Resources to learn IRIS, F-Stop, Lenses, Shutter Speeds, ND Filter, DoF, etc...

[Jon McGuffin]

I'm having difficulty finding a good resource on learning the various critical functions necessary to shoot good video. I own a pair of HDR-FX1 cameras and I know well enough how to set the following fuctions, I'm just still unsure what they specifically do and why I would change them from whatever the camera wants to do...

Iris
I understand this has to do with how much light to let into the camera, and I also know how to adjust it on my camera but what do the values mean and what specifically can I achieve by modifying this. Frankly, other than the image being a little darker or lighter, I don't notice much difference in the footage I shoot while moving this around.

Shutter Speed
Similiar to above, I get the fact you set this on a fast shutter speed for fast action stuff. What I don't understand is that if I'm shooting in 60i mode, how can the shutter be anything other than 59.97fields per second? Don't get this at all.

35mm - 70mm, etc, etc..
I know I have a 72mm ring on my camera that I use to put clear lenses on for protection. I also know that Leitus makes that cool 35mm adapter that suddenly makes everybody with a camera look as though they are shooting a feature film. What does all this mm stuff mean and don't I adjust this while zooming in and out? When they say a movie is shot at 35mm, does that mean I should just adjust my zoom to match that width and then never zoom in or out and all of a sudden, my shots will have that "movie" feel. Kinda frustrating.

Depth of Field..
I know there have been a ton of references to this subject here at DVInfo. Most seem to say, open up the iris all the way, move your subject as far away from the background as possible, and then move your camera as far away and zoom in. Well, maybe my camera is just too good because it still manages to pick up focus perfectly in these scenerios... I'm lost.. and how often is this technique really used in movies? Can it be overdone?

F-Stop
Not sure what this means either but I have a good feeling it has something to do with the Iris above. How does this relate?

ND Filter
What is the ND filter and why is it when I'm outside in bright sunlight my camera flashes ND1 at me, so I switch to ND1 and then it flashes ND2. What's frustrating more than anything is the fact the image quality doesn't appear to change much or at all while moving this around.

I'd really like to understand these various settings so that I can get used to using my camera in manual mode just about everywhere. Anybody have a good book or resource I can find that will bring me up to speed!?!?!

Thanks...

Jon, frustrated...

[Chris Barcellos]

Here's my primer..


Iris

Iris is one of the primary controls over letting light get to the sensors. Iris can affect the quaility your image too. One of the primary uses of iris is to affect the depth of field in an image. The lower the f stop (iris setting), the wider the lens is deemed to be open. More light enters because more of the lens is used. However, at same time, depth in which the lens can focus decreases-- especially when that same lens is in telephoto mode. You can try this simply yourself by setting the camera in manual, and zoom all the way in. It you need to adjust light for exposure, adjust the shutter or the add ND filter by switching it on. Now pick an item 20 feet away, and manually adjust focus, you should see that area of focus will be very shallow. NOw if you adjust your iris, to all most all the way closed, readjusting the other things for exposure, you should see the depth of field increase. Most film maker like the shallow depth of field effect, because it is more like 35mm depth of field.


Shutter Speed

I shoot most of my FX1 stuff at 1/60, but a higher speed can be set to give a more staccato effect.... and produce a sharper image feel. And at the lower end, say 1/15, you get a surreal effect. Mostly, though, shutter speed is another method of adjusting light reaching the sensor. I use 1/60 because it provides a natural feeling rendition of motion.


35mm - 70mm, etc, etc..

35mm is the size of film used in the standard slr camera. 72 mm ring is simply the size of the opening to accept the filter for sizing purposes. Your FX1 has a 72mm thread on it.

Focal lenghts of lenses are also expressed in mm. The FX1 zoom ring shows the focal length of the lens on it as 4.5-54. 4.5mm is the wide angle and 54mm is the telephoto end. If you do a lot of still photography, you will know that the common focal lengths on 35mm slr cameras run from about 24 mm to 500mm. In the case of the FX1, the zoom is equivalient to 32.5 mm to 390 mm in a 35 mm SLR camera.



Depth of Field..

I brought up depth of field above, but you are noticing one of the draw back s of video cameras. Here's a brief though simple explanation. Depth of field not only is affected by F Stop and the effective focal length you are shooting at, but it is also tremendously affected by the size of the image sensor. Our FX1 have a 1/3 inch sensor. The 35mm slr film frame is much larger. And with the 35mm camera, we see a corresponding reduction in depth of field at equivalent focal lengths. 35mm film slr camera have similar depth of field characteristics as do the movie camera which have made all the moview we have gotten used to. On a 2 1/4 inch medium format camera, the depth of field is even shallower. (Remember this is with corresponding equivalent focal lenght that provide the same field of view.)

