Thanks, Brian. I don’t think there is much point in me using optical filters. I don’t have enough knowledge to really know when to use them successfully.
I have all this stuff stored in my head and taking up space. I would be happy to share what I know if you have any specific things you want to know. There have been plenty of books written on the topic. There are probably books written on individual categories of filters.
Any use of filters in photography is easier if you have the classic RGBCMY photographic colour wheel firmly in your mind. If you have the painter’s colour wheel in mind nothing will make sense. The same is true for any type of colour correction on images. Photography involves both additive and subtractive primaries. Filters pass certain colours and absorb or block certain colours.
It is worthwhile knowing that the image sensor in a digital camera is sensitive to light well beyond the range you can see - infa-red and ultra-violet. Even though there are filters in the camera narrowing the range of light hitting the sensor to the visible spectrum it is still possible for light we can’t see to affect the image. In particular, making the sky turn out lighter than the way we see it. Yes, the UV filter actually has a purpose beyond protecting the lens.
Another thing worth knowing is that our eyes are exposed to far more “far blue” light in recent years from computer screens, TVs, LED light sources, phones and tablets, and this type light has been found to be a causative factor in early onset macular degeneration so make sure your computer glasses, or whatever glasses you use for screen work are protected for far blue wavelengths. As of the last 3 to 5 years you won’t have any trouble getting them at any place selling glasses.
There are a few basic categories of filters used in photographic applications.
• A huge number of the old colour balancing filters are irrelevant with digital cameras now anyhow. This was one of the biggest categories of filters and professional or commercial photographers still need a working knowledge of this, especially for location work.
Proper use of white balance (and a white balance reference card) eliminates them. But knowing how to use a white balance card is very useful. Control of white balance is very poor in most camera apps, and it shows. It is much easier to set the correct white balance at the time of exposure than it is to figure out how to fix it later. The only saving grace in this dilemma is that people often want to change the white balance for dramatic effect rather than accuracy.
• The Polarizer filter is in a class of its own since it doesn’t change colours directly but can enhance colours and clarity by reducing the effects of glare, reflections, and haze. It can have a big effect or none, depending on the angle of the light. The biggest danger is overdoing the polarizing effect on skies, making them too dark. Also, the angle of the light changes the polarizing effect and in normal lenses this is no problem. However, with ultra-wide lenses the angle of view relative to the angle of the light will be drastically different from one side of the image to the other, so the polarizing effect could be very different on each side of the image, which isn’t pretty. Something to watch for.
The classic use of the polarizer is to enhance the contrast between blue sky and clouds. I tend to use it more to improve colour on overcast and rainy days by removing the white of the sky that is reflected by every shiny or wet object (like leaves) in the scene. Great for woods and waterfall scenes. Also to remove glare on some surfaces in a close-up that would otherwise lead to exposure problems. For instance the glare off the shiny top of a mushroom, or leaves, etc. Perfect for water photos, especially when you want to see into the water and not the haze reflected on the water. Close-ups of frogs, etc. Eliminate reflections that appear on glass in some scenes (such as your own reflection).
• Close-up lenses are not really filters in the usual sense but might as well be grouped here since they are mounted the same way. Their main purpose is to allow closer focusing than would otherwise be possible. Like reading glasses for your camera. It takes a stronger power of close-up lens the closer you want to get. Typically sold as +1, +2, +4, etc. sets.
• Neutral density filters simply reduce the amount of light coming through your lens. They don’t change any colours in your photo. Their main use is in reducing the amount of light, allowing you to use different camera settings than would otherwise be possible.
On a DSLR or video camera they are use in bright light to allow a wide open aperture to be used in the lens, giving shallow depth of field in order the blur the background. The iPhone always has its aperture wide open so that doesn’t help us in that case.
The other use is to reduce the light quite a bit, allowing a much slower shutter speed for motion blurring effects, like flowing water in waterfalls. ND filters come in a variety of strengths to block different amounts of light. 2 stops, 4 stops, 6 stops, etc.
The word “stop” in photography refers to one complete step in the shutter speed or aperture setting. This has become blurred because modern cameras now offer almost continuously variable shutter speed and aperture settings. So it is important to remember what the steps used to be. Each step is a doubling, or halving, of the setting, whether shutter speed or light.
So the traditional shutter speed scale goes, 1 second, 1/2 sec, 1/4, 1/8, 1/15, 1/30, 1/60,1/125, 1/250, 1/500, 1/1000, 1/2000. It’s easy to see in this scale that each step is double or half the time depending on which way you are advancing in the scale. Each step is “1 stop”.
