Not a good pic, but tech questions related

[Tom3]

I was playing with the camera the day the moon was very big:

None of the pictures turned out good...

The pics were done using a D200 with the following settings: f/5.6, 4sec exposure, 100 iso, 98mm focal length.

The blue halo you see was caused by the polarizer on the lens.

but this triggered two questions:

- When I did shots with a wide aperture, I got a warm tint, while when I did it with a small aperture, it was very monochromatic; Could it be that the wider opening lets street light come in more? I mean the same trees (lit by street light) were in the same shots, but the yellow tint was gone with the small aperture.

- I assumed that I could do manual focus and set the lens on infinity: it turns out that no... at infinity the moon is blurry! a tiny tiny bit less and it was sharp; what is the reason? (it's a sigma 18-200, could it be the cheap construction?)

 

[Big Mike]

When I did shots with a wide aperture, I got a warm tint, while when I did it with a small aperture, it was very monochromatic; Could it be that the wider opening lets street light come in more? I mean the same trees (lit by street light) were in the same shots, but the yellow tint was gone with the small aperture.

Good question. I'd guess it has something to do with the different angles that the light is bent at for different aperture. Diffraction and maybe some chromatic aberration etc.

- I assumed that I could do manual focus and set the lens on infinity: it turns out that no... at infinity the moon is blurry! a tiny tiny bit less and it was sharp; what is the reason? (it's a sigma 18-200, could it be the cheap construction?)

By setting it on infinity...did you just turn the ring all the way? On many lenses, this would indeed be past infinity focus....just a little bit. This might have something to do with lesser quality lenses, zoom lenses or maybe it's there to allow for expansion/contraction or slight misalignment.

 

[OldClicker]

Way over exposed if you actually want to see the moon. Try f/16 at 1/125 sec. - TF

 

[Josh66]

By setting it on infinity...did you just turn the ring all the way? On many lenses, this would indeed be past infinity focus....just a little bit. This might have something to do with lesser quality lenses, zoom lenses or maybe it's there to allow for expansion/contraction or slight misalignment.

I think it's like that to prevent the autofocus system from taking/causing any damage from hitting the mechanical stops. Every lens I own will go past infinity (the 135 f/2.8 will go waayyyy past infinity).

On Canon lenses the vertical part of the infinity mark (looks like an "L") is infinity.

 

[Tom3]

right, now thinking of it, it makes sense to have a little slack at the end.

oldclicker: yes, it is overexposed; this shot was to demonstrate that the wide aperture will let the street light in; I should have posted the ones with the small aperture: you can only see the light from the moon, absolutely no tint from the street.

 

[Josh66]

Did you change the shutter speed and the aperture?

If you changed the aperture without also changing the shutter speed, that is why the wider aperture is letting you see more of the street light.

If the small aperture shots used the same shutter speed as this, they would be exposed less.


The above photo was at f/5.6, 4 sec. If the other photo was at f/16 the shutter speed would have to be 32 seconds to get the same exposure. If it was f/16 and 4 seconds, it would be exposed 3 stops less that this one.

 

[Tom3]

yes, I did somewhat match the shutter speed / aperture to get constant exposure, so the street light did not come from the longer exposure, the color was really tied to the wider aperture.

Big Mike's theory is probably right.

I would venture to guess that, with a large aperture, it gives more opportunity for light that wouldn't have made it to enter sideways and possibly bounce a couple times inside the lens before hitting the sensor.

 

[Josh66]

Interesting...

Can you post the other one too?

 

[Tom3]

this one:

Is still 100 ISO, but f/40 and 30 sec exposure

As you can see, there is no tint from the street

 

[Sw1tchFX]

Shoot at f/8, spot meter on the moon, ISO100 or 200 and get that polarizer off, it has no buisness being on there when you're in anything but broad daylight and want to cut out reflections.

You do that, you'll have great shots.

 

[Josh66]

f/5.6 to f/40 is a difference of about 5 1/2 stops, while 4 seconds to 30 seconds is only about 3 stops. Your f/40 picture would have had to be somewhere around 140 seconds to get the same exposure.

That's why it's so much darker, even at 30 seconds it's still exposed about 2 1/2 stops less than the first one.

 

[Tom3]

it makes sense now; so I guess, with a 140 seconds exposure, the same tint would have appeared.

 

[Josh66]

Yes, I'd say that would be the case. The shutter simply wasn't open long enough for the light from the street lamp to register.

 

[AlexColeman]

The reason for infinity not being the best focus, is that it has alot of play, to adjust for temp. humidity, pressure. Shutter wasn't open for street light, only for moon.

 

[shed301]

I have been told numerous times to take your polariser off when shooting at night or moon shots. not sure if it would be useful in this situation though..... just try it and see i guess