So now we can bring up the Letus. The Letus (or a Brevis, or a Redrock M2) is an adapter that screws on the front of you camera. On that adapter, you mount a standard 35 mm lens. It throws an image on a "sensor" in the adapter. The senser in this case is ground glass. You use the FX1 other video camera to capture that image. The ground glass is vibrated, or in the case of the M2, spun, to eliminate the grain of the ground glass. The resulting image has the simalar film depth of field that film makers crave.

Have we overdone that issue ? That a personal choice issue. But it seems like everyone who wants to make films look like films, want to do something in this area... RED, and new camera, is using a very large sensor that will address that.

F-Stop

Iris open is designated in F Stop increments. The wider open the iris, the lower the F stop.


ND Filter

ND mean neutral density. This is simply a filter that is used because the chip in the camera would blow out brighter images without it. In auto or manual mode, The FX1 is suggesting you use the ND to help the other settings in the camera be in reasonably standard exposure. In auto, the camera still adjusts to try to get you the best image. If you switch into manual mode, you will see the effect.


So slide the camera out of auto lock to check it out. Turn off the auto and start messing with it, and learn how to adjust each of the controls.

Gain

You didn't bring this up, but it is important with the FX1. Gain is electronic enhancement introduced by the camera when light is low. Though you don't see it on the LCD, when gain is employed, grain is added to the image. The more gain, the grainier the picture. Try to keep it low. In fact, I shoot in manual, and I have the gain switch set at L. I then go into the menu, and select gain setup, setting L to 0, M to 9 or 6. and H to 12-18. If you shoot in auto, you won't have gain control. You also have to push the gain button to access the slider control to give L, M, H gain selections. By the way, if you turn on the ND filters in auto, one of the things that makes the picture not change is the addition of gain to the picture....


Hope this helps.

[Jim Andrada]

In what follows, I'm totally ignoring shutter speed

If the numbers on the F-stops look rather strange, it's because the F-stop represents the ratio between the open diameter of the iris and the focal length of the lens

This is so that the same F stop number represents the same amount of light hitting the film (or sensor) regardless of the focal length of the lens.

The system helps make it possible (for example) for a light meter to only have to relate the shutter speed and iris opening to the sensitivity of the film. You don't have to also input the focal length of the lens to calculate the exposure.

The numbers in the F-stop series look strange because they are relating a linear dimension (the diameter of the iris opening) to the area of the opening, which is the real gate controlling the amount of light that gets to the film/sensor.

Since the square root of two is roughly 1.4, increasing the diameter by a factor of 1.4 lets in twice the total amount of light.

So an F stop of 1.4 lets in a grand total of twice as much light as an F-stop of 1, and an F-stop of 2.0 lets in twice as much light as an F-stop of 1.4.

Now, to further complicate things, lenses are usually labeled to show their focal length and maximum aperture as a fraction of that length.

In overly simplistic terms, a 50mm f 2.0 lens would indicate that the maximum iris diameter is 5 /2 or 25mm. If you wanted a 500 mm telephoto lens with an f 2.0 maximum aperture, it would have to be 10" in diameter - not too practical for hand held still photography. Please take all this with a large grain of salt - the actual optical design of the lens has a lot to do with this and a real 500 mm f 2.0 lens would probably not be 10" in diameter. But it would be a truly humongous chunk of glass!

Please don't take any of this literally as it's meant to try to get across the basic concept that long focal length and large aperture run up against practical problems which dictate that the longer the focal length (ie the more telephoto) the less light you can get through a practical lens. And as you zoom in on a subject (increasing focal length) the image inevitably gets dimmer at maximum iris opening. Because the physical iris is of a fixed size maximum opening, ie a smaller fraction of the focal length of the zoomed out lens.

Yes, this means that in low light situations you may not be able to get good exposure zoomed in, but can still get good exposure when zoomed out.

[Ervin Farkas]

One place to start learning: http://www.cybercollege.com/indexall.htm and there are others as well - Google is your friend.

[Jon McGuffin]

Wow! Excellent replies... It is helpfull. I'm still uncertain of many catch phrases and I'm definately confused regarding lense width's, etc. I am going to go to the site suggested though and do some more reading.

