Understanding ISO - The Sensitivity of Your Sensor

In a discussion about creating deliberate slow exposure the discussion got around to ISO. I put in this answer (slightly edited for here) and I thought my answer might be useful more widely so hence this post.

ISO is the measure of sensitivity of the sensor in your camera. The lower the ISO the better the quality of the image.

The acceptable ISO settings are quite camera dependant and can be subjective. Some will give little or no noise up to 6400 iso. As a general guide here is what I use:
Landscape on a tripod 50 - 100 iso
Landscape hand held 100 - 400 iso
Street daytime bright 100 iso
Street dull day 400 iso
Street evening whatever it takes - I can shoot at night with 6400 with acceptable hand held results
Stop action (eg. bike racing) what ever ISO it takes to get the fast shutter and long dof that I want
Panning action 100 iso generally
In old school photography with film (aka classic) we were taught the relationship between shutter and aperture but rarely discussed ISO because we couldn't change it (except with advanced dark room techniques). Today we need to think of shutter aperture and ISO as part of a balanced triangle because we can change the ISO whenever we want.

When you change one of the tips of the triangle it affects the other tips and your image. The higher the ISO the less light you need to activate the sensor to create an image but the more noise (unattractive colour or brightness changes from pixel to pixel). If you need a faster shutter to stop the action and have a sharp image at an acceptable depth of field then the only element you can change to achieve that is the ISO.

As an example, during the some street photography I was photographing a dancer at night, I wanted sharp, she was moving quickly and I wanted a short dof - I wanted to see the crowd watching her but I wanted attention on her. I didn't mind a bit of noise and I didn't want to distract her with flash. I needed at least 1/20th to hand hold with acceptable sharpness and I wanted f/1.8 for very low depth of field. I changed the ISO until my light meter indicated an acceptable exposure at ISO of 500 and took the shot. Well a series, never rely on just one! Between the first few I checked my outcome and made minor adjustments. I was actually very lucky with this. 1/20th is nowhere near what I usually need for a hand held with sharp results - as a general rule of thumb for my and my shaky hands I double the focal length so in this case I should use 170 - I braced my body against a light pole to minimise my movement.

Light, Patterns, Textures, Flows and Creativity

Everywhere around us there is light. The light reveals patterns, textures and flows in everything. We can add to the light and we can use the patterns and textures. We can represent the flows as a moment in time or as a time frame through long exposure.

Recently I started to play with glass spherical orbs, I've got several different sizes and colours. They are a lot of fun and you can really unleash some creativity with them. They can be shot with themselves as the subject and a part of the composition as I have above where the orb rests on the granite edifice of a building. The harsh tungsten lighting thrown from right of camera highlights the shape and form of the granite and is relit by the light cascading from the orb itself. I really enjoy the contrast between the harsh hand chipped rock wall blocks and the smoothness of the orb.

Moving away from the orb for a moment, we come to flows. At Monash University I found a small stepped fountain lit from under the water. I spent some time with this small flow of water - you're looking at around a 10cm drop in height with a very gentle cascade barely able to promote any bubbles. Grubbing around on the floor with my lens just about in the water got me the angle and appreciation of the light I was looking for. This gentle golden flow is reminiscent of many things from beer through a bubbling stream. When I first posted it on g+ people were guessing what it was for quite a while. This to me is the mark of a successful abstract. It's part of a greater whole or context but when this small unrecognisable part is removed from that context it becomes abstract.

Still with the flows and textures is another image I've shared on Google+ which kept people guessing for most of a Saturday. A harmless game encouraging people to recognise what is going on. At the time of posting this image no-one has figured out yet that this is the result of a laser beam being shot into an acrylic block. The shape, flow, form and texture is amazing. It reminds me of a rocket climbing into the sky or the water blast of a powerful fountain. Shortly people will read this and know what it is. How do I know? Too many late night stints on Discovery Channel and Discovery Science. When a laser is built for a laser cutter the laser generator is tested and tuned using acrylic blocks as they allow the test engineer to see the beam and how it's focused and directed.

There are wonderful textures and shapes everywhere, while walking through Melbourne Observatory with some friends late one night we found this interesting orb of dark material with stainless steel bands. Spectacular. Capturing the texture at night was an interesting experience. I tried to balance the depth of field to capture the spots dots and fingerprints of curious children from during the day with the need to capture the feel and colours overall.

