Tag Archives: Sensor

Understanding depth of field

• Pebbles •
Depth of field helps the background lose distinguishing features which makes the subject stand out.

It is an important artistic tool.

Depth of field (DoF) provides a way to isolate the subject from the background. Sharpness inside the DoF is of critical importance to our image. The blur outside the DOF helps divert the eye to the sharp subject. So how does DoF work?

Depth of field defined…

The depth of field (DoF) is an easy concept to remember. The DoF is the sharp part of a picture. It is defined by the out of focus parts of the picture on either side of it. A lens can only focus at one point. It is at that point that the image is sharpest. However, on either side of the sharp point is a zone where, to the naked eye, the sharpness is still good enough to be convincing.

The eye can distinguish sharp detail inside the DoF and we want to ensure our subject is in that zone. However, we also want to compose our picture so the unimportant parts of the picture are out of focus. To achieve these things we need to know how to control the DoF. In fact there are three ways to control it…

• Change the size of the Aperture.
• Change the focal length
• the distance of the subject from the lens

As the aperture changes size so does the zone of sharpness we call the DoF. As we open up the aperture wide the DoF gets shallow. As we make our aperture smaller the DoF gets wider and eventually we get sharpness all through to the horizon. Similarly changes to the distance from the subject affects DoF. If I walk away from a subject (and do not change the aperture) the depth of field gets wider (and visa versa). Focal length changes affect the DoF in a similar way.

A Simple Guide to Depth of Field

In this video Dylan Bennett provides the best explanation I have seen on why the DoF changes with the three factors I mentioned above. He uses a simple explanation and some great diagrams to show what is involved.

Dylan Bennett

Just to clear up a point…

The analogy that Dylan Bennett uses, “toothpaste squeezed in a tube”, works well for most people learning the idea of DoF. What really causes the DoF to elongate is related to something called the Circle of Confusion (CoC). For more detail you can see information in: “Definition: Circle of Confusion”.

The CoC projected onto the sensor is (notionally) a tiny point of light representing one point of light from the subject. When that part of the subject is in focus the CoC is very small and individually indistinguishable from those around it. Like this it’s a sharp representation of the subject point of light. However, at a point outside the DoF an individual point of light can no longer be represented by a sharp point on the sensor and begins to blend with points around it. It has lost its sharpness. This is because the lens focuses points outside the DoF slightly before the sensor or slightly after it.

Diagram showing various sizes of Circles of Confusion (CoC) on the sensor sized according to the focus (not to scale). Only CoCs projected from within the Depth of Field are sharp. Our eyes cannot perceive them well as they form sharp points. Ones projected from outside the depth of field are unsharp to our eyes.

Here is the reason why the DoF widens as the aperture gets smaller. When the aperture is small the angle of light that can pass through it from the subject is also small. As a result the CoC for each point of light is relatively small. This is because it can only originate from a small angle of light.

A very small aperture means that the circles of confusion are never big enough for our eye to see. This allows the lens to focus at infinity (say f11 and smaller). When the aperture is wide open the reverse is true. The circles of confusion can be much bigger. Only those rays that the lens will naturally focus will be sharp (a shallow depth of field).

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By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

Facts about focus… how much do you know?

• Orb – Out Of Focus •

Focus is more complex than at first sight.

Focus, and the associated functions on a camera, take a little time to sort out in your mind. Here are some fundamentals and some obscure facts for you to think about.

