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lots of noob questions, depth of field, etc...
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PostPosted: Sat Jan 05, 2013 11:21 am    Post subject: lots of noob questions, depth of field, etc... Reply with quote
Hello,

I'm getting back into photography, currently film only but perhaps digital soon, and have many amateur questions. Thanks to anyone who chooses to answer any of them, I've had trouble answering them myself via google.

I want to better understand the role of aperture and have several questions on the subject. For instance, lower f/stop-larger opening means less depth of field and higher f/stop number-smaller opening means greater depth of field. I guess this is due to what size opening light enters the lens, which is how our eyes work (though I don't really understand this optically very well). If someone could explain this in a simple way it'd be great. I have some other questions on aperture..

1) When I drastically change the aperture from say 3 to 22, why can I not see a change in the depth of field through the viewfinder? Is this not supposed to be visible through the viewfinder? Or is it that I have a cheap prime lens? I tested this on my Vivitar V4000 as well as a friend's Canon (can't remember the model or lens he had) and couldn't see the DOF change.

2) What does aperture affect besides depth of field?

3) Let's say I have a low aperture setting and a fast shutter speed, and a high aperture setting with a slow shutter speed (I think that's right) and my light meter tells me both are perfectly exposed. The depth of field will be different right? But the exposure's of both pictures will be the same? And should the sharpness be the same although there are different shutter speeds (assuming I manually focus both well)? I hope this makes sense.

I then have a few random questions.

4) What are the benefits or limitations, or perhaps the applications, of all lens, catadioptric, or all mirror lenses?

5) What exactly is an AF confirm adapter? I understand or assume that this tells you how well something is in focus, but can these be used on film cameras with various lenses?

6) What does it mean when a camera has a fixed prism?

7) I understand the common sensor on DSLR's are of a different size than a 35mm film sensor, so a film lens used on a DSLR has the focal length magnified 1.5x, and a DSLR designed lens on a film camera would reduce the focal length. Are there any other problems with using new DSLR lenses on film cameras or old film lenses with DSLR cameras?

I'm sure some of these questions are quite elementary, as I said I'm a total amateur. Thanks to anyone in advance.


PostPosted: Sat Jan 05, 2013 1:02 pm    Post subject: Re: lots of noob questions, depth of field, etc... Reply with quote
ftngrave wrote:
Hello,
....
7) I understand the common sensor on DSLR's are of a different size than a 35mm film sensor, so a film lens used on a DSLR has the focal length magnified 1.5x, and a DSLR designed lens on a film camera would reduce the focal length. Are there any other problems with using new DSLR lenses on film cameras or old film lenses with DSLR cameras?


Easy one first.

The focal length of a lens is an intrinsic property and can be measured quite independently of any camera. When a lens of focal length 50 mm (for example), designed for a film camera with a film size of 36 x 24 mm, is attached to a DSLR camera with a sensor size of 24 x 16 mm the focal length remains unchanged at 50 mm. However the angle of view changes. Since the angle of view* depends upon the ratio (d/f) where d is a film/sensor dimension (commonly the diagonal, but it might the horizontal or vertical) and f is the focal length, we find that our 50 mm lens used on the DSLR (as described above) will give the same angle of view as a lens with a focal length of 75 mm used on the film camera (again as above). This leads some people to say that the focal length of the lens has been magnified by a factor of 1.5. But it hasn't. It's unchanged at 50 mm. d has changed but not f. Unfortunately it's a widely held and used misconception.

Wikipedia (use Google) is sometimes a useful source of info.

Focal length : http://en.wikipedia.org/wiki/Focal_length

Angle of view : http://en.wikipedia.org/wiki/Angle_of_view

* for a rectilinear lens


PostPosted: Sat Jan 05, 2013 1:59 pm    Post subject: Re: lots of noob questions, depth of field, etc... Reply with quote
ftngrave wrote:
1) When I drastically change the aperture from say 3 to 22, why can I not see a change in the depth of field through the viewfinder? Is this not supposed to be visible through the viewfinder? Or is it that I have a cheap prime lens? I tested this on my Vivitar V4000 as well as a friend's Canon (can't remember the model or lens he had) and couldn't see the DOF change.