These stainless steel roof panels were suspended on tight cables with an interesting wave pattern over head. The panels interlock and provide protection from the sun and during the day are somewhat mundane - but at night - a whole new magic. Each panel is sitting differently to its neighbor so the qualities of the light and reflection are different on each. Good fun. While this is still a traditional thirds type of composition it moves away from the regular thirds using the diagonals to break the image up. The central star attracts the eye away from the brightest and darkest areas and holds it captive.

So the point of this blog entry? It pays to look all around you including up in the quest for light, patterns, textures and flows. Use them to help with your creative thinking, your ability to see something that others don't. You are a photographer, your role is to find the interesting within the mundane and expose it to other people. Good luck with your quest.

As always, write and comment if the mood strikes you. All feedback and criticism is welcome.

Too Much LIght? Neutral Density May Be The Answer

What are Neutral Density Filters?

Neutral Density (ND) filters are simply a filter that mounts in front of your lens either by themselves or in combination that reduce light over some or all of the frame. They can come as screw mount or as square filters that use a mounting system to hold them. They can be made of plastic or glass. There are many brands. You tend to get what you pay for. Cheap Chinese versions can be had on ebay for a few dollars or you can utilise brands such as Cokin and Lee. The cheaper filters cut the light but introduce colour casts (chromatic aberation) in the resuling image. They can introduce softness too.

They come in various ratings, the most common being one stop ND2, two stop ND4 and three stop ND8. There are others right up to ND400 otherwise described as black glass.

You will noticed in the four images above which are the same composition but with no filter, ND2, ND4 and finally ND8 that the exposure of the sky changes but if left to its own devices the camera will change other areas too as it compensates for the darker area.

Generally speaking the best way to make use of these filters is on manual mode. Yes I know that's scary but it does not have to be. Expose an image so that it's right for the darker foreground, in this case the sand. Then add the filter without changing the exposure. In the examples above I left the camera on AV and as it was using evaluative metering across the frame it over compensated for the dark band of graduation at the top. This is especially noticeable in the right hand image where the sand is quite dramatically whiter and brighter.

Rating	Amount of light passed	f/stops lost
ND2	50%	1
ND4	25%	2
ND8	12.5%	4
ND400/Big Stopper	0.1%	10

Note that the above table is not strictly accurate - especially the ND400 (it's really about 8.75 stops), I've tried to keep it simple so use this information as a exposure guide to find your starting point then trial and error from there.

Full

Full frame ND filters are used where you want to reduce the overall light. They have many uses, but the most common is where you wish to use a large aperture such as f/1.8 on a bright day or bright scene. Choose a full frame when the lighting over the scene is even.

Graduated Hard and Soft

When the scene is not evenly lit and there is a lot of contrast from one area to another a graduated filter can be helpful. Half the filter reduces the available light and the other half is clear. The boundary between the halves can be hard or gradual. Hard edge filters are most useful where the edge itself is hard, in architecture and from sky to sea for example. Gradual filters are better where the transition is also more gradual, such as the forest canopy or a rolling meadow. The filters in the image above are gradual.

Black Glass

Black glass also known as big stopper or ND400 are a special purpose ND. They can allow you to manage a daylight exposure so long you can smooth the sea, eliminate traffic or even see a world without people.

In this image I'm holding my black glass to obscure the sun and it's light trail across the water. The exposure is made for the overall area then I held the filter in place. I felt this was the best way to show the dramatic affect that this black filter has on the available amount of light. It doesn't let you expose the sun properly but it does cut out an enormous amount of light.

How Are They Used?

Once affixed to your camera you use them as though you would use any lens. 

Be aware that in camera automatic exposure software often fails when you have an full ND fitted. Generally you'll have to calculate your own exposure values changing one or more variables of the exposure triangle (ISO, aperture or shutter speed) to increase your exposure by the number of stops of the ND.

For graduated ND you expose for the dark area and let the ND cut down the light in the bright area. 

Be aware that auto focus systems often don't work as you would expect with ND filters fitted. Some cameras and lenses manage better than others. I find that my camera manages a good focus lock with all but the black glass. With that it might work in extremely bright environments.

