Manually lowering the ISO speed (<100)

I read that some of the Canons have an option to turn the iso speed down to 50, maybe the Nikon's have this too? Anyway, mine only goes down to 100, which is probably low enough. But I am thinking about doing starscapes, or at least trying to capture something half decent with what I have. I think the technique is to take a series of 30s exposures and merge them together somehow. Also, no reason why I can't take a shot of the foreground using a different method and layer it onto the starscape.

Anyway, maybe there will be a time when lower ISO speeds for longer exposures will come in handy...

So I was just wondering if this can be achieved by simply placing a filter on the lens? ie. a filter which reduces the intensity of the light by 50% would surely effectively also reduce an ISO setting of 100 to 50, or maybe not to the same scale. There are obviously going to be issues with uniformity over the spectrum of light but in theory that should work, right?

Sounds like you want a neutral density filter. They come in various strengths. Using one will slow down your shutter speed to create certain effects such as blurred waterfalls, but it won't alter the ISO. For star photos, I don't think you will want an extended length of time because of the star movement.

Some Nikons have an additional feature to lower the ISO below the normal range although after checking, it doesn't appear to be an option on your camera.

Below is a photo I shot with my D600 at ISO Lo1. Flickr reads it as ISO 50 although that isn't how the camera lists it. Unfortunately at the time I did not own an ND filter. I used a Circular Polarizer to slow down my shutter speed as much as possible. Now I own a 6-stop and a 9-stop neutral density filters. If you ever decide to buy one, make sure to buy it to fit the largest diameter lens you own. Some Nikon lenses take a 77mm size filter while others take a smaller size. You can always use step up rings to fit a larger filter on a smaller lens. That way a filter can be used on all the lenses you own.


Neshaminy Creek Waterfalls - Autumn by *Hark*, on Flickr

Any ISO other than "Native ISO" involves either amplification or attenuation of the sensor data, which has the potential to introduce noise. So, going to ISO 50 does not, as you might assume, lower your noise level. What it does allow is for you to shoot at a lower shutter speed than you might otherwise be able to do on a bright day. As Hark mentions, using an ND and staying in native ISO is a better solution, but just like high ISO's, it's there when you need it and it's your only option.

A 2x ND filter will effectively drop that ISO 100 to 50. I often have to use them when doing portraiture outdoors because although the synch speed of my D700 is 1/250, I have to drop it below 1/200 to synch with my wireless strobes. It also lets me shoot my 180mm f/2.8 Nikkor at f/4 for minimal depth of field.

dr_eyehead said:

I read that some of the Canons have an option to turn the iso speed down to 50, maybe the Nikon's have this too?

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You are thinking too hard. Do what hark did with her settings which is ISO Lo1 = ISO 50 if that is something that you need.

hark said:

Sounds like you want a neutral density filter. They come in various strengths. Using one will slow down your shutter speed to create certain effects such as blurred waterfalls, but it won't alter the ISO. For star photos, I don't think you will want an extended length of time because of the star movement.

Some Nikons have an additional feature to lower the ISO below the normal range although after checking, it doesn't appear to be an option on your camera.

Below is a photo I shot with my D600 at ISO Lo1. Flickr reads it as ISO 50 although that isn't how the camera lists it.

Click to expand...

gqtuazon said:

You are thinking too hard. Do what hark did with her settings which is ISO Lo1 = ISO 50 if that is something that you need.

Click to expand...

You're not thinking hard enough. I don't have ISO Lo1 on my camera. Hark has a different camera, D600.

Use a neutral density filter as others have mentioned. They can be purchased in strengths anywhere from less than one stop up to 10 stops. Here is a cheat sheet with information on ND filters. Choosing the best ND filter: remember these 4 tips (and save this cheat sheet!) | Digital Camera World - page 2 Even if your camera had the Lo1 setting I would strongly recommend against using it especially for star shots. It reduces you dynamic range and overall picture quality. You are better off using ISO100 and ND filters.