Jon

[Jim Andrada]

Hope you can find the info you're looking for.

And don't hesitate to ask questions here if you're still confused.

[Jon McGuffin]

Chris B, Jim & Ervin.. Thanks again.. I've had some time to play around with the cameras tonight and here are my remaining questions and comments..

#1) I was able to achieve this "Depth of Field" look!! I place a shampoo bottle about 2 meters away, then zoomed in on it, put the camera in manual mode. It was indoors at night with one single light bulb, so I opened up the Iris all the way (low F-Stop, 1.7 I believe) and then manually adjusted the focus until I had a clear shot of the bottle and everything in the background (about 10' away) was completely blurred. Sweet!!! I hadn't been able to achieve this before.

#2) Shutter Speed........ Why would the speed at which the camera takes pictures have such a great deal to do with light sensitivity?!? As I turned the shutter speed up, the camera got MUCH darker. This had a MUCH bigger effect than the Iris control knob incidentally. I couldn't tell much difference between everything at 60 and above up to 1000 as far as the image was concerned (other than being darker). At 30, the image was much brighter. 15 was unusable. Shooting on a bright sunny day, might it be smart to move this up to 120 - 250 range? Would that temper some of the bright light coming into the camera?

#3) Gain... I hadn't mentioned gain before because that was the one thing I was pretty familiar and comfortable with. Since then though, I have manually adjusted Gain to go from 0db, 6db, then 12db. I'll try to shoot at 0db whenever possible. Would sure love to see this camera have a little better light sensitivity. I'm here in a fairly well lit room and everything is just flat dark unless I jam the gain, open the iris, and decrease the shutter speed.

#4) I'm completely lost on the mm "stuff". so 35mm is the size of film in a traditional camera. Am I correct to assume that while holding the camera and moving the manual zoom all the way in (close up to far away image) and then all the way back is in fact moving through the different mm sizes? So zoomed in might be 25mm and zoomed out might be 200mm? If that's the case, then what is the purpose and meaning of a 35mm lense if my camera can do variable between 28mm and 200mm? I already have that covered...

#5) Back to F-Stops... Jim, you said that the larger the F-Stop the more light comes in. Meaning an F-Stop of 2.0 let in twice as much light as 1.4, etc. Playing with my camera, it appears the exact opposite. The lower the number, the more bright the picture is. Also, the lowest setting, say 1.7, is barely brighter than say 2.4.. What am I doing or seeing wrong here?

Thanks again guys.... I gotta believe I'm not the only guy out there with these questions...

Jon

[Chris Barcellos]

#2) "Shutter Speed........ Why would the speed at which the camera takes pictures have such a great deal to do with light sensitivity?!? As I turned the shutter speed up, the camera got MUCH darker. This had a MUCH bigger effect than the Iris control knob incidentally. I couldn't tell much difference between everything at 60 and above up to 1000 as far as the image was concerned (other than being darker). At 30, the image was much brighter. 15 was unusable. Shooting on a bright sunny day, might it be smart to move this up to 120 - 250 range? Would that temper some of the bright light coming into the camera?"

Though not a perfect analogy, light affects a sensor in the same way water might fill a cup- the full being the perfect exposure. Look at the shutter as a valve. If you open it for 1 sec, you might get a quarter cup, if you open it 2 sec, a half cup. 4 sec give you the perfect exposure. Now if you change the aperature of the valve and make it twice as big, (like you would an iris on a camera, then you would fill the cup in two sec. Finally, the pressure of the water is the same as light brightness. If the water is pressurized to produce twice the flow rate, then if you adjust the shutter speed faster, the flow rate can be returned to the perfect fill up...




#3) Gain...

The VX and PD from Sony have been the best low light 1/3 chip cameras, but they are only SD. The FX1 does a decent job..but has to rely on gain in situations where those cameras do not.

#4) 35mm in the term as we know it refers to the size of the film image. Any 35mm lens, whether it be a 200 mm focal lenght telephoto, or a 28 mm wide angle throw an image that will fit on a 35 mm piece of film at the focal plane.



#5) The lower the f stop, the more light that is let in.

[Jim Andrada]

Sorry - I re-read my post and I wrote it backwards. F stop of 1.4 lets in twice as much as an F stop of 2.0 and half as much as an F stop of 1.0

That's what I get for posting in the wee hours of the morning. Hope I didn't confuse you too badly!

The problem with the mm designation is that it's used for at least two things - the width of the film and the focal length of the lens.

