DOF for the Rest of Us

[Ritchey Cable]

Maybe I'm alone, but in reading all of the different threads, I'm trying to discern something that I thought I understood. When I first started using the Canon XH-A1 and I wanted DOF I just set up the camera way back from my subject, made sure the background was a good bit of distance from the subject and zoomed in to the subject and got the desired DOF I was looking for.

A little while after that, I bought a piece of equipment I thought was incredible, a 35mm adapter. I used the Letus Extreme for about a year with a handful of nikon lenses. I admit I was pretty much "shooting from the hip" in trying to get the desired effect, but my understanding at the time was use a lens that is pretty fast because you lose some light with the adapter. That was pretty much the only thing I looked at when purchasing lenses.

Now, as I prepare to transition to the Canon 7D, I'm reading all different things that apparently have a significant effect upon DOF. Could someone please speak in a pretty elementary language about the effects of ISO, F-Stops, and ND filters upon the DOF. Also, if there are any other settings I'm missing, I'm up for it.

EDIT: **To clarify I understand that a faster f-stop lets in more light, and that changing the ISO effects light sensitivity, and ND filters block out light, but what I don't know is if a faster f-stop effects DOF. Does more light have an optical effect that creates the depth of field? So what I'm really asking is what settings are worth paying attention to when trying to achieve a shallow depth of field.**

As I see it, I think we are seeing a real shift to DSLR as a standard option for filming and honestly I could use a little help making the transition. Thanks for any help.

[Bruce S. Yarock]

"Now, as I prepare to transition to the Canon 7D, I'm reading all different things that apparently have a significant effect upon DOF. Could someone please speak in a pretty elementary language about the effects of ISO, F-Stops, and ND filters upon the DOF. Also, if there are any other settings I'm missing, I'm up for it."

Ritchey,
I've pretty much taken the same route as you. I was really excited when I got my Letus Extreme, but due to the size of the rig and miserable low light performance, I haven't used it a lot lately. Enter the 7D...
I just got mine yesterday, and ordered some basic accessories (viewfinder, nikon to canon lens adapter, etc.)
I'n no dof expert, but I'll fill you in on what I've picked up and figured out.

Iso- on a dslr iso is like "gain" on a video camera. The higher the iso, the more digital or artificial light you have. With the letus, it's real simple. you have to use lots of light or the image sucks. The 7d , on the other hand, lets you jack up the iso, and still get a nice clean image. So the higher iso/lower noise ability of the 7D lets you shoot in lighting conditions that would be impossible for the Letus.

F stop- the smaller (faster) number your f stop, the wider open and the more light you let in. more open ( in addition to telephoto) gives you shallower dof. Stopping down your aperture (larger f srop number) gives you a deeper dof, but also makes your image darker.

I did a little testing today with the 7D that relates to this issue. I had a subject sit on the couch in a room with only basic ambient light. I put a dummy head a few feet behind on a stand. i used a canon 85mm f 1.8 lens, f stop at 3, shutter 50, and iso around 300. The subject was nicely exposed, in focus, and the dummy was blurred and darker . Then I decided to try to create deeper dof, just by changing the f srop and iso. I jacked the f stop up to about 6, which put the dummy more in equal focus with the subject. But the scene was now a lot darker, so I raised the iso to about 700. Bingo! The subject is again exposed correctly,
both the subject and dummy are in focus, and the dummy is a hair darker.
This kind of adjustment would be impossible with a 35mm adapter.

Nd filters- Our cameras have internal nd filters wich we use when there is too much light. We can keep the iris wider while lowering the exposure with the internal nd filter. Unfortunately dslr's don't have internal nd filters ( at least my Nikons don't). So....if I want to keep my iris wide ( for shallower dof) I need to use an nd filter . You can have filters for each lens size, but I'm raelizing that 's a pain. I'm going to have to break down and get a mattbox, which lets you use the same filters for any of your lenses. It also protects your camera from flare.
I'm sure i left some things out, and I'm learning as I go along. Hopefully this info will be helpfull.
bruce Yarock
Yarock Video and Photo

[Bill Pryor]

Ritchey, f-stops go from wide open to stopped down. A wide open f-stop on a fast lens, say an f1.8, lets in more light. It also will create a shallower depth of field. The more you stop down, ie., go to the smaller f-stops, like 5.6, 8, etc., the less light you let in but the greater your depth of field becomes.

Depth of field is dependent on f-stop, negative or chip size and distance. The kit lens that comes with the 7D package only opens up to a 3.5, so it is not going to give you as shallow a depth of field as a faster lens that opens up to an f-1.8. However, if you are in close enough to the subject and the background is far enough away and you are shooting as wide open with that lens as you can, you will minimize depth of field.

