array, the first you are stuck with the second (provided lenses are interchangable) can be modifed by swapping lenses of different focal lengths. Cameras with a large sensors and corespondingly large ‘body depth’ project more light on to the sensor than small ones.

Photoscale = f/h (in my case h=60000mm)

The higher we go the more we see but less the we can reslove. At 60m the photo-scale will vary according to the focal length and resoloution by sensor size.

Here are the photo-scales for the cameras and lenses  I have flown over the last 3 years. .  The image pixel density expressed as pixels per square metre gives an idea of the resolution achieved. The table shows the X1 is equivillent to the D5100+35mm lens in this respect:

K A P Image resolution by photo-scale at 60m*

f

PS

Sensor size

 mPixel

Image Pixels per m2
Nikon DSLR

D70

D5100

D70/ D5100

D70/ D5100

50.0

1:1200

23.7 x 15,5

23.5

x

15.7

6

16.2

5,000 13,500

35.0

1:1700

3,500 9,500

28.0

1: 2000

3,000 8,100

10.5

1: 6000

1,000 2,700
Leica X1

24

1:2,500

23.6 x 15.8

12.2

9,760

Olympus EP1

17

1:3,500

17.3 x 13

12.3

3,500

42

1:4,500

2,700

Panasonic LX3

5.1

1:12,000

8.07 x 5.56

10.1

841

Canon Ixus 7

5.8

1:10,000

6.17 x 4.55

7

700

*Theoretical photo-scales for a height of 60m: all PS and pixel per m2 values are approximated to nearest 100.

The difference between photoscales at (roughly) 60m with 35, 24 and 10.5mmm focal lengths is just about discernable in these 3 image extracts:

Individual cow turds are distinct in the 24 and 35mm shots but not in the 10.5mm coverage!

Field of View The wider the view the bigger the image area captured at a given height but this is at a cost of distortion at the image edges, with wide angle lenses the rays of light follow a bent path through the glass such that as much a 2/3rds of the image is lost with my widest (10.5mm) lens.  In practice, at 60m AGL, the narrow field of view (FoV) from big focal length lenses makes them inappropriate for nadir work and the weight/ stabilisation issues make long lenses impractical for KAP.

The difference in the field of view is clearly visible in this montage of the the shots above overlaid at the same scale. The background is the 10.5mm focal length, purple the 24mm and yellow the 35mm:

Close inspection of the edges of 10.5mm image reveals (as well as the photographer at the end of the string!) the downside of the big field of view is distortion at the edges:

This isn’t too surprising when you consider the ‘off camera’ state of the image:

Even so the coverage is very useful compared to the tighter, ‘sharper’ lenses and taking overlapping shots means the best can be made of the combined  ‘sweet spots’ to get a decent mosaic of a big subject  in relatively few shots.

The exposure: illumination at capture is probably the biggest challenge for low level aerial photography: to get a good exposure with a fast shutter, reasonably high ISO(400-800) and get sharp, blur free shots from a camera rocking about under a kite is a bit of a tall order but provided shutter speeds don’t drop below 125th and its its not blowing a howling gale it’s bright sunlight that will get the best out of the effort. With overcast skies getting good exposures is difficult, on ‘Auto’ the shutter speed will slow down and shots will be susceptible to motion blur. Pushing the ISO runs the risk of getting ‘noisy’ images.  Good exposures tend to be those with slow shutter speeds, nice low ISO and some careful bracketing – all fine if you are working on a tripod!

Other considerations in choosing a camera are image/lens stabilisation,  durability and control-ability. Image stabilisation is very effective in reducing the number of blurred images, there is some debate over effect of stabilisation on image quality but it’s a positive option for KAP in my view. Control-ability is important in terms of fitting a shutter servo, using a video relay (HDMI interfaces are not helpful) or setting an intervalometer on the camera (a strong benefit of opting for Canon cameras is scripting with CHDK) .

Getting an idea of what to expect from a camera becomes less confusing once you realize the bigger  sensor ups the pixel density far more than the effect of focal length on photo-scale. ‘DX’, APS-C  or full frame – there’s only one serious KAP ‘FX’ contender in a sensible weight range – the mighty Leica M9!

Of course there is much to be said about image quality at the darkroom end too but getting the best resolution possible at capture has to be the staring point!

Discussion of the post processing of one of my KAP images is here: https://billboyheritagesurvey.wordpress.com/2011/11/04/dslr-kap-image-quaity-what-do-i-know/

About billboyheritagesurvey

Heritage worker
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