It would help to know what camera and lens you are using. With a simple camera and lens then you would indeed see the depth of field change as you adjust the aperture. I would guess the reason why you are not seeing it is that your camera is more advanced, and holds the aperture fully open, even though you adjust the setting, until the moment you fire the shutter. To see the DOF you need to activate the Depth of Field Preview button if you camera has this feature, many cameras don't.

Quote:
2) What does aperture affect besides depth of field?

The primary function of the diaphragm is to adjust the amount of light entering the camera. Assuming average light, by using a wide aperture you can set the shutter speed at fast setting, which is useful for action shots, or a small aperture and slow shutter for greater depth of field, for landscapes for instance.

A secondary function, which applies to cameras set on automatic exposure, is that adjusting the aperture setting varies the depth of field and the degree of background blur while the camera adjusts the shutter speed to maintain the same exposure.

Quote:
3) Let's say I have a low aperture setting and a fast shutter speed, and a high aperture setting with a slow shutter speed (I think that's right) and my light meter tells me both are perfectly exposed. The depth of field will be different right? But the exposure's of both pictures will be the same? And should the sharpness be the same although there are different shutter speeds (assuming I manually focus both well)? I hope this makes sense.

Yes, the exposure should be the same. The different aperture settings and shutter speeds are, in general, all in divisions of one "stop". Opening up the aperture from, say, f5.6 to f4 gives one more stop exposure, so to compensate you would need to increase the shutter speed by one stop, from say 1/125 to 1/250.

Lenses are at their optimum sharpness at around f5.6 or f8. At wide open and fully closed aperture settings, all lenses display aberrations of some kind, the better quality lenses displaying less than poor quality, obviously. I was always taught to use lenses at mid-aperture and open and close this only as a last resort if the light level demands it.


PostPosted: Sat Jan 05, 2013 11:40 pm    Post subject: Reply with quote
Quote:
3) Let's say I have a low aperture setting and a fast shutter speed, and a high aperture setting with a slow shutter speed (I think that's right) and my light meter tells me both are perfectly exposed. The depth of field will be different right? But the exposure's of both pictures will be the same? And should the sharpness be the same although there are different shutter speeds (assuming I manually focus both well)? I hope this makes sense.


In addition to Peter's answer, I would note that colors will be different based on those settings. If you ever shoot HDR, that uses different aperture settings to catch different colors and tones. But two photos exposed ideally with different aperture and shutter combinations can change how an image appears.

Here is an example. The first image was taken wide open (f2, I think) at a fast shutter speed. The second was at f8, slower shutter speed. Both were ideally exposed according to the light meter.




PostPosted: Sun Jan 06, 2013 12:19 am    Post subject: Reply with quote
There are 3 variables that determine the exposure:

- aperture value of the lens
- shutted speed of the camera
- support speed (of film or sensor)

This means that for every exposure value that you decide to record, there are theoretically something like 700 or more combinations that you can use.
Of course 99,9% of the times you will end up choosing amongst the most common dozen ones.
The different combinations do not provide the same result.
The major causes of differences are two: the different reactions of the support to different shutter speeds (for instance, most film require an increase in time after the 1 second of exposure),
and the changes in depth of field determined by different lens apertures. This second aspect being the most immediately visible.

The choice of the "best" lens aperture to use is a complicated issue. There are the technical aspects to keep in mind:
not all lens apertures render the same image quality. Every lens has it's own curve.
It is said, that the technically optimal aperture for a lens equals to two stops down it's widest value. So for instance, if your lens' widest is f/1.4, the optimal aperture would be f/2.8,
if lens widest is f/2, optimal would be f/4, and so on. This of course is not true for a specific lens, but it's a good starting point to test your lens, because even if not exactly true, it is usually not so far from truth.

However, the technical performance is not the only aspect to consider when choosing an aperture. Your subject, and it's relation to the background, also matter a lot - actually, in my opinion, they matter more.
Depending on the result that you want to achieve, you should choose the appropriate aperture to obtain the desired depth of field.
See this photograph of mine:



My purpose was to represent the lion in it's entirety, and to separate it from the background.
In order to obtain that, I choosed an aperture of f/3.5.
If I choosed a wider aperture, say an f/2, I would have been unable to get the whole lion into an acceptable focus: some parts of it would have been blurred.
If I choosed a smaller aperture, the background would have been rendered with more detail, and I would have gotten less separation of the subject from the background. Separation was essential because both
the subject and the background are made from the same materials (rocks) and are lightened by the same light (the skylight). Losing the separation would have meant to have the subject confused with the background.