Focus facts…
1. When you point your camera at a single colour, blank space, solid blue sky or night sky the auto-focus will ‘hunt’ for some contrast. The lens will be unable to focus but keep trying. If it detects a difference of contrast in the frame it’ll be able to balance it out and use it to create a focus. If your lens is ‘hunting’, find something with a bit of colour or light contrast. Then it will stop hunting and focus. More…
2. The light from your lens focuses on the “image plane”, the flat surface that is also the surface of the Digital Image Sensor. The circle of light it creates is bigger than the sensor. The light that falls outside the sensor area is normally inferior to the light which falls on the sensor surface. This is because the edges of the lens creates an inferior quality of light compared to the middle part of the lens. More…
3. There is such a thing as a “fixed focus lenses”… they have no focus capability. They are put in the camera at time of manufacture and set to give a deep focus from a few meters through to infinity. Can you imagine why such a lens would be used in a camera? More…
4. Bokeh appears in out-of-focus areas. Some people think that the prominent little circular highlights are the bokeh. This is incorrect. In fact bokeh is a statement relating to the quality of the out-of-focus blur. The blur itself is called… blur! More…
5. Macro lenses are highly effective at focusing at short distances from the lens and to enlarging the image. Actually they will also focus at other distances too. Macro lenses produce a great portrait. More about macro lenses…
6. When a lens lets light through the light follows something called the optical path. The focus is achieved on the optical path. However, in a DSLR when the mirror is down, the optical path ends up at the eye. The optical path is not the same all the time. More…
7. Where you focus affects your exposure. Most people assume focus and exposure are two separate things. And, in fact, they are. But, your camera senses the light conditions where you are focusing and sets that focus at the same time as taking readings for the light meter. Change your point of focus and you may change the light reading. Even a tiny movement may change your exposure readings if your focus changes from a lighter to darker surface. Useful to know if you are in a darker environment with bright light sources around. Especially useful if you are trying to learn manual control.
8. The ‘sensor plane’ is the surface on which the image is projected by the lens. It is the surface of the sensor itself. In former times it was the same surface that the film was laid upon. Any idea what other names might be applied to the same surface in a camera? More…
9. Sometimes it is important to measure your focus distance precisely. For example if you want to repeat shots or vary the distance form camera to focus-point by a measured amount in a series of shots. Such measurements are often used in forensic photography, macro work and for working out scene sizes and distances. There is only one place where you can take a true measurement from on a DSLR. It is clearly marked. Know what it’s called and where it is? More…
10. Consider turning off your auto-focus when using a tripod. The motor that works the auto-focus creates vibration that may affect your shot. It is less likely to have an effect on long exposures, but if you get into the habit of doing it you may help to sharpen your shots. Reducing any vibration is good when you are trying to sharpen your shots.
11. The Plane of focus is a very important concept – it is about where you are focussing. It gives you an idea of what your focus is doing at any angle of the camera. You should really have a good idea about it if you want to make sure you are getting a proper focus on the subject of your shot. More…
Lots more to learn…

Focus is big subject and can become extremely technical. There is a lot more you can learn. These points are just some fun ideas to help you learn about different aspects of focus in general. If you have any comments, other ideas or want me to write about another aspect of focus – leave a comment at the bottom of the page.

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By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

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Lenses and designations? Confused? An easy guide

• Lenses •
Buying lenses optimised for your sensor is confusing.

Lenses are a big investment…

It is difficult to know which lens is optimised for your digital image sensor. There seems to be so many different designations. Here is a guide to which lens designation you want.

Explaining the differences

Brands like Canon and Nikon have their own lenses range. Third party manufacturers, like Sigma, Tokina and Tamron etc. manufacture lenses for brands like Nikon, Canon and others. if buying lenses the third party manufacturers have lenses which are equivalent to the Camera brand manufacturer or possibly better. Look around at online reviews to see what standard of lenses and prices are available.

Make sure you buy lenses fitted with the correct lens mount for your camera. Older models of cameras may have the correct mount but some of the more recent lenses might not be suitable to work with the camera. So check the mount and camera are compatible before buying.

Why are lens mounts specific to brands? It’s mainly historical – the development paths of the manufacturers differ. However, they also want their customers to stay loyal to the brand. This unfortunate situation means you have to reinvest in a new range of lenses if you change your camera body. Hmmm! Expensive.

There are two types of camera sensor. There are cropped sensors – which is a small size. These are more often referred to as APS-C format.

The other sensor format is full frame sensor. These are the size equivalent of the old film SLR frames on a roll of film.

Full frame digital sensors are less common than cropped sensors. The cropped sensors are easier and cheaper to manufacture. However, in recent years we are seeing an increase in full frame releases of new cameras. The higher resolution (more pixels) and potentially bigger print sizes are attractive to consumers. As full frame format gets cheaper they are likely to become more common.

