Depth of Field

Wow, this is a tough subject. So many people get so confused by this subject.

While researching to try to write up my own thoughts on it I ran across a site by Paul va Walree that already had all the answers. You can find it here.

Paul brings in things other sites overlook, such as print size or projected size and viewing distance (see the Visual Acuit section). He demonstrates how depth of field (DOF) does not normally vary with focal length (see the Background Blur section). I say normally because the construction of some lenses (symetrical, retro focus, telephoto, etc.) can have a little effect. Many folks are surprised to learn that some longer focal length macro lenses sometimes offer more depth of field than a shorter focal length lens. He also talks about why DOF scales on lenses are inadequate (see the Hyperfocal distance section).

For more on DOF scales and DOF tables I suggest reading some things from Harold M. Merklinger. Start with his Depth of Field Revisited article. Then you can follow it up by reading Merklinger's book The Ins and Outs of Focus (available as a PDF file).

Going back to Paul van Walree, he has a few other articles online that are going to save me from doing a lot of writing. I highly recommend these to all serious photographers interested in the technical side of things:

http://www.vanwalree.com/optics.html
http://www.vanwalree.com/optics/misconceptions.html
http://www.vanwalree.com/optics/filterflare.html

If you want to go even deeper into diagrams and math you can check out Norman Korens article on Depth of Field. A couple of summaries from that site:

"DOF is much more closely related to magnification and f-stop; DOF expressed in distance is nearly independent of focal length."

"Those who use depth of field scales, tables, and formulas (e. g. for hyperfocal settings), restrict themselves – most probably without knowing why – to the image quality potential of an average pre-World-War-II emulsion."
(above is actually a quote from Carl Zeiss Camera Lens News at http://www.zeiss.de/C12567A8003B8B6F/EmbedTitelIntern/CLN01e/$File/CLN1.pdf (PDF File))

I hope the above referenced web sites help bring things into focus!

Martha Casanave Workshop in Coupeville, WA

Just a few weeks until Martha's "The Portrait and the Figure" workshop (June 14-17, 2007). I will be the class assistant for this workshop...

This four-day workshop will address the following issues: What exactly defines a portrait, and how is it different from the portrayal of the nude? What are the similar issues involved in both approaches? What are 'traditional' portrait and figure photography, and what are some alternative, non-traditional approaches? How is the relationship with your model affected by the exchange of money? Using readings, discussion and camera work with each other and nude models (male and female); we will explore topics such as helping sitters relax, posing, nonverbal communication, and the relationship between model and photographer. Casanave will present practical techniques for dealing with these issues, and will also demonstrate, using Polaroid materials, an on-location indoor environmental portrait. The class will build the portrait together, 'from scratch,' solving, one by one, the problems of natural lighting, composition, background, placement of subject(s), and posing. Anyone interested in photographing people is welcome to join this workshop; previous experience with this subject matter is not necessary, though a basic knowledge of photography is required.

Martha's bio is at:
http://www.coupevillearts.org/asp-bin/inst_bio.asp?InstructorID=334

Cost is $400 plus a $100 fee for the class models. But if you tell them I referred you to the class you can get a $20 discount.

http://www.coupevillearts.org/ or call them at 360-678-3396

Light, Gesture, Color

I just spent a great three days with photographer Jay Maisel. Lots of things still going through my head. I thought I'd jot down some of the things I got out of these wonderful days...

Figure and ground
Shape is the enemy of color
You can't step into the same river twice-stop and take the photo
Form over concept
No wallpaper
You're not as bad as you think
You're not as good as you think
Have lots of patience
Nothing's neutral. It either adds to or hurts the image.
Get in closer
No triangles in the corners
You are responsible for every square millimeter of the frame
Watch the edges
Everything has gesture--find good jesture
Forget the rules of composition
Make the image yours
Symbol vs anomoly
Interaction of colors alters the colors
Interruption of pattern
Boring or involving?
Accept Accidents
Don't get boxed in by preconceived ideas
If partially out of focus it competes
Out of focus foregrounds need to be completely out of focus so they become abstract elements
When you think you are done, do some more
Have an affect on the rest of your life
Learn to see better
Open your eyes
Carry a camera with you everwhere
Don't take the picture if it doesn't move you
Look behind you

