Just a quick note to let readers know that Peter Burian and Jack Neubart have both published their new reviews of the Pentax K-x and the Panasonic Lumix DMC-GH1 on Photocrati. We have not covered a lot of camera gear from those manufacturers below, so this represents the opening of a new line of reviews on the site. For other great reviews of these cameras, try the Pentax K-x or Lumix DMC-GH1 pages on Testfreaks. They link to most of the major review shops on their camera pages. It's been a busy fall for new cameras from most of the major manufacturers!

Jack Neubart, one of the Senior Technical Editors at Photocrati, has just published a nice, user-friendly Canon T1i Review. Some of his thoughts:

"Small and compact, a good choice-if you don't mind some compromises.

I'm not really sure that Canon knows exactly what to do with this camera. It seems simple enough, judging by weight and size, but is otherwise a complex tool with lots of oomph built in. If you're looking to move up to a digital SLR without making a heavy investment or simply want an inexpensive and lightweight DSLR that you can tool around with on your next vacation, the Rebel T1i would be a good choice.

However, don't buy this camera if you expect to use it extensively to shoot video-HD or otherwise, as you may be disappointed. Buy it as a still digital, and use the video capture as an adjunct feature to capture special moments when a camcorder isn't handy...."

 

Dear readers, we've recently moved our extensive collection of digital slr camera and lens reviews to Photocrati, the pro photography blog and photographer community. Here are quick links to relevant sections: Nikon Digital SLR Camera Reviews, Canon Digital SLR Camera Reviews, Nikon Lens Reviews, Canon Lens Reviews, Sigma Lens Reviews, Tamron Lens Reviews, Camera Bag Reviews, Tripod Reviews.

Polarizing filters eliminate glare on non-metallic surfaces like glass or water. Additionally they can be used to increase the contrast and color saturation of your images. I use my polarizer not only to eliminate glare, but also to make skies a nice shade of blue or to get "pop" out of clouds.  But when it comes to choosing the right polarizing filter for your needs, there are several options to consider. Common options include, linear vs. circular, film or digital versions, or quality vs. price.

 

Switching to digital sensors from film brought a lot of advantages.  But, with that switch we picked up one big problem, contamination on the sensor.  When using a film SLR contaminants like: dust or pollen would occasionally  fall on the film.  However, the contaminant would quickly move out of the picture when new film advanced with each photo taken.

With digital there is no such advantage.  Any debris that falls on the sensor might just stick causing a spot to cover part of the sensor and hence our image.  Since each sensor pixel is only about 6 microns wide and typical dust or dirt particles are 20 microns you can end up with a dark spot on your photo where the light was blocked by the contamination.

When I first saw an add for the FireFly I thought it was a myth or gimmick so I had to order one and try it for myself.  I really don't like cleaning a sensor as it's possible to end up with more of a mess then you had in the first place.

I had been using a plain blower bulb and sticking it in as far as I dared to try and clean the sensor with mixed results and it's not something I would recommend anymore.  Also, I had heard horror stories of the wet method and dry contact methods causing either damage or contaminating the sensor even more.

First I tried my old blower bulb and took a sample photo at F/22. Then I used the FireFly system (see FireFly in use photo below).

My daughter using the FireFly to clean a Canon 40D sensor.


I was amazed at how the FireFly got all the contaminants off my sensor.  Using a regular blower bulb I just seemed to either be blowing the dust around or not moving it at all.  But the FireFly removed all the particles!  It was at this point I decided to delve deeper into just how the FireFly works and I discovered 3 main areas of it's design.

Sensor photo after using regular bulb blower, dust spec on bottom left of sensor remains.


Sensor photo after using FireFly dust spec is gone!

1. Using a 9-voltt battery the FireFly creates a corona discharge field which effectively ionizes air that passes through the upper chamber of the FireFly, this produces positive (+) and negative (-) air ions in the air.  This effectively neutralizes static charges for both the contaminants and the surface of not just the sensor but the mirror box and shutter.  This allows the dust, pollen and other particles to just fall out off the surfaces and then just be blow away down and out of the camera.

2. The blower bulb for the FireFly not only has a cap in place so no contaminants  enter while not in use but also has an inline replaceable 20-micron filter that helps cut down on any re-contaimination by air introduced by the blower.