As in a 35mm camera with a 35mm lens (sorry - I couldn't resist!)

Film sizes (widths) are often expressed in mm and a camera that accepts 35 mm wide fim is known as a 35mm camera. Nothing to do with the lens size.

Lenses intended for 35mm cameras, as has been said, are designed to cover a standard picture on the 35 mm wide film. They'll actually cover a circle about 46mm in diameter within which the rectangular 35mm picture size has to fit

Unfortunately this whole exercise is sort of like going to the lumber yard and trying to buy a piece of wood 2" by 4"

The film in a 35mm camera is actually 35mm wide. There are also sprocket holes punched along both sides, so the usable clear space inside the holes is about 24mm which is the height of the photo. The image width is about 36mm

So the image size of a 35mm camera is about 24 by 36 mm. Are we thoroughly confused yet. In order to cover this rectangular image, the lens has to cover a circular area with a diameter equal to the diagonal of the picture size, or a hair under 46mm

So if you use a lens intended for a 35mm camera, this is the size of the image it can produce

OK, back to lenses. You have it exactly right - as you zoom from telephoto to wide angle, the focal length starts at (using your example) 200 mm to 28mm

Re shutter speeds - in still cameras a shutter speed of 1/30 lets in twice as much light as a shutter speed of 1/60. So a change of 1 shutter speed is exactly the same as a change of 1 f-stop.

Put another way, an exposure of 1/30 at f 2.8 lets in exactly as much light as a shutter speed of 1/60 at f 2.0. So once you know the required exposure, the trade off beween shutter speed and f stop required for that exposure (amount of light) is completely up to you. Want to stop action --- use a higher shutter speed and a larger (numerically smaller) iris opening. You'll get stopped action, but at the expense of shallower depth of field.

Want more depth of field? Then smaller opening at slower shutter speed = better depth of field at the cost of more motion blur.

This all assumes theoretically perfect film, but real film doesn't exhibit such nice reciprocity between exposure time and aperture. Real film becomes less sensitive at long exposure times and the package for professional films will tell you the range within which the aperture and shutter speed can be traded off against each other. For example a given film may be rated for exposures of less than 1/10 second whereas other films may be rated for longer exposures.

[Jon McGuffin]

Okay, this is all starting to come together for me a little bit more...

Let me get some clarification though..

Chris, your water analogy is a good one as it relates to shutter speed and this makes a lot of sense however...

I'm just a little confused on the timing. In other words, 1/1000th shutter may be letting in TINY drops of water, but because it's doing it with so much more speed, shouldn't the net effect be the same? 1/60th shutter let's in half as much light as 1/30th but because it's flickering twice as fast, the same amount of light is coming in during the same period of time, no?

Also, if my camera is in HDV 1080i/60 mode.. snapping 59.97 fields/second. How can the "shutter" of the camera operate at anything other than 60 frames per second? Wouldn't every value have to be some multiplier of 60 at the least and if I am capturing at 60i then isn't the 1/60th shutter speed the logical and most natural shutter speed for my camera? (Kinda like the "native" resolution on an LCD monitor?)

Jim..

Thank you for the clarification and explanation on F-stop's, shutter speed, etc. I'm going to have to memorize this stuff... What does the term "telephoto" mean? Can I use this interchangeably with close up? Now, I'm really going to throw you.. I have a Wide Angle lens adapter for my HDR-FX1.. It says 0.8x on the side of it. What does that mean?

Your help is awsome guys,

Thanks

[Jim Andrada]

Jon,

I think you're starting to get shutter speed and frame rate mixed up. Before we get there, let's just think about a single picture and be sure we have the concepts down. I remember that in your first post you were asking about this, but take it easy for the moment. Pretend your camera only takes one picture at a time.

Then think about the water analogy a little

While you're thinking - a telephoto lens is a long focal length lens - think about when you zoom in on a distant object and it gets bigger - now you're in the telephoto range. Strictly speaking telephoto has a specific meaning re the design of the lens but don't get into that quite yet.

Lets say that your lens is zoomed in to get a close up. And lets assume it's a regular camera lens, not a video lens (same idea, just different focal lengths)

Let's say for sake of argument that you have a whopping 500mm lens on your camera.

You stick on your wide angle adapter (.8) what this does is play games with your base lens so that the 500 mm lens acts like a 400mm lens (400 =- .8 X 500). And at the other end of the range, say 100 mm your wide angle adapter makes it look like an 80mm lens. Wide angle is the opposite of telephoto.

