Flash & Exposure

[allendehl]

Hi friends,

Another of my newbies questions,

I'm starting in this hobby and I'm using manual mode. (I think when you have to take care of everything is when you really learn and understand whats going on)

In manual mode, the flash wont go off by itself, so I need to know when to use it and when not to....and thats the question:

1)
If I'm shutting a closeup of a person at night, probably in my own medium-lighted home..should I use the flash?...How will this affect the exposure?. If I can't have settings for a correct exposure(I mean meter in position 0) due to the light/lens relationship and I'm forced to shut underexposure...is the flash the solution?

2)
For close-ups to objects, for instance a coin or an apple in top of a table, and looking for a short DOF to have a blur background...should I use a flash if I don't have enough light?..or am I forced to look for a better moment or lighting conditions to shut?

3)
For portrait photos at night, forced to underexposure due to lighting conditions, lens, etc..should I use flash?...will this improve the exposure?

Thanks a LOT

 

[rdzmzda]

well obviously in poor lighting conditions flash will illuminate the area....you will need to watch out for the fact that especially on closeups you will start to overexpose them....what kind of camera are you using p&s or slr? read up on bounce flash if thats even an option for you.

 

[rdzmzda]

oh yeah haha before its even said ill just say it now...get understanding exposure....truly a great book

 

[allendehl]

Hi rdzmzda(oh boy!..I had to copy & paste it LOL)

I'm using my new Canon Rebel XS/18-55mm with 18-55mm 1:2.5-5.6 and 75-300mm f/4-5.6 lenses and no external flashes.
About the book...sure I will. I'm just waiting for having time to go to BN in the weekend.

 

[Village Idiot]

You can press the flash button on the front of the camera and it will pop up. AFAIK, the flash exposure is automatically set no matter what mode you're in when using the popup flash.

Pop up flashes suck btw. If you really want your flash to look good, you should look at a speed light of some sort.

 

[Dao]

You may want to learn more about exposure. Whether you have enough light is based on the amount of exposure to your photos (film or sensor).

A darker environment can have under, correct or over exposure photos. Same thing apply to a bright environment.

How to obtain an correct exposure? It is mainly based on the combination of shutter speed, aperture as well as the ISO sensitivity(Settings on digital or Film speed).

Take a look at this page

Exposure value - Wikipedia, the free encyclopedia

From the above link, you will find "table 1", the EV table and "table 2", some typical exposure value.

For example, you want to take a picture of an office area, from table 2 you will find a EV number of 7-8.

What that number means is when you look up on table 2, look at row EV8, you will find a lot of shutter speed/aperture combinations from each column on that row and any of those combinations will give you the same exposure. Please note that the settings are based on ISO100.


So now, you are taking a picture with your camera using the focal length of 18mm. So set your camera aperture to F/4 and shutter speed to 1/15 with ISO100 and take a shot. (according to the table) If you see camera shake type of blur in your photos, increase your shutter speed from 1/15 to 1/30. In general, to avoid camera shake, you need at least 1 / (focal length) of the shutter speed. So 1/30 should be fast enough to avoid camera shake blur.

However, according to table 2, in order to have 1/30, you aperture will need to set to F/2.8. But your lens can only go all the way to F/3.5. (Limitation of the lens). In order to have the exposure taken at EV8, you have no choice but change the ISO from 100 to 200.

In other words, exposure wise ...


Aperture F/4, Shutter Speed 1/15, ISO100 = Aperture F/4, Shutter Speed 1/30, ISO200


And if you really do not want to bump up the ISO value, you can use a flash to add additional light. If your flash is able to give you 3 stops of light then ...


Aperture F/4, Shutter Speed 1/15, ISO100 = Aperture F/4, Shutter Speed 1/125, ISO100, 3 stops of light from the flash

You may noticed the shutter speed move 3 slots to the left on the table.


Same thing apply if you want to use a smaller aperture ... for example

Aperture F/4, Shutter Speed 1/15, ISO100 = Aperture F/5.6, Shutter Speed 1/15, ISO200.


So, as far as flash or no flash, it really depends on the situation and your equipments. In some case, due to the limitation of the equipment, you may need to use a flash to make the correct exposure. And most of the time, I would rather not to use it if my equipments are able to take a good photo without it. Of course, once you step into the Strobist area ... things will change.

In the example above, if you have a fast lens, such as 17-50mm F/2.8 from Tamron, you can take the same shots with correct exposure based on the following settings.

Aperture F/2.8, Shutter Speed 1/30, ISO100.


Hope this help. Of course, if anyone think the above information are not correct, please feel free to say so because I am still learning.

 

[allendehl]

Hi Dao, thanks a lot for this great posting.

"And if you really do not want to bump up the ISO value, you can use a flash to add additional light. If your flash is able to give you 3 stops of light then ..."

How do I know how many stops does my flash incorporates?...I guess that applies only for those shuts in the flash range.

My questions are intend to those situations where my lenses are not able to expose correctly. Last night I was trying to make some shuts outside in the garden...and it was a nightmare to get that exposure meter to stick in the middle at a decent speed(the slower possible without blurring), even raising the ISO. I was just trying normal portrait shuts....