Why do I need a light meter?
The only way to perform incident exposure metering is with a separate light meter, also called exposure meter. It is essential for a photographer to work some time with incident metering if they want to wise-up regarding everything exposure.
A light meter can perform reflective light metering and some models even very accurate spotmetering by sliding the white cap aside and using the light sensitive cell just like with your camera. But a separate light meter shines with the white cap over the cell. Now you can go to your subject and measure the light actually falling on them. Now you read the exact exposure values needed to expose any subject correctly in that spot.
It may be necessary to correct the exposure values when focusing close or performing extreme movements with your viewcamera, in which case less light reaches the sensor / film than with everything straight and focused at infinity. There are tables and formulas for that.
What is a ‘normal’ lens?
A lens is considered normal when it shows approximately the same angle of view on the photographic surface it was intended for as your own vision.
Accepting some variation in ratio of the resulting image the angle of view of a person, in which they can see detail, is between 50 and 60 degrees. As such this excludes periferal vision.
For regular 135mm ‘full frame’ camera’s this means a focal length of about 50 mm. In practice anything between 40 and 58 mm is considered ‘normal’. Shorter focal lengths are called ‘wide angle’, and longer ones ‘tele’.
On medium format camera’s traditionally 80 mm has been considered normal. And 90 mm for 6×7 to 6×9 film camera’s. I could go on but you get the gist.
How does DOF relate to sensor size?
The short answer is: it does not.
The longer answer: since DOF (Depth Of Field) depends on angle of view combined with the aperture, it seems to depend on sensor size through the incorrect assumption that focal length equals angle of view. When you look at focal length as a given in stead of angle of view, then indeed the DOF depends on the size of the recording surface you stick behind it because it influences the angle of view.
The other way it seems to depend on sensor size is when you fix pixel density in your (thought) experiment and vary focal length, aperture and sensor size to project a constant image on different sensor sizes: of course the circle of confusion becomes smaller on larger sensors then for the same scene, simply because there are more pixels to record the image and so blur from one pixel to the next happens sooner. However, it is not the sensor which determines DOF, but the 2d image that reaches your retina. When you consider a fixed display size (e.g. an 8×10 print or full screen on your monitor) the larger sensors image data will be enlarged less, hence display the same DOF as the photo from the smaller sensor. Now you’re back to the short answer.
I’m planning to write a blog post with clarifying imagery on this, as soon as I find the time.
What is a ‘fast’ lens?
A lens is usually considered ‘fast’ when its maximum aperture is relatively large.
For longer focal lengths and zoom lenses on regular 135 mm ‘full frame’ camera’s f/2.8 is considered fast. For shorter primes (135 mm and shorter) f/2 is fast. For a normal lens f/1.4 or lower is fast. In the normal focal length range (roughly 40 – 58 mm) there are some offerings at f/1.2. Canon even made a 50 mm f/1.0 lens. This means the opening is as wide as the focal length. No mean feat, the lens contacts even had to overlap the rear element.
The projected image circle plays a role here as well. Since a smaller circle shows less abbreviations of the lenses it is easier to design a lens with a larger aperture. This is also why large format lenses rarely have an aperture below f/4.5, they have a very large image circle, where light and sharpness need to be even.
Other things that can be ‘fast’ about a lens are the speed with which it focuses, or the time it takes the lens from closest focus to infinity, for instance. There are huge differences there, more expensive lenses generally performing better with their fast ring-type motors and especially performing well on pro bodies that can deliver the electrical current needed to move quickly. But this is generally not what is meant by a fast lens.
A case can be made (as I have done in the past) that the lens on the Hubble is a fast lens. It goes 28.000 km/h orbiting the earth.
What is Depth Of Field (DOF)?
Depth Of Field is the distance measured from the film / sensor plane things are considered sharp. It inspires a lot of debate in the photographic community.
Depth Of Field depends on the Circle Of Confusion which depends on the enlargement and viewing distance, ultimately on the transfer of the original scene via a flat image onto your retina, which even depends on your own eyesight.
For practical purposes it is good to know that DOF depends on the aperture solely. The smaller the aperture, the more DOF your image has. Prime lenses often have a printed scale near the focusing ring indicating what is usually considered ‘sharp’ at different aperture settings. This assumes an adequate viewing distance for each size of enlargement (read: the larger the print, the further away it assumes you to be).
What is a leaf shutter?
A leaf shutter is a centrally opening shutter located in the lens, much like an aperture but fast. Because it has smaller moving parts it can be a lot faster than old fashioned shutters with curtains. Also because it is a small circle opening and closing it always exposes the entire frame in all stages of its cycle. This allowed for faster flash-sync speeds essential to fashion and portrait photography back when curtain shutters were at 1/60th.
Before the (recent) advent of electronic shutters large format lenses used leaf shutters exclusively because view camera’s did not have a shutter other than the sheet that you would remove from the cassette when you were ready to take your photo. Any shutter with curtains the size of 4×5 inch or even 8×10 inch would be expensive and unwieldly. Leaf shutters in the lens were more practical.
