Archive for the ‘Lenses’ Category

Polarizing Filters

Tuesday, June 19th, 2012

Polarizing Filters – You can’t mimic these in Photoshop!

A polarizing filter is one of the few filters that is equally effective with color imaging and with black-and-white.  It can:

•  Minimize or eliminate reflections in glass, water or most any surface except metal.

•  Darken skies in color photos as well as in black-and-white

•  Cut through haze

•  Increase the saturation of colors

Types: There are two basic types of polarizer, the original “linear” polarizer and the comparatively new “circular” polarizer.  They essentially accomplish the same thing, but linear polarizers pose problems with most modern through-the-lens metering systems, so circular polarizers were developed to minimize or eliminate metering issues.

Filter Factor: Most filter manufacturers list polarizers as having a variable filter factor, usually 2 to 4 depending on the effect of the polarization.  I personally just use a factor of 2.5 (1.3 stop correction) because any further darkening, say, of a sky is an effect I want, and do not want to override.

Use:  The great thing about using a polarizer is that you can actually see the effect before taking the picture.  When held up to the eye or placed on a lens and rotated in a clockwise or counter-clockwise direction, you can see gradual lightening or darkening as you rotate.  You can pick just the degree of effect you want.  At its lightest orientation, it is essentially just a 1.3-stop neutral density filter.  If you are using aa non-through-the-lens camera (rangefinder, Holga, etc) you will need to hold the filter up to your eye and note what part of the filter is at “12 o’clock” for the effect you want, then put the filter on your lens in the same orientation.

 

Above, a polarizer effectively eliminating reflections in glass.  Note that the reflections in the polished table are only moderately affected – that light was polarized in a different direction.  Below, note how the polarizer has darkened the sky, increased saturation in the colors and reduced haze.

 

Using Polarizers with Wide Angle Lenses: Polarizers work by darkening light in the subject that is already polarized.  If there is no polarized light, the filter has no effect other than neutral density.  The light in a sky is not evenly polarized, so if you are using a wide-angle lens, you will capture part of the sky that may be highly polarized AND also include part of a sky that is less polarized.  If you are using a wide-angle lens on a subject that does not include sky, you may not notice any odd effect.

In the end, no well equipped camera case should be without the versatile polarizing filter!

 

 

Filters – How to Choose and Use

Friday, April 6th, 2012

Filters – How to Choose and Use

Part 1

 

I think the thing I like most about working in black-and-white is the fact that it’s much more an expression of how I feel about a subject than a representation of “reality.”  The world doesn’t exist in Black-and-White (my mother told me that…) so a b/w image is by its very nature an abstraction of the things we see.

The judicial use of filters can greatly enhance the impact of how a subject appears, and in black-and-white we can even skew the way colored subjects relate to each other.

I normally like to be fairly subtle about my use of filters; a photograph shouldn’t look like a filter was used, just as a print shouldn’t look like it was dodged and burned!  One of the most generally popular choices, a #8 Yellow, is usually so subtle that I don’t see much point in using it.  Another popular choice, the #25 Red, is often too strong, rendering skies and day-lit shadows illogically dark.

My two favorite filters, a #12 Yellow (“minus blue”), and a #23 Red, respectively, have both more strength and finesse than the ones found in most camera bags.  The #12 yields an effect almost as strong as a #15 orange, but with only a 1 stop filter factor, only slightly greater than the #8.  The #23 tends not to make skies quite so artificially dark as the #25.

Understanding the relationships of different colors of light to each other is key to choosing a filter.  A standard color-wheel is shown below.  The numbers in various color areas are Wratten filter-number designations, an industry standard utilized by many filter manufacturers.  A #12 filter, for example is pure yellow, a #8 is a light yellow. The capital letters in bold are called Additive Primary colors, and the lower-case letters are Subtractive Primaries.

Red is opposite Cyan

Green is opposite Magenta

Blue is opposite Yellow

 

In Black-and-White photography the practical effect of a filter is to lighten its own color and darken its opposite color.

In purely scientific terms, a filter has no effect on its own color and darkens everything else, including “neutral” colors.  When we apply a “filter factor” to the exposure, neutral colors remain unchanged and then the filter’s own color becomes lighter and its opposite becomes darker.