In the first times, you will have to make experiments to verify the needed aperture. But after some months of frequent photography,
you'll start to develop the "sixth sense" and start to "feel" the best aperture without making tests or take measurements.


PostPosted: Sun Jan 06, 2013 12:22 am    Post subject: Re: lots of noob questions, depth of field, etc... Reply with quote
One of the nice things about digital is playing around with settings and snapping shots for free, with immediate feedback.

It sounds like you might not have a digital SLR to mess around with, here's a couple of simulators that let you change settings and see how the shot turns out -

http://camerasim.com/camera-simulator/
http://www.kamerasimulator.se/eng/?page_id=2

Quote:
5) What exactly is an AF confirm adapter? I understand or assume that this tells you how well something is in focus, but can these be used on film cameras with various lenses?


I can only speak to Canon bodies. The adapter has a chip to interface with the body's autofocus mechanism, and when the autofocus thinks it has something in focus the autofocus confirmation light activates in the viewfinder. I believe this would still work on Canon EF film bodies.

Quote:
7) ... a DSLR designed lens on a film camera would reduce the focal length.


In the case of Canon EF-S lenses, they're designed so they won't mount onto full-frame bodies, because they wouldn't cover the whole film or sensor.


PostPosted: Sun Jan 06, 2013 1:36 am    Post subject: Re: lots of noob questions, depth of field, etc... Reply with quote
Carrying on from where I stopped Wink

ftngrave wrote:
4) What are the benefits or limitations, or perhaps the applications, of all lens, catadioptric, or all mirror lenses?

I have very little experience of reflex lenses and none of catadioptric lenses. You'll get better answers from others I'm sure.

Quote:
5) What exactly is an AF confirm adapter? I understand or assume that this tells you how well something is in focus, but can these be used on film cameras with various lenses?

An AF chip on an adapter is used to fool the camera into thinking an auto-focus lens has been fitted. The benefit is that the auto-focus circuitry in the camera will sound a beep and a make a flash signal in the viewfinder when focus is accurate. So they only work on auto-focus cameras.

Some people find them useful, others (me included) find them not sufficiently accurate. I think a lot depends upon the quality of the camera.

Quote:
6) What does it mean when a camera has a fixed prism?

The image reflected onto the focus screen by the mirror in an SLR or DSLR will be reversed and upside down. To correct this in the viewfinder the image is passed through a prism (sometimes called a pentaprism) or sometimes a series of mirrors. Mirrors are cheaper and lighter weight, but less efficient so they make a slightly darker image in the viewfinder. As far as I'm aware, all prisms are fixed, I've never heard of any other type.

Quote:
7) I understand the common sensor on DSLR's are of a different size than a 35mm film sensor, so a film lens used on a DSLR has the focal length magnified 1.5x, and a DSLR designed lens on a film camera would reduce the focal length. Are there any other problems with using new DSLR lenses on film cameras or old film lenses with DSLR cameras?

John has answered this well. Various different film sizes have been used over the years, the most common being 135 film, which is 35mm wide and has a standard frame size of 24 x 36mm. A more recent innovation was APS film, which was 24mm wide but the frame size could be set in the camera -
H for "High Definition" (30.2 × 16.7 mm; aspect ratio 16:9)
C for "Classic" (25.1 × 16.7 mm; aspect ratio 3:2)
P for "Panoramic" (30.2 × 9.5 mm; aspect ratio 3:1)

You will see reference to "full-frame" or FF in many threads on the forum. This refers to the full frame size of 135 film (24x36mm). Digital cameras with FF sensors are generally very expensive, so to keep the cost manageable for consumers, the sensor sizes used on the majority of cameras are smaller in size. The most common is the APS-C sensor (roughly 24x16mm), but even smaller sensors are used on the 4/3 and m4/3 cameras by Olympus, Panasonic etc.