3. The very design of the FireFly means you can safely get the blower as close as possible to the sensor without ever touching it.  This maximizes the clean ionized air flow to the surface of the sensor.  In fact the nozzle is short enough that it's not physically possible to touch the sensor with the end of the tip even with the FIreFly as far into the sensor shutter box area as possible.  The upper part of the FireFly is so wide that it will not fit into this area.  Someone put a lot of thought into this design to protect this most fragile area.

Use:

It's very simple to use and my daughter got it right the first try.

1. Take a sample control photo first of a clean white object such as piece of white paper at F/22 with the focus at infinity.   Make sure only the paper is showing in the photo.

2. Move your camera, tripod and FireFly into a controlled clean environment as free from contaminants as possible.  Wipe down the outside of the camera with a lint free cleaning cloth to make sure all dust on the outside of the camera is removed.

3.  Following the instructions remove the upper FireFly compartment cover and insert a new 9 volt battery.  If the FireFly was not stored in a dust proof container carefully remove all dust using a lint free camera lens cleaning cloth.

4. Remove the protective cap from the FireFly blower and insert the male end onto the upper housing coupler.

5. Place your camera securely on a tripod and face the camera downwards so that any particles blown loose will tend to fall away from and out the camera.

6. Following the instructions in your camera operator manual remove lens or camera lens hole cover.

7. Set your camera for manual sensor cleaning mode.  This will move the mirror into the upward position.  Make sure you have a freshly charged battery when doing this as you don't want the battery to fail and the mirror to fall back downwards onto the FireFly while it's in use.  This may damage your camera.

8. With the FireFly AWAY from the camera activate the ion system by pressing and holding the button making sure the function light illuminates and give the bulb several puffs of air.  This will clear any contaminated air out of the system before putting it into your camera.

9. Carefully place the cleaning nozzle end up into the camera body and at each quadrant of the sensor give the bulb a couple of squeezes to blow any contaminants from that area.

10. Remove the FireFly and return the camera to normal operation using the cameras instructions.

11. Remount a lens and take a test photo as in step one.

12. Zoom in on the photos and switch back and forth between the pre-cleaning photo and post cleaning photo to see the results.  The contaminants should be gone.  If not carefully follow the steps above and re-clean the sensor.  Store your FireFly in a dust proof bag or case.

If your sensor is very dirty and/or has not been cleaned in a long time then the FireFly may not get off all of the contaminants.  In that case you may have to send your camera in and have it cleaned or use a more invasive method.  Since it is so safe to use I find myself using it on a regular basis just to keep even the smaller particles off.

Conclusion: The FireFly is a non-contact safe and effective way to keep your sensor clean if used correctly and on a regular basis.

From my many conversations with other photographers there seems to be two thoughts on how to protect the front lens element.  There are those that believe filters offer the best protection, while others claim filters reduce image quality making lens hoods the best option.

In the old days if you needed a sharp photo at slow shutter speeds on dark cloudy days or at night all we had were: tripods, leaning against a wall or buying really fast film. Thanks to technology from years of advancements in military, space and consumer products we now have Image Stabilization (IS) to assist in getting impossible shots and make them insanely sharp. Normally a shutter stop speed as high or higher then the reciprocal of the lens is required to make a sharp photo. For instance a lens with a focal length of 60mm would require 1/60s to produce a sharp image. A 300mm lens would require a shutter speed of 1/300s. Some of the top IS systems today can allow a shutter speed as much as 4-5 stops slower then normal. For example let's take the 60mm lens with the 1/60s shutter speed. Divide the 60 by say 4 and we have a new minimum shutter speed requirement of only 1/15th of a second. These IS systems literally take the shake out of your photographs by steadying the shot to make it sharp.

The IS systems for SLR's fall into three main types: 1. Lens mounted 2. Sensor mounted (inside the camera) and 3. Gyro mounted (camera and lens attached to Gyro mount).

 

1. Lens mounted stabilization has a system of lens elements that float inside of a lens and are attached to vibration and movement correction devices such as coils, springs and dampers that use electromagnets to correct the image so it is stable (see opening photo example of Canon 70-200mm F/2.8 IS L lens IS. There is typically one for vertical movement and one for horizontal. These devices also have gyroscopic (small gyros) sensors attached that feed information to the stabilizer system to correct movements. The gyro is simply a spinning rotor and frame assembly within a Gimbal that let's the entire assembly rotate in any direction. Once spinning the gyro resists changes in movement and hence the system can reference the difference between the stable gyro and unstable lens and correct any movements.

