Lens dew heater

[daz]

Looking for a bit of a sense check here .....

Have now (finally) got what I think are the correct resistors 4.7 ohms and I think they are 0.5 watts.

So the plan is to make a chain of 6 of these and connect via a fused plug to the 12v output from the power pack.

This gives me the following:

12v

25.28 - 31.02 ohms resistance (based on 10% tolerance)

0.48 - 0.39 amps

4.6 - 5.7 watts

Is this enough power, or too much?

How hot will the resistors get, do you think?

Any input appreciated before I go and melt something vital!!!

Cheers

[Astroman]

How big is the scope?

[daz]

Sorry, didn't make this clear - it's for my SLR camera, 50mm lens.

The plan is is afix the chain to a short cardboard tube to wrap round the lens

[FLO]

... it's for my SLR camera, 50mm lens.

The plan is is afix the chain to a short cardboard tube to wrap round the lens

Just use a longer black cardboard tube and you aren't likely to need a dew heater.  (Its about how much air is held stationary in front of the objective; the tube should project at least the diamater of the objective). 

Steve

[Astroman]

Steve's right. But add your get-up, you shouldn't have any problems. You want about 5 watts or so, so you're right there.

[daz]

Excellent - thanks both

[FLO]

The plan is is afix the chain to a short cardboard tube to wrap round the lens

Daz, if you tell me which 50mm lens you have I might be able to find a proper lens hood for you - no charge. 

Steve

[daz]

Hi Steve

That is very kind of you. It's a 28-90mm Ultrasonic AF lens for a Canon EOS. I don't have it with me, but can get whatever other details are needed.

Cheers

Daz

[Ant]

I made a lens hood for the 300D - a filter ring adapter and some matt black plastic.

The downside is that it does cause some vignetting at the widest FOV...

Ant

[FLO]

It's a 28-90mm Ultrasonic AF lens for a Canon EOS.

No probs,

If you send me your address I shall send one out next week.

Steve

[FLO]

It'll be with you tomorrow Daz - post willing.

As with all hoods for wide-zooms, its too shallow to act as a dew-shield on its own but it can be extended so long as you accept that it will vignette at the wide setting.

Steve

[daz]

Many thanks, Steve - look forward to playing

[daz]

Steve - it duly arrived today

Thanks buddy