What is flash-sync speed or x-sync speed?
Flash (or X- from the old connector) sync speed is the maximum shutterspeed (actually the minimum time) at which the shutter reaches it’s fully open state.
To understand you need to know that a shutter consists of two curtains slightly behind eachother physically, just in front of the sensor or film. When it fires the first curtain moves out of the way to let light through. Depending on the speed setting the second curtain follows after a certain time. Because the curtain has to travel a distance, it is not instantly open, this means that for short times (high shutterspeeds) e.g. 1/2000 the second curtain starts to close while the first curtain is not yet fully open. This leaves a kind of slit that travels along the plane of the film / sensor very fast.
Since a flash needs to reach all of the sensor / film plane to expose correctly there is no right time to trigger it, there will only be a slit visible where the flash light hits the plane behind the shutter. Using the flash for correct exposure of the whole frame only makes sense when the second curtain is not yet closing while the first curtain is fully open. The shortest time (highest speed) where this happens is called the flash sync speed. This is typically 1/200 or 1/250 in a modern camera.
Ways to overcome this are currently:
- High speed sync: the flash emits a kind of continuous light or burst to expose evenly during the traveling of the slit. This greatly reduces the output power of the flash.
- A leaf shutter. The opening and closing do not affect parts of the image, so at every speed all the parts of the image are exposed. This limits options somewhat as the fastest speeds of a leaf shutter are usually 1/500 or 1/800.
- An electronic shutter. Such a system does not use a traditional shutter, but triggers the photograph by turning the sensor on for a certain time. This way extremely short flash sync speeds are possible (e.g. 1/1600 for current Phase One systems).
What is 18% gray?
18% gray or ‘middle gray’ is a neutral tint used to calibrate exposure and also useful to calibrate white balance.
A gray card reflects 18% of the light that reaches it. Reflective exposure meters like the one in your camera are calibrated to expose an 18% gray card correctly. Since it is a neutral color and by default correctly exposed, it is an ideal sample point to set the white balance as well. Either in camera or (preferably) afterwards in your raw processing software.
Contrary to what the name suggests 18% gray is exactly the middle between black and white, as we humans perceive bightness logaritmically. The same reason exposure works with stops, not with linear values. Because it is the middle, it was chosen early in the development of exposure meters and camera’s to be the reference for exposure metering.
What is incident metering?
Incident metering measures the light that falls on a subject.
It is the opposite of reflective metering implemented in most camera’s that measures the light reflecting off a surface. Reflective metering allows for metering at a distance and so-called spotmetering, but you have to correct the measured values for dark, bright or reflective surfaces.
Incident metering can only be performed in the place of the object you want to photograph. This is the typical assistant holding the light meter with the bright white cap on it in front of a subjects face. It measures directly the light falling on the white cap (and therefore on the face of the person when the assistent moves the light meter out of the way). It always yields the right exposure values. A light subject will be light on the photograph and a dark subject will be dark.
Of course parts of the subject or scene can still be outside of the dynamic range of the film or sensor.
What is ISO (in photography)?
ISO stands for International Organization for Standardization. As such they have standardized the hell out of this world. In the realm of photography ISO is a measure of how sensitive a surface is to light. It used to be film, film was rated for instance 100 ISO or 400 ISO. Nowadays we talk about electronic sensors, whose ‘native’ ISO usually is somewhere between 25 (old sensors) and 200. Higher sensitivities are available by either analog amplification in the sensor reading electronics or digitally enhancing the raw data before committing to storage.
ISO is one of three variables that determine exposure. Unlike the other two (shutterspeed and aperture) it has no direct effect on the sharpness of the resulting photograph. However, the higher the ISO, the more amplification so to speak, the more noise is introduced up to a point the noise can interfere with the (perceived) sharpness. Noise can blur sharp lines in dark areas for instance.
As with shutterspeed and aperture, ISO numbers are typically one stop apart. Usually starting at 100 you get this: 100 – 200 – 400 – 800 – 1600 – 3200 where each number means the surface is twice as sensitive to light (there is one stop more exposure all other things being equal). Most camera’s offer values in between e.g. 320 ISO.
What is aperture?
Aperture is a measure to indicate the amount of light reaches your sensor or film through a lens. It is denoted by f/ and then a number, indicating it is a fraction.
The aperture is one of three (the other two being shutterspeed and ISO) variables that determine exposure. The larger the aperture (the lower the fractional number) the more light reaches the sensor or film.
Aperture (or F-number) is the lens’s focal length divided by the diameter of the entrance pupil. As such this takes into account the distance the light has to travel as well, so no matter what lens you use, for the same aperture (F-number) the same amount of light reaches the light sensitive surface.