What we commonly call a “blue” sky is technically a bit more cyan, which is why a red filter will darken the sky more than a yellow filter.  Orange is in between.  Keep in mind that outdoor shadows are illuminated by the sky, not the white light of the sun.  Any filter that darkens the sky will also darken the shadows!

Green or red filters can be quite useful in the Southwest, for example, where we might come across a brilliant green plant in front of a red sandstone wall.  With no filter used, the b/w film will see the green and red as being largely the same:  gray mush.  A strong green filter will make the plant light and the sandstone dark, the red filter will do the opposite.

For workers using digital cameras for b/w, my tests indicate that it is better to use a computer-simulated “filter” after a RAW capture, rather than an actual filter for the capture itself.  While this may only approximate the effect of using a filter with film, the effect ought to be similar – without any need for exposure compensation for the filter’s own density.

Polarizing filters are also extremely useful for both B/W and color work – but we’ll cover that in another post!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Understanding Lenses – Depth of Field

Friday, March 16th, 2012

Where do you focus, and how does the aperture affect an image?

In a certain way, the opening question should be the other way around!   There is a law of physics that governs the relationship between shutter speed and aperture (f-stop).  Shutter speeds are pretty easy to understand: 1/60 second is one half as much time as 1/30.  F-stops are a little different:  f8 is one half the light of f5.6, which is half the light of f4!  The point is, for any shutter speed/f-stop combo, one-half the exposure time with twice the light equals the same total amount of light given to the film – or pixels. 1/60 @ f4 = 1/30 @ f5.6 = 1/15 @ f 8.

There is always the inescapable relationship between exposure time and aperture.  If you are photographing a sports event, you will likely go with a fast shutter speed and let the aperture fall as it will.  This article will “focus” on aperture as primary.

F-stop #s versus depth-of field. A lens can only truly focus on ONE plane.  With a perfect lens, that plane would be equally sharp at any aperture – but everything nearer or farther would rapidly become unsharp.  Increasingly smaller apertures reduce this apparent unsharpness, increasing what is called depth-of-field. The smaller the aperture (f16 is smaller than f4), the greater the apparent sharpness.

 

In the example above, figure A is focused approximately on the line of traffic in the foreground.  The chain-link fence is way out of focus, as is the distant railing.  The wide-open aperture (f 1.4) necessitated a very fast shutter speed resulting in the cars frozen in time.  Figure B is with the lens stopped down 4 stops (f 5.6).  The point of focus was not changed, but the fence is now a good deal sharper, as is the distant railing, but at the now much longer exposure time, the nearby car, while still sharp in focus, is blurred in time.  Figure C is still focused in the same place but the lens is now stopped down 3 more stops to f 16. The fence now appears to be quite sharp as does the distant railing, but the car is now quite blurred at 1/15 second. (Note: it is a total coincidence that the images seem to show the same car!)

 

Most fixed-focal length lenses have an engraved scale allowing you to evaluate how much apparent sharpness (depth-of-field) you can get at various apertures.  The example above shows a Hasselblad 80mm lens set at f 22.  As the lens aperture is stopped-down, the depth of field increases in the proportion of 1/3 toward the lens from the plane of critical focus and 2/3 beyond the plane of focus.  Figure A above shows the lens focused at about 3.3 feet, and at f 22 the depth of field runs from 3 feet away to 4 feet.  Figure B shows what would happen if we did a landscape with the lens focused on infinity.  The image would only be “sharp” from about 17 feet away to distant mountains. If we instead focused at 17 feet (this is called the “hyperfocal” distance) the image would now be sharp from about 9 feet to the mountains (Figure C).

There are two ways to plan how to make this work.  One way is to choose your aperture first and see how much depth-of-field you get, and the other is to find out what aperture you need to work with and then see how much depth of field you need to work within.  Let’s say your camera is on a tripod, and you want as much as possible near-and-far to be sharp.  Take the Infinity mark on the lens and place it over the engraving for your smallest aperture.  The lens is now focused automatically at the hyperfocal distance and you can read the depth of field on the focus scale of the lens.  In this example (Figure C), f 22 gives you a pretty sharp image from about 9 feet to infinity.  Lets say the camera is NOT on a tripod, and you can’t manage to stop down to f 22, but only to f 8.  In this case, you would place the Infinity mark over the f 8 index.  You would now see that the image would only be sharp from about 20 feet to infinity (see green arrows, figure C).