 

Since the entire system is designed for each size of lens it can be optimized for that lens focal length by being as large as needed. As noted above the longer the focal length of the lens the more stabilization required so the larger the movement required to correct it. And longer focal lengths like 400mm or longer the lens can be as large as required to enclose the IS system allowing corrective movement to be very large. The disadvantage to this systems the requirement to have it in every lens used which can be very expensive.

2. Sensor mounted stabilization moves the IS from out on the lens to the sensor inside the camera. It works the same way as the lens system above but because the sensor inside the camera can only move so much it works best with shorter focal length lenses. The advantage to this type of IS is the fact it will work with all of our lenses to some extent. I say to some extent because once the lens become long enough say 400mm the required movement of the sensor to stabilize the shot is so great that the sensor housing would have to be huge to move enough to stabilize the shots. Hence the advantage of a Lens or external Gyro mounted type IS.

 

3. External gyro stabilization goes to another extreme by allowing you to have truly huge gyro and stabilization systems allowing for huge amounts of correction to movements and vibration. The camera and lens mount to this type of IS using the tripod mounts on the bottom of the camera or lens. The other advantage of this system is that it will work with any camera and lens made so long as it has a tripod mount. Systems with built in IS will provide even more of a boost by providing additional IS to the stabilization already provided by the external Gyro system. The disadvantages to this type of system are: bulk, size and cost. Disadvantages that are all mute when you need the sharpest possible shot in impossible situations such as mounting a camera system to a helicopter which produces huge amounts of vibration and movement. Systems like these are mainly used by professionals due to the costs and size.

 

Also note that some older IS systems do NOT take into account that they are mounted on a tripod or some other solid object and if left ON will produce blurry photos when the camera and lens are not moving at all. Most newer systems have software algorithms designed to detect the system is tripod mounted and will compensate for vibration in the camera such as shutter and mirror movements. These modern systems can be left ON when tripod mounted. If using and older system be sure to turn OFF the IS when using a tripod.

 

Enough talk, what do images look like with the IS both ON and OFF.

 

The first photo below with IS OFF was taken at 400mm at 1/50th of a second well below the required reciprocal shutter speed of 1/400th of a second for a sharp photo. As you can see by looking at the birds eyes the photograph is blurred both in the vertical axis and some in the horizontal axis.

 

 

The second photo with IS ON was taken at 400mm at 1/40th of a second is even lower below the required shutter speed but is very sharp thanks to the wonders of IS.

 

 

Pushing your IS to the extreme:

 

Think that you can only pull off a shot that's 4-5 times slower then the reciprocal of the focal length? Think again. By using the tips below I have been able to take sharp shots at 400mm with a shutter speed as slow as 1/20th second.

 

Put your camera in the highest continuous shutter mode so that when you press the shutter button it takes as many photos as fast as possible. Brace your and support your elbows against a solid object if possible or against your side if not. Time your breathing so that you are close to maximum inhale or exhale and press and hold the shutter button until you take at least 4 or 5 photos in rapid succession and then let the shutter button go. Try to be as smooth as possible and practice this until you are as smooth as possible. When you review your photos later chances are the second or third photo in your group will be sharp even at extremely slow shutter speeds and well below the reciprocal focal length. With this technique I have been able to squeeze sharp photos as slow as 1/20 a second out of a 400mm focal length even with an older style IS only designed to give a 2 - 3 stop advantage.

 

Conclusion, when used correctly an IS system will help you pull off that impossible shot and is an indispensable tool for the modern photographer.

Filters have been around for almost as long as cameras.  Not only are there many types of filters, there are potentially unfamiliar terms and naming to go with them.  Those that are new to filtering may find the options confusing.  Hopefully I can help alleviate some of the confusion.

 

DOF or Depth of Field is the depth of area in a photograph that is in focus (such as one fence post being in sharp focus).  DOF is often used in portraits and macro photos to emphasize the subject while displaying non-essential items as a non distracting backdrop.  By sharply focusing on your subject and blurring out everything else you center all interest on your intended viewer area.  The DOF area can be very shallow as in the opening photo of the fence post.  Images made by a camera lens such as the Canon 85mm F/1.2 II L with: wide apertures and longer focal lengths can produce a very shallow DOF which gives you maximum control over what's in and out of focus.

