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Canon 70D & 18-55 STM kit lens


I am the proud owner of a new 70D !  The problem I have encountered is with the 18-55 STM lens attached the night flash pics are very dark , testing outside starting at 25 ft . Moving up to 4-5 ft there is enough light & the focus is GOOD .

Different Canon flashs made no difference 420EX & 270EX 11 . 

I also have a Canon T4i , put the same lens on the T4i with the same camera settings & the lighting is very nice from 30 ft all th way up to 4 ft with nice lighting & focus .  Please help  ?

I frogot to add that all my other Canon lens work very well with lots of light in flash pics , just the 18-55 STM is the problem & then only on the 70D  !!



What ISO are you using on each camera and what mode are you using when shooting?


The 70D built-in flash has a "Guide Number" of 12 meters.  


A bit about what a "Guide Number" is... 


This is a measure of the distance that a flash can adequately light a photo based on a BASELINE (very important to keep that in mind) of f/1.0 and ISO 100.


This means that if you use a 70D with the flash and it fires at full power while the camera is set to ISO 100 and you've got an f/1.0 lens which is set to use f/1.0, THEN... it can adequately light up a subject 12 meters away (about 39 feet.)


Ok... right away you might be wondering where you get this "f/1.0" lens.  The answer is... you don't.  Nobody makes one.  Canon used to make a 50mm f/1.0 lens but they haven't made that in years.  But it doesn't matter.   


The REASON they use "f/1.0" (and this isn't Canon's standard.. it's an industry standard) is because it makes the math very easy.  All you do is divide the guide number by the f-stop you actually plan to use when you shoot.


Suppose you are using the 18-55mm f/3.5-5.6 lens at a 50mm focal length.  The best focal ratio you are allowed to use is f/5.6.  So you DIVIDE the 12 meter guide number by 5.6.  This gives you your effective distance.


12 ÷ 5.6 = 2.14 meters.  If you convert that to feet, it's 7 feet.  


Of course this still assumes ISO 100 and if it's dark, you probably want to bump up the ISO.  Each time you bump up the ISO by 2 full stops you get to double that distance.  If you go up just 1 full stop you get to increase the distance by a factor of 1.4x (it's actually based on the square root of 2, but that's very close to 1.4).  


This means if you use ISO 400 (instead of ISO 100) then you can adequately illuminate a subject 14 feet away.  If you go up 2 more full stops to ISO 1600 then you can get to 28 feet away.  


I'm wondering if your Rebel T4i was either at higher ISO or if it was set to auto-ISO.  This would explain the difference.


BTW... you might be thinking 12 meters isn't a very high guide number... and you'd be right.  Most pop-up flashes are not particularly power.  This is why there are a lot of external flash options available.


Canon Speedlite models numbers actually tell you the guide number right in the model number of the flash.  If you eliminate the trailing 0 in the speedlight model, the numbers that remain actually ARE the guide number of the Canon flash.  WARNING:  This only works for Canon.  No other company that I know of does this and some make it difficult to figure out what their guide number is.


So... a 270EX II has a guide number of 27 meters (you can see already that this is a bit more than twice as powerful as the pop-up flash).  A 430EX II has a guide number of 43.  A 600EX-RT has a guide number of 60.


Remember... divide the guide number by the f-stop you actually plan to use when you shoot and that's the distance that the flash can handle... AND... guide numbers can be in either meters OR feet... Canon uses meters.


Lastly... do make sure you haven't somehow dialed down the "flash exposure compensation".  That's a setting that allows you to deliberately ask the flash to decrease it's power (or increase it's power assuming you aren't already at the max.)  This would imply that the flash can actually do much more, but you've asked it to under-illuminate the shot.



Tim Campbell
5D III, 5D IV, 60Da

First thanks for the very informative responce !!  In this test both cameras were set in AV mode auto ISO  F8  the flash comp. was set to + 1/3 on both cams . I don't understand the vast difference in lighting in the two cams ? 

That being said I put on a 40MM f2.8 lens on the 70D without changing any cam settings & the lighting was great at 30 ft. with the same 420EX flash . Also very nice as close as 4 ft.

My thinkng is in order to use the 18-55 STM lens I will need to increase the flash comp. from + 1/3 up to 1 1/2 or 2  which is very hot at 6 ft.  Thanks  BK