The full frame sensor size is the same size as a 35 mm (36mm ×24mm) film frame in old SLR cameras. Because of the historical significance of the 35mm format modern DSLRs are based on the same standard. Lenses are normally designed to fit either the full frame format or the cropped format.

Lenses designed for the full frame sensor have an image circle that covers the whole 35mm sensor. These lenses tend to be more expensive because they need a wide circle of light thorough them to cover the sensor. They have bigger glass elements as a result.

Full-frame sized lenses are able to fit a camera with the same mount and a cropped sensor. The image circle from the lens remains constant. The smaller sensor size (APS-C) is therefore only able to process the light from the centre of the circle – the rest of the light spills over the side of the sensor. The resultant photograph is like a zoomed-in crop of the image that would have otherwise been taken with a full frame sensor.

This image-cropping effect of smaller sensors is known as the “crop factor”. It represents the ratio of the size of the full-frame 35 mm sensor to the size of the smaller format. The apparent zooming effect also gives rise to an alternative name – the “focal-length multiplier”.

The ratio of full-frame to crop tends to lie in the range 1.3–2.0 for most cropped sensor DSLRs. You might say that a 100 mm lens on a camera with a 1.5 crop factor creates an apparent zoom multiplying the focal length by 1.5. A 100mm lens would then appear to produce the same picture as a 150mm lens. This is not a true magnification since the focal length of the lens is the same on both cameras. Instead the cropped sensor is likely to produce a lower quality result than than the full frame sensor while revealing a closer result.

You can use lenses designed for full frame sensors on cropped sensors. It does not work the other way. A lens designed for a cropped sensor creates an image circle smaller than the full-frame sensor. It would create a circular image with very strong vignetting around the sides. Manufacturers recommend not using lenses designed for cropped sensors on full frame cameras.

Designations

To ensure that buyers purchase the correct lenses for full frame or cropped sensor manufacturers designate them with specific marques. Here is the breakdown of the most common designations…

Manufacturer  Full frame
(and APS-C)
APS-C
(cropped)
Canon           EF pEF-S
Nikon          FX DX
Sigma          DG DC
Tokina          FX DX
Sony     Various‑incl.
3rd party mounts
DT
Tamron          Di Di-II
Samsung   Not available‑2013 NX
Pentax Check manufacturer
specification
DA
Konica‑Minolta Check manufacturer
specification
DT
Other related sources…

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By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

Can you write? Of course you can!

We would love to have your articles or tips posted on our site.
Find out more…
Write for Photokonnexion.

How to make a digital camera

How to make a digital camera

A lot goes into making a digital camera.

Canon have made this video to explain the key processes and ideas that lie behind camera design and production. The video is Canon focussed, but the concepts apply to any digital camera.

All the key processes are mentioned in the video. It is particularly interesting to see the considerable technology that backs the manufacturing itself. Camera production is very high tech and the degree of precision involved is considerable.

Despte the complexity of the actual technology the video is a simple and easy watch. The ideas are explained in easy steps and the diagrams make it easy to see what is going on. No mad scientists were involved in making this video! Simple clear ideas and demonstrations are used throughout.

I found this a very useful video to watch. I am sure you will find it interesting and eye opening. Enjoy!

Skylight and UV filters

• UV and skylight filters •
There is a debate about how useful they are…

What are UV and Skylight filters?

The keen starter in photography wants to protect their investment… Filters protect your lens – right? Or, is it that they stop damage from the sun? I want to clear up some myths and explain some half truths in this article. You may also save some money.

What are these filters for

Skylight and UV (UltraViolet) filters have a single purpose… to reduce ultraviolet light reaching film. The only difference between them is that skylight filters have a slight pink colour. Both filters prevent the slight tendency of some chemical films to acquire a slightly blue colour cast under some light conditions. (Yes, we are talking about film).

That was simple, wasn’t it?

Now the myths cleared up

UV and skylight filters have a number of myths surrounding them.