OK, lets think about a real video camera. You have to think about it as if it is really two machines. The first machine takes a picture and puts it in memory and the second machine reads whatever is in memory and writes a field out to tape every 1/60 of a second.

If your shutter speed is say 1/100 of a second, it sort of acts like time lapse camera. If you do time lapse with a still camera you take one picture (for example) every minute. Each exposure might only be a fraction of a second, so the shutter clickes once every minute and stays open for lets say 1/100 of a second.

The video analogy would be that you take a picture every 1/60 of a second instead of once a minute but each exposure still only takes 1/100 of a second. So you take this picture in 1/100 of a second and it stays in memory for 1/60 of a second at which point the #2 machine writes half of it (eg the even scan lines) to tape. Then machine #1 takes another picture (exposure 1/100 of a second) and stuffs it into memory, and in a little while machine # 2 comes along and grabs the odd scan lines of the frame and writes them to tape.

As long as the shutter speed is faster or the same as the field rate it's easy.

What do you think would happen though if the shutter speed were 1/4 second. Think about Chris' water analogy. It takes 1/4 second for the sensor to fill up with enough light to take the picture. So even if machine # 2 asks for a new picture please, machine # 1 can't respond because the sensor isn't full of light (water in Chris' analogy) yet

But machine # 2 has to write SOMETHING to tape every 1/60 second.

So what will it look like? Why don't you do it and see and then get back and post what it looks like and tell us what you think is going on.

[Warren Kawamoto]

There are 4 factors that influence depth of field. Know these and you can pretty much achieve the shallow depth of field look without any cumbersome 35mm adaptors.

What is depth of field? It is the area of focus in front of and behind the subject you are focusing on. A shallow depth of field means that your subject is in sharp focus, but the foreground and background is blurry. A deep depth of field means that the foreground, subject, and background is all sharp and in focus.

Here are the 4 factors you need to remember to control depth of field:

1. Imaging chip size. The larger the sensor on your camera, the shallower your depth of field. For video cameras, the common sizes are 2/3 inch, 1/2 inch, 1/3 inch, and 1/4 inch chips. From this list, 2/3 inch chips will provide the shallowest depth of field. 1/4 inch sensor has the deepest depth of field.

2. F-stop. Also called iris, diaphram, or aperature, and these terms are used interchangeably. Here is a list of f-stops common on video cameras (except 1.4, most lenses aren't that fast)
1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22
Numbers on the left side (1.4, 2) will yield a shallow depth of field, while numbers on the right (16, 22) will yield a deep area of focus.

3. Focal length. Controlled by your zoom control. When you are wideangle, your depth of field is very deep and everything may appear in focus, no matter how near or far your subject is. Zoom in and your depth of field becomes very shallow. Focusing becomes very critical at the telephoto end.

4. Camera to subject distance. The closer your subject is to your lens, the shallower your depth of field.

Know and understand the above. You can create the shallowest depth of field by opening your iris all the way, using the longest focal length possible, and bringing your subject closest to the lens as possible. You can achieve that shallow depth of field look without any expensive adaptors!

I have 2 little secrets for shooting shallow depth of field. Macro focus. Put your subject a foot away from the front of your lens, zoom in half way, then manually macro focus your subject and the background blurs out completely!
My other secret weapon is a variable neutral density filter. It's called Singh-Ray. I use it outdoors. Put the filter on, leave the iris wide open. Use the Singh Ray as an aperature control. It is extremely smooth!

Hope this helps...
Warren

[Jon McGuffin]

Thanks again Jim!,

Okay, here are my findings. While putting the camera into a shutter speed of 15 (1/4 of 60fields) I get what I consider to likely be an unusable mode where everything is blurry or slow motion like when I move the camera or the subject matter moves. Other than this setting allowing the picture to brighten a little, I can't think of too many scenerio's in which this would be usefull other than while shooting something that flat out isn't moving at all. If this were the case, I'd probably be better of shooting with a still camera.

What I did find interesting though is that the shutter speed rate of 30 doesn't exhibit nearly the motion blur that 15 does and to me, assuming you have little movement in your field of view and might be useable in lower light situations. It brightens the image up noticeably over a shutter speed of 60...

Warren,

This is a great explanation of Depth of Field and the more I've been playing with it with Chris & Jim's suggestions above, as well as yours, it's becoming quite obvious how this works. I went to check out the Singh-Ray filters.. Wow.. $350... I'd sure like to see those in action..