Good photographers are blessed with the ability to extract passionate joy from things totally overlooked by most people

Shooting when you have nothing to say is worse than talking when you have nothing to say--In the first case you still have to go through the pictures at a later time

Adjusting your viewfinder diopter setting

This is one of those questions that came up in a recent class. Some people never looked at this setting. Some just assumed to set it at zero. Some said they always wondered, but were afraid to ask. Some thought that it was too obvious to ask. Just about everyone was wrong.

The diopter control helps you focus on the viewfinder in the camera. If you wear eyeglasses you are probably familiar with the term diopter. Your eyeglass prescription may be described in diopters. If you are nearsighted your eyeglasses will have a negative diopter adjustment (i.e., -1.5). If you wear reading glasses they may be marked with a positive number (i.e. +2.25).

Camera viewfinders can be adjusted through a range of diopters, but the range may be different between models and brands of camera. -2 to +1 is a typical range.

So..., just how do you adjust this dial?

The first temptation is to look through the viewfinder and try to adjust it so that the image in the finder looks its best. Nope.

What you want to do is adjust it so that the items in the viewfinder (the etchings on the focusing screen, for instance) look clearest. Pay no attention to the image you are pointing the camera at when adjusting. Put the lens in manual focus mode, point the camera at a plain white wall, and throw the image completely out of focus. Now you can concentrate on the focusing screen. Does it have a circle etched in it? Or small boxes to show the autofocus points?

Put your diopter control all the way at one end of its range. Look at those etchings on the screen and slowly turn the dial until the screen is sharply focused. You might find two or three click settings that all look pretty good. It is like sitting in the chair at the eye doctors where they examiner is saying #1 or #2... #2 or #3...? Which looks better? Sometimes there is a big difference. Sometimes it is very subtle. Pick the one that looks the best.

Setting the diopter will not affect autofocus. But it will affect manual focusing. If set wrong, an in focus image will look out of focus in the viewfinder. But I don't think that you can do the reverse. That is, I don't think that you can make an out of focus image look in focus by adjusting the diopter.

Which macro lens should I get?

So, you want to get a macro lens. Which one should you get? There are a variety of different focal length macro lenses on the market. Typically they are aroun 50mm, 65mm, 100mm, and 180mm. Why select one focal length over another?The first thing that might come to mind is working distance. With a longer focal length lens you can be further from your subject at the same magnification. If you are photographing a flower or insect at 1:1 with a 50mm lens the distance between the camera and the subject is going to be very short. You will have little room for lights and skittish subjects will probably crawl or fly away. With the 100mm lens you will have a more comfortable working distance. With the 180 you will have lots of room.

Next, you might think about depth of field. But depth of field is more related to magnification than the focal length. If your subject is being recorded at 1:1 the depth of field is going to be just about the same with any of the lenses.

There is another consideration that many might not think of. The field of view. Field of view determines what you are going to see in the background and is, I think, the most important consideration affecting the final look of your image.

With a short focal length lens, which has a wide field of view, you will get a lot of the background in your image. Here are some examples. All were taken at an aperture of f/8.

The first image was taken with a 15mm full-frame fisheye lens with a 12mm extension tube to allow it to focus as close as possible...

In this image the subject was literally touching the front element of the lens. And, as you can see, there are a lot of potentially distracting background elements. The main subject here is a bit smaller than in the other two images, but this was as large as this lens could reproduce the subject.

The next image was taken with a 50mm lens with both a 12mm and a 25mm extension tube, again at its closest focusing distance. Less background, but still distracting. You can see a chair, a white house, a brown fence, some green grass, etc.

The final image was taken with a 180mm lens set for 1:1 reproduction. With its limited field of view the background is just the green grass.


In the 50mm and 180mm versions the subject is appears almost identically. But the overall look is a lot cleaner with the longer lens.

Resolution??