For a longer focal length (e.g. 400 mm) the fysical opening through which the light travels is a lot bigger than for a shorter focal length (e.g. 50mm). At an aperture of f/2.8 the entrance pupil in a 50 mm is 17.8 mm (50 / 17.8 = 2.8). In the 400 mm it is 142.8 mm (now you know why those tele-lenses are so big).
The entrance pupil is the virtual opening the lens group behind the aperture ‘sees’, so it is not the exact physical opening of the aperture blades, but nonetheless directly related.
Aperture influences sharpness of a photograph through DOF.
What is shutterspeed?
Shutterspeed is the amount of time the shutter of your camera stays open. Exactly during that time the sensor or film is exposed to the scene projected by the lens. Shutterspeed is one of three variables that determine exposure (the other two being aperture and ISO). The shorter it stays open, the less you expose.
Shutterspeeds are measured in (fractions) of a second. 200 usually means 1/200 of a second. A shutterspeed of 30 means 1/30 of a second. 30″ means 30 seconds (long exposure).
Every doubling of the time naturally increases exposure by one stop. Hence you often see the following series of shutterspeeds that are logically (approximately) one stop apart each: 1″ – 2 – 4 – 8 – 15 – 30 – 60 – 125 – 250 – 500 – 1000 – 2000 – 4000. Most camera’s allow one or two values in between, sometimes depending on settings (1/2 or 1/3 stop) in the menu.
Most shutters have a setting of B, then the shutter stays open as long as you press (hold) the shutter release. This allows for very long exposures for instance when shooting night scapes.
Shutterspeed influences sharpness of photographs through motion. Motion blur means there was relative displacement between the blurred object and the camera while the shutter was open.
What is backfocus?
Back focus is a term from cinema where it applies to zoomlenses only. The better (expensive…) cinema lenses are designed in such a way that the focus distance does not change while changing the focal length (zooming). This enables camera persons to zoom in on a scene, focus precisely on a subject, and zoom back out to the full scene again resting assured the focus is tack sharp.
Most zoomlenses for photocamera’s do not offer this design for it is considered less relevant and therefore not worth the added cost. While zooming the focus distance can slightly change, just like the focal length slightly changes during focusing…
Recently ‘back focus’ has been applied to camera – lens combinations that consistently focus (slightly) behind the subject. The inverse is labeled ‘front focus’. Several photo camera’s offer correction settings for it in the menu. If you ask me it is a load of bullocks.
What is a prime lens?
A prime lens is a lens that offers a single focal length, unlike a ‘zoom’ lens that offers a continuous range of focal lengths.
Note that focal length usually changes during focusing, even with a prime lens. This is not considered zoom however, since it is a side-effect only.
What is the most expensive photograph?
The most expensive photograph is ‘Phantom’ by Peter Lik, it cost a whopping $ 6.5 million in 2014. But many say he was artificially driving up the price as a marketing stunt. The second most expensive photograph is Rhein II by Andreas Gursky of which a print fetched $ 4.3 million at auction in 2011. In 2018 Andreas Gursky produced Rhein III.
Who is the greatest photographer?
Joeri van Veen.
When should I underexpose?
Probably contrary to your intuition you need to underexpose when you are measuring off a dark (matte) subject or your scene is predominantly dark. You are correcting for the exposure meter assuming everything it measures is a neutral grey (also called 18% grey).
It is also a good idea to (slightly) underexpose when your scene contains some bright spots, for example a traffic light in twilight, or a bright red dress that catches a last ray of sun in a city scape. This is because even though the exposure meter (that sees everything in black and white) thinks all is well, one single color (one of the ‘channels’ in your digital camera, they are red, green and blue) may overexpose already thereby shifting the color of the subject. A bright red dress becomes pale orange when overexposed.
When should I overexpose?
Probably contrary to your intuition you need to overexpose when you are measuring off of a light tint in your scene, or the entire scene consists of light tints. For example a snow landscape, a beige dress on a caucasian person, a sunlit beach. Overexposing is nothing less than correcting the exposure meter that assumes everything is a neutral grey (also called 18% grey).
Notice that when you use incident metering you never correct, since it always yields the correct exposure for the amount of light in that spot. Dark subjects will automatically appear dark, and light subjects light.
What are stops (in photography)?
What is the ‘sunny sixteen’ rule?
The sunny sixteen rule is a mnemonic for photographing without exposure metering. According to it outside in the sunlight, the shutterspeed must be the inverse of the ISO you are shooting with when the aperture is f/16.
So at ISO 100 your shutterspeed should be 1/100, at ISO 400 it should be 1/400.
Of course you can use a different aperture as well, you need to adjust the shutterspeed as well the same amount of stops.
What is a viewcamera?
A viewcamera is a photo camera with movements. This means the front part (with the lens) and the back part (with the film or sensor) can move independently from each other. This enables perspective correction (commonly called shift) and having the focal plane not parallel to the sensor (called swing or tilt). Viewcamera’s are relatively large because of those movements. They are mostly used on a tripod as handholding them is very uncomfortable.
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