To see exactly how much DOF you are getting at any combination of settings there are on-line DOF field calculators such as the one at http://www.dofmaster.com/dofjs.html and pocket guides you can carry with you.  Simply enter in the required information such as: camera type, focal length of lens used, F/ Stop used and focus distance.  Up pops your in focus items or DOF: near limit, far limit and total depth of DOF.  In this way you can quickly determine if you will have the required DOF for your subject.  Get it wrong and part or almost all of your subject could be out of focus which is actually much worse then having everything in focus.  If your subject is out of the DOF area then people can not see your subject!  Also very important is putting the center of focus in the exact area you need it on your subject.  Take several photos of your subject and review them making sure important items such as eyes for portraits and critical areas of macro items are in sharp focus.

Bokeh:  What is it, what does it do, how do I get it and what lenses have the most?   Bokeh comes from the Japanese noun boke which means fuzzy or blurred when it comes to the out of focus areas in a photograph.  Images made by a camera lens such as the Canon 85mm F/1.2 II L with: wide apertures and longer focal lengths produce a shallow depth of field which produces this blurred area.  Also required is a large enough film plain or sensor to produce this effect.

Different lenses produce both different amounts of Bokeh and different qualities of Bokeh.  The number of blades in the aperture also plays a part in great Bokeh because having more of them produces a very round aperture opening which produces pleasing Bokeh by producing very round points of light in the blurred area.  Cheaper lenses typically have less blades in the aperture and this can produce out of focus lights that are more of a distracting pentagon shape instead of pure round points of light.  The more distracting the Bokeh the worse its quality is considered.  The Bokeh effect is often used in portraits and macro photos to emphasize the subject while displaying distracting backgrounds as a pleasant backdrop.

As camera manufacturers are still battling out who can stuff more megapixels into their bodies, there is a much more important battle going on in the strobist world in regards to sync speed.

What is sync speed?

Sync speed is the amount of time that your camera's shutter speed can talk to your flash — and be in sync. Overtime, this number has been getting faster and faster. Most shutters on digital SLR's that we're familiar with (these days) have a high speed sync hovering around 1/200s. For example, the Nikon D300/700/3 family have sync speeds up to 1/250s as well as the Canon 10-50D family. The 5D and 5Dmkii have sync speeds of 1/200s.

Be warned by the title, this is not a direct comparison of every feature per feature pinning the Canon 5D mkii versus the Nikon D700; that's been done countless times before. Nor is this a brand debate between Canon and Nikon. I am firmly against any argument that says "brand X is better than brand Y" solely on the basis of a name, or how well it sells. With that said, we can move on.

So, what's this about anyway?

It's about flash photography, and how each stacks up for the aspiring inner-strobist. So for those of you looking into awesomeness of well-lit photography via speed-lights, read on. We'll look at a few variables that aid in flash photography and what each camera stacks up in those categories. And just for reference during this article, I will call the Canon 5D mark ii as "5D2", and the Nikon D700 as "D700", only for simplicity sake.

Shutter sync speed

The 5D2 has a high speed sync of 1/200s. The D700; 1/250s.

Winner: D700 by a 1/3 stop.

Why is this important?

Sync speed is directly tied to your shutter, and how fast your flash can communicate with it. The faster, the better. It allows you to fine-tune ambient light and especially useful in outdoor / sunny sutuations. 1/3 of a stop of light is marginal, but just know it for reference.

Shooting in the dark

A common request among a strobist is to kill all ambient light (if at all possible), and within these situations, auto-focus can be a pain. Luckily the D700 has a built-in AF assist illuminator on the front of the camera that will help you get focus lock 4 out of 5 times over the 5D2 which has none.

Winner: D700

Why is this important?

It's not terribly important, but it's easy to miss a shot just because of a simply auto-focus error. The 5D2 just requires a little more patience in the dark. It has been said that sometimes "live view" can help fine-tune your focus when looking through your viewfinder is difficult.