The short form:

#1, First thing, you should ignore resolution.
On its own, it ismeaningless. You have to combine it with a physical size.

For example, take a file that is 2000x1000 pixels. What is the resolution?

The answer is 72 dpi, or 100 dpi, or 200 dpi, or 300 dpi, or whateveryou want it to be. The DPI by itself means nothing.

That 2000x1000 pixel image will be:
at 72 dpi = 28 x 14 inches
at 200 dpi = 10 x 5 inches
at 300 dpi = 6.5 x 33 inches
That's all from the same file with no changes to it.

Typically, a 6 megapixel image will be something like 3072x2048pixels. If you set that to 180 dpi the size in inches will be 17.067x 11.378 inches. If you set it to 300 dpi it will be 10.24 x 6.827 inches.

As there are a number of 8 (and greater) megapixel cameras out now,here are the numbers for an 8mp image:

3504x2336 pixels
19.467 x 12.978 inches @ 180 pixels per inch
11.68 x 7.787 inches @ 300 pixels per inch

Some camera files claim to be 72 dpi. Others claim to be 180 dpi.Others might claim some other resolution. The amount of pixels is the important number. You can change the resolution at any time without affecting the file size/quality. Go to Image > Image Size and uncheck the RESAMPLE box. Then you can type in whatever resolution you want. The document size will change, but the pixel dimensions will not.

The Longer Version (click here).

Outdoor Fill Flash

How to mix strobe with sunlight

There are two basic concepts to keep in mind---
* The flash/strobe exposure is controlled by the aperture
* The ambient light exposure is controlled by the shutter speed
You are creating a double-exposure in one frame from two light sources.

The first think you need to know is the highest flash sync shutter speed for your camera. This is typically between 1/125 and 1/250 of a second, but varies by camera and could be higher or lower. This information is in your camera manual.

Start by metering the scene using the shutter speed about two stops lower than the max sync speed. For example., if your camera can sync at up to 1/250 of a second start the process by metering at 1/60 of a second (two stops lower than 1/250). Note the aperture required at that shutter speed. For example, let's say the meter said F/8 at 1/60.

Meter the flash/strobe with a hand-held flash meter. If the meter has a shutter speed setting set it to the max sync speed for your camera. For this example, let's say that the flash meter reads F/11.

Adjust the output of the flash to get a reading that matches the ambient light reading from the previous step. In this case we would lower the power of the flash 1 stop so that the meter reads F/8. If you cannot lower the flash power you will need to move the flash further away from the subject or add some diffusion material or neutral density filters over the flash to lower the output.

Set your camera (in Manual mode) to F/8 and 1/125. The scene should be balanced between the ambient and flash.

Now you can adjust your shutter speed for effect. A faster shutter speed will darken the background without making the flash-lit subject darker. But keep in mind that you can't set a shutter speed higher than your flash sync speed. A slower shutter speed will start to overexpose/wash out the background (make it brighter than the subject) But if you go too far you may also end up brightening the subject, too. And you also risk having some motion blur and ghosting around the subject (which is not necessarily bad, it can be an interesting effect).

But what happens if your flash exposure is F/22 but you want to photograph at F/8 or F/11? You have to lower the output of your flash unit. This can be done by lowering the power on the strobe. If you cannot lower it any more you can try moving the lights back (not good if you are trying to keep the light close and soft, moving them back may give too much contrast), or you can change the light quality by adding more diffusion, or you can use neutral density filters over the flash.

Another option is to use a Neutral Density (ND) filter on the camera. With a 3-stop ND filter your flash exposure will go from F/22 to F/8. Meter again for the ambient light with the ND filter in place to determine your balancing shutter speed. If you are using a hand-held meter for the ambient light remember to adjust it to take into account the ND filter. Again, you can adjust the shutter speed up/down to affect the look of the background (remember it has to be lower than the max sync speed of your camera).

Example:

Flash exposure f/22
Ambient exposure 1/60 @ F/22 (or 1/500 @ F/8)
Desired aperture F/8

Use a 3-stop neutral density filter:
Flash exposure F/8
Ambient Exposure 1/60 @ F/8

A Matter of Perspective

A common misconception is that lens focal length affects perspective.

