Tuesday, May 29, 2012

Is it in Focus?

I recently wrote about lenses and perspective. In there I mentioned following up with a post about depth of field. I assume that most of my readers are familiar with the term. If you need a refresher, I suggest starting with this Tutorial from Cambridge in Colour.

The main control we have over depth of field is the aperture of our lenses. Wide open (lower number f/stops) we get less depth of field. As we stop down (larger f/numbers) the depth of field increases. But stopping down the lens can have negative effects, too. Small apertures lead to an effect called diffraction that lowers the image quality. Think of what happens when you put your finger over the end of a faucet. The smooth flow of the wide open faucet turns into a wild spray. So, you can stop down too far and get a lot of depth of field, but with an overall loss of image quality. What aperture you start to suffer diffraction at will vary with sensor size and specifications. Too much math to go into here, so I suggest you take a visit to Cambridge In Colours excellent tutorial on diffraction. While there, read all the other tutorials. This guy is good!

I just wanted to cover some of the questions that might come up when looking at the images in my most recent post about focal length and compression. Consider these two images

24mm at 48"
85mm at 48"
They were both taken at f/5.0, but the top one, taken with a 24mm lens shows the grid patter in the background more in focus than the lower image taken with the 85mm lens. Well, of course. We've all been told over and over that you get more depth of field with a shorter lens. But is that really true?

It is partly true. In this case it is true because the images were taken from the same camera position and the one taken with the 24mm lens was cropped in from this original 

24mm not cropped
So, we do see more depth of field, but at a considerable loss in resolution. The image taken with the 85mm lens is 3744x5616 pixels, while the crop from the 24mm lens is only 995x1492 pixels. That's fine for a 3x5 print or for use online. But not for much more. If you need to print an 8x12 or larger the lack of pixels is going to be a much bigger issue than the extra depth of field you gained by using a shorter lens--unless that non-cropped version of the photo is really what you wanted.

What if you decided to move in closer with the 24mm lens to get the subject to be about the same size in the frame? Then you do get your full 3744x5616 pixel image. But it is a very different image. The look of the subject is totally different as seen here...

And the depth of field has shrunk back to where it was with the 85mm! What's with that? The shorter lens is supposed to give more depth of field, isn't it? Yes, when used at the same camera to subject distance. But when you move in to get the same framing you counter that.

The fact of the matter is that focal length alone is not a contributing factor in calculating depth of field. You need to combine focal length with the camera to subject distance. So, it is more like magnification in the original image is the contributing factor. If the main subject is the same size on the sensor with a long or a short lens the DOF will be approximately the same.

Let's look at these images, which were all taken at f/22...

35mm full frame
200mm full frame
35mm cropped
For these the camera was moved to try to maintain about the same size head in both frames (same magnification). Here we see that the images taken with the 35mm lens and the 200mm lens exhibit similar depth of field. The yellow text on the black disc in the background is blurred about the same. The overall effect of the 200mm vs the 35mm appears that the subject stands out more in the 200mm version. But that is more about the limited field of view behind the subject. The blurriness is about the same in each, but there is a lot less background to be noticed in the photo taken with the 200mm lens. In the photo taken with the 24mm lens there is a lot going on in the background, and despite being equally out of focus there is more context available for our brain to tell us what is back there (fence, table, my wife Kim, etc.).

And overall, the image quality on the 35mm photo is lower, as seen below...

200mm full frame, no cropping
35mm full frame, no cropping
Both of these images were taken at f/22. As noted in the last post, the perspective is the same (camera and subject weren't moved, I just changed lenses). But if we enlarge the center section of the 35mm shot we get this...

35mm cropped to same framing as the 200mm
Here we can see the 35mm image has more depth of field (the text on the black disc is not as out of focus), but suffers a lack of contrast and overall loss of edge detail that I am chalking up to diffraction. Here is another comparison. This time both images are taken with the same lens in the same position, but one is at f/16 and the other at f/2.8. While the f/16 version has greater depth of field, the f/2.8 version appears more crisp and contrasty. I admit I added to the challenge here by working in less than ideal conditions with strong backlight. But this is a situation we often find ourselves in.


Anyway, I just want you to stop and think about the generalizations we hear every day as we learn about photography. Telephoto compression, stopping down for better images, focal length and depth of field, etc., etc. All of them have an element of truth to them. But they are often only half the story.

I end this as I started, by suggesting you visit the Depth of Field and other tutorials at Cambridge In Colour where they go into great technical details.

And after that please go out and do your own experiments. Then share what you learn in the comments here.


Monday, May 28, 2012

Lens "compression?"

Or, why should you get up and move around when making a photograph.

Just a quick question... Which compresses a scene more, a short or a long lens? Don't be so quick to come up with your answer. In each pair of photographs below one was taken with a wide angle lens and the other with a telephoto. Big thanks to one of my favorite models* for obliging me here.

24mm lens at 48 inches
85 mm lens at 48 inches
35mm lens
200 mm lens

While the lens was changed between each image, the camera and subject were stationary. If the camera to subject distance stays the same, the perspective (relationship between objects in the scene) stays the same. The wide angle lens lets you get more of the scene into the frame. The telephoto lens lets you fill the frame more selectively. But for the part of the scene that is common to both the perspective will be the same.

Here's what the original full frame images look like from the short lenses. The images taken with the longer lenses were not cropped.

24mm at 48 inches, no crop
35mm, no crop. Same distance as with the 200mm
However, if you do move the camera, things change dramatically, as seen in each of these non cropped images. By moving in close to fill the frame with the 24mm lens we get a much different drawing of the face than we do filling the frame with the 85mm lens. It isn't the lens, however, that is causing the different looks--it is the camera to subject distance. The lens just lets us fill the frame the with the subject's head at the different distances.