They prevent sunlight damaging my digital image sensor.
• No, they don’t. Sensors are UV insensitive or have built in filters (for both infra red and ultraviolet). UV (and IR) light has no effect on them.

They prevent the blue colour cast on sunny days.
• Not true. It is about 25 years since ultraviolet sensitive film was on sale. Even then, the film brands that were sensitive tended to only be sensitive in relatively few conditions; eg. when it was sunny at high elevations or beside the sea.

They provide more clarity in bright sunlight or at high elevations (over say, five thousand feet).
• Once upon a time… some colour film brands used a chemical that was sensitive to UV light. Around 30 years ago an ultraviolet inhibitor was developed that reduced the sensitivity of the film. Problem solved. The slight lack of clarity caused by the sensitivity went away.

They prevent lighter greys being over-bright when in black and white mode.
• Silver-based chemical black and white films were affected by UV. This is not a problem in digital cameras.

The skylight filter has slight pinkness that warms the picture up.
• No it doesn’t – pink is not a warming filter colour. Pink reduces blues in the image. Anyway, if you use auto-white balance any colour effect will be wiped out. If you use RAW there is no need for a filter as you can adjust in developing.

Actually these filters have problems

It turns out that UV and skylight filters can cause a few problems. Poor quality filters; inappropriate filter materials and lack of special coatings all take their toll…

Image effects…
Affects are created by using these filters. In particular over-exposure haze, flare and ghosting are created. The haze results from light bouncing between filter, lens elements and the sensor inside the body of the lens/camera. This creates a slight haze of over-exposure in very bright conditions. Flare, and therefore reduced contrast in the image, is sometimes caused by a beam of bright light being scattered by the filter. More expensive filters reduce this by having chemical coatings on (lens glass has coatings too). Ghosting is where spots of light appear in the image that were not in the scene. They originate from back reflection off the sensor onto the other lens elements or the filter. Usually this happens in low light situations stimulated by bright lights like car headlights.

Adding another glass (or resin/plastic) element…
Additional elements degrade the image. Cheaper filters can cause chromatic aerations, creating colour banding in an image. There may be additional light scattering. Some filters significantly reduce the light getting through (maybe as much as 1/3rd of a stop of light) leading to underexposure. Optical aberrations may be caused by poor alignment of the filter element (not flat/parallel) in its place. This causes loss of definition, particularly in some places where sharpness would be expected.

Are there any reasons to buy them?

Yes, but not many.

Protection:
UV and skylight filters do provide protection, creating a barrier against mechanical damage to your lens. The front elements glass or coatings on the surface are protected from dust, dirt, splashes and possible scratches or breakage from a bump, scrape or blow.
• Alternatively, consider a proper lens hood. They prevent angular light beams straying into the lens which can improve the image. They also greatly reduce the probability of damage to the lens too. Lens hoods are cheaper than filters, and don’t cause optical problems.

Supporting your dealer:
Filters are expensive to buy, but are profitable to sell. In these hard economic times you will be providing a rich return for your dealer and helping him survive a tough market.

A mistake to clear up

Somebody told me recently, “I always have this polarising filter on the front of my lens”. Wow! (It was actually a skylight filter when I looked). Polarising filters are great for reducing some reflections from some surfaces and may darken skies in some light conditions. Some people mix them up with UV and skylight filters. Just let me say for now, don’t keep a polarising filter on your camera.

By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

How Safe Is Your camera sensor from dust?

The camera sensor is incredibly sensitive to dust. Your shots can be ruined!
Image Sensor picture curtesy of Wikipedia

Your image sensor is your camera!

Without it, you have no means of taking pictures. It makes sense to look after it and to prevent dust from getting onto the working surface. What can you do if it does get dust or other substances on it?

How does dust get onto your sensor?