As the above post said, you need ND filters to allow you to shoot with your lenses more open, but you may have to shoot at a higher gain (ISO) which will create more grain (though the 7D looks pretty good with higher ISOs than most cameras). You do not want to change to a faster shutter speed to allow you to open up your aperture. Anything other than the standard 1/50 for 24 fps will give you some funkiness that may or may not be seen (stuttering at faster speeds, blurring and strobing at slower speeds, and if you go fast enough a fan blade will even appear to turn backwards).

In most cases, unless I'm trying to hide something in the background, I like the depth of field to look normal. In other words, on wide shots, the background is only slightly soft, on closer shots it is softer but I can still tell what's there. To me, overdoing the shallowness of depth of field is an effect and should be used judiciously like any other effect that can draw attention to itself. Look at, for example, how some TV shows shift back and forth between the characters in a close 2-shot. You can do that focus shift once to introduce the background person, but back and forth is very annoying--as bad as a zoom, in my opinion.

The nice thing about a camera with a big chip is that you have more control over the depth of field than you do with smaller chip cameras, so you can do what you want with it. However, just because you can doesn't always mean you should.

[Chris Barcellos]

The way I see it, the three big factors regarding obtaining a shallow depth of field are:

1. Size of sensor (bigger sensor, shallower depth of field)

2. Focal length of lens ( the longer the the lens or zoom setting, the shallower the apparent depth of field)

3. Aperature (The wider the aperature on a given lens, the shallower the depth of field.)

[Bruce Foreman]

Most of us with new 7D's are probably gonna want to buy some ND filters. In testing for shallow DOF with my T1i in open shade in my backyard I got lucky with an F1.8 lens wide open. I didn't note what shutter speed the camera chose but it had to be fast, there was no real subject motion except my model pushing her hair back and she does stuff like that smooth and graceful.

I got no "stuttery" motion.

Yesterday, shooting the DVC17 entry with the 7D, several times I tried to use fairly wide aperture but gave up when I saw the shutter speed of 4000 knowing what would happen with any real motion.

Gonna have to hit up B&H when they open up again. Spare battery, second media card, filters (my credit card is still reeling from the 7D purchase).

[Nathan Moody]

Quick advice on filters: screw-in ones are super expensive and specific to lens diameter. Better solution is to get a filter holder (Cokin, Lee) and buy resin or glass filters. The challenge is that the smaller filters are cheaper (the holders, too), but the larger ones (4x4, 4x6) are more compatible with matte boxes. Something to consider so that when you purchase ND's and other filters (everyone should own 1-2 grad ND's, IMO, if they shoot outdoors) you don't waste money buying the same filter twice.

*ahem* Not that i speak from experience, heavens no. :)

[Bruce S. Yarock]

nathan,
A friend told me that someone makes an inexpensive screw in adapter that you could buy for each lens. That way you could have one set of filters, that fit in any of the adapters. Don't know who makes them.
My other option will be a mattbox.
Bruce Yarock

[Bill Pryor]

You can use Series 9 filters. You buy the Series 9 filters with a set of retaining rings and stepdown adapters for each different size lens. The filters are expensive, all over $100 each, but cheaper than 4 X 4, and the rings are cheap, like around 10 bucks apiece. Then you'll need a bigger lens hood with appropriate adapter ring to hook onto the Series 9 outer threaded ring. Also, when stacking more than 1 filter onto a wide angle lens, you'll get corner vignetting at your full wide angle, most likely, although you might get by with 2 filters. Overall it's probably easier to use something like a Lee Filters lens hood, which is like a matte box without rails, but is a bellows instead of with a french flag.

[Bruce S. Yarock]

Bill,
Is this the Lee model you mentioned? What else would I need to attach it to my lenses?
Bruce Yarock

LH Lee Standard Lens Hood

[Daniel Browning]

Originally Posted by Ritchey Cable

When I first started using the Canon XH-A1 and I wanted DOF I just set up the camera way back from my subject, made sure the background was a good bit of distance from the subject and zoomed in to the subject and got the desired DOF I was looking for.

There is an important difference between "background blur" and DOF. They are frequently confused, but they are most definitely not the same thing. For example, you can have lots of background blur and deep DOF at the same time.

Originally Posted by Ritchey Cable

Could someone please speak in a pretty elementary language about the effects of ISO, F-Stops, and ND filters upon the DOF. Also, if there are any other settings I'm missing, I'm up for it.

F-stops affect DOF. ISO and ND filters, by themselves do not affect DOF. But sometimes, adding an ND filter will allow you to use a faster f-stop. Other times, using a lower ISO lets you use a faster f-stop.

There is a common misconception that telephoto lenses have thinner DOF than wide angle lenses, leading people to use the "back up zoom in" technique to make DOF thinner, or the "zoom out walk in" technique to make DOF deeper. In actuality, these actions do not change the DOF (except at near-hyperfocal distances), but they do change the background blur.

For example, say you are doing a headshot at 50mm f/5.6, and the eyes and hair are both in focus. The background, too, is somewhat in focus and quite distracting. If you switch to 100mm f/5.6 and frame the headshot the same way, you'll find the eyes and hair are both still in focus: the DOF did not change. However, the background did get magnified greatly and the blur increased. If you switch to 200mm f/5.6, you'll find the eyes/hair are still in the same focus. DOF again did not change, but the background is even more blury now. Even 400mm f/5.6 has no change.