24mm at 12 inches
85mm at 48 inches
Getting physically closer to the subject slims the subject's face by making the relationship between nose to eyes to ears stretch out. As you take your camera and move back away from your subject the nose/eyes/ears are all relatively closer to each other, resulting in a flatter overall look which makes the subject look wider.

This works outside as well as in the studio.

35mm cropped from the image below
35mm full frame
Compare the 35mm** crop here with the 35mm crop in the third image from the top of this post to compare the compression. Same lens, but different camera to subject distances.

Keep in mind that objects close to the camera will appear larger in your photographs. But it is all relative. At 48 inches away your subject's nose, eyes, and ears are all relatively the same distance from your camera. When you move in so the nose is only 12 inches from the camera it is relatively a lot closer to the camera than the eyes and ears, so the eyes and ears are proportionally further away and look much smaller.

Consider this illustration (not to scale)...

In reality (above)

What it appears like from being in close

Pay attention to the relationship between the yellow and green shaded areas above. Note that close or far, the green shaded area remains the same size. In this illustration I have shown the green area as nose/eyes/ears at 12 inches and 48 inches. However, you can also think of the subject as a rock, trees, and a mountain and 12 feet and 48 feet. What I want you to see is that you move closer to or further from your subject the elements in the scene maintain their relationship between each other (nose/eyes/ears or rock/trees/mountain). No lens is going to change that relationship. If you switch from a 100mm lens to a 20mm lens without moving the camera, everything in the frame is rendered at 1/5 the size it was before. The relationship between those objects stays the same, but we see more of their surroundings.

At the top we see the camera to the nose is 12" and to the eye is 13" and it is 16" to the ear. When we pull back those numbers change to 48", 49", and 52". The tip of the nose to the ear in each situation remains about 4" and isn't going to change whether we're close or far away, or whether we use a short or long lens. What has changed is the yellow shaded area. Up close that 4" is relatively similar to the 12" camera distance. The ears are 1/3 again as far from the camera as the nose is. Moving back that relationship changes so that the ears are now relatively the same distance from the camera as the nose is (just 1/12 again as far). The further you move back from your subject the more pronounced the effect. Hence the appearance of "compression" or flattening of the view caused by backing away from your subject.

As noted, the same thing happens in landscape photography. Just substitute rock/trees/mountains for nose/eyes/ears. Move in close to an object and it becomes big in the scene and everything behind it starts to fall away in size. But if you back up far enough the relationship between the objects gets tighter (compressed). It doesn't matter if you have a short or long lens, the compression is the same. What the lens choice does is allows you to get more into the frame (short lens) or to zero in on a particular part of the scene (long lens).

If you stand in one spot and use a zoom lens the perspective stays the same. In many cases this is exactly what you want, and there is nothing wrong with that. But if you take the opportunity to move around your subject and change the camera to subject distance, whether with a prime lens or a zoom lens, you open yourself up to more variations and possibilities. Move in close to emphasize size differences and select a shorter lens to take it all in. Move back to de-emphasize size differences and select a longer lens to bring it all closer to you. You could use a short lens and get the same compression, as shown above, but that leaves you with tiny subjects that you will need to blow up to billboard size to recognize, with an inherent loss of quality.

You have probably noticed differences in the depth of field and overall image quality in the outdoor examples. I'll discuss that in a future post. For now, just start moving around and view things in different perspectives.

Then come back and let us know what you discovered in the comments below.


*I use a mannequin for these examples for a variety of reasons. One is that I can make sure the model doesn't move between camera setups. Another is that I hope the audience won't be distracted by the look of the model and facial expressions that might otherwise influence the look of the image.

**The short lens used in close was actually zoomed to 32mm. Oops! But it doesn't change anything about the lesson.

Tuesday, May 22, 2012

Charity Photo Session for Cancer Survivors

It has again been a while since I've updated the blog. I've been super busy the past two months with things like Moisture Festival (50 shows over a four week span), creativeLIVE online workshops, reconfiguring and painting my home studio, etc., etc. In between I was taking time to get some sleep and let the blog wallow for a while.

Today I have a great event to talk about. I had the honor and pleasure to work with a great crew of photographers, assistants, makeup artists, Lightroom, Aperture, and Photoshop experts and other volunteers from the Seattle Smug Mug Users' Group and the Virginia Mason Medical Center in Seattle, WA. Together we held a 2-day event creating portraits for cancer survivors.

Fellow photographer, Paul (Pablo) Conrad has done a great post listing all the people involved on his blog. He also has some great behind the scenes photos there. Check them out.

I need to make a special call out to Erin Kohlenberg and Earnie Glazener for pulling this all together. And a big thanks to Bay Photo for donating a 16x20 or 16x24 metal mounted print to each of our photo subjects.

Part of our group on day 2

These portrait events can be very emotional. Some of our subjects have never had makeup applied before in their life. Others may have never had a good photo made of them or of them with their family. I'm still processing my side of the story. Right now I wish there were two or three of me at the event so I could have listened in during the hair and make up sessions or could have seen the faces of the people as they got to see their images on the screen. Again a big thanks to Pablo for capturing those moments in photos. He has the photo journalist gene that I'm sorely lacking.

Here are some of the photos from the day. I had the pleasure to make portraits of both the survivors and the volunteers. They range from individuals to the family of 9 shown above. Here are some of the survivor photos. A few more photos are on my Facebook page:

And some of our volunteers, some of whom are also survivors:

David Loseno
Jesse Villanueva
Paul Gibbons
Anne Mills
Kelly Hasenoehrl
Henry Lingat 
Nat Seymour
Melissa Wax
Ilona Berzups
Arnie Cohen