The aperture scale (also called f-stops) follows the same pattern but the numbers don’t make sense like the shutter speed numbers do. f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16. The weirdness of the numbers is because the f-number is a measure of the diameter of the opening, or aperture, while their effect comes from the area. Once again, each step is referred to as 1 stop and represents either a doubling or halving of the exposure. But the iPhone doesn’t have an adjustable aperture so you can ignore that part.
• Gradient, or graduated, neutral density filters are dark at the top and gradually lighten towards the bottom. The transition from dark to clear can be more or less gradual. Also the degree of darkness might be 1 stop or 2 stops, etc.
Most often used when the sky is very bright and the ground is dark, like sunsets , allowing a single exposure to give a correct exposure for the sky and ground at the same time. It is usually necessary to adjust the height position of the filter relative to your scene so these filters are usually rectangular rather than the round screw-on variety. They are mounted in a special filter holder that allows sliding the filter up/down to the desired position. These filters were more popular with film but still are used in high end photography. An alternative is to combine pictures of different exposures for sky and ground in post or use HDR.
• Then there are the colour filters used for black and white photography. Fairly strong yellow, orange, red, green, blue. Unless you are using film there are apps on the iPhone that internally simulate the use of these filters. So you don’t need any of those either but it helps to understand why they were used so you know how to use the B&W apps.
Even though you end up with a B&W image the film itself is still colour sensitive and the coloured filters are used to alter the relative balance of certain colours to prevent tone mergers.
For instance, you take a picture of a red flower on a green bush but in the B&W photo the red and green look exactly the same shade of grey so you can’t see where the flower begins and the leaves end - the flower disappears. You have a tone merger. This is a very common problem in B&W. It could be any colour/tone combination. The solution is to lighten or darken one or the other colour so there is a bigger difference in how red or green (in this example) is rendered in the final B&W photo. A filter passes it’s own colour and blocks others. It would be your choice whether you decided to have a dark flower against lighter leaves, or a light flower against dark leaves but you need one or the other in order to see the difference in tones.
A lot of people used a yellow filter for B&W if the were doing a lot of landscape photos to make the sky look more the same tone we see with our eyes. Blue skies come out lighter than expected in B&W.
The understanding of how coloured filters controls tonal separation in B&W photos is the single most important thing needed for successful B&W photos. Without that, B&W becomes a crap shoot - sometimes you’re lucky and sometimes you’re not and you have no idea why. You see this all the time in casually made B&W images.
The difficulty when shooting B&W film is that you can’t see beforehand the effect your filter will have in the final image. You just have to know in advance what is going to happen based on experience (and info charts).
There are 2 routes to making B&W on digital cameras. The dumb way is to simply discard the colour information at the start and manipulate the grey-scale image contrast to get the look you want. Avoid any apps that work this way. There are too many situations where nothing you try to do can make the image any good.
The best digital B&W is made from using all 3 RGB colour layers and digitally filtering the layers to manipulate the colours to give the desired tonal separation in the final B&W. This is where an understanding of the colour filters is very useful.
• Tricolour filters: a set of red, green, blue filters in exactly the right densities. This is the earliest form of colour photography. In the early days before colour film, a special camera had 3 B&W negatives, as well as mirrors or prisms, etc, and each negative had one if the 3 tricolour filters. A single press of the shutter made 3 pictures representing the red, green & blue parts of the image. In the darkroom those 3 B&W negatives were each printed in turn, in register, with the associated red, green or blue filter to make a colour print. Imagine the work involved!
Those same 3 filters are the basis for the Harris Shutter apps. The Harris Shutter was first made in the days of the earliest colour images on separate B&W negatives. But much later on, if you made a triple exposure on colour film, or a digital camera, through the tricolour filters, you end up with a completely normal colour photo. Boring so far. But... what if things move between exposures... then things get interesting.
For instance, I once made a picture of fall leaves on the ground in the woods, using a film camera - no chance for error! With my camera set on multiple exposure (on a tripod of course), I made my first exposure with the first colour filter. Then I raked the leaves a bit to change their locations. Also I waited half an hour for the shafts of sun and shadow to move. Then I changed to the next filter and made another exposure, raked the leaves again, and waited another half an hour for the sun to move. I made my last exposure with the last filter. I only had time for the one shot. The resulting picture is quite interesting and unique. At first glance is looks normal enough. A fall scene with coloured leaves. But as you look closer you see a terrific range of colour in the leaves and colours on the edges of trees, shadows, etc. as the sun moved. I’ve seen people study that picture quite a while trying to figure it out, getting more and more perplexed. Why are there some blue leaves? And so on.
And it looks like I’m writing a book, too, so I had better stop now and have some breakfast.