Useful info thanks. Like the look and the price of this variable ND filter here>

52mm Variable ND Neutral Density Filter for Nikon: Amazon.co.uk: Electronics

Just be aware that some variable ND filters (especially the cheap ones) can give you uneven density at some settings. I have seen some where they create a cross pattern that is darker than the rest of the picture. Just like any filter, cheap quality filters are not worth it. Pay the price to get a decent one.

dr_eyehead said:

Useful info thanks. Like the look and the price of this variable ND filter here>

52mm Variable ND Neutral Density Filter for Nikon: Amazon.co.uk: Electronics

Click to expand...

This product doesn't state what range of densities it offers. I would be a little leery of this one.

Steve B said:

Just be aware that some variable ND filters (especially the cheap ones) can give you uneven density at some settings. I have seen some where they create a cross pattern that is darker than the rest of the picture. Just like any filter, cheap quality filters are not worth it. Pay the price to get a decent one.

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Surely by design a variable filter that uses two polarised filters at different angles should have the same density across all settings?

Variable ND's are OK, but you need to buy expensive before they give you good performance beyond 6 stops (after which you get streaking and hot spots). If you do buy one, I highly recommend buying a 77mm and step down rings - this way you have one filter that can be used on all your lenses. Buy small and you buy again.

dr_eyehead said:

I read that some of the Canons have an option to turn the iso speed down to 50, maybe the Nikon's have this too? Anyway, mine only goes down to 100, which is probably low enough. But I am thinking about doing starscapes, or at least trying to capture something half decent with what I have. I think the technique is to take a series of 30s exposures and merge them together somehow. Also, no reason why I can't take a shot of the foreground using a different method and layer it onto the starscape.

Anyway, maybe there will be a time when lower ISO speeds for longer exposures will come in handy...

So I was just wondering if this can be achieved by simply placing a filter on the lens? ie. a filter which reduces the intensity of the light by 50% would surely effectively also reduce an ISO setting of 100 to 50, or maybe not to the same scale. There are obviously going to be issues with uniformity over the spectrum of light but in theory that should work, right?

Click to expand...

I can't understand why you would want the ISO so low for shooting starscapes. Take a look at some of the examples in the low light forum. It's very common to see ISO in the 3200 range.

SteveL54 said:

I can't understand why you would want the ISO so low for shooting starscapes. Take a look at some of the examples in the low light forum. It's very common to see ISO in the 3200 range.

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Is this the same ISO setting one should use for city lights from a distance?

Here is a cityscape of NYC ISO 2000 shutter 1/4 sec f/4.0




And another ISO 3200 Shutter 1/20 sec. f/5.0

WOW!!! Beautiful shots Steve! So the higher ISO allows more light in?

Increases light sensitivity. But you must pay attention to your exposure triangle, as they all depend on one another.
Google it, or search here for a better explanation than I could possibly give.

It all depends what you want to achieve. You want somewhat fast shutter for fixing the stars sharply. You want slow or very slow shutter (minutes) for start trails and for exposing the foreground if it's interesting enough.

When you go very slow, however, like a night shoot with low ISO and a 10 stops ND filter, you risk not even capturing the low light emitting stars AND you get another kind of noise dubbed 'hot pixel' which you need to remove. That can be done in camera but it means for every exposure you do the camera does another one that's exactly as long (see long exposure noise reduction) which means your star trails become dotted lines. It can also be done in post process, you'll need a 'dark frame' which you'll get by shooting for the same shutter speed with the lens cap ON.

BTW, placing a filter on the lens will not "decrease ISO" a anymore than boarding a plane will make you fly ; for practical purposes it might accomplish the same but there will be a day where "I can fly" will only cause confusion. - ISO sets the sensors sensitivity. ND filter reduces the light that gets in, but so does faster shutter speed or smaller aperture.

Tom Grove said:

WOW!!! Beautiful shots Steve! So the higher ISO allows more light in?

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It is important to understand how a digital camera generates ISO values other than the base ISO value. It does not let more light in. It amplifies the signal and by doing so gives the appearance of higher sensitivity. This comes at a cost though of increased noise and decreased dynamic range. There are a ton of articles about this out on the internet.

Here is a star shot. Shutter open for 20sec. ISO 3200 f/5.6
I've found that any longer than this will usually start to blur the stars.