Understanding ISO, shutter speed and F stop

[HowardinOregon]

In a recent forum entry a photo of the moon taken when it was close to the earth included the metadata showing a relatively high iso, lens stopped to f16 and a fast shutter speed (image taken on a tripod). I understand the aperture setting to permit a deeper depth of field. However, I don’t understand why the iso was set so high when the shutter speed could have been set much slower to obtain the same EV. Assuming motion blur and depth of field can be controlled, wouldn’t one obtain better resolution at iso 100 rather than 1600 by using a slower shutter? What is the benefit of not using the lowest iso possible given that the desired depth of field and motion blur or camera shake can be controlled while properly metering (using a tripod for example) by simply slowing the shutter?

 

[ARShutterbug]

Using an aperture of f/16 on anything but large film sizes can cause a decrease in image sharpness due to diffraction. Using an aperture of f/11 or f/8 would be better.

The lowest possible ISO Speed does not necessarily mean the best sharpness. Some cameras like ISO Speed 200 better, which also depends on how you are processing the negative.

The exposure time should be set fast enough to freeze the motion of this object, and because it's normally so bright, there's little point in dragging the shutter.

Why a person uses, or appears to use, some such exposure settings isn't really relevant to you. Don't try copying other people, especially based on their apparent EXIF data. Learn what works best by trying it yourself.





--
http://www.alexanderrogge.net/arshutterbug

 

[AnthonyL]

I'll just add to Alexander's comments that setting and aperture for DOF at these distances is irrelevant. Try punching in the numbers at http://www.dofmaster.com/dofjs.html

The aperture is generally set to hit the sweet spot of the lens for optimum sharpness. Generally many lenses are soft wide open and you will hit diffraction issues at small apertures.

With a modest ISO, and bearing in mind that the moon is a "midday" lit lump of rock, the resulting shutter speed will be quite sufficient to avoid motion blur on a tripod mounted camera, and even with a carefully hand held camera with stabilisation (on a long focal length lens it might be worth upping ISO to ensure any camera shake blur is minimised).

 

[Barrie Davis]

I understand the aperture setting to permit a deeper depth of field.

The aperture setting of f/16 doesn't make any sense for a Moon shot. Great Depth of Field is not needed... everything in shot is many millions of miles away, sufficient to be considered a flat subject without depth, meaning ANY aperture would do....

... the sharpest aperture being about two stops down from maximum, whatever that is.

However, the lens in question may have been restricted to an effective f/16 by the fitting of some tele-extender device, or another. If the lens was an f/8 maximum previous to installing a 2x extender, then its maximum would become limited to f/16 when fitted.

[This exact topic has been discussed, here....]

http://forums.dpreview.com/forums/read.asp?forum=1018&message=41484710

However, I don’t understand why the iso was set so high when the shutter speed could have been set much slower to obtain the same EV.

Slow shutter speeds are not appropriate to shooting the Moon. When the disk of the Moon is large enough to fill the frame, its apparent motion is also magnified. To get sharp pictures at long shutter speeds the camera therefore needs to track the moon, automatically keeping it in frame. This is done with a special clockwork/electric device mounted between camera and tripod called an 'equatorial'.

If you haven't got an equatorial, the exposure has to be brief. With reasonably wide aperture lenses being quick is not so difficult. The Moon is a landscape in full sunshine, after all...

... even if it is dark here on Earth, it is very light on the Moon!
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

 

[Graystar]

It would help if you can give a link to the image so we can see it.

The aperture used should be the sharpest aperture for the lens. When I shoot the moon with my 70-300mm zoom at 300mm, I use f/10, as that seems to give the best sharpness. Using a smaller aperture only makes the image worse due to diffraction.

The ISO used should always be the lowest possible to minimize noise. The only reason to raise ISO is if there's a shutter-speed requirement.

The moon is in motion, and so you actually need to freeze it when shooting. But it's not moving very fast, and 1/125s is good enough to freeze it at 300mm. The longer your focal length, the faster the shutter speed necessary. You can also use a mount that tracks the moon. Such a mount will let you use much longer focal lengths and you can shoot at whatever shutter speed you want.

This moon shot was taken with a Nikon D90 at 300mm, f/10, 1/125s, and ISO "L1.0", which is actually one stop below my base ISO of 200.





