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Problem w strange light

nick carulli , Mar 07, 2007; 03:47 a.m.

I have acanon 1ds mark 11,16.7 mp. Recently while shooting indoor sports I noted 1/4-1/2 frame of random photos there was a yellow discoloration along the long axis of the image.( It was not a dirty sensor or software proble w photoshop.) it was not on each image. It also could not be from a different temperature of light since 3 frames shot at the same time and only 1 frame had this. It was sort of like when the flash sync was off on film camrea. I was shooting asa 1000, and it was most apparent on the 85mm 1.2. It was not always on the same side of the image, it was sometimes in the middle. Anyone else seen this?It does not seem to happen on outdoor subjects. The lenses are clean as well.

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Raphael Emond , Mar 07, 2007; 04:14 a.m.

Under fluorescent lighting, it's quite possible. The faster the shutter, the worse the problem. Try to stick to 1/60 or 1/120s

Jim Strutz - Anchorage, AK , Mar 07, 2007; 04:30 a.m.

The lights are flickering with 60 cycle current, and your faster shutter speed used with the f/1.2 lens caused it to show. This happens with many types of efficient lighting systems commonly used in indoor sports buildings.

As Raphael said, you have to keep the shutter slower if you want to avoid it. But I think 1/120 is still too fast. Unfortunately, this will likely cause motion blur with many subjects, and you can only use so many panning shots.

Rainer T , Mar 07, 2007; 05:04 a.m.

-- "The lights are flickering with 60 cycle"

Actually, the light is flickering with twice the frecuency of the supplying AC. That is because the emitted power is the product of current and voltage.

The voltage is U=Uo * sin(wt)

The current is I=U/R or I = Uo * sin(wt) / R

The power is P = U * I = Uo*Uo * sin(wt) * sin(wt) / R

that is Uo*Uo * (0.5 - 0.5*cos(2wt)) / R

as you see, the term in the cosine function now is 2wt (instead of wt in the sine function). Therefore, the frequency has doubled.

This is also easy to visualise ... the emitteds power is the product of two sine functions. The peaks in emitted power are at the spots where the sine functions reach their positive maximum as well as where they reach their negative maximum. So, in one cycle of the sine function, the power function had already two cycles (aka double frequency).

I wouldn't have mentioned this, because it seems to be a bit nitpicking ... nevertheless, the frequency is important to choose a correct shutterspeed.

Rainer

Mark U , Mar 07, 2007; 06:17 a.m.

With fluorescent lights, I'm not sure that Rainier's analysis is strctly correct, since at lower voltages some of the phosphors are not emitting at all at higher wavelengths - which gives rise to the changing colour balance. He is correct that peak output happens on each positive and negative portion of the cycle, so the effective frequency is twice the mains frequency. There's a nice example posted in this thread:

http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00EFxL&tag=

Rainer T , Mar 07, 2007; 07:04 a.m.

My analysis is only correct for the electrical power consumed, not for light being emited ... (because light emission doesn't go back to zero immediately when electrical power consumption reacheds zero) ... I was just commenting on the frequency.

There are some interesting comments in the thread you mentioned ... which in afterthought bring me to the persuasion, that one should stay a bit below X-sync speed of the camera used, if tube/fluorescent-light is used as a main source of light. That way, variations in lightstrength and color can influence the complete frame. Using a very short/fast setting like 1/4000sec will not help, because the shuttercycle still takes about X-sync time to complete ... A faster/shorter time will only narrow the horizontal band. The trick is to use a slower/longer time, and thereby spread the band over the whole frame.

Rainer

Mark U , Mar 07, 2007; 09:51 a.m.

Actually, a fluorescent tube has a resistance that falls with increasing voltage - hence why they are operated with a ballast that limit the current. Probably more than you want to know about fluorescent tubes:

http://home.howstuffworks.com/fluorescent-lamp6.htm

(link)

(link)

X sync isn't the key variable here - the important thing is to capture the output from an exact cycle or number of cycles of light output. If the shutter curtains move slowly across the frame so that only a strip of the sensor/film is exposed at the shutter speed, so long as each portion of the sensor/film is exposed for an exact cycle or multiple the lighting will appear even, although different parts of the sensor/film will start and end their exposure at different points of the cycle.

Rainer T , Mar 07, 2007; 10:25 a.m.

Mark,

on modern cameras X-sync is always around 1/100sec or faster (thereby X-sync is always near the timing of one power-cycle of the tube or X-sync is faster ... but X-sync is never slower.

Shorter times than X-sync do expose each individual location of film/sensor shorter than X-sync-time (and thereby never for one or more power-cycles), the whole exposure takes a bit longer than X-sync-time.

Therefore, not shutter times shorter than X-sync exist, where only a small strip of film/sensor is exposed for at least one (or more than one) cycles.

So, only times of at least X-sync-time will help.

An exception would be a camera with very slow X-sync of say 1/30 sec, where you probably get a good picture with 1/50sec (since your assumption now applies).

Rainer

Russ Savage , Mar 07, 2007; 12:21 p.m.

maybe someone could give him some actual useful advice regarding his situation. any other options besides use a longer shutterspeed?

Bob Atkins , Mar 07, 2007; 01:15 p.m.

I doubt it. If you shoot at high speed under fluorescent lights, this can happen.

The only advice is to use a slower shutter speed or not to shoot under flickering fluorescent lights, neither of which helps a great deal.

There are some things you just can't do.


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