What is GN ?????

So what the hell is that GN thing ...???  .... Any wild guess ???

Ok lets start from the scratch .... 

GN term is basically belongs to the world of FLASH PHOTOGRAPHY. A world which is not liked by many (lets be honest here) .... Don't like not because of the hardware it self or price .... people hate it because they not able to control the flash at the beginning and thats where the turning point it. At the beginning when you see trouble .... exactly at that point either you make it or break it :) .

So i was telling the GN thing here .... Let me get in my comfort zone by copy pasting the definition from the WIKIPEDIA :) 

"The guide number (GN) for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film or sensor sensitivity and angle of view. A higher guide number indicates a more powerful flash."

The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose film or a digital sensor with the specified sensitivity."

GN = distance × f-number

Even it took me some time to get what Wikipedia is saying here .... Lets get practical now gentlemen.

So what is Guide Number?

In simple mathematical terms, it is the 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 35mm flash head position, at full 1/1 power. 

Lets take a basic Nikon Flash (yes i am proud Nikonian) for an example. The SB700 has a GN of 92. To make our math easier, we'll around it off to 100. Lets take a look at our diagram to see what this means. We have a flash and a subject.They are 20 feet away in distance.

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

Because .... f/5 x 20' = 100GN

Now what happens if we move our subject 20' further away ...... !!!!

To compensate for this additional distance, we would need to let in more light! This means we will have to open up our camera aperture to f/2.5 

f/2.5 x 40' = 100GN

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

Note that all of the above calculation are done based on the assumption of ISO100 (already highlighted in blue above). So what if we have ISO more then 100 ????

Converting Guide Number for ISO

The Guide Number charts are typically always printed showing ISO 100 values, but we can just multiply ISO 100 GN values by 1.41 to get ISO 200 GN values, or multiply by 2 to get ISO 400 GN values, etc. Or we can divide if converting going the other way.

ISO	25	50	100	200	400	800	1600	3200	6400
Factor	x0.5	x0.71	x1	x1.41	x2	x2.83	x4	x5.6	x8

Note that all values in the "Factor" line is the hundredth of the Square root of the ISO.

Doubling Guide Number doubles distance range, or doubles f/stop number, which is two more stops.

Doubling flash zoom number should theoretically multiply Guide Number by 1.414, but only as an approximation, because of course, it depends on the individual reflector design.


Lets have some example here ..... Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400.

So we know we need flash power of f8 x 12 feet = GN 96 (feet) at ISO 400. The charts are always for ISO 100 that comes with the flash manual for any flash. Below is the GN chart of SB800 as an example here  where distance of the flash with subject is given in meters/feet.

We know the ISO 400 factor is x2 (from above converting guide factor). So now we need to scale down ISO 400 to ISO 100 to check the flash power from the above chart and that we can do by dividing ISO400 by 2.

Converting this to ISO 100 is GN 96/2 = GN 48 (feet, ISO 100). We search the guide number chart in the speedlight manual (ISO 100), and maybe we find this value at 24mm zoom and 1/4 flash power to be say GN 49 feet. More than close enough to 48. This SB-800 GN chart below (for an example) says that 24mm flash head zoom and 1/4 flash power is GN 49, almost exactly the GN 48 that we need.

The charts show GN as both meter and feet values (as meters/feet), so use either one, so long as you are consistent with units. Duplicate this situation by setting flash to 24mm zoom, 1/4 power, and set the camera at ISO 400 and f/8, and you're very close on first try for a 12 foot flash distance (direct flash). Or, the chart includes several other combinations of power level and zoom which are near GN 49. It need not be exact, GN varies with f/stop numbers so that another GN value of about 12% difference is roughly within 1/3 stop (and 41% is one stop). We don't necessarily have to match flash zoom to lens zoom values, we can use any wider flash zoom - which may waste a little light if the flash is wider than the lens, which is no problem if not at maximum power (some flashes do not zoom anyway). FX flash on DX cameras is already 1.5x wider anyway. To adjust results of this method, you can simply adjust the power level by 1/3 stop, or adjust the aperture by 1/3 stop. Or zooming the flash head makes steps sometimes roughly about 1/3 stop (no bets, there is no actual relationship). 

I hope we all use this GN thing at some point in our flash photography .... practically speaking.