In a variety of ways… The main entry point is the lens hole. Every time you open your camera, you expose the inside to dust. It is impossible to avoid this. However you can minimise the potential for dust entry…

• Avoid any opening of your camera unless necessary.
• Avoid dusty environments. Wipe down your camera with a slightly damp cloth and wipe dry before opening.
• Avoid windy environments. Get out of the wind before opening.
• Avoid damp environments: dry before opening. Moisture kills cameras and takes dust inside.
• Hold your camera with the lens hole pointed toward the ground so that dust/skin/hair cannot fall into it.
• Avoid having an open lens hole for any longer than necessary.
• Do not breath or blow directly into the lens hole!
• Avoid opening the camera in the warm soon after coming in from the cold. The same when entering air conditioned places from hot out-doors. Temperature differences cause air to be sucked in and moisture to condense inside the camera. Allow several hours to equalise temperatures inside and out before opening.

Some cameras leak in other places too… The battery cavity, plugs and jack holes for cables may allow air and dust inside. Every time you put a battery into the camera you are creating a piston to push air (and dust) into your camera. Beware – apply the same rules above for all plugs, batteries and cards movement.

I suspect dust on my sensor – how can I tell?

It’s sometimes difficult to tell if dust is on the lens or on the image sensor, or even if it is on the viewfinder. Here is how it is done.

A dust spot most often shows up on your pictures in the lightest areas. If you are looking for dust, hairs and specks then photographing blue sky does the trick. First, thoroughly clean your lens with lens cleaner and a proper lens cloth. It is best to do the photographs in aperture priority mode. Choose f22 or the smallest aperture. Zoom out to the maximum amount. Then, manually focus to infinity (the camera will probably not be able to auto-focus in a clear sky). Take a few pictures of the sky – the blue areas only.

Download the pictures to your computer. Enlarge them on-screen to the full size. Any dark spots, specks or lines you see on every picture in the same place are dust, other detritus or hairs. Birds, aircraft and UFO’s will be in different positions on the pictures.

What can I do about dust I find?

The simple answer is, clean it. This sounds terrifying. It really is not. It can be done in a few seconds with one simple piece of equipment. You will need to buy a blower. They are simple pump-action nozzles that expel fresh air against the sensor and blow off the dust. Here is the one I recommend… I have used them for years without any problem.

The Rocket Blower

Once you have your blower find out how to lock the mirror up on your DSLR. The mirror lock-up function will be somewhere in your menus. Check in your manual.

The procedure below is best with the camera on a tripod. You can look into the camera with your hands free to use the blower. There will be no danger of moving the camera while doing the blowing.
To blow-clean the sensor with the Rocket Blower:

1. Mount camera on a fixed position/tripod away from contaminants.
2. Take off your lens so the lens hole is open.
3. Turn on your camera.
4. Set mirror lock-up to enabled in your menu.
5. Activate mirror lock-up for cleaning (as advised in the camera manual).
6. There will be a click and you will now be able to see the sensor.
7. Hold the blower so the nozzle points at the sensor, at about 60mm to 100mm from it.
8. Squeeze the bulb of the blower about 3 to six times, directing the air at the sensor.
9. Unlock the mirror lock-up (as advised in the camera manual).
10. The mirror will click and drop.
11. Turn off the camera.
12. Replace the lens.
13. Retest for dust – re-take sky pictures and view on the computer.
14. Repeat if dust is still present. (Normally once or twice is enough).

The whole procedure for one test and clean-up should be about five to ten minutes.

Check that the procedure is compatible with instructions in your camera manual before proceeding. Follow any variations suggested.

Common Sense

Be sensible. While you are cleaning your sensor make sure you…

• Use only air.
• Use only the type of blower shown above (any similar brand).
• Do not use compressed air (especially canned). It may damage your camera beyond repair.
• Do not use other gases (they may contain particulates, corrosives or moisture).
• Do not use anything to directly touch the sensor.
• Do not use your mouth to blow into the hole – grease and bodily fluids will be forced onto the sensor surface and will not come off again.
If after three or four attempts you still have dust…

The next level of cleaning involves wiping the sensor with a sensor cleaning fluid and using a cleaning implement. This is a much more delicate operation. It should only be carried out if you are confident dealing with delicate electrical components. I do not cover this operation here. If not confident, take it to a properly accredited service agent for your camera manufacturer.

By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.