As John said, the focal length of a lens is set, whatever the size of film or sensor. The image projected onto the film or sensor doesn't change as a result of the size of the film/sensor - but the outer edges of the image are cropped more, it's like looking through a smaller window opening. This means that when images from an APS-C camera are enlarged up to a similar size as images from a FF camera, the field of view is reduced, which gives the appearance of having been taken with a lens of longer focal length. The ratio between FF and APS-C is around 1.6 or 1.5 to 1. So a picture taken with a 50mm lens on an APS-C camera will have a similar field of view to a picture taken with an 80mm lens on a FF camera.

Lenses designed specifically for APS-C cameras will generally not be able able to cast an image over the whole frame of a FF camera. Most manufacturers make it impossible to mount these lenses on their older FF cameras, either film or digital. Old film lenses, on the other hand, are designed to cover a larger area than an APS-C sensor. However, there are restrictions on what make of lens can be mounted on what make of camera.


PostPosted: Sun Jan 06, 2013 12:26 pm    Post subject: Reply with quote
Just attempt to further clarify the effects of aperture, shutter speed and ISO:

Aperture and shutter speed control the amount of light reaching the film/sensor

ISO is how quickly the film/sensor records that light.

Aperture numbers get smaller as the opening gets larger and on a lens where the aperture is set manually, the numbers are usually marked at these "stops": 22, 16, 11, 8, 5.6, 4, 2.8, 2. (some lenses don't close to f22 and some open wider than f2). Each of those f numbers allows half as much light to pass than the number to the right and twice as much light as the number to its left. ie: f5.6 allows twice as much light as f8 and half as much as f4.

Shutter speeds on a camera that allows manual adjustment will be marked in a sequence similar to this: B, 2, 4, 8, 15, 30, 60, 125, 250, 500 1000. Leaving "B" for the moment, each number represents a fraction of a second - half, quarter, eighth, fifteenth etc, and each allows either half as much or twice as much as its neighbours. "B" is short for "Bulb" (an old type of cable release or trigger) and is used for long exposures you would time with a watch.

Using aperture and shutter speeds creatively can seriously enhance your photography. Aperture can be used to control how much is in focus and shutter speed controls motion blur. 500th and 1000th can freeze the droplets in a crashing wave. Slower speeds can be used for creative blur, ie: shooting while panning the camera to follow a racing car will give a sharp car but a blurred background to illustrate the speed.

In these days of automation, where cameras make decisions for you, it is easy to miss out on the fun you get from the creative process of manual control, but gaining an understanding of how exposures work and the effects of aperture and shutter speeds will reap great rewards.

Last edited by skida on Sun Jan 06, 2013 11:26 pm; edited 2 times in total


PostPosted: Sun Jan 06, 2013 10:34 pm    Post subject: Reply with quote
skida wrote:
ISO is how quickly the film/sensor records that light.

I think that it's more about film/sensor sensitivity during exposure to light, and subsequent amplification, during the development of a film negative or signal processing in a DSLR. A nice summary is given at the top of this link.


PostPosted: Sun Jan 06, 2013 11:28 pm    Post subject: Reply with quote
sichko wrote:
skida wrote:
ISO is how quickly the film/sensor records that light.


I think that it's more about film/sensor sensitivity during exposure to light,


I think for the purpose of clarification, my statement does not contradict the scientific explanation you linked to.


PostPosted: Sun Jan 06, 2013 11:36 pm    Post subject: Re: lots of noob questions, depth of field, etc... Reply with quote
ftngrave wrote:
....When I drastically change the aperture from say 3 to 22, why can I not see a change in the depth of field through the viewfinder? Is this not supposed to be visible through the viewfinder? Or is it that I have a cheap prime lens? I tested this on my Vivitar V4000 as well as a friend's Canon (can't remember the model or lens he had) and couldn't see the DOF change......


I don't know which camera model you are using, but most with manual focusing will have a DOF preview button. Sometimes near the lens and sometimes on the lens. On automatic aperture camera/lenses, the aperture doesn't close until you click the shutter, so changing the setting on the lens won't show any change through the viewfinder.