Happy learning Guys :)


Babar-Photography
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How to get blurry background ....

This is one of the first thing that we all wants when we hold our first DSLR. Some time we also think before we know any thing about photography usually is, blurry background only possible from expensive cameras. 

Well if you know a little about photography .... above mentioned assumptions are totally wrong. Blurry BG is depend on the following 3 things,

1- Aperture value

2- Distance from the subject/object

3- Focal Length

Let me explain them one by one in detail,


1- Aperture: 

This is the first thing which make any BG blur. The unit of the aperture is define in "f/?" means if you have 50mm f/1.4 lens (where f stands for focal length of the lens & 1.4 is the ratio by which aperture will open) So question is .... what "?" is? 

"?" is the ratio which divide the "f" (focal length) and gives us the opening of the aperture as an answer. Higher that number means, less aperture will be open & less BG will be blur (more Depth of Field). Smaller that number means, more aperture will open more BG will be blur (less/shallow Depth of Field). Simple one-on-one math ... any number divided by smaller number give higher value and vice versa. 

Another question is ... what is Depth of Field. Answer is simple .... it is focal plane/area in which anything will be focus by the camera. Rest will be blur and it will be measure in distance. If you check below image, i adjust the aperture in such a way that I focus the Lamborghini only. Anything before and behind the car is totally out of focus. So the total length of the car is the total focus area. I can very that length by changing the aperture from f/1.2 ~ f/22 in general. f/1.2 means more light will get inside the lens. f/22 means less light will get inside the lens. Check below some more example of the aperture value.

Following are the aperture types depending on the lens you got,

a- Variable aperture lens: These lenses are general "ZOOM Lenses" and their aperture varies from f/3.5~f/5.6. For example you have kit lens which varies the zoom between 18-55mm and its aperture range is between f/3.5~5.6. So what happens is when you set the zoom @ 18mm, by default you lens aperture is f/3.5. Ones you zoom out to 55mm, by default your zoom is f/5.6. You cannot have aperture f/3.5 @ 55mm to get more light thats why these lenses are cheap. 

b- Fixed aperture lens: These lens are also "Zoom Lenses" but their aperture remains fix throughout the zoom length. For example there is a famous 3 Nikon trinity lenses as follows,

14-24mm f/2.8
24-70mm f/2.8
70-200mm f/2.8

All above 3 lenses are best of the best but very expensive because there aperture all wide open @ f/2.8 all the time regardless the focal length. Making such lenses is difficult thats why they are very expensive. They range around 1800~2200 USD per piece. Below are the example of some blur BG using all above 3 lenses,

14-24mm f/2.8 lens

24-70mm f/2.8 

70-200mm f/2.8

There are also cheap lenses which are from 24-120mm f/4 lens means they can provide f/4 constant aperture at all the zoom range. Reasonable bargain if you cannot afford the famous Nikon Trinity. 


c- Prime Lens: These lenses are having "Fixed" focal length (means no zoom). But the biggest advantage is that you can have the widest aperture ever possible in them. For example, 

14mm f/2.8
24mm f/1.4
35mm f/1.4
50mm f/1.2
50mm f/1.4
85mm f/1.2
85mm f/1.4 (this focal length is the addition of 35 & 50mm lens)
105mm f/2.8 (macro lens)
200mm f/2
300mm f/2.8

So as you can see, all above examples are having the widest aperture available and they are capable to provide the most blur BG every possible way. Check below examples and see why Prime lens are more used to blur BG.


35mm f/1.4 Samyang/Rokinon lens

50mm f/1.2 Nikon AIS lens

85mm f/1.8 G Nikon lens

So as you can see, prime lenses are the best source to blur BG. The lower the f/? .... the more the BG is going to blur. As there is no zoom feature in them thats why they can produce the maximum optical quality. 



2- Distance from the subject/object

It is simple math .... the far you are from the subject, the less BG will going to be blur. The more close you are from the subject/model the more BG is going to blur. 

So there is a chances that you will be more BG blur with kit lens of 18-55mm f/3.5~5.6 zoomed all the way to 55mm (f/5.6) close to the subject/model in compared to 50mm f/1.8 prime lens which is 5 meters away. 