Rear/second curtain sync

The good news is, both cameras have the ability to sync at the end of an exposure. This is generally a nice effect for shutter speeds longer than 1/8s since it freezes your subject at the final moment and makes blur seem less noticeable. The bad news is that with the 5D2, you can't access or change any of your flash settings unless you have a Canon EX-related flash sitting on top of your camera's hotshoe. Talk about a buzz-kill! So much for your wonderful single-off-camera-flash setup. Not even your brilliant Pocket Wizard Tranceivers mounted on your hotshoe will fix it. With the D700 you all-access to your flash settings (via Custom settings → E-1-7), however to the point, you can change your curtain settings to via the on-camera flash button by holding it down, and twisting a dial.

Winner: D700

Why is this important?

As mentioned, rear curtain allows you to reduce the amount of blur as it freezes the very end of your scene, which is most useful on longer shutter speeds. But moreover, you should generally be able to change all your flash settings without having a flash attached. That's just silly.

Remote flash capability

This setup requires more than just your camera, as of course you need a speed-light. So with the 5D2 we can assume you have a brand new shiny 580EXii, and with the D700, let's pair it with a SB-900. On-camera, both of these pairs work as intended. But of course, not the way a strobist intends to use them (off-camera). Straight out of the box with the Canon setup, you're out of luck. But with the Nikon pair you can use the built-in pop-up flash, set it to commander mode (Custom Settings → E-3), drop the power setting to "--", and you're golden! You can even control the power settings on your remote flash from your camera!

Winner: D700

Why is this important?

For the ameatur, this is great. Granted, professionals won't use this setup, but it's always there in a pinch for D700 users. With the Canon setup, you'd have to get an extra speedlight transmitter, but at that price, why not binge for another speedlight?

ISO

Now we're not talking about crazy high ISO, we're talkin' "how low can you go" ISO. If we look at the unbiased comparison from DXOMark, we can see that the 5D2 has a low ISO of 73 (set on both ISO 100 & 50), whereas the D700 struggles to get down to 162 (set on both ISO 200 & 100). And yes, if you were wondering, both camera manufacturers are lying to you. But for the record, the 5D2 is clearly able to dip lower.

Winner: 5D2

Why is this important?

Similar to having a high sync speed, reducing your ISO can also help reduce ambient light albeit it also taking away flash light too. ISO is essentially "light sensitivity," which is "all light." However we can conclude that this "sort of" makes up for the 5D2's 1/3 stop loss in shutter sync speed.

In Conclusion

It would be too easy to say that the Nikon D700 is the clear winner in regards to the needs of a strobist. Most of the things are small issues, but Nikon has essentially won in four out of the five categories. Nevertheless a counter argument can be made for each. For example with high speed shutter sync, you can pop on a new Mini TT1 and even the playing field at 1/500s shutter syncs. Or if you're shooting in the dark, carry around a tiny $7 Maglite from Radioshack. And as for rear/second curtain sync? Hehe. Buy another flash. And as for doing remote flash, get some Pocket Wizards or a pair of Cactus 2's for $32. Not to mention a simple PC cable will do the job for a few meezly dollars.

So to the basic point, money adds up for 5D2 shooters. And if you're a professional studio shooter, then the money will add up no matter what camera system you're using. This comparison is an "out of the box" experience with no extra accessories. In the end, you make the decision.

Admittedly, one of the most boring subjects in photography is the inverse square law. But before you begin scratching your head and yelling out "the inverse-what?" Just hold on a sec. First off, one does not need to memorize the laws of light to go and start taking pictures, or even to become a professional. But anyone with an SLR that truly wants to master all the variables in an exposure, you should at least know about it, and have a good sense of how it works. And of course the main purpose to know about the inverse square law (if you haven't figured it out already) is when making the dive into flash photography.

Therefore, if you're one of those who claim "I don't shoot with flash, I only use available light," then have fun on your little pedestal making up excuses on why you pretend not to be interested in flash photography. Sure your brand new Canon 5Dmkii or Nikon D3 with a 50mm f/1.4 lens is going to destroy the darkness with stunning images at ISO 3200, but we are talking about professional studio portraiture / magazine quality photography that utilizes flash to create amazingly sharp, colorful, and beautifully lit photos. Not to mention, your flashes are "always available" — use them to your advantage.

Alright, now that we have that out of the way, let's look at some basics that we might already know.