What if your lens doesn’t have markings?  A lot of modern zoom lenses have distance scales, but no depth-of-field markings.  If this is the case, you can find the hyperfocal distance by putting the nearest subject and distant subject marks on the lens an equal distance from the central focus mark.  If your camera has a “depth of field preview” button, this can be a useful aid in seeing just how much is sharp – or not!  But the actual depth of field for any given f-stop will just be a guess.

One bit of fun with f-stops: Selective Focus!  Sometimes, you can make a stronger statement by limiting how much is in focus.  Just leave the lens at its widest aperture.  The figure on the left was done with a 200mm lens at f 4.5 focused exactly on the near marker, and the figure on the right was done at f 22 with the lens set at the hyperfocal distance.

One last thing I’d like to comment on in this writing: lens quality.  Photo gear can be expensive, no doubt about it.  Especially at an entry level, the prospect of getting an off-brand lens for a lot less than the brand that has your camera’s name on it can be awfully tempting.  In these days of computer-aided engineering design a “Brand X” lens can be quite good – but there is an equally good chance that it will not measure up to the quality or durability of a top brand.  One of the reasons being that the Brand X lens manufacturer can cut a lot of production cost by using much looser manufacturing tolerances than the top brands.  The glass itself may well be of lesser quality.  If you need to save dollars, look for quality used gear from a reputable source.

Hopefully, all of this will help you have a better understanding of the relationship between your vision, your lens, and your results!

Choosing a Lens – How Does Focal Length Affect Your Image?

Thursday, January 12th, 2012

It comes as quite a surprise to a lot of the photographers I work with that the only thing that changes when you use a different focal-length lens is the cropping of your image!

Optical aberrations aside, short focal-length, or “wide-angle,” lenses do not distort close subjects, and long focal-length, or telephoto, lenses do not compress subject features.  What really causes these familiar effects discussed so often in popular texts is a change of perspective:  a change in the camera’s physical position relative to your subject.

When you move in close to a subject, it becomes very large in relation to its background.  So, that over-large nose you get with a wide-angle-lens portrait is because you’ve probably moved in very close to the subject in order to fill the frame, and the nose, being closest to the lens, is now very large in relation to the ears.  This is a matter of your proximity to the subject and has nothing to do with the lens itself.

When you look at a distant scene through a long focal-length, or telephoto, lens elements in the scene may appear compressed, almost right on top of each other.  Once again, this has nothing to do with the lens, but is simply a matter of the tight framing on the subject.  If you put the camera down and frame the scene just as tightly with your hands, the elements of the scene will appear as “compressed” as they did through the lens.

What about zoom lenses as opposed to fixed focal-length lenses?   Do they help make your choice easier?  Well, yes and no.  Assuming the optics are up to snuff, a zoom can provide a great deal of convenience – it’s a zillion focal-lengths in one piece of hardware.  But that convenience can lead to overly casual, rather than critical, vision.  Imagine a photographer out for a walk.  He (or she) comes across a detail or a scene that interests him. Camera goes up to eye, hand zooms lens to frame the subject, auto-focus and metering do their jobs, shutter goes click, and it’s on down the path.  Would the image have been more powerful if our photographer had moved in close to some boulder in the foreground, making it monolithic in relation to the background?  Maybe.  Or maybe backing up a bit might have let some tree branches frame the scene.

The point in all this is that to maximize the impact of a visual statement it is important to give thought to the image structure first.  Is the composition better closer in?  Farther back? Up, down left or right?

Once you pick your camera position, then choose the focal length that gives the cropping you want.  If your first guess is too tight, use a shorter lens, if it’s too loose, use a longer lens. If you don’t have a lens that is quite right, use one slightly shorter than you would like and crop. That’s the lens to use!