3- Focal Length

The more wide angle lens you have, the less BG is going to blur. Check above image i shot with 14-24mm f/2.8 lens shot at 14mm f/2.8.

The more zoom lens (telephoto) you have the more BG is going to blur. Check above image i shot with 70-200mm f/2.8 lens shot at 200mm f/2.8.



About Focus Area:

Focus area in Wide angle lens: In wide angle lens, if you are setting aperture between f/5 ~ f/13 .... your focus plane distance will be in tens of meters .... towards the camera

Focus area in medium angle lens: Lets say 50mm, if you are setting aperture between f/5~f/13 ... your focus plane distance will be in feets .... towards the camera.

Focus are in Telephoto lens: Lets say 85~500mm (or above) ..... if you are setting aperture between f/5~f/13 ... your focus plane distance will be in centimetres (may be in millimetres) .... towards the camera.

Note: The distance range i mentioned above for 3 categories for "Focus Area" can be vary using Point 1, 2 & 3 but the value will be some where near to the range i mentioned. You can use the combination of Point 1, 2 & 3 to get your desire BG blur ....

Happy Learning guys .... 

Babar-Photography

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How to get black background in flash Photography ???

Today my long end was started which span over 3 days so i woke up late. The next thing i was doing after wake was checking my mobile screen to see any notification of Facebook (lets be honest ... who doesn't do this :) ) .... after cruising quickly, i found one image from one of my friend from Facebook where he used 2 flash light and struggled to get Black Background (BG) .... So i decided to help him which i did by suggesting quick tips which you will read shortly .... 

Question what i am asking you all ..... why you need black BG? what so special about it ? why can't we deal with white BG? Why we want Black only

Answer is simple .... black (or under expose image) always bring Drama and feeling into the image condition provided if subject/model is properly expose using the flash or studio strobe. We technically call them "LOW-KEY IMAGE". Of course you can bring the charm of fantasy by using White BG (technically call HI-KEY IMAGE) but you really need to play with light & "Colours" to stand out your image in HI-KEY photography unlike LOW-KEY where single ray of light can be difference maker if properly placed :) .... So lets dive in how to get LOW-KEY image or image with Black BG.

About getting black BG .... Its very simple but most photographer failed to get it initially

There are the following ways to get the black BG,

1- Use black sheet as a BG  .... To do so, use any thing  who's surface is black and less shinny ... Black card board. .. Black velvet piece of cloth which "absorb" the light .... Not reflect it. And this goes to any black material which absorb the light. So try to use Black material to get Black BG.

2- Try to use back ground with more distance from the subject/model. Why is that??? .... Answer is simple .... The more the distance you have behind the subject ... The quicker light is going to fall off/fade behind the subject and you will see less amount of background expose. The formula is simple. Double the distance of the subject with the BG ... Four times light is going to be fade/fall off. Thats call inverse square law.

Check below table for further detail illustration

3- Use high speed sync flash which works at 1/8000 of a shutter speed. This will kill any ambient light coming from the backside of the subject trying to get enter into the lens. When u go higher shutter speed, it will kill ambient light but you may need to increase your flash power a bit. Or open up your aperture (say f/2.8 or f/1.4) or increase ISO a bit (say upto ISO 400) ... You also may requires external battery pack for flash to get more juice out of your flash for extra push of light at higher sync speed. If some of you read my last blog about flash sync speed ... i did mentioned about flash power decrease at the higher shutter speed. The reason is that, at higher shutter speed, you shutter opens for very little time so the only light sensor able to get is the one which is near. In this our case it is flash light which reflect from the subject and then goes into the camera .... so when you have less time to absorb the light, you may need to push more light (means more flash power) to properly lit the subject.

Remember .... Aperture controls the Flash exposure ..... Shutter Speed controls the ambient exposure .... :)

4- Use light modifiers to restrict the light in a specified direction and don't get spill. Use softbox instead of umbrella. Umbrella spills the light all over .... Softbox always confine the light in a direction where you put in.

Check the following link and see how far the light spills ...

Light Spilling Image

Also I would like you all to see below link as well which is a practical implementation of the Point 2, 3 & 4 above.

Hi Speed Sync Image


For any question, you can email me at "Babar.photography@gmail.com".

Thanks,

Babar-Photography
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