Shutter Speed, Aperture, and ISO sensitivity

Shutter Speed

By now everyone knows that the longer the shutter speed, the more light you let in. Inversely, the faster the shutter, you let in less light. And of course, stops of light (in terms of shutter speed) work in factors of two. That means if you double a shutter speed of 1/100, it becomes 1/50. That is 'one stop of light' brighter. Inversely, if you cut it in half from 1/100 → 1/200, you have made your exposure 'one stop darker'.

Great, let's move on.

Aperture

We should all have our f stops memorized. If not, it's a simple scale where we can double our numbers, starting at f/1, and each time we double, we will increase by two stops of light. Therefore:

1 → 2 → 4 → 8 → 16 → 32

To calculate our increments in-between, we simply multiply each stop by our magic number 1.4 (which is, coincidentally, the rounded number of the square root of 2 (which is ~ 1.414)). This makes our first stop easy. 1 x 1.4 = 1.4! We can now fill in our gaps accordingly!

1.0 → 1.4 → 2.0 → 2.8 → 4 → 5.6 → 8 → 11 → 16 → 22 → 32

We now have an aperture scale displaying full one stop increments. Just for kicks, let's have a brief look at what a classic third aperture scale looks like (since many of you will be working with these numbers on your SLR):

... → 1.4 → 1.6 → 1.8 → 2.0 → 2.2 → 2.5 → 2.8 → ...

Ok, I get the aperture numbers, remind me of the exposure relationship?

Let's say we had a correct exposure of 1/125 at f/8. Your model sitting their patiently is waiting for their beautiful portrait to be taken. You (the photographer) have made a design decision to go for a more shallow depth of field. So you drop your aperture to f/4. That's two stops of light brighter. f/8 → f/5.6 → f/4. So to compensate, you speed up your shutter by two stops. 1/125 → 1/250 → 1/500. Easy peezy, makes sense, been there done that.

ISO Speed

We all know that our lowest ISO produces our cleanest files. Our light stops will look fairly similar and our scale usually looks something like this:

100 → 200 → 400 → 800 → 1600

Some of the latest cameras like the Nikon D700/D3, or the Canon 5Dmkii go up to crazy ISO's like 25,600, which is four stops brighter than ISO 1600!

Jumping quickly back to our example, if we were originally at ISO 400 and needed to drop two stops of light, we could have simply went from ISO 400 → 200 → 100

Ok, now what about this flash thing?

Flashes have stops of light too! These increments are measured in half increments, just like shutter speed. This is what we call the "flash power," or rather, how much light it spits out. Our scale looks like this:

1/1 → 1/2 → 1/4 → 1/8 → 1/16

From left to right, we say "full power," "half power," "quarter power," etc. Some flashes can let out light as dim as 1/128 like the SB-800/900 or the Canon 580EXii.

One of the main things you should consider when buying a flash, is to look at how powerful they are. Meaning, "how bright will this be at full power compared to this other flash at full power?" We can tell how powerful they are by looking at the flash Guide Number. This number is extremely important to know when buying a flash. Almost nearly as important as knowing how many millimeters your lens is. For example, you wouldn't go out and buy a 200mm lens without knowing it's, well, 200mm! Right?

What is a Guide Number?

In simple mathematical terms, it is aperture multiplied by distance, in which your flash can properly expose a subject at a given distance (aperture and distance). The standard Guide Numbers you should be looking for are measured at ISO 100 (film speed), at the 35mm head position, at full 1/1 power.

Let's take a basic Nikon flash, for example the SB-600 has a G.N. of 98. To make our math easier, let's just round it off to 100. What this means is the following. Take a look at our ascii diagram. We have a flash (F), a subject (S). They are twenty feet apart in distance.


/
(F)——————————20'——————————(S)
\

To properly expose our subject, we would need an aperture of f/5.

f/5 × 20' = 100 G.N.

Now if we moved our subject 20' further...

/
(F)——————————————————————40'——————————————————————(S)
\

To compensate or this distance, we would need to let in more light! This means we'll have to open up our aperture to f/2.5

f/2.5 × 40' = 100 G.N.

If we kept our aperture at f/5, we would underexpose our subject. Likewise, if we opened up to much, for example, to f/1.4, we would overexpose our subject. Got it?

Ok, I'm ready for that inverse square thingy

Alright, if you insist: The inverse square law states that "the intensity of light radiating from a point source is inversely proportional to the square of the distance from the source."