If you don't have the manual for your camera you may find a copy on this site:

http://butkus.org/chinon/index.html

edit: I re-read your post and looked up the Vivitar V4000 manual on the above site. It would seem your Vivtar doesn't have a DOF preview facility, but it may close the diaphragm if you press the shutter half way, like the Russian Zenits did.


PostPosted: Mon Jan 07, 2013 1:40 pm    Post subject: Reply with quote
skida wrote:
sichko wrote:
skida wrote:
ISO is how quickly the film/sensor records that light.


I think that it's more about film/sensor sensitivity during exposure to light,


I think for the purpose of clarification, my statement does not contradict the scientific explanation you linked to.

Perhaps I should have quoted more of your original post ...

skida wrote:

Just attempt to further clarify the effects of aperture, shutter speed and ISO:

Aperture and shutter speed control the amount of light reaching the film/sensor

ISO is how quickly the film/sensor records that light.


Aperture and shutter speed control the amount of light reaching the film/sensor

Agreed. However not all of the light which reaches the film/sensor is used to make an image. In the case of film, some will be reflected at the front. Some will pass through the emulsion and be absorbed on the acetate backing. Some will be absorbed by the silver halide crystals and be dissipated as heat. Only a part of the light incident on the film will be absorbed by silver halide grains in a way which leads to the formation of latent image centres and ultimately, after development, to the negative image itself.

So the statement ...

ISO is how quickly the film/sensor records that light.

is untrue because that light refers to the light incident on the film, and not all of it is recorded.

Most sources discuss ISO in terms of sensitivity. If a film converts a large proportion of the light falling on it into latent image centres the amount of light it needs to produce an image is less than that needed by a film which converts only a small proportion of the incident light - into the latent image centres. The first film needs less light than the second - it is said to be more sensitive.

For similar light intensities the first film can collect the amount of light it needs to form the required number of latent image centres in a shorter time than the second film. The film is said to be faster and we talk about film speed. However Film speed = Film sensitivity.


PostPosted: Mon Jan 07, 2013 2:15 pm    Post subject: Reply with quote
Many apologies. I will modify my statement to:

"ISO is how quickly the film/sensor records the light that reaches it"


PostPosted: Mon Jan 07, 2013 2:18 pm    Post subject: Reply with quote
Thanks to everyone for their responses! I wanted to clarify some things various people contributed.

"Lenses designed specifically for APS-C cameras will generally not be able able to cast an image over the whole frame of a FF camera. Most manufacturers make it impossible to mount these lenses on their older FF cameras, either film or digital."

I'm actually looking at older Pentax DSLRs. Let's say a DSLR designed lens fits on a K mount film camera--with a telephoto lens, the entire frame would be in the lens if one zoomed in a bit. Am I right in the way I'm thinking of this? There would be vignetting if the lens was zoomed out all the way.

"In addition to Peter's answer, I would note that colors will be different based on those settings. But two photos exposed ideally with different aperture and shutter combinations can change how an image appears."

This is very interesting, along with the two photograph examples. So why is David's second photo more of a wash with less contrast in colors? Is it more due to the higher aperture, or does it have more to do with the slow shutter speed? I'm not sure how aperture exactly affects color, but I would think it's more due to the slow shutter speed.

"The major causes of differences [in terms of shutter speed, aperture, and ISO] are two: the different reactions of the support to different shutter speeds (for instance, most film require an increase in time after the 1 second of exposure)."

I think this is in terms of the relationship between film/sensor speed and shutter speed? What is "an increase of time"? Is this to say that a higher ISO speed often requires a 1 second or faster/shorter shutter speed to provide proper exposure?

"not all lens apertures render the same image quality. Every lens has it's own curve."

If someone could explain this in simple terms, it'd be very helpful. I think the aperture openings of different lenses have their own unique shape, some more with an edgier hexagon shape, and others more curved. I think I read this somewhere. But how does the shape of the aperture's opening affect the photo? Or is a lenses' curve something else entirely?

Thanks again to everyone for their responses, I think I'm starting to better understand the basics.


PostPosted: Mon Jan 07, 2013 2:36 pm    Post subject: Reply with quote
ftngrave wrote:
"not all lens apertures render the same image quality. Every lens has it's own curve."

If someone could explain this in simple terms, it'd be very helpful.