Perspective is the image relationship between objects in a scene. This relationship is determined by the distance between the camera and the subject. You select the camera/subject distance to set the relationship you want and then you select the focal length of lens that will fill the film format you are using. Pretty basic—but often confusing.

Everyone knows that wide angle lenses make things look expansive with a lot of space between objects in the image and that telephoto lenses condense everything and make items in the scene look compressed and close to each other. And that is true. But it is not because of the focal length of the lenses being used.

When you are close to your subject you generally need a short (wide angle) lens to capture the scene. In the scene you may notice that objects very close to the camera appear to be much larger than similar sized images just slightly further away from the camera. This is not caused by the lens. It is a result of the front objects being relatively much closer to your camera than the further objects. When you are farther away from the subject you often use a long focal length (sometimes called telephoto) lens. In those images the relative distances between the camera and the subject and the distance between various objects in the scene are greater (the objects in the frame are closer to each other than they are to the camera) and the objects in the scene appear to be compressed closer to each other. Again, this is because of the distance between the camera and those objects, not because of the lens. The lens is just selected to fill the frame. Take the photo created with the short lens and enlarge it and you will see that objects in the distance have the same perspective as if they were taken with a longer lens and enlarged less.

Let's take an example of shooting a portrait. You typically want to be at least 6 feet from your subject when taking portraits. This sets up the relationships between you and your subject and between your subject's features (such as their nose, eyes, and ears). Let's say that the distance between the tip of your subject's nose and their ear is 6 inches. The camera to nose distance is 6 feet and the camera to ear distance is 6.5 feet. The ratio is 12:1. This gives a pleasing relationship and looks good. Now if you move in closer to your subject, let's say 3 feet, the nose to ear distance has not changed. The nose is now relatively much closer to the camera (ratio is 6:1) than the ear and it will appear to be too large giving your subject a distorted appearance. The distortion is caused by the closer subject distance. If you were able to focus a telephoto lens on the camera at the 3 foot distance you would see the same distortion as you do with a wide angle lens.

The difficulty in comprehending this situation is that you can’t focus the telephoto lens that close and, even if you could, the angle of view of the telephoto wouldn’t show enough of the subject to see the problem clearly. It might be easier to understand this by saying that a short/wide angle lens will allow you to see this perspective distortion because you can get so close, you can’t do that with a long/telephoto lens. Photographers tend to use a wide angle lens in situations which causes them to blame the lens for the distortion when the lens is just showing what it sees.

It is easier to demonstrate this by turning the situation around. Set up your camera on a tripod and have a subject seated 6 feet away from your camera. Take two photos; one with a long focal length lens and the other with a short focal length lens—do NOT move the camera or the subject between the shots. You should end up with one image (take with the long lens) having a larger subject than the other. Now open both images side by side on your computer and enlarge the image taken with the short lens so that the subjects face is the same size in both images. Note that the face looks exactly the same. No distortion "caused" by the wide angle lens. As stated earlier, you select your distance from the subject to set the relationship between the nearest and furthest objects in the scene, and then you select the lens whose focal length will fill the format of your film. Images taken with a short focal length lens will have a greater feeling of space and images taken with a long focal length lens will have a compressed feeling of space.

Remember that wide-angle is relative to the size of your film or sensor. If you are using a large format 8x10 view camera a 150mm lens is considered wide angle. On a 35mm camera 150mm is not.

(click on the images below to see larger versions)

The images above were all taken from the same camera position. The camera was on a tripod and not moved between exposures, only the lenses were changed. The first shot is with a 16mm lens, the second with a 35mm, and the third with a 135mm lens. If you enlarge the section of the photos showing the head (as seen below) you will see that the relationship between the size of the objects (nose to eyes to ears, head to the window behind it, etc.) do not change (though depth of field does change, more is in focus in the image taken with the shorter lens). Even though we’ve used a super wide angle (16mm) and a telephoto (135mm) the perspective has not changed because the camera to subject distance has not changed.