Use your camera to measure your light and set the focus and change to manual focus then carefully fit your ND filter(s) and adjust your exposure to suit. Note that if your ND is screw in and your focus ring turns the barrel of your lens then you have to be very careful not to accidentally change focus when fitting the filter.

I find that while the filter may claim 1, 2, 4 or 10 stops of light reduction this rarely turns out to be quite so simple. Adding 10 stops either by opening the aperture  increasing the length of the exposure or boosting the ISO by 10 stops usually does not yield a well lit image. If you've lost colour and your histogram is flat and not full width then you have not exposed sufficiently, try again with allowing more light in.

When would you use them?

Full Frame and Black Glass

The most common reason for me to dig an ND out of the bag is to increase exposure time. I use ND for making milky waterfalls, eradicating pesky people from a scene or perhaps creatively adding drama by having your model stand still while everyone else gets blurred.

This image by fellow Melbourne photographer +Trace McLean shows a creative use of the ND. His camera is set for a long exposure and he stands still while the peak hour people stream around him.

This creative use gives an impression of a rock washed by tides that shall not be moved.

Other interpretations include being unchanging in a world of change.

Of course you could replicate this in photoshop but that would spoil the fun of actually standing in the path and getting in the way unmoving.

The image was published on g+ and is used here with permission from Trace and shows the steps of Flinders Street Station in Melbourne under the famous clocks.

The image to the right is a sunset image from the groyne at Mentone near the end of Warrigul Rd.

The use of the ND400 allowed for a very long exposure that would normally not be possible at that time of day. This allows the eye to view the foreground detritus, the groyne itself and the path of light to the sun as the normally distracting wavelets of the sea are gone.

The black glass is also used to remove unwanted distraction and to add serenity to a scene. In the left image below there is a dog by the rock, a number of birds in the air and chop on the surface of the water. In the right image these are simply gone leaving the oily feel of the smooth water leading the eye to the distant shore and pastel sky.

The black glass also lends itself to creating characteristic mist that occurs in long exposures where there is bright light and waves.

All the ND is doing here is allowing for that wonderfully long expoure letting the magic of light and nature take care of the rest.

My final example use is astronomical  in 2012 Venus transited the Sun. The planet crossed between the sun and earth occluding a small area of the sun.

To achieve this shot I utilised two of the ND400's stacked together and my 70-200 at f/64 as well as the 2X extender which eats another 2 stops. This enormous reduction in light enabled me to catch this rare event in bright daylight. That black area of the image was bright blue sky.

I must urge caution when photographing the sun. Ever seen an ant fried with a magnifying glass? Now imagine that heat concentrated onto your camera mirror, sensor shutter curtain and the sensor itself or right into your eye through the view finder. Always use live view on the LCD to avoid blinding yourself.  When shooting the sun I keep the lens and camera covered with a dark towel until I'm ready to shoot. I shoot then re-cover before any noticeable heat build up can occur.

This image created during the White Night festival in Melbourne shows hundreds of thousands of people that have been turned into a brown mist by a black glass assisted long exposure of several minutes. The filter basically made them vanish.

Imagine what you could do with less people? Want to empty Bourke St at lunchtime? Get a clear image of the Flinders St clocks during the day? Black glass and a long exposure is the answer.

Mixing white cap waves with long exposure and good light yields a wonderful misty effect showing where the wave heights are as they come through. You get a base level which is the normal sea level without waves and a layer of mist the height of the waves. Black glass with a long exposure during the day or during the blue (pre dawn) or golden (pre sunset) hours will help you to generate this effect in your images. At the height of the day you may need to stack two or more black glass filters.

As a final image for this blog you can do wonderful things with waterfalls. Even small ones like this at Shiprock falls near Gembrook.

The black glass along with a long exposure develops the water into a white mist which looks so much better than stop motion water. The mist conveys the movement and even the sound of the waterfall to the viewer.

Graduated

I utilise graduated ND a lot. I often use them at the beach and when creating landscapes. I've also used them to reduce the glare off water and to mask bright spots when photographing the interior of a structure with big windows. You can also use them in architecture where you want a blue sky with a reasonably well exposed building.