Therefore an object twice as far away, receives only 1/4 the amount of light. Or if it's twice as close, then it's 4x as bright. Each of these is a difference of two stops.

This is confusing, I know. But luckily, and coincidentally, we already know a little about this already with our aperture scale. Just know that light has "depth" in the same way that our focal plane does. By now you know that the closer you get to an object, the shallower your depth of field looks, of which we can conclude that all objects behind your subject "quickly fall out of focus." This same exact rule applies to light (Thank heavens!). Let's look at a simple example, and we'll use the numbers on our aperture scale to make it easy.

In this illustration, we added a background (B):

/
(F)——————————4'——————————(S)——————————4'——————————(B)
\

Let's assume these settings:

ISO 100
f/4
flash is at 1/4 power

It is safe to assume that our background is two stops underexposed. (4' → 5.6' → 8')

Now what if we moved our subject two feet closer toward the flash?

/
(F)————2'————(S)————————————————6'—————————————————(B)
\

Since we just moved our subject "twice as close" we made it two stops brighter! Therefore to make up for this overexposure, we need to do one of two things:

A) Dial down the flash two stops
1/4 → 1/8 → 1/16
or
B) Close down the aperture
f/4 → f/5.6 → f/8

Each will keep your subject properly exposed, but it's an artistic decision for you to make if you want to keep your shallow depth of field (option A), or remove more ambient light and get a slightly sharper image (option B). Also pay close attention that due to the inverse square law, our background is now "four stops" underexposed!

8' → 5.6' → 4' → 2.8' → 2'

Cool! What flash do I buy? The one with the higher Guide Number, right?

Well, kind of, yes. There are some cases where newer, more expensive flashes will have lower Guide Numbers than say for instance, the 30 year old Vivitar 285HV for $89 (via B&H) that has a G.N. of 120 whereas we'll see Canon selling their flagship 580EXii for over $400 which has the same G.N. Or even the new Nikon SB-900 has a G.N. of 111. What's up with that? Well, clearly if you buy a new Nikon or Canon speedlight for your system, it will talk to your camera with all the latest and greatest i/e-TTL technology, the stops of light will range from full 1/1 power to 1/128, whereas the Vivitar only has stops of 1/1 → 1/2 → 1/4 → 1/16 (yes, it skips 1/8). The SB-900 will have a 17-200mm range of light to cover larger or tighter spaces. It has gel detection, three different patterns to throw at your subjects, it even has super quick recycling time.

But hey, don't knock on the Vivitar; off camera, that's a helluva bargain for that much power! Hook it up to your camera with a PC chord, or a pair of Pocket Wizards, and you're golden.

With all that's been said, go out, get a flash (if you don't already have one), throw it on manual, and experiment by taking a ton of pictures. Questions are warmly welcomed. You can find my contact information on my Colophon at http://photography.dustindiaz.com/colophon/

*If you own or have used this camera, let us know what you think! Add your own review or comments to this article. We welcome your views.*

This is the second in a series of camera comparisons. The last article compared the Nikon D90 and Canon 50D. Now we look at the 50D and 40D.

The fact that we're even comparing the 40D and 50D says something. It should be a simple, open and shut  case. One is newer, better and more expensive; one is older, a bit less attractive but also less expensive. You get the newer one if you can afford it, or if not, get the older one which is still a good value and good camera. That's how it usually works.

Surprisingly, it's not that easy. The fact is that reviews seem to be ambivalent about the image quality advantages of the 50D. Despite the fact that the 50D has 5 extra megapixels (15 versus 10), the 40D manages to keep up remarkably well, and in some respects (such as noise) even out performs it's newer and more expensive sibling.

*If you've used either of these cameras, let us know what you think! Add your own reviews or comment below. We value your views.*

*For individual reviews of these cameras, see SLR Geek's product review pages for the Nikon D90 and Canon 50D.

I've had several people raise this or similar questions in the last week, so I thought I'd write a post. I'll be doing a series of comparison posts over the next two weeks for various Canon and Nikon digital cameras.

Much of the early online discussion of these two cameras (Canon 50D and Nikon D90) was of the opinion that they weren't really comparable, mostly because of the different price points. The 50D was a more expensive camera targeting the "pro-sumer" / light professional market, while the D90 was a less expensive, upper end consumer camera. By now (January 2009), however, the price gap has narrowed somewhat.