Backfocus

[k_jensen 0]

Hello guys,

 

We are having a discussion if it is necessary to adjust the focus with a filter on a dome camera.

 

I mean, we cannot adjust the backfocus - why use a filter then?

 

Any arguments?

 

Best regards

Kenneth

[rory 0]

Hi, whether you are backfocusing or doing a regular focus, for it to stay in proper focus in day and night you should still focus it in low light .. or simulated low light.

[k_jensen 0]

Hi rory,

 

But if the lens is IR corrected why should the forcus change?

If I want to focus on a specific spot, and I get the correct focus under normal light conditions - then I will still get the focus in low light conditions.

 

Correct?

 

Best regards

Kenneth

[VST_Man 1]

agreed..................

[Cooperman 0]

Hi Kenneth,

 

The IR corrected bit is something of a red herring.

 

Depending on the lens arrangement, if it is a fixed or 'no iris' lens, with the cameras 'Electronic Iris' controlling the exposure, then focussing in any light level will produce the same result.

 

If however, the lens is an 'auto iris' type (either video or DD), then you need to force the lens to maximum aperture before setting the focus(as rory has suggested).

 

If you focus an AI lens in bright lighting, the aperture will be quite small, and the depth of field will be significantly increased; so you could effectively be setting the focus at any point within the enlarged Depth of Field.

 

When the light level drops and the iris opens, the image could then go out of focus as the Depth of Field contracts.

 

The purpose of a light reducing filter is simply to allow a reasonably exposed image to be focussed, when the iris has been set to maximum aperture. Without the benefit of a light blocking filter, there may only be the slightest of shadow areas visible in a white image, which isn't the easiest thing to focus upon.

[k_jensen 0]

Hi Cooperman,

 

thank you for your reply.

I can see your point, but are still not fully convinced

Maybe I don’t understand it

 

Picture a room with bright sunlight. In the middle, I have a chair to focus on.

 

If I focus without filter I have a focused picture with bright sunlight.

If the light drops, the DOF moves, I agree. But it will only be front DOF and back DOF that moves?

 

Like this: http://www.donaldnealmckay.com/sharedImages/DepthOfField.gif

 

I agree that it could be easier to make correct focus with a filter, but it can be done without!

 

regards

Kenneth

[rory 0]

If not convinced, give it a try ...

[k_jensen 0]

he he, could do that.

 

Kenneth

[jisaac 0]

it can definitly be done without. But why do it without? Makes it a whole lot less adjustments (possiblilty).

[Airick 0]

In bright light, you get a small aperature from the auto iris. Images will appear more focused through a smaller aperature. Therefore, if you set your focus in bright light and it looks good, the image may go out of focus when the light level drops.

 

Basically, in bright light, you do not have to be as precise with your focusing controls because there is a wider range where the image will appear in focus. So, most people will tell you to focus in low light and the image will remain focues in normal and bright lighting.

[k_jensen 0]

In bright light, you get a small aperature from the auto iris. Images will appear more focused through a smaller aperature. Therefore, if you set your focus in bright light and it looks good, the image may go out of focus when the light level drops.

 

Basically, in bright light, you do not have to be as precise with your focusing controls because there is a wider range where the image will appear in focus. So, most people will tell you to focus in low light and the image will remain focues in normal and bright lighting.

 

ok, thats why "I cant get it" - nomally you have a bigger DOF in bright light.

So if you get the focus right in bright light, then it would be right when the light drops. See my picture linke above.

 

I agree that it is much easier to focus with a filter - but not required.

 

Kenneth

[jisaac 0]

TRY ABF ON YOUR NEXT ORDER.

[rory 0]

Kenneth are we on IR corrected Lenses or Regular Lenses?

[k_jensen 0]

Kenneth are we on IR corrected Lenses or Regular Lenses?

 

Panasonic Vandal proof dome SDII chip - day/night, cant remember the number CW-474 maybe. Im sure they use IR corrected lenses.

 

Kenneth

[jisaac 0]

use a filter the sdII and even the SDIII with abf still need you to get it set right

[rory 0]

Kenneth are we on IR corrected Lenses or Regular Lenses?

 

Panasonic Vandal proof dome SDII chip - day/night, cant remember the number CW-474 maybe. Im sure they use IR corrected lenses.

 

Kenneth

 

The lens is seperate normally, but I dont know that camera so cant say for sure.

[k_jensen 0]

use a filter the sdII and even the SDIII with abf still need you to get it set right

 

the dome do not have abf.

Only body camera CP-480

[rory 0]

my bad i didnt see the dome part .. generally Day Night domes dont use IR corrected lenses, but it depends on the manu .. Extreme does on certain domes .. Pano may also but it would have to say in the specs.

[rory 0]

Lens doesnt say it is IR Corrected, in fact it doesnt appear to be a very low lux camera if you check the specs, pretty much similar in lux levels as the Sanyo.

If you really want a dome with an IR corrected lens then you may want to check out the Extreme models.

 

0.8 lx (0.08fc) at color imaging mode, 0.1 lx (0.01fc)

at B/W mode (with optional clear dome cover)

F1.4 (Wide) to F176, close / F1.8 (Tele) to F176, close

[Cooperman 0]

For what it's worth k_jensen, the link you posted shows Depth of Field diagrams which are fundamentally wrong.

 

If you've always managed to get away with focussing cameras with the lens stopped down, then obviously you're a much luckier man than I.

[k_jensen 0]

For what it's worth k_jensen, the link you posted shows Depth of Field diagrams which are fundamentally wrong.

 

If you've always managed to get away with focussing cameras with the lens stopped down, then obviously you're a much luckier man than I.

 

Wrong? DOF is big in bright light, and small in "no light" - that can not be wrong.

 

I am a lucky man!

[kandcorp 0]

So what do your guys use for focus filters. A piece of tinted glass or is there a specific material that works the best.

[rory 0]

best thing is low light like 6pm ish ///

[jisaac 0]

#5 welders glass. Works Perfect. As along as you can put it all the way against the glass without any light getting through.

[rory 0]

if you can buy that in your country .. noone down here ever heard of it, tried polorizing filters but they are useless in our bright sun .. tried a tinted glas also and stilll useless ... in fact, the biggest problem is the glare on the CCTV Monitor from the Sunlight // so i always focus in evening time (just when its getting dark) for Day Night cameras (and in BW mode), or at least overcast for all other outdoor cameras. This ofcourse is to get a perfect image.