The above images are selections from the original photos in the first row. They have been enlarged so that the head appears at approximately the same size in each frame. They show that the relationship between the head and the background does not change when the camera remains in place and just the lens is changed. There is no change in perspective.

In this set of images the camera was moved closer to the subject for the 16mm and 35mm versions to approximate the same head size. This shows how the perspective does change when the camera to subject distance changes (from 6 feet for the 135mm photo to a foot or less for the 16mm photo. The perspective change is because the camera distance has changed, not because of the focal length of the lens.

The image below attempts to simulates what it might be like to be able to take a photo from this close with a long lens. The full frame is taken with a 16mm wide angle lens. The box in the center shows what a longer focal length lens would see IF you could focus this close. The distortion would still be there, even though you are using a telephoto lens.

Testing a teleconverter

This is a test of an older lens and teleconverter on a current digital body.
The body is the Canon 5D. The lens is a Canon 300mm F/4 L (not the current Image Stabilization model) and the converter is the Canon 2X converter (not the current Version II model). Click on the image below to see a full-size version (1000 pixels tall).

Overall, it works pretty well. This image was taken wide open (f/4 on the lens, effectively F/8 due to the use of the converter). The camera was on a monopod, not a tripod. Shutter speed was 1/500 second.

The file was created as a raw file and converted with Adobe Camera Raw 4.0. No sharpening has been applied in ACR or in Photoshop for this test.

There is noticeable red fringing in the tripod color locking knob in the lower left corner of the image, but this can be helped with the Lens Correction filter in Photoshop (under Filter/Distort/Lens Correction/Chromatic Aberration).

Digital Sensor Sizes and crop factors

Talking with some students and other friends, I still see/hear confusion about what it means to have a 35mm SLR style digital camera that has a sensor that is smaller than a 35mm film frame. So I'm trying to explain that here. What does it mean to have a camera that has a 1.6 focal length conversion factor?

Does that mean the camera crops the image? Does the camera magnify the image? Does it change the focal length of the lens? Something else going on?
Well, right off the bat we can eliminate the magnification or focal length questions. The focal length of the lens is a property of the lens and is not changed by what camera body it is attached to. A 50mm lens is a 50mm lens. No special voodoo magic in a camera body is going to change the focal length of the lens. What does change is the field of view of a lens. The lens projects an image circle. Different cameras make use of different angles of view within the projected image. Smaller sensor cameras discard the outer range of the circle. Is that a crop? You have to decide for yourself. I don't think it really matters. But some folks get very religious about this. Look at the figure here and decide for yourself if you want to say the camera is cropping out the outer range of the image circle or if you want to describe it some other way...

In the above example, the full rectagle is what your eye sees. The larger black circle represents the image circle projected by the lens. The blue rectangle represents what a full-frame 35mm size image sensor records. The red rectangle represents what a 1.6 crop-factor camera sees.

I think the entire situation is better comprehended by folks who grew up using a variety of different camera formats--especially a view camera with different size film backs, like my Deardorf 5x7 that also takes a 4x5 back. When you change the camera back from 5x7 to 4x5 without moving the camera or changing the lens you simply get less of the image in the frame. The lens didn't magically get larger. The image didn't magically get larger. The image projected by the lens is still exactly the same. It is just the area of it that is covered by film got smaller.

For those folks who never used a camera before, I don't really think it makes much difference in your taking photos. It does, though, make a difference if you are reading a book or article that assumes you are using a 35mm film (or full frame sensor) camera with lenses designed for those cameras. You have to take into account the crop/conversion factor. When a text says that you need to use a 70mm or greater lens for "normal" perspective in a portrait you have to convert that to say 44mm or greater if you have a 1.6 crop sensor (typical of many Canon dSLR cameras) or 47mm or greater with a 1.5 crop sensor (Nikon). It isn't the lens that distorts, it is the distance between the camera and the subject. But that is the subject for another blog entry.

As for how to describe the small sensor effect, you decide for yourself if you want to call it a crop of the full data or something else. Me? I'll just go out and make more photographs and not worry about it.