Category Archives: photography

Infrared camera trigger experiments

Wouldn’t it be nice if I could take a picture of a bird as it passed through an invisible beam of light? The idea’s not original, these things exist, but they are quite dear, so I am experimenting with making these.

The most obvious way is a light source and a photocell, and indeed many years ago at secondary school I developed an analogue circuit 1 using OC71 transistors scavenged off postwar computer boards to make up monostable multivibrators for the delay elements  and one with the black scraped away from the housing to act as a phototransistor.

Not bad for a school project from 1976. It looks better as an animation than a sequence of stills.

This gonzo technology of 40 years ago triggered the flash for the source negatives used in the animation – you set a very slow drip, and as the drop passes the photocell it triggers the delay. By increasing the delay between the drop passing and the flash going off you get the progressive animation, assuming each drop makes a similar pattern.


OC 71

This was done with a manual camera, a new Canon AE1 ISTR that one of the other kids had. But the trick is to do all this in the dark, click the camera on Bulb and use the trigger to trip the electronic flash, which responds within milliseconds and has a short duration of about a millisecond if you reflect some of the flash back into the photocell of the flash (to turn it off as early as possible).

So there’s nothing incredibly hard about doing this, in controlled conditions, in a darkened room. If I were doing it again, I’d do it in the same basic way, using a phototransistor and a CD4538 CMOS dual monostable rather than a discrete monostable – one half to give the delay controllable with a pot and the other to make a pulse off the falling edge to go into a NPN transistor to trip the flash. There’s no need to muck about with PIC microcontrollers or Arduinos though you could do it that way if you really have to for a higher cost plus the aggravation of writing code, plus the jitter of the Arduino sampling the sensor/responding to the interrupt. In high-speed photography sub-milliseconds matter.

Everything gets harder outdoors

Outdoors you have massive and variable amounts of light from the sun, distances are longer, there’s just a whole lot more hurt all round.

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  1. people normally consider monostables as digital but mine was built using discrete transistors and resistors, and the time delay was infinitely variable, as it would be with a CMOS 4538, so I consider it analogue

Using near IR to look for photosynthesis and plant health with NDVI

The NoIR Raspberry Pi camera comes with a blue filter to do near infrared photography – the blue filter ices the visible red but passes near IR which records as red, apparently.

NDVI image of something in the polytunnels

NDVI image of something in the polytunnels. Should have made a not of what this plant is 😉 Anyway, more red and going to magenta white overload=more photosynthesis

NDVI (Normalized Difference Vegetation Index) is the near IR plus red divided by near IR minus red. Take a look at this image for the meaning of the colours – red, magenta and white is more photosynthesis, cool colours and black are less. Chlorophyll uses red but doesn’t use near IR which it reflects, hence the difference carries useful information.Lots more at Public Lab. Continue reading

Canon EF 100-400 L lens fungus attack

note – this is a Mk 1 version of the Canon EF 100-400 L

A working photographer uses their lenses all the time and probably never runs into this. I was into bird photography for a while, about eight years ago, and had the Canon EF100-400 IS L like every other wannabe bird photographer. In between now and then the field has separated the sheep from the goats – real bird photographers use longer primes, because the birds are always at the long end of any zoom. Or they use astro scopes on manual focus 😉

Lapwings landing

Lapwings landing

Anyway, I take time out from birds and photography, because life gets in the way, and I stow the lenses in a relatively cold room. A couple of years back I figured I’d take some long lens pics, and get greeted by this

Lens fungus. Nasty

Lens fungus. Nasty

which makes me curse. Mainly on the front element, though a starting spot on the inner element, which is part of the IS mech. The inner part is magnified by the biconvex front element. The spotty crap is on the inside of the front element, the fine filigree round the edge on the front of the front element. Continue reading