With most lenses, the image quality isn't as sharp when wide open. Some lenses are better than others. The technical reasons are quite unimportant (so I haven't bothered studying them).

My recommendation would be to get out there shooting, jotting down camera settings for each shot. Experience is the best teacher.

Film cameras with manual focus lenses usually have a depth of focus scale next to the focus ring and this can be very helpful in giving an idea of how much will be in "acceptable" focus.


PostPosted: Mon Jan 07, 2013 2:43 pm    Post subject: Reply with quote
ftngrave wrote:

I think this is in terms of the relationship between film/sensor speed and shutter speed? What is "an increase of time"? Is this to say that a higher ISO speed often requires a 1 second or faster/shorter shutter speed to provide proper exposure?


ISO numbers are confusing. I remember it by thinking of higher ISO numbers as more sensitive to light.

ISO, aperture, exposure time are intertwined, it's hard to explain them in isolation. Here's a good tutorial:

http://www.cambridgeincolour.com/tutorials/camera-exposure.htm

The good news is that once you get the concept it quickly becomes second nature.


PostPosted: Mon Jan 07, 2013 4:12 pm    Post subject: Reply with quote
ftngrave wrote:
Thanks to everyone for their responses! I wanted to clarify some things various people contributed.

"Lenses designed specifically for APS-C cameras will generally not be able to cast an image over the whole frame of a FF camera.
Most manufacturers make it impossible to mount these lenses on their older FF cameras, either film or digital."

I'm actually looking at older Pentax DSLRs. Let's say a DSLR designed lens fits on a K mount film camera--with a telephoto lens, the
entire frame would be in the lens if one zoomed in a bit. Am I right in the way I'm thinking of this? There would be vignetting if the
lens was zoomed out all the way.


Let me use Canon lenses to explain what I meant, I'm less certain about Pentax lenses.

The mount on Canon film cameras and FF DSLRs is called the EF mount. The APS-C DSLRs have an adapted version of this called
EF-S. Lenses designed for the EF cameras can be fitted on the EF-S mount too, but EF-S lenses can only be used on EF-S mount
cameras. They cannot be physically mounted on an EF camera. Canon decided this because they cast a smaller image onto the
sensor, and on the FF sensor this would cause vignetting.

The equivalent Pentax lenses specifically for APS-C DSLRs are designated DA. I don't know for certain whether or not they can be
physically mounted on older film cameras - someone else might confirm that. However, like Canon lenses, they don't have a manual
aperture control, so they can't be used on a film camera anyway, unless it has electronic aperture control feature.

Zooming in or out makes no difference to the size of the image on the sensor - this is static whatever the focal length. Zooming
adjusts only the angle of view in front of the lens, not behind it.


PostPosted: Mon Jan 07, 2013 7:03 pm    Post subject: Reply with quote
ftngrave wrote:
"The major causes of differences [in terms of shutter speed, aperture, and ISO] are two: the different reactions of the support to different shutter speeds (for instance, most film require an increase in time after the 1 second of exposure)."

I think this is in terms of the relationship between film/sensor speed and shutter speed? What is "an increase of time"? Is this to say that a higher ISO speed often requires a 1 second or faster/shorter shutter speed to provide proper exposure?


This looks like Reciprocity Failure.


PostPosted: Mon Jan 07, 2013 7:34 pm    Post subject: Reply with quote
ftngrave wrote:
Or is a lenses' curve something else entirely?

I think - but I might be wrong - that "curve" simply means the line you get when you plot one variable against another. Of interest here it the effect of aperture on things like vignetting, resolution and chromatic aberration. Here's an example. The review site uses bar charts but if you imagine putting a point on the top of each bar, and then joining the points, you can see how you can get a "curve".


PostPosted: Wed Jan 09, 2013 5:58 am    Post subject: Reply with quote
"In addition to Peter's answer, I would note that colors will be different based on those settings. But two photos exposed ideally with different aperture and shutter combinations can change how an image appears."

This is very interesting, along with the two photograph examples. So why is David's second photo more of a wash with less contrast in colors? Is it more due to the higher aperture, or does it have more to do with the slow shutter speed? I'm not sure how aperture exactly affects color, but I would think it's more due to the slow shutter speed.

Can anyone talk about the causal differences between David's two photographs?
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