Now another example. Say you're doing a headshot at 200mm f/2. The eyes are in focus, but the nose and ears are out of focus, and you want them to be in focus. If you change to 100mm f/2, the background is now less magnified (less blurry), but the ears and nose are still out of focus. If you try 50mm f/2, the background is now more distracting, but still the ears and nose are OOF.

I hope that helps to illustrate the difference between background blur and DOF.

For me, the most helpful way to remember DOF is "framing and f-number". For a given sensor size (e.g. 7D), you can forget about focal length, subject distance, and all that jazz: just remember framing and f-number.

For example, if you want to shoot a headshot, it doesn't matter whether you use a super telephoto 500mm lens from 100 feet away or a 50mm lens right up in their face: both are going to give you the exact same DOF. (They will, of course, give you different background blur.)

One important weakness of the "framing and f-number" rule of thumb is that it is inaccurate when the focus distance is near the hyperfocal distance, such as when you're shooting landscapes. But for other types of shooting it is a very good approximation.

[Terje Rian]

Hello Ritchey,

I think ND filters are a much needed tool to control your exposure during DSLR shooting. I own a matte box and 4x4 ND filters, which I use with my Sony EX-3. I hoped to use the mattebox with my Canon's also, but I ended up rather frustrating trying to get it "rigged" for my DSLRs (without spending a lot of money). I decided to go for a more mobile and flexible solution and bought a Vari-ND filter (and a few step-down rings) for my 5D Mark II and 7D. I'm very pleased with the results I get. A Vari-ND filter is two circular polarizer filters stacked. When you rotate one of them you get up to 8 stops of light reduction, perfect when you need to get the right shutter-aperture-ISO ratio. I use a the Fader Vari-ND 86mm, which will make me able to use the filter on all of my lenses (due to the large filter size). For me a Vari-ND filter really makes DSLR shooting even more exiting.


Best wishes,
Terje Rian

[Christopher Lovenguth]

Originally Posted by Daniel Browning

There is an important difference between
For example, if you want to shoot a headshot, it doesn't matter whether you use a super telephoto 500mm lens from 100 feet away or a 50mm lens right up in their face: both are going to give you the exact same DOF. (They will, of course, give you different background blur.)

This is true but the compression of distance will be affected by focal length. So that someone's facial features at 500mm will be "compressed" as opposed to right up in their face at 50mm will distort their features.

Here is a great site with visual examples to some of these questions: kevinwilley.com - Educational Information

Look at topic 4 and 2.

[Kin Lau]

Originally Posted by Daniel Browning

One important weakness of the "framing and f-number" rule of thumb is that it is inaccurate when the focus distance is near the hyperfocal distance, such as when you're shooting landscapes. But for other types of shooting it is a very good approximation.

For Ritchie,

This last statement by Daniel is why it's so confusing. With most video cameras before VDSLR's, shooting at most non-telephoto lengths meant that you were fairly close to the hyperfocal distance. That's why you have to "back-up and zoom in" on the XH-A1.

The wide end of the 20X zoom on the XH-A1 is 4.5mm or 32.5mm equiv for 35mm photography. Even wide open at f1.6, hyperfocal distance is only 3.8ft, meaning if you focused at 3.8ft, everything from 2.8ft to infinity is in focus.

Even if you backed up and shoot at 9mm (2x zoom) or 65mm photographic equiv, at f1.6 and your subject at 10ft, DOF is still about 23ft.

The key is to remember/understand that focal length is a major factor in determining DOF _because_ of the hyperfocal factor as you switch from a small sensor video camera to a large sensor DSLR. The shorter the focal length, the faster you'll run into the hyperfocal distance. On the XH-A1, the 20x lens is 4.5mm to 90mm, so you needed to get into the longer focal lengths to get away from the hyperfocal distance, but then you need to shoot further away.

In the same example if you shoot with a Sigma 30/1.4 (50mm equiv in photographic terms) @ 10ft on the 7D, DOF would be only 1.8ft, that's the advantage and disadvantage of shooting with a aps-c sized sensor.

[Ritchey Cable]

Thanks for the information. I learned quite a bit in reading these responses. Ever since I bought my Letus Adapter, I always thought the optics of DOF were simply in the glass itself. Interesting what differences these different settings have on the final image. Thanks again, very helpful.

[Scott Brickert]

Originally Posted by Kin Lau

In the same example if you shoot with a Sigma 30/1.4 (50mm equiv in photographic terms) @ 10ft on the 7D, DOF would be only 1.8ft, that's the advantage and disadvantage of shooting with a aps-c sized sensor.

Hey Kin,
I'd like to understand how you figured that out, so I could do it myself. In particular, estimating the DoF for a given focal length, aperture and distance.