.

 

[BlackMagic]

The aperture setting of f/16 doesn't make any sense for a Moon shot. Great Depth of Field is not needed... everything in shot is many millions of miles away, sufficient to be considered a flat subject without depth, meaning ANY aperture would do....

The moon averages about 237,000 miles away, not "many millions." Nonetheless, the point stands.

 

[Leonard Migliore]

This moon shot was taken with a Nikon D90 at 300mm, f/10, 1/125s, and ISO "L1.0", which is actually one stop below my base ISO of 200.

What is the benefit of using "LO 1.0" in this situation? I thought that the LO settings reduced dynamic range without improving noise performance.

--
Leonard Migliore

 

[Barrie Davis]

The aperture setting of f/16 doesn't make any sense for a Moon shot. Great Depth of Field is not needed... everything in shot is many millions of miles away, sufficient to be considered a flat subject without depth, meaning ANY aperture would do....

The moon averages about 237,000 miles away, not "many millions." Nonetheless, the point stands.

Thanks! I'll try to be more precise about that kind of thing, in future.

Yeah... the Moon is strictly "local".... just around the block! ;-)
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

 

[Graystar]

This moon shot was taken with a Nikon D90 at 300mm, f/10, 1/125s, and ISO "L1.0", which is actually one stop below my base ISO of 200.

What is the benefit of using "LO 1.0" in this situation? I thought that the LO settings reduced dynamic range without improving noise performance.

What the expanded ISO range does is trade highlight dynamic range for improved noise.

Essentially, the expanded ISO function is like having built-in ETTR. All it's doing is overexposing the scene by one stop, and remapping the signal levels so that the levels that are usually associated with one stop over mid gray, are now rendered as mid gray. It's exactly the same as overexposing by one stop and then applying -1 EC in post processing. This reduces noise.

I find that it does help in the darker areas because of the nature of the image. First, because of the size of the moon, this image is usually viewed at 100% so any bit of noise is visible. second, due to the softness of the Nikon 70-300mm VR lens (even though it's the best of the 70-300mm lenses,) this image needs lots of precise sharpening. Reducing noise helps keep the smooth areas from getting too many sharpening artifacts.

I've also found that using Lo 1.0 is good for portrait shots where lighting is controlled. I can easily give up the stop of highlight room for reduced noise levels. Again, ETTR is all it really is...but it saves the step of having to fix the exposure, and that works well for image review when shooting. There's a definite improvement in noise in the shadow areas of a face...especially anything blurred.

.

 

[HowardinOregon]

Okay,

1. The moon is in motion so I need to stop it by using a faster shuttter e.g. 125/sec. @ 300 mm.
2. f16 may be stopped too far becasue of defraction.

3. Assuming an f4 lens stopped to f8 for best sharpness, wouldn't one want to have the lowest iso with resulting shutter speed (whatever that turns out to be) to obtain the optimal exposure? My thinking is lower iso=better noise reduction. So why would one increase iso with what appears to be an excessive shutter speed of say 1/1000 when the same Ev can be obtained by reducing shutter speed to match the lowest iso at f8?

4. Thanks for the informtion about an equitorial. What other practical uses does such a device have (moon shots are relatively infrequent)?

 

[Graystar]

3. ...My thinking is lower iso=better noise reduction.

And that's correct. There's no valid reason for increasing ISO beyond the base ISO of the camera. Like I said in my other post, I actually go one step below base ISO. However, that's not true for all types of astrophotography. There are some techniques where high ISO is used.

4. Thanks for the informtion about an equitorial. What other practical uses does such a device have (moon shots are relatively infrequent)?

Equatorial mounts are used for astronomy. When you're viewing objects in the sky, it helps to have an equatorial with a clock drive. I have a 1500mm telescope and at the highest magnification you can see objects moving across the field of view. The clock drive holds the image steady by rotating the mount opposite the rotation of the earth.

For serious high-magnification shots of the moon you actually need a computer driven mount. An equatorial with a simple clock drive is great for viewing objects, but the moon is in orbit around the earth and so its rate of movement is different than other objects in the sky. This is important for long-exposure shots as well